0001453687-25-000120 | 10-Q | XBRL Viewer | Cartesian Therapeutics INC

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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Liabilities:
Warrant liability	$ 848	$ 3,836
Recurring
Assets:
Total assets	40,357	39,088
Liabilities:
Warrant liability	848	3,836
Contingent value right liability	369,000	395,500
Total liabilities	369,848	399,336
Recurring \| Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	40,357	39,088
Recurring \| Level 1
Assets:
Total assets	40,357	39,088
Liabilities:
Warrant liability	0	0
Contingent value right liability	0	0
Total liabilities	0	0
Recurring \| Level 1 \| Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	40,357	39,088
Recurring \| Level 2
Assets:
Total assets	0	0
Liabilities:
Warrant liability	0	0
Contingent value right liability	0	0
Total liabilities	0	0
Recurring \| Level 2 \| Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	0	0
Recurring \| Level 3
Assets:
Total assets	0	0
Liabilities:
Warrant liability	848	3,836
Contingent value right liability	369,000	395,500
Total liabilities	369,848	399,336
Recurring \| Level 3 \| Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	$ 0	$ 0

Accrued Expenses

9 Months Ended

Sep. 30, 2025

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):


	September 30, 2025		December 31, 2024
Payroll and employee related expenses	$	3,060	$	3,534

Accrued patent fees	126		813
Accrued research and development costs	2,078		2,987
Accrued professional and consulting services	1,717		3,674

Property and equipment	107		782
Other	614		286
Accrued expenses	$	7,702	$	12,076

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Current assets:
Cash and cash equivalents	$ 143,384	$ 212,610
Accounts receivable	722	872
Prepaid expenses and other current assets	3,154	3,144
Total current assets	147,260	216,626
Non-current assets:
Property and equipment, net	12,394	9,912
Right-of-use assets, net	4,972	5,535
In-process research and development assets	150,600	150,600
Goodwill	48,163	48,163
Long-term restricted cash	1,735	1,669
Investments	2,000	2,000
Long-term prepaid expenses and other assets	5,551	518
Total assets	372,675	435,023
Current liabilities:
Accounts payable	2,225	288
Accrued expenses and other current liabilities	7,702	12,076
Lease liabilities	3,876	2,851
Contingent value right liability	0	7,761
Total current liabilities	13,803	22,976
Non-current liabilities:
Lease liabilities, net of current portion	8,727	11,133
Warrant liability	848	3,836
Contingent value right liability, net of current portion	369,000	387,739
Deferred tax liabilities, net	16,141	16,141
Total liabilities	408,519	441,825
Commitments and contingencies (Note 17)
Stockholders’ deficit:
Preferred stock, value, issued	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized as of September 30, 2025 and December 31, 2024; 26,003,606 and 25,767,369 shares issued and outstanding as of September 30, 2025 and December 31, 2024, respectively	3	3
Additional paid-in capital	698,537	689,887
Accumulated deficit	(729,797)	(692,071)
Accumulated other comprehensive loss	(4,587)	(4,621)
Total stockholders’ deficit	(35,844)	(6,802)
Total liabilities and stockholders’ deficit	372,675	435,023
Series A Preferred Stock
Stockholders’ deficit:
Preferred stock, value, issued	0	0
Series B Preferred Stock
Stockholders’ deficit:
Preferred stock, value, issued	$ 0	$ 0

Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Details) - 3SBio License $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15.0

Collaboration and License Agreements - Genovis (Details) $ in Millions	1 Months Ended
	Feb. 28, 2023 USD ($)
Genovis AB
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 4.0

Fair Value Measurements - Schedule of Fair Value Measurement Inputs and Valuation Techniques for CVR Liability (Details) - Level 3	Sep. 30, 2025	Dec. 31, 2024
Estimated probability of success \| Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.950	0.950
Estimated probability of success \| Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	1.000	1.000
Expected volatility of future revenues
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.225	0.220

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2025	Dec. 31, 2024
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	9,427,168.437	9,427,168.437
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	26,003,606	25,767,369
Common stock, shares outstanding (in shares)	26,003,606	25,767,369
Series A Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	134,904.563	134,904.563
Preferred stock, shares issued (in shares)	120,790.402	120,790.402
Preferred stock, shares outstanding (in shares)	120,790.402	120,790.402
Series B Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	437,927	437,927
Preferred stock, shares issued (in shares)	437,927	437,927
Preferred stock, shares outstanding (in shares)	437,927	437,927

Equity - 2024 Private Placement (Narrative) (Details) - July 2024 Purchase Agreement $ / shares in Units, $ in Millions	Jul. 02, 2024 USD ($) $ / shares shares
Class of Stock [Line Items]
Sale of stock, price per share (in dollars per share) \| $ / shares	$ 20.00
Sale of stock, gross consideration received on transaction \| $	$ 130.0
Common stock
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	3,563,247
Series B Preferred Stock
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	2,937,903
Series B Preferred Stock \| Directors and Executive Officers
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	2,359,500

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

Segment Information (Details) $ in Thousands	3 Months Ended						9 Months Ended
Segment Information (Details) $ in Thousands	Sep. 30, 2025 USD ($)	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2025 USD ($) segment	Sep. 30, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment							1
Revenue:
Collaboration and license revenue	$ 0			$ 0			$ 400	$ 39,111
Grant revenue	452			387			1,450	561
Total revenue	452			387			1,850	39,672
Operating expenses:
Research and development	13,802			11,400			43,345	33,799
General and administrative	7,716			6,562			23,271	23,039
Net loss	(35,902)	$ 15,886	$ (17,710)	(24,183)	$ 13,836	$ (56,824)	(37,726)	(67,171)
Operating Segment
Revenue:
Collaboration and license revenue	0			0			400	39,111
Grant revenue	452			387			1,450	561
Total revenue	452			387			1,850	39,672
Operating expenses:
General and administrative	7,716			6,562			23,271	23,039
Other expense (income), net	14,836			6,608			(27,040)	50,005
Net loss	(35,902)			(24,183)			(37,726)	(67,171)
Operating Segment \| Legacy Selecta programs
Operating expenses:
Research and development	0			297			0	6,644
Operating Segment \| Descartes-08 for MG
Operating expenses:
Research and development	5,431			4,377			17,502	8,588
Operating Segment \| Early stage programs
Operating expenses:
Research and development	1,523			571			4,228	1,226
Operating Segment \| Research and development employee expenses
Operating expenses:
Research and development	3,971			2,947			11,927	8,372
Operating Segment \| Research and development stock-based compensation expense
Operating expenses:
Research and development	1,005			799			4,090	2,287
Operating Segment \| Research and development facilities and other expenses
Operating expenses:
Research and development	$ 1,872			$ 2,409			$ 5,598	$ 6,682

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Sep. 30, 2025
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Equity - “At-the-Market” Offerings (Narrative) (Details) - At-The-Market Offering - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024	Dec. 13, 2024
Class of Stock [Line Items]
Aggregate maximum gross sales proceeds			$ 100.0
Sale of stock, number of shares issued in transaction (in shares)	0	0

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Equity - Preferred Stock (Narrative) (Details) - shares	Sep. 30, 2025	Dec. 31, 2024
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock, shares issued (in shares)	0	0
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	120,790.402	120,790.402
Preferred stock, shares issued (in shares)	120,790.402	120,790.402
Convertible preferred stock, common stock issuable upon conversion (in shares)	4,464,273
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	437,927	437,927
Preferred stock, shares issued (in shares)	437,927	437,927

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Stock Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2025

Share-Based Payment Arrangement [Abstract]

Schedule of Stock-Based Compensation Expense

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss, was as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Research and development	$	1,005	$	799	$	4,090	$	2,287
General and administrative	1,585		896		4,287		2,430
Total stock-based compensation expense	$	2,590	$	1,695	$	8,377	$	4,717

Schedule of Weighted Average Assumptions Used

The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions:


	Three Months Ended September 30,						Nine Months Ended September 30,
	2025			2024			2025			2024
Risk-free interest rate	3.95		%	4.05		%	4.42		%	4.04		%
Dividend yield	—			—			—			—
Expected term (in years)	6.25			6.23			6.20			6.20
Expected volatility	93.79		%	94.98		%	97.19		%	95.07		%
Weighted-average fair value of common stock	$	12.48		$	17.97		$	16.87		$	19.79

Schedule of Options Activity

The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan for options for common stock:


	Number of common stock options	Weighted-average exercise price ($)		Weighted-average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)









Outstanding at December 31, 2024	1,706,035	$	11.99	7.59	$	12,025
Granted	1,055,709	$	16.87
Exercised	(82,944)	$	3.29
Forfeited	(245,144)	$	17.74
Outstanding at September 30, 2025	2,433,656	$	13.83	7.25	$	5,087

Vested at September 30, 2025	895,577	$	7.76	4.83	$	4,698
Vested and expected to vest at September 30, 2025	2,189,301	$	13.40	7.18	$	5,085

Schedule of Restricted Stock Units Activity

The following table summarizes the Company’s restricted stock units under the 2016 Plan and the Old Cartesian Plan:


	Number of shares	Weighted-average grant date fair value ($)
Unvested at December 31, 2024	444,238	$	19.86
Granted	256,790	16.93
Vested	(153,293)	19.86

Forfeited	(24,481)	17.07
Unvested at September 30, 2025	523,254	$	18.55

Leases - Schedule of Components of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Leases [Abstract]
Operating lease cost	$ 557	$ 1,058	$ 1,712	$ 2,789
Variable lease cost	438	380	1,270	1,129
Short-term lease cost	131	1	150	5
Less: Sublease income	0	(250)	0	(1,010)
Total lease cost	$ 1,126	$ 1,189	$ 3,132	$ 2,913

Investments (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	Sep. 30, 2021	Sep. 30, 2025	Dec. 31, 2024
Marketable Securities [Line Items]
Investments		$ 2,000	$ 2,000
Cyrus Biotechnology, Inc.
Marketable Securities [Line Items]
Equity securities, stock purchase agreement, shares purchased (in shares)	2,326,934
Investments	$ 2,000	$ 2,000
Cyrus Biotechnology, Inc. \| Series B Preferred Stock Purchase Agreement
Marketable Securities [Line Items]
Equity securities, stock purchase agreement, par value per share (in dollars per share)	$ 0.0001
Equity securities, stock purchase agreement, purchase price per share (in dollars per share)	$ 0.8595

Summary of Significant Accounting Policies	9 Months Ended
Summary of Significant Accounting Policies	Sep. 30, 2025
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company disclosed its significant accounting policies in Note 3, “Summary of Significant Accounting Policies” included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. There have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2025. Recent Accounting Pronouncements Not Yet Adopted In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning with the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company is currently in the process of evaluating the impact of the standard’s adoption on its consolidated financial statements and related disclosures. In November 2024, the FASB issued ASU 2024-03, Income Statement Reporting - Comprehensive Income - Expense Disaggregation Disclosures (ASU 2024-03), which requires public companies to disclose, in interim and annual reporting periods, additional information about certain expenses in notes to financial statements, including purchases of inventory, employee compensation, depreciation, amortization of intangible assets, and selling expenses. This guidance will be effective for the annual period beginning the year ended December 31, 2027 and for interim periods beginning January 1, 2028, with early adoption permitted. The Company is currently in the process of evaluating the impact of the standard’s adoption on its consolidated financial statements and related disclosures.

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2025

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):


	September 30, 2025		December 31, 2024
Payroll and employee related expenses	$	3,060	$	3,534

Accrued patent fees	126		813
Accrued research and development costs	2,078		2,987
Accrued professional and consulting services	1,717		3,674

Property and equipment	107		782
Other	614		286
Accrued expenses	$	7,702	$	12,076

Stock Incentive Plans

9 Months Ended

Sep. 30, 2025

Share-Based Payment Arrangement [Abstract]

Stock Incentive Plans

The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board of Directors. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board of Directors formally terminated the 2008 Plan.

In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 40,341 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants.

Additionally, pursuant to the terms of the 2016 Plan, the Board of Directors is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board of Directors or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board of Directors established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In June 2024, the Company’s stockholders approved an amendment and restatement of the 2016 Plan to reserve an additional 3,466,544 shares of the Company’s common stock for issuance. In January 2025, the number of shares of common stock that may be issued under the 2016 Plan was increased by 1,030,694. As of September 30, 2025, 3,614,406 shares remain available for future issuance under the 2016 Plan.

In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan, was adopted by the Board of Directors without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 39,166 shares of its common stock for issuance. In March 2019, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 66,667 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 60,833 shares of the Company’s common stock for issuance thereunder. In June and December 2024, the Board of Directors approved amendments and restatements of the 2018 Inducement Incentive Award Plan to reserve an additional 360,000 and 450,000 shares, respectively, of the Company’s common stock for issuance thereunder. As of September 30, 2025, there are 496,639 shares available for future grant under the 2018 Inducement Incentive Award Plan.

In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of September 30, 2025, there are 36,179 shares available for future grant under the Old Cartesian Plan.

In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 776,865 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre- and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.1 million and $0.3 million was recognized during the three months ended September 30, 2025 and 2024, respectively, and $0.5 million and $1.0 million was recognized during the nine months ended September 30, 2025 and 2024, respectively, as research and development expense in the consolidated statements of operations and comprehensive loss. Following the stockholder approval of the conversion of the Series A Preferred Stock into shares of common stock, the options exercisable for shares of Series A Preferred Stock became exercisable for shares of common stock.

Stock-Based Compensation Expense

In April 2025, the Company entered into a separation agreement and release, or the Separation Agreement, with the Company’s former Chief Technology Officer, Metin Kurtoglu. Dr. Kurtoglu’s employment with the Company ended effective May 2025, and Dr. Kurtoglu will serve as a consultant to the Company from May 2025 through April 2026, or the Consulting Period. Pursuant to the Separation Agreement, certain of Dr. Kurtoglu’s stock options and restricted stock unit awards were modified to accelerate the vesting of a portion of the awards, continue the vesting of the remaining awards during the Consulting Period, and extend the post-termination exercise period of the modified stock options. The services performed during the Consulting Period do not qualify as substantive services under ASC 718 and therefore, the continued vesting of these awards represents a modification to the original award. The modification resulted in the recognition of $0.7 million of stock-

compensation expense during the nine months ended September 30, 2025 which is reflected in research and development expenses on the consolidated statements of operations and comprehensive loss.

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss, was as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Research and development	$	1,005	$	799	$	4,090	$	2,287
General and administrative	1,585		896		4,287		2,430
Total stock-based compensation expense	$	2,590	$	1,695	$	8,377	$	4,717

Stock Options


	Three Months Ended September 30,						Nine Months Ended September 30,
	2025			2024			2025			2024
Risk-free interest rate	3.95		%	4.05		%	4.42		%	4.04		%
Dividend yield	—			—			—			—
Expected term (in years)	6.25			6.23			6.20			6.20
Expected volatility	93.79		%	94.98		%	97.19		%	95.07		%
Weighted-average fair value of common stock	$	12.48		$	17.97		$	16.87		$	19.79

The expected term of the Company’s stock options granted has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. Expected volatilities are based on the Company’s historical volatility.

The weighted-average grant date fair value of stock options granted during the three months ended September 30, 2025 and 2024 was $9.82 and $14.23, respectively, and $13.54 and $15.75 during the nine months ended September 30, 2025 and 2024, respectively.

As of September 30, 2025, total unrecognized compensation expense related to unvested common stock options was $13.7 million, which is expected to be recognized over a weighted average period of 2.8 years.

The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan for options for common stock:


	Number of common stock options	Weighted-average exercise price ($)		Weighted-average remaining contractual term (in years)	Aggregate intrinsic value (in thousands)









Outstanding at December 31, 2024	1,706,035	$	11.99	7.59	$	12,025
Granted	1,055,709	$	16.87
Exercised	(82,944)	$	3.29
Forfeited	(245,144)	$	17.74
Outstanding at September 30, 2025	2,433,656	$	13.83	7.25	$	5,087

Vested at September 30, 2025	895,577	$	7.76	4.83	$	4,698
Vested and expected to vest at September 30, 2025	2,189,301	$	13.40	7.18	$	5,085

Restricted Stock Units

During the nine months ended September 30, 2025, the Company granted 256,790 restricted stock unit awards with a weighted-average fair value of $16.93 per share based on the closing price of the Company’s common stock on the date of grant under the 2016 Plan, which generally vest over a four-year term. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock unit awards based on historical experience.

Unrecognized compensation expense related to the restricted stock units was $5.9 million as of September 30, 2025, which is expected to be recognized over a weighted-average period of 2.5 years.

The following table summarizes the Company’s restricted stock units under the 2016 Plan and the Old Cartesian Plan:


	Number of shares	Weighted-average grant date fair value ($)
Unvested at December 31, 2024	444,238	$	19.86
Granted	256,790	16.93
Vested	(153,293)	19.86

Forfeited	(24,481)	17.07
Unvested at September 30, 2025	523,254	$	18.55

Leases

9 Months Ended

Sep. 30, 2025

Leases [Abstract]

Leases

7495 New Horizon Way Leases

On February 28, 2024, the Company entered into a lease agreement with 7495 RP, LLC, or the Landlord, pursuant to which it agreed to lease from the Landlord the manufacturing space located at 7495 New Horizon Way, Frederick, Maryland, or the Frederick Lease Agreement. The space consists of 19,199 leasable square feet of integrated manufacturing and office space. The lease commenced on May 1, 2024 which was the date the Landlord delivered full possession of the premises to the Company. The Frederick Lease Agreement will terminate approximately 7.2 years following the commencement date. The Company will have one option to extend the term of the Frederick Lease Agreement for a period of five years at a cost of 100% of the then-fair market value, not to exceed 103% of the then-current base rent.

Base rent, which was due beginning on July 1, 2024, is $0.9 million annually and is subject to an annual upward adjustment of 3% of the then-current rental rate. In addition, the Company is obligated to pay its share of operating costs and

taxes related to the property. The Company paid the first month’s rent of $0.1 million upon execution of the Frederick Lease Agreement.

The Company assessed the classification of the lease at the commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset of $3.6 million and $3.7 million, respectively, on the commencement date. The Frederick Lease Agreement includes a tenant improvement allowance of up to $0.7 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. Additionally, the prepaid rent was included as an adjustment to the right-of-use asset. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods.

Effective May 7, 2024, the Company and the Landlord entered into the first amendment to the Frederick Lease Agreement, or the First Frederick Lease Agreement Amendment, providing for the expansion of the premises leased pursuant to the Frederick Lease Agreement by approximately 7,842 square feet. In connection with the expansion of the leased premises, the Company is obligated to pay $0.3 million in additional annual base rent for the first year of the term, which is subject to an annual upward adjustment of 3% of the then-current rental rate, as well as its share of operating costs and taxes. The lease commenced on May 7, 2024 which was the date the Landlord delivered full possession of the premises to the Company and will be coterminous with the Frederick Lease Agreement. The rent commencement date was September 1, 2024.

The Company assessed the classification of the lease at the September 1, 2024 commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset each of $1.2 million on the commencement date. The First Frederick Lease Agreement Amendment includes a tenant improvement allowance of up to $0.1 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term.

Effective August 30, 2024, the Company and the Landlord entered into the second amendment to the Frederick Lease Agreement, or the Second Frederick Lease Agreement Amendment, providing for the expansion of the premises leased pursuant to the Frederick Lease Agreement and First Frederick Lease Agreement Amendment by approximately 2,009 square feet. In connection with the expansion of the leased premises, the Company is obligated to pay $0.1 million in additional annual base rent for the first year of the term, which is subject to an annual upward adjustment of 3% of the then-current rental rate, as well as its share of operating costs and taxes. The lease commenced on September 1, 2024, which was the date the Landlord delivered full possession of the premises to the Company and will be coterminous with the Frederick Lease Agreement and the First Frederick Lease Agreement Amendment. The rent commencement date was September 1, 2024.

On March 13, 2025, the Company and the Landlord entered into the third amendment to the Frederick Lease Agreement, or the Third Frederick Lease Agreement Amendment, providing for the expansion of the premises leased pursuant to the Frederick Lease Agreement by approximately 6,439 square feet. In connection with the expansion of the leased premises, the Company is obligated to pay $0.2 million in additional annual base rent for the first year of the term, which is subject to an annual upward adjustment of 3% of the then-current rental rate, as well as its share of operating costs and taxes. The lease commenced on November 1, 2025, which was the date the Landlord delivered full possession of the premises to the Company and will be coterminous with the Frederick Lease Agreement, the First Frederick Lease Agreement Amendment and the Second Frederick Lease Agreement Amendment.

The Company secured a letter of credit from Silicon Valley Bank, a division of First-Citizens Bank & Trust Company (successor by purchase to the Federal Deposit Insurance Corporation as Receiver for Silicon Valley Bridge Bank, N.A. (as successor to Silicon Valley Bank)), or SVB, for $0.3 million for the Frederick Lease Agreement, the First Frederick Lease Agreement Amendment and the Second Frederick Lease Agreement Amendment, which is recognized as long-term restricted cash as of September 30, 2025 and December 31, 2024 and renews automatically each year.

65 Grove Street Lease

In July 2019, the Company entered into a lease with BRE-BMR Grove LLC for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Watertown Lease Agreement. As part of the Watertown Lease Agreement, the Company incurred $0.8 million in on-reimbursable construction costs. The lease began in March 2020,

when the Company took control of the office space, and the lease term is eight years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods.

On September 1, 2022, the Company entered into an amendment to the Watertown Lease, or the Watertown Lease Agreement Amendment, to expand the Company’s laboratory and office space located at 65 Grove Street, Watertown, Massachusetts by approximately 7,216 square feet. The lease term began on September 1, 2022, consistent with when the Company took control of the office space and expected lease term is 5.7 years. The discount rate of 11.3% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. Rent payments began in November 2022, and the base rent for the first year is $0.1 million per month. The Company recorded the right-of-use asset and operating lease liabilities of $3.2 million during the year ended December 31, 2022 as control of the premises was transferred to the Company during such year.

On October 6, 2022, the Company entered into a sublease agreement to sublease 7,216 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts. The sublease commenced on October 24, 2022. The term of the sublease expired on March 31, 2024 with no option to extend the sublease term. Sublease income is included within other (expense) income, net in the consolidated statements of operations and comprehensive loss.

As a result of the sublease agreement, rent payments to BRE-BMR Grove LLC for the lease of the office space increased. The change of consideration in the contract was accounted for as a lease modification and the right-of-use asset and lease liability were remeasured at the modification date of October 24, 2022. The discount rate of 11.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods as of October 24, 2022, resulting in a decrease of less than $0.1 million to both the right-of-use asset and lease liabilities.

In May 2023, the Company received notice from BRE-BMR Grove LLC that the requirements to reduce the amount of the letter of credit for the Watertown Lease had been met. In connection therewith, in June 2023, the Company secured a letter of credit from JPMorgan Chase Bank, N.A. for $1.4 million, which is recognized as long-term restricted cash as of September 30, 2025 and December 31, 2024, and renews automatically each year.

On October 31, 2023, in connection with entering into Amendment No. 1 to the Sobi License as described in Note 13, “Revenue Arrangements”, the Company entered into a sublease agreement with Sobi to sublease approximately 5,600 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts for which Sobi paid $1.0 million upfront rental payment. The sublease commenced on November 6, 2023. The term of this sublease expired on November 5, 2024. Sublease income is included within other (expense) income, net in the consolidated statements of operations and comprehensive loss.

As a result of the expiration of the sublease to Sobi in November 2024 and the Company’s decision to cease use of its office and laboratory space at 65 Grove Street, Watertown, Massachusetts, the Company assessed the right-of-use assets and related furniture and fixtures associated with the Watertown Lease Agreement and Watertown Lease Agreement Amendment for impairment. The carrying value of each asset group was compared against the future net undiscounted cash flows projected to be generate over the remaining lease terms. These projections included management’s estimates of cash inflows from potential sublease income. The carrying amount of the asset groups was found to be unrecoverable, thus the Company assessed the fair value of each asset group. The fair value was determined using the income approach, whereby the Company discounted the estimated net cash flows using a rate commensurate with the Company’s estimated incremental borrowing rate. As a result of this assessment, which included unrecoverable operating and maintenance costs, the Company determined that each asset group was fully impaired. As such, an impairment charge of $7.6 million was recognized during the year ended December 31, 2024, $7.4 million of which related to the right-of-use assets and $0.2 million related to property and equipment.

704 Quince Orchard Road Leases

In connection with the Merger, the Company acquired two operating leases for office and laboratory space in Gaithersburg, Maryland. The leases expire in January 2027 and do not contain any renewal rights. The discount rate of 11.5% was determined based on the Company’s incremental borrowing rate adjusted for the lease term.

For the three and nine months ended September 30, 2025 and 2024, the components of lease costs were as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Operating lease cost	$	557	$	1,058	$	1,712	$	2,789
Variable lease cost	438		380		1,270		1,129
Short-term lease cost	131		1		150		5
Less: Sublease income	—		(250)		—		(1,010)
Total lease cost	$	1,126	$	1,189	$	3,132	$	2,913

The maturity of the Company’s operating lease liabilities as of September 30, 2025 were as follows (in thousands):


	September 30, 2025
2025 (remainder)	$	1,118
2026	4,538
2027	4,345
2028	2,314
2029	1,409
Thereafter	2,188
Total future minimum lease payments	15,912
Less: Imputed interest	3,309
Total operating lease liabilities	$	12,603

The supplemental disclosure for the statements of cash flows related to operating leases was as follows (in thousands):


	Nine Months Ended September 30,
	2025		2024
Cash paid for amounts included in the measurement of lease liabilities:	$	2,527	$	2,548

Other than the initial recording of the right-of-use assets and lease liabilities for the Frederick Lease Agreement and Amended Frederick Lease Agreement, which were non-cash, the changes in the Company’s right-of-use assets and lease liabilities for the nine months ended September 30, 2025 and 2024 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows.

The following summarizes additional information related to operating leases:


	September 30,
	2025		2024
Weighted-average remaining lease term	4.0 years		4.7 years
Weighted-average discount rate	11.9	%	11.6	%

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 3,060	$ 3,534
Accrued patent fees	126	813
Accrued research and development costs	2,078	2,987
Accrued professional and consulting services	1,717	3,674
Property and equipment	107	782
Other	614	286
Accrued expenses	$ 7,702	$ 12,076

Fair Value Measurements - Schedule of Roll-Forward of Fair Value for the Company’s Level 3 Warrant Liabilities and CVR Liability (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2025 USD ($)
Warrant liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 3,836
Change in fair value	(2,988)
Ending balance	848
CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	395,500
Distributions	(7,754)
Change in fair value	(18,746)
Ending balance	$ 369,000

Stock Incentive Plans - Schedule of Weighted Average Assumptions Used (Details) - Options - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.95%	4.05%	4.42%	4.04%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (in years)	6 years 3 months	6 years 2 months 23 days	6 years 2 months 12 days	6 years 2 months 12 days
Expected volatility	93.79%	94.98%	97.19%	95.07%
Weighted-average fair value of common stock (in dollars per share)	$ 12.48	$ 17.97	$ 16.87	$ 19.79

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)
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+ Details

Related-Party Transactions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended
	Mar. 26, 2024	Nov. 13, 2023	Sep. 30, 2025	Sep. 30, 2024
Related Party Transaction [Line Items]
Weighted average exercise price (in dollars per share)			$ 46.96
Proceeds from exercise of common warrants			$ 0	$ 2,877
2023 Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		149,330.115
Related Party
Related Party Transaction [Line Items]
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)	65,681
Issuance of CVRs upon exercise of warrants (in shares)	1,970,443
Related Party \| 2019 Warrants
Related Party Transaction [Line Items]
Warrants exercised (in shares)	65,681
Weighted average exercise price (in dollars per share)	$ 43.80
Proceeds from exercise of common warrants	$ 2,900
Related Party \| 2023 Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		149,330.115
Total aggregate purchase price		$ 60,250

Restructuring (Tables)

9 Months Ended

Sep. 30, 2025

Restructuring and Related Activities [Abstract]

Schedule of Change in Accrued Restructuring Balance

The following table summarizes the change in the Company’s accrued restructuring balance included in accrued expenses and other current liabilities on its consolidated balance sheets (in thousands):


	December 31, 2024		Charges		Cash Payments		September 30, 2025
Severance liability	$	80	$	—	$	(80)	$	—

	December 31, 2023		Charges		Cash Payments		September 30, 2024
Severance liability	$	3,896	$	777	$	(4,586)	$	87

Net Loss Per Share Allocable to Common Stockholders - Schedule of Computation of Basic and Diluted Net Loss Per Share Allocable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Numerator:
Net loss	$ (35,902)	$ 15,886	$ (17,710)	$ (24,183)	$ 13,836	$ (56,824)	$ (37,726)	$ (67,171)
Denominator:
Weighted-average common shares outstanding - basic (in shares)	26,002,892			21,471,408			25,962,302	14,561,613
Weighted-average common shares outstanding - diluted (in shares)	26,002,892			21,471,408			25,962,302	14,561,613
Net loss per share allocable to common stockholders:
Basic (in dollars per share)	$ (1.38)			$ (1.13)			$ (1.45)	$ (4.61)
Diluted (in dollars per share)	$ (1.38)			$ (1.13)			$ (1.45)	$ (4.61)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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Reference 27: http://www.xbrl.org/2003/role/disclosureRef
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Reference 29: http://www.xbrl.org/2003/role/disclosureRef
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Reference 30: http://www.xbrl.org/2003/role/disclosureRef
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Reference 31: http://www.xbrl.org/2003/role/disclosureRef
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Reference 32: http://www.xbrl.org/2003/role/disclosureRef
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Reference 33: http://www.xbrl.org/2003/role/disclosureRef
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Reference 34: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 35: http://www.xbrl.org/2003/role/disclosureRef
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Reference 36: http://www.xbrl.org/2003/role/disclosureRef
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Reference 37: http://www.xbrl.org/2003/role/disclosureRef
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Reference 38: http://www.xbrl.org/2003/role/disclosureRef
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Reference 39: http://www.xbrl.org/2003/role/disclosureRef
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Reference 40: http://www.xbrl.org/2003/role/disclosureRef
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Reference 44: http://www.xbrl.org/2003/role/disclosureRef
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Reference 48: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Related-Party Transactions (Tables)

9 Months Ended

Sep. 30, 2025

Schedule of Related Party Transactions

The below issuances and sales to related parties of the Company were made during the nine months ended September 30, 2024.


Name	Shares of Series B Preferred Stock purchased	Total aggregate purchase price (in thousands)
Timothy A. Springer, Ph.D.	1,636,832	$	32,737
TAS Partners LLC, affiliate of Timothy A. Springer, Ph.D.	721,361	$	14,427
Chafen Lu, Ph.D., wife of Timothy A. Springer, Ph.D.	1,307	$	26

The 2023 Private Placement included a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the nine months ended September 30, 2024.


Name	Shares of Series A Preferred Stock purchased	Total aggregate purchase price (in thousands)
Timothy A. Springer, Ph.D.	99,140.326	$	40,000

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2025

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis

The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of September 30, 2025 and December 31, 2024 (in thousands):


	September 30, 2025
	Total		Level 1		Level 2		Level 3
Assets:
Money market funds (included in cash equivalents)	$	40,357	$	40,357	$	—	$	—





Total assets	$	40,357	$	40,357	$	—	$	—

Liabilities:
Warrant liability	$	848	$	—	$	—	$	848
Contingent value right liability	369,000		—		—		369,000
Total liabilities	$	369,848	$	—	$	—	$	369,848


	December 31, 2024
	Total		Level 1		Level 2		Level 3
Assets:
Money market funds (included in cash equivalents)	$	39,088	$	39,088	$	—	$	—





Total assets	$	39,088	$	39,088	$	—	$	—

Liabilities:
Warrant liability	$	3,836	$	—	$	—	$	3,836
Contingent value right liability	395,500		—		—		395,500

Total liabilities	$	399,336	$	—	$	—	$	399,336

Schedule of Cash and Cash Equivalents

The Company’s consolidated statements of cash flows include the following as of September 30, 2025 and 2024 (in thousands):


	September 30,
	2025		2024
Cash and cash equivalents	$	143,384	$	219,198

Long-term restricted cash	1,735		1,669
Total cash, cash equivalents, and restricted cash	$	145,119	$	220,867

Schedule of Restricted Cash

The Company’s consolidated statements of cash flows include the following as of September 30, 2025 and 2024 (in thousands):


	September 30,
	2025		2024
Cash and cash equivalents	$	143,384	$	219,198

Long-term restricted cash	1,735		1,669
Total cash, cash equivalents, and restricted cash	$	145,119	$	220,867

Schedule of Fair Value Measurement Inputs and Valuation Techniques for 2022 Warrants Liability and CVR Liability

A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows:


	September 30, 2025		December 31, 2024
Risk-free interest rate	3.60	%	4.25	%
Dividend yield	—		—
Expected life (in years)	1.53		2.28
Expected volatility	97.44	%	92.92	%

The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows:


	September 30, 2025		December 31, 2024
Estimated cash flow dates	2025 - 2038		2025 - 2038
Estimated probability of success	95.0% - 100.0%		95.0% - 100.0%
Expected volatility of future revenues	22.5	%	22.0	%

Schedule of Roll-Forward of Fair Value for the Company’s Level 3 Warrant Liabilities, CVR Liability and Forward Contract Liabilities

The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 11, “Equity” to these unaudited consolidated financial statements) for the nine months ended September 30, 2025 (in thousands):


	Warrant liability
Fair value as of December 31, 2024	$	3,836


Change in fair value	(2,988)
Fair value as of September 30, 2025	$	848

The following table reflects a roll-forward of fair value for the Company’s Level 3 CVR liability for the nine months ended September 30, 2025 (in thousands):


	CVR liability
Fair value as of December 31, 2024	$	395,500

Distributions	(7,754)
Change in fair value	(18,746)
Fair value as of September 30, 2025	$	369,000

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 16,518	$ 12,269
Less: Accumulated depreciation	(4,124)	(2,357)
Property and equipment, net	12,394	9,912
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	8,283	7,295
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	417	415
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	4,177	3,427
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	269	268
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	170	169
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,202	$ 695

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders’ Deficit (Parenthetical)	Apr. 04, 2024
Statement of Stockholders' Equity [Abstract]
Reverse stock split conversion ratio	0.0333

Collaboration and License Agreements - Biogen M.A., Inc. (Details)	Sep. 08, 2023
Biogen M.A., Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Written notice for agreement termination	30 days

Description of the Business	9 Months Ended
Description of the Business	Sep. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the Business	Description of the Business Cartesian Therapeutics, Inc., or the Company (formerly known as Selecta Biosciences, Inc., or Selecta), was incorporated in Delaware on December 10, 2007, and is headquartered in Frederick, Maryland. The Company is a clinical-stage biotechnology company pioneering cell therapy for the treatment of autoimmune diseases. Unlike DNA cell therapies, the Company’s cell therapy method degrades naturally over time without integrating into the cell’s genetic material. Therefore, the Company’s cell therapy is distinguished by its capacity to be dosed repeatedly like conventional drugs, administered in an outpatient setting, and given without pre-treatment chemotherapy required with many conventional cell therapies. On November 13, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger, or the Merger Agreement, the assets of the Delaware corporation which, immediately prior to the Merger (as defined below), was known as Cartesian Therapeutics, Inc., or Old Cartesian, as disclosed in Note 4, “Merger” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. The transaction was structured as a stock-for-stock transaction pursuant to which all of Old Cartesian’s outstanding shares of capital stock were exchanged based on a fixed exchange ratio for consideration of 224,099 shares of the common stock, par value $0.0001 per share, of the Company, or the common stock, and 384,930.724 shares of the newly designated Series A Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series A Preferred Stock. The Series A Preferred Stock is intended to have economic rights similar to the common stock, but with only limited voting rights. Additionally, the Company assumed all outstanding stock options of Old Cartesian. The common stock and Series A Preferred Stock related to the Merger were issued on December 5, 2023. In connection with the Merger, the Company entered into a definitive agreement, or the 2023 Securities Purchase Agreement, for a private investment in public equity transaction, or the 2023 Private Placement, with the Investors (as defined below). The 2023 Securities Purchase Agreement provided for the issuance to the Investors of an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of approximately $60.25 million. See Note 11, “Convertible Preferred Stock” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024 for further discussion of the 2023 Private Placement. In connection with the Merger, a contractual contingent value right, or CVR, was distributed to the holders of record of the Company’s common stock and 2022 Warrants (as defined below) as of the close of business on December 4, 2023, but was not distributed to holders of shares of common stock or Series A Preferred Stock issued to stockholders of Old Cartesian or the Investors in the transactions. Holders of the CVRs will be entitled to receive certain payments from proceeds received by the Company, if any, related to the disposition or monetization of the Company’s legacy assets following the issuance of the CVRs. For additional information, see Note 7, “Fair Value Measurements” to these unaudited consolidated financial statements. On March 27, 2024, the Company’s stockholders approved the conversion of shares of Series A Preferred Stock into shares of common stock. For additional information, see Note 11, “Convertible Preferred Stock” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. Additionally, on March 27, 2024, the Company’s stockholders approved an amendment to the Company’s restated certificate of incorporation, as amended, or the Charter, to effect a reverse stock split of the Company’s issued and outstanding common stock, at a ratio in the range of 1-for-20 and 1-for-30, with such ratio to be determined at the discretion of the Company’s board of directors, or the Board of Directors. The Board of Directors subsequently approved a final reverse stock split ratio of 1-for-30, and the Company effected the Reverse Stock Split on April 4, 2024. As a result of the Reverse Stock Split, all figures in this Quarterly Report on Form 10-Q relating to shares of the Company’s common stock (such as share amounts, per share amounts, and conversion rates and prices), reflect the Reverse Stock Split for all periods presented, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Shares of common stock underlying outstanding stock options, restricted stock units and warrants were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with their terms. Additionally, the conversion ratio of the Company’s Series A Preferred Stock was proportionally adjusted. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. On July 2, 2024, the Company entered into a securities purchase agreement, or the 2024 Securities Purchase Agreement, for a private investment in public equity financing, or the 2024 Private Placement, which provided for the issuance of 3,563,247 shares of common stock and 2,937,903 shares of Series B Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series B Preferred Stock, each at a purchase price of $20.00 per share. The 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses. On September 20, 2024, the Company’s stockholders approved the conversion of shares of Series B Preferred Stock into shares of common stock. For additional information, see Note 11, “Convertible Preferred Stock” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in preclinical and clinical development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and nine months ended September 30, 2025 and 2024 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America, or U.S. GAAP, have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2024 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 13, 2025. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial position as of September 30, 2025, the consolidated results of operations for the three and nine months ended September 30, 2025, and cash flows for the nine months ended September 30, 2025. Such adjustments are of a normal and recurring nature. The results of operations for the three and nine months ended September 30, 2025 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2025. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through public offerings and private placements of its securities, funding received from research grants, collaboration and license arrangements and a credit facility. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, the Company’s revenue has primarily been from collaboration agreements. The Company has devoted substantially all of its financial resources and efforts to developing its existing product candidates, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any product candidates. As of September 30, 2025, the Company’s cash, cash equivalents, and restricted cash were $145.1 million, of which $1.7 million was restricted cash related to lease commitments. The Company believes the cash, cash equivalents and restricted cash as of September 30, 2025 will enable it to fund its current planned operations for at least the next 12 months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing collaborations with other companies. Management’s expectations with respect to its ability to fund current and long term planned operations are based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any of these strategic or financing opportunities will be executed on favorable terms, and some could be dilutive to existing stockholders. Further, the liability associated with the CVR Agreement (as defined below) will be settled solely through cash flows received under the Company’s License and Development Agreement, or as so amended, the Sobi License, with Swedish Orphan Biovitrum AB (publ.), or Sobi, and any other Gross Proceeds (as defined in the CVR Agreement) net of certain agreed deductions. Under the CVR Agreement, 100% of all milestone payments, royalties and other amounts paid to the Company or controlled entities under the Sobi License, and any other Gross Proceeds will be distributed, net of specified deductions, to holders of the CVRs. There is no obligation to the Company to fund any amount related to the CVR liability. See Note 7, “Fair Value Measurement”, for more information on the CVR. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates. As of September 30, 2025, the Company had an accumulated deficit of $729.8 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2025
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, Selecta Biosciences Security Corporation, a Massachusetts securities corporation which the Company dissolved in December 2024, and Cartesian Bio, LLC, a Delaware limited liability company, which is a variable interest entity for which the Company is the primary beneficiary. All significant intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: fair value of the intangible assets acquired in connection with the Merger, fair value of the CVRs, deferred income taxes, revenue recognition, accrued research and development expenses, stock-based compensation expense, fair value of the liability-classified warrants, and impairment of long-lived assets. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information	Segment Information Operating segments are defined as components of an enterprise for which separate and discrete information is available for evaluation by the chief operating decision maker, or the CODM, for the purposes of assessing performance and allocating resources. The Company views its operations and manages its business in one operating segment, which prior to the Merger related to the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases and subsequent to the Merger relates to the research and development of cell therapy product candidates. The Company’s CODM function is fulfilled by its Chief Executive Officer. The CODM function assesses performance for the segment and decides how to allocate resources based on consolidated net income (loss) that is also reported on the consolidated statements of operations and comprehensive income (loss). The CODM function uses net income (loss) to monitor budget versus actual results to assess performance of the segment. Segment assets are the same as total assets on the Company’s consolidated balance sheets. All long-lived assets are located in the United States. Long-lived assets consist of property and equipment, net, and operating lease right-of-use assets.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Not Yet Adopted In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning with the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company is currently in the process of evaluating the impact of the standard’s adoption on its consolidated financial statements and related disclosures. In November 2024, the FASB issued ASU 2024-03, Income Statement Reporting - Comprehensive Income - Expense Disaggregation Disclosures (ASU 2024-03), which requires public companies to disclose, in interim and annual reporting periods, additional information about certain expenses in notes to financial statements, including purchases of inventory, employee compensation, depreciation, amortization of intangible assets, and selling expenses. This guidance will be effective for the annual period beginning the year ended December 31, 2027 and for interim periods beginning January 1, 2028, with early adoption permitted. The Company is currently in the process of evaluating the impact of the standard’s adoption on its consolidated financial statements and related disclosures.

- Definition

Disclosure of accounting policy for segment reporting.

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- Definition

Disclosure of accounting policy for the use of estimates in the preparation of financial statements in conformity with generally accepted accounting principles.

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+ Details

Leases (Tables)

9 Months Ended

Sep. 30, 2025

Leases [Abstract]

Schedule of Components of Lease Costs and Additional Information Related to Operating Leases

For the three and nine months ended September 30, 2025 and 2024, the components of lease costs were as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Operating lease cost	$	557	$	1,058	$	1,712	$	2,789
Variable lease cost	438		380		1,270		1,129
Short-term lease cost	131		1		150		5
Less: Sublease income	—		(250)		—		(1,010)
Total lease cost	$	1,126	$	1,189	$	3,132	$	2,913

The following summarizes additional information related to operating leases:


	September 30,
	2025		2024
Weighted-average remaining lease term	4.0 years		4.7 years
Weighted-average discount rate	11.9	%	11.6	%

Schedule of Maturity of Operating Lease Liabilities

The maturity of the Company’s operating lease liabilities as of September 30, 2025 were as follows (in thousands):


	September 30, 2025
2025 (remainder)	$	1,118
2026	4,538
2027	4,345
2028	2,314
2029	1,409
Thereafter	2,188
Total future minimum lease payments	15,912
Less: Imputed interest	3,309
Total operating lease liabilities	$	12,603

Schedule of Supplemental Disclosure for the Statement of Cash Flows Related to Operating Leases

The supplemental disclosure for the statements of cash flows related to operating leases was as follows (in thousands):


	Nine Months Ended September 30,
	2025		2024
Cash paid for amounts included in the measurement of lease liabilities:	$	2,527	$	2,548

Related-Party Transactions

9 Months Ended

Sep. 30, 2025

Related-Party Transactions

2024 Securities Purchase Agreement

On July 2, 2024, the Company entered into the 2024 Securities Purchase Agreement with the Purchasers. The Purchasers included (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors; (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Dr. Chafen Lu, Dr. Springer’s wife. See Note 11, “Convertible Preferred Stock” to the consolidated

financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024 for further discussion of the 2024 Private Placement. The below issuances and sales to related parties of the Company were made during the nine months ended September 30, 2024.


Name	Shares of Series B Preferred Stock purchased	Total aggregate purchase price (in thousands)
Timothy A. Springer, Ph.D.	1,636,832	$	32,737
TAS Partners LLC, affiliate of Timothy A. Springer, Ph.D.	721,361	$	14,427
Chafen Lu, Ph.D., wife of Timothy A. Springer, Ph.D.	1,307	$	26

2023 Securities Purchase Agreement

On November 13, 2023, the Company entered into the 2023 Securities Purchase Agreement with (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors, (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, a member of the Company’s Board of Directors, or the Investors, in which the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million. See Note 11, “Convertible Preferred Stock” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024 for further discussion of the 2023 Private Placement. The 2023 Private Placement included a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the nine months ended September 30, 2024.


Name	Shares of Series A Preferred Stock purchased	Total aggregate purchase price (in thousands)
Timothy A. Springer, Ph.D.	99,140.326	$	40,000

Exercise of Amended 2019 Warrants

On March 26, 2024, TAS Partners LLC exercised 65,681 Amended 2019 Warrants, paid the per-share exercise price of $43.80 in cash for an aggregate exercise price of $2.9 million, and received 65,681 shares of common stock and 1,970,443 CVRs.

During the three and nine months ended September 30, 2025, there were no related party transactions.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

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-Topic 850
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+ Details

Leases - Schedule of Supplemental Disclosure for the Statement of Cash Flows Related to Operating Leases (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 2,527	$ 2,548

Equity - Common Stock (Narrative) (Details)	Apr. 04, 2024
Equity [Abstract]
Reverse stock split conversion ratio	0.0333

Property and Equipment

9 Months Ended

Sep. 30, 2025

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment consists of the following (in thousands):


	September 30, 2025		December 31, 2024
Laboratory equipment	$	8,283	$	7,295
Computer equipment and software	417		415
Leasehold improvements	4,177		3,427
Furniture and fixtures	269		268
Office equipment	170		169
Construction in process	3,202		695
Total property and equipment	16,518		12,269
Less: Accumulated depreciation	(4,124)		(2,357)
Property and equipment, net	$	12,394	$	9,912

Depreciation expense was $0.6 million and $0.3 million for the three months ended September 30, 2025 and 2024, respectively, and $1.8 million and $0.7 million for the nine months ended September 30, 2025 and 2024, respectively

Restructuring

9 Months Ended

Sep. 30, 2025

Restructuring and Related Activities [Abstract]

Restructuring

In April 2023, in light of then-current market conditions, the Board of Directors took steps to extend the Company’s cash runway by pausing further development of the Company’s product candidate, SEL-302, for the treatment of methylmalonic acidemia, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of the Company’s product candidate, SEL-212, and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan that resulted in an approximate 90% reduction of the Company’s headcount as of April 2023.


	December 31, 2024		Charges		Cash Payments		September 30, 2025
Severance liability	$	80	$	—	$	(80)	$	—

	December 31, 2023		Charges		Cash Payments		September 30, 2024
Severance liability	$	3,896	$	777	$	(4,586)	$	87

The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs. The Company recorded these restructuring charges based on each employee’s role to the respective research and development and general and administrative operating expense categories on its consolidated statements of operations and comprehensive loss. For the nine months ended September 30, 2025, the Company recognized no restructuring charges. For the nine months ended September 30, 2024, the Company recognized $0.2 million in research and development expenses and $0.6 million in general and administrative expenses. Payments for the restructuring plan were completed in the first quarter of 2025.

Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Nov. 13, 2023
Business Combination [Line Items]
Goodwill	$ 48,163	$ 48,163
Intangible assets	$ 150,600	$ 150,600
Old Cartesian
Business Combination [Line Items]
Goodwill			$ 48,200
Intangible assets			150,600
Old Cartesian \| Descartes-08 for MG
Business Combination [Line Items]
Intangible assets			93,900
Old Cartesian \| Descartes-08 for SLE
Business Combination [Line Items]
Intangible assets			$ 56,700

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

- Definition

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

+ References

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+ Details

Segment Information

9 Months Ended

Sep. 30, 2025

Segment Reporting [Abstract]

Segment Information

The following table presents selected financial information with respect to the Company’s single operating segment for the three and nine months ended September 30, 2025 and 2024 (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Revenue:
Collaboration and license revenue	$	—	$	—	$	400	$	39,111
Grant revenue	452		387		1,450		561
Total revenue	452		387		1,850		39,672
Less
Operating expenses:
Legacy Selecta programs	—		297		—		6,644
Descartes-08 for MG	5,431		4,377		17,502		8,588
Early stage programs	1,523		571		4,228		1,226
Research and development employee expenses	3,971		2,947		11,927		8,372
Research and development stock-based compensation expense	1,005		799		4,090		2,287
Research and development facilities and other expenses	1,872		2,409		5,598		6,682
General and administrative	7,716		6,562		23,271		23,039

Other expense (income), net (1)	14,836		6,608		(27,040)		50,005
Net loss	$	(35,902)	$	(24,183)	$	(37,726)	$	(67,171)


(1) Includes interest income; foreign currency transaction, net; gain on change in fair value of warrant liabilities; (loss) gain on change in fair value of contingent value right liability; loss on change in fair value of forward contract liabilities; and other income (expense), net.

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

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+ Details

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details)	Sep. 30, 2025 shares
Class of Stock [Line Items]
Total (in shares)	12,306,725
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	4,193,019
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	523,254
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	2,433,656
Series A Preferred Stock
Class of Stock [Line Items]
Total (in shares)	4,026,346
Series B Preferred Stock
Class of Stock [Line Items]
Total (in shares)	437,927
Exercise of warrants
Class of Stock [Line Items]
Total (in shares)	692,523

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Fair Value Measurements - Narrative (Details)				1 Months Ended	9 Months Ended	12 Months Ended
Fair Value Measurements - Narrative (Details)	Dec. 13, 2023 USD ($)	Dec. 06, 2023 USD ($) shares	Nov. 13, 2023 shares	Feb. 11, 2024 USD ($)	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Jun. 28, 2024 USD ($)	Jun. 30, 2020 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net					$ 0	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net					$ 0	$ 0
Obligation to issue stock (in shares) \| shares			148,710.488
2023 Securities Purchase Agreement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares			149,330.115
Issuance of stock for settlement of liability	$ 14,800,000			$ 35,200,000
SOBI \| License
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contract liability							$ 30,000,000	$ 75,000,000
Old Cartesian
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed		100.00%			100.00%
Fixed semi annual amount		$ 750,000
Old Cartesian \| 2022 Warrants \| Contingent Value Right
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Class of warrant or right, number of securities called by each warrant or right (in shares) \| shares		30
Dividend yield \| Valuation technique, option pricing model
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input					0
Dividend yield \| Valuation technique, option pricing model \| 2022 Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input					0	0

Net Loss Per Share Allocable to Common Stockholders (Tables)

9 Months Ended

Sep. 30, 2025

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss Per Share Allocable to Common Stockholders

The following table sets forth the computation of basic and diluted net loss per share allocable to common stockholders for the three and nine months ended September 30, 2025 and 2024 (in thousands, except share and per-share data):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Numerator:
Net loss	$	(35,902)	$	(24,183)	$	(37,726)	$	(67,171)



Denominator:
Weighted-average common shares outstanding - basic and diluted	26,002,892		21,471,408		25,962,302		14,561,613



Net loss per share allocable to common stockholders:
Basic and diluted	$	(1.38)	$	(1.13)	$	(1.45)	$	(4.61)

Schedule of Potential Dilutive Shares of Common Stock Excluded from the Computation of the Diluted Net Loss Per Share Allocable to Common Stockholders

The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net loss per share allocable to common stockholders for all periods presented, as the effect would have been anti-dilutive:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2025	2024	2025	2024
Common stock options and restricted stock units	2,956,910	2,420,400	2,956,910	2,420,400
Warrants to purchase common stock	692,523	974,954	692,523	974,954
Series A Preferred Stock	4,026,346	5,544,719	4,026,346	5,544,719

Series B Preferred Stock	437,927	437,927	437,927	437,927
Total	8,113,706	9,378,000	8,113,706	9,378,000

Collaboration and License Agreements	9 Months Ended
Collaboration and License Agreements	Sep. 30, 2025
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration and License Agreements	Collaboration and License Agreements Biogen MA, Inc. On September 8, 2023, the Company entered into a non-exclusive, sublicensable, worldwide, perpetual patent license agreement, or the Biogen Agreement, with Biogen MA, Inc., or Biogen, to research, develop, make, use, offer, sell and import products or processes containing or using an engineering T-cell modified with an mRNA comprising, or encoding a protein comprising, certain sequences licensed under the Biogen Agreement for the prevention, treatment, palliation and management of autoimmune diseases and disorders, excluding cancers, neoplastic disorders, and paraneoplastic disorders. The Company is not obligated to pay Biogen any expenses, fees, or royalties. The Company may terminate the Biogen Agreement for any reason or no reason, and Biogen may terminate the agreement after a notice-and-cure period of 30 days if the Company fails to pay a fee owed to Biogen or for any other material breach of the agreement. The Biogen Agreement will otherwise expire when all claims of all issued patents within the patents and patent applications licensed to the Company under the Biogen Agreement have expired or been finally rendered revoked, invalid or unenforceable by a decision of a court or government agency. The Biogen Agreement encompasses patents and patent applications in the PCT/US2010/026825 patent family, which was filed March 10, 2010. In general, all patents that issue in this family have an expected expiration date of March 10, 2030, subject to potential patent term adjustments and/or extensions. For the U.S. patents and applications in this family, U.S. Patent 9,034,324 was awarded 677 days of patent term adjustment, which would extend the expiration date of this patent to January 16, 2032, absent any challenges to the patent term. The other issued patent in this family was not awarded any patent term adjustment, so its expected expiration date is March 10, 2030. National Cancer Institute of the National Institutes of Health Effective September 16, 2019, the Company entered into a nonexclusive, worldwide license agreement, or the NCI Agreement, with the U.S. Department of Health and Human Services, represented by the National Cancer Institute of the National Institutes of Health, or NCI. Under the NCI Agreement, the Company was granted a license under certain NCI patents and patent applications designated in the agreement, to make, use, sell, offer and import products and processes within the scope of the patents and applications licensed under the NCI Agreement when developing and manufacturing anti-BCMA CAR-T cell products for the treatment of MG pemphigus vulgaris, and immune thrombocytopenic purpura according to methods designated in the NCI Agreement. In connection with the Company’s entry into the NCI Agreement, Old Cartesian paid to NCI a one-time $0.1 million license royalty payment. Under the NCI Agreement, the Company is further required to pay NCI a low five-digit annual royalty. The Company must also pay earned royalties on net sales in a low single-digit percentage and pay up to $0.8 million in benchmark royalties upon the Company’s achievement of designated benchmarks that are based on the commercial development plan agreed between the parties. Under the NCI Agreement, the Company must use reasonable commercial efforts to bring licensed products and licensed processes to the point of Practical Application (as defined in the NCI Agreement). Upon the Company’s first commercial sale, the Company must use reasonable commercial efforts to make licensed products and licensed processes reasonably accessible to the United States public. After the Company’s first commercial sale, the Company must make reasonable quantities of licensed products or materials produced via licensed processes available to patient assistance programs and develop educational materials detailing the licensed products. Unless the Company obtains a waiver from NCI, the Company must have licensed products and licensed processes manufactured substantially in the United States. Prior to the first commercial sale, upon NCI’s request, the Company is obligated to provide NCI with commercially reasonable quantities of licensed products made through licensed processes to be used for in vitro research. Additionally, the Company must use reasonable commercial efforts to submit a BLA with respect to a licensed product by the fourth quarter of 2026 and make a first commercial sale of a licensed product by the fourth quarter of 2028. The NCI Agreement terminates upon the expiration of the last to expire of the patent rights licensed thereunder, if not sooner terminated. The NCI Agreement encompasses patents and patent applications in the PCT/US2013/032029 patent family, which was filed March 15, 2013. In general, all patents that issue in this family have an expected expiration of March 15, 2033, subject to potential patent term adjustments and/or extensions. For the U.S. patents and applications in this family, only two patents were awarded patent term adjustments. U.S. Patent 9,765,342 was awarded 297 days of patent term adjustment, which would extend the expiration date of this patent to January 6, 2034, absent any challenges to the patent term. The other patent, U.S. Patent 10,876,123, was awarded three days of patent term adjustment, but this patent is subject to terminal disclaimers filed against other family members, so this patent will not extend beyond the March 15, 2033 date. The other issued patents in this family were not awarded any patent term adjustment, so the expected expiration date for these patents also remains March 15, 2033. There is also a pending patent application which, if issued, will expire on March 15, 2033, but could also be subject to patent term adjustment and to any potential future terminal disclaimers. NCI has the right to terminate the NCI Agreement, after giving written notice and providing a cure period in accordance with its terms, if the Company is in default of a material obligation. The Company has the unilateral right to terminate the agreement in any country or territory by giving NCI 60 days’ written notice. The Company agreed to indemnify NCI against any liability arising out of the Company’s, sublicensees’ or third parties’ use of the licensed patent rights and licensed products or licensed processes developed in connection with the licensed patent rights. Genovis AB (publ.) License Agreement In October 2021, the Company entered into the Genovis Agreement with Genovis. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to the Xork enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Genovis was eligible to earn from the Company development and sales-based milestones and sublicensing fees. The Genovis Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. In February 2023, the Company made a $4.0 million payment to Genovis as a result of the sublicense of Xork to Astellas. See Note 13, “Revenue Arrangements” to these unaudited consolidated financial statements for further discussion on the Astellas Agreement. In March 2024, the Company notified Genovis of its intention to terminate the Genovis Agreement, which occurred effective September 13, 2024. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of September 30, 2025. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.

Property and Equipment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.6	$ 0.3	$ 1.8	$ 0.7

Goodwill and Intangible Assets	9 Months Ended
Goodwill and Intangible Assets	Sep. 30, 2025
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets On November 13, 2023, the Company merged with Old Cartesian in accordance with the terms of the Merger Agreement, by and among Selecta, Sakura Merger Sub I, Inc., a wholly owned subsidiary of Selecta, or First Merger Sub, Sakura Merger Sub II, LLC, a wholly owned subsidiary of Selecta, or Second Merger Sub, and Old Cartesian. Pursuant to the Merger Agreement, First Merger Sub merged with and into Old Cartesian, pursuant to which Old Cartesian was the surviving corporation and became a wholly owned subsidiary of Selecta, or the First Merger. Immediately following the First Merger, Old Cartesian merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. In connection with the Second Merger, Old Cartesian changed its name to Cartesian Bio, LLC. As a result of the Merger, Selecta changed its corporate name to Cartesian Therapeutics, Inc. See Note 4, “Merger” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024 for further discussion of the Merger. Goodwill The Merger resulted in goodwill of approximately $48.2 million. There were no changes to the carrying value of the Company’s goodwill during the nine months ended September 30, 2025. Intangible Assets The Merger resulted in total indefinite-lived intangible assets of $150.6 million, of which $93.9 million is related to Descartes-08 for myasthenia gravis, or MG and $56.7 million is related to Descartes-08 for systematic lupus erythematosus. There were no changes to the carrying value of the Company’s indefinite-lived intangible assets during the nine months ended September 30, 2025.

Leases - Schedule of Maturity of Operating Lease Liabilities (Details) $ in Thousands	Sep. 30, 2025 USD ($)
Leases [Abstract]
2025 (remainder)	$ 1,118
2026	4,538
2027	4,345
2028	2,314
2029	1,409
Thereafter	2,188
Total future minimum lease payments	15,912
Less: Imputed interest	3,309
Total operating lease liabilities	$ 12,603

Related-Party Transactions - Schedule of Related Party Transactions (Details) - USD ($) $ in Thousands			9 Months Ended
	Jul. 02, 2024	Nov. 13, 2023	Sep. 30, 2025
July 2024 Purchase Agreement \| Series B Preferred Stock
Related Party Transaction [Line Items]
Shares issued (in shares)	2,937,903
Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement
Related Party Transaction [Line Items]
Total aggregate purchase price	$ 32,737
Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement \| Series B Preferred Stock
Related Party Transaction [Line Items]
Shares issued (in shares)			1,636,832
Timothy A. Springer, Ph.D. \| Related Party \| 2023 Private Placement
Related Party Transaction [Line Items]
Total aggregate purchase price		$ 40,000
Timothy A. Springer, Ph.D. \| Related Party \| 2023 Private Placement \| Series A Preferred Stock
Related Party Transaction [Line Items]
Shares issued (in shares)		99,140.326
TAS Partners LLC, affiliate of Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement
Related Party Transaction [Line Items]
Total aggregate purchase price	14,427
TAS Partners LLC, affiliate of Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement \| Series B Preferred Stock
Related Party Transaction [Line Items]
Shares issued (in shares)			721,361
Chafen Lu, Ph.D., wife of Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement
Related Party Transaction [Line Items]
Total aggregate purchase price	$ 26
Chafen Lu, Ph.D., wife of Timothy A. Springer, Ph.D. \| Related Party \| July 2024 Purchase Agreement \| Series B Preferred Stock
Related Party Transaction [Line Items]
Shares issued (in shares)			1,307

Equity - Warrants (Narrative) (Details)	9 Months Ended
Equity - Warrants (Narrative) (Details)	Sep. 30, 2025 shares
Equity [Abstract]
Warrants issued (in shares)	0
Warrants exercised (in shares)	0
Warrants cancelled (in shares)	0

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2025
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of September 30, 2025, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Other As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at the Company. Through September 30, 2025, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established. Additionally, as permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect on the Company’s business, financial position, results of operations or cash flows

Basis of Presentation (Details)	9 Months Ended
Basis of Presentation (Details)	Sep. 30, 2025 segment
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of operating segments	1

Net Loss Per Share Allocable to Common Stockholders

9 Months Ended

Sep. 30, 2025

Earnings Per Share [Abstract]

Net Loss Per Share Allocable to Common Stockholders


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Numerator:
Net loss	$	(35,902)	$	(24,183)	$	(37,726)	$	(67,171)



Denominator:
Weighted-average common shares outstanding - basic and diluted	26,002,892		21,471,408		25,962,302		14,561,613



Net loss per share allocable to common stockholders:
Basic and diluted	$	(1.38)	$	(1.13)	$	(1.45)	$	(4.61)


	Three Months Ended September 30,		Nine Months Ended September 30,
	2025	2024	2025	2024
Common stock options and restricted stock units	2,956,910	2,420,400	2,956,910	2,420,400
Warrants to purchase common stock	692,523	974,954	692,523	974,954
Series A Preferred Stock	4,026,346	5,544,719	4,026,346	5,544,719

Series B Preferred Stock	437,927	437,927	437,927	437,927
Total	8,113,706	9,378,000	8,113,706	9,378,000

Description of the Business (Details) $ / shares in Units, $ in Thousands						9 Months Ended
	Jul. 02, 2024 USD ($) $ / shares shares	Apr. 04, 2024	Mar. 27, 2024	Dec. 06, 2023	Nov. 13, 2023 USD ($) $ / shares shares	Sep. 30, 2025 USD ($) $ / shares	Dec. 31, 2024 USD ($) $ / shares
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share) \| $ / shares					$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, par value (in dollars per share) \| $ / shares					$ 0.0001
Reverse stock split conversion ratio		0.0333
Cash, cash equivalents, restricted cash and marketable securities \| $						$ 145,100
Restricted cash and cash equivalents \| $						1,700
Accumulated deficit \| $						$ 729,797	$ 692,071
Minimum
Cash and Cash Equivalents [Line Items]
Reverse stock split conversion ratio			0.05
Maximum
Cash and Cash Equivalents [Line Items]
Reverse stock split conversion ratio			0.0333
Securities Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares					149,330.115
Total aggregate purchase price \| $					$ 60,250
July 2024 Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, price per share (in dollars per share) \| $ / shares	$ 20.00
Sale of stock, gross consideration received on transaction \| $	$ 130,000
Common stock \| July 2024 Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	3,563,247
Series B Preferred Stock
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001
Series B Preferred Stock \| July 2024 Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	2,937,903
Old Cartesian
Cash and Cash Equivalents [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed				100.00%		100.00%
Old Cartesian \| Common stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares) \| shares					224,099
Old Cartesian \| Preferred stock \| Series A Preferred Stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares) \| shares					384,930.724

Fair Value Measurements - Schedule of Fair Value Measurement Inputs and Valuation Techniques for Warrants Liability (Details) - Valuation technique, option pricing model	Sep. 30, 2025	Dec. 31, 2024
Risk-free interest rate \| 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0360	0.0425
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
Dividend yield \| 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Expected life (in years) \| 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	1 year 6 months 10 days	2 years 3 months 10 days
Expected volatility \| 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.9744	0.9292

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 2,590	$ 1,695	$ 8,377	$ 4,717
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	1,005	799	4,090	2,287
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,585	$ 896	$ 4,287	$ 2,430

Collaboration and License Agreements - National Cancer Institute of the National Institutes of Health (Details) - NCI Agreement $ in Millions	Sep. 16, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
License and option agreement, royalty payment	$ 0.1
License and option agreement, benchmark royalties, maximum	$ 0.8
Written notice for agreement termination	60 days

Segment Information (Tables)

9 Months Ended

Sep. 30, 2025

Segment Reporting [Abstract]

Schedule of Segment Reporting Information, by Segment

The following table presents selected financial information with respect to the Company’s single operating segment for the three and nine months ended September 30, 2025 and 2024 (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2025		2024		2025		2024
Revenue:
Collaboration and license revenue	$	—	$	—	$	400	$	39,111
Grant revenue	452		387		1,450		561
Total revenue	452		387		1,850		39,672
Less
Operating expenses:
Legacy Selecta programs	—		297		—		6,644
Descartes-08 for MG	5,431		4,377		17,502		8,588
Early stage programs	1,523		571		4,228		1,226
Research and development employee expenses	3,971		2,947		11,927		8,372
Research and development stock-based compensation expense	1,005		799		4,090		2,287
Research and development facilities and other expenses	1,872		2,409		5,598		6,682
General and administrative	7,716		6,562		23,271		23,039

Other expense (income), net (1)	14,836		6,608		(27,040)		50,005
Net loss	$	(35,902)	$	(24,183)	$	(37,726)	$	(67,171)


(1) Includes interest income; foreign currency transaction, net; gain on change in fair value of warrant liabilities; (loss) gain on change in fair value of contingent value right liability; loss on change in fair value of forward contract liabilities; and other income (expense), net.

Net Loss Per Share Allocable to Common Stockholders - Schedule of Potential Dilutive Shares of Common Stock Excluded from the Computation of the Diluted Net Loss Per Share Allocable to Common Stockholders (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Potential common shares
Total (in shares)	8,113,706	9,378,000	8,113,706	9,378,000
Common stock options and restricted stock units
Potential common shares
Total (in shares)	2,956,910	2,420,400	2,956,910	2,420,400
Warrants to purchase common stock
Potential common shares
Total (in shares)	692,523	974,954	692,523	974,954
Preferred stock \| Series A Preferred Stock
Potential common shares
Total (in shares)	4,026,346	5,544,719	4,026,346	5,544,719
Preferred stock \| Series B Preferred Stock
Potential common shares
Total (in shares)	437,927	437,927	437,927	437,927

Leases - Narrative (Details) $ in Thousands									1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
Leases - Narrative (Details) $ in Thousands	Mar. 13, 2025 USD ($) ft²	Aug. 30, 2024 USD ($) ft²	May 07, 2024 USD ($) ft²	May 01, 2024 USD ($) extension_option	Feb. 28, 2024 USD ($) ft²	Nov. 13, 2023 operating_lease	Oct. 31, 2023 USD ($) ft²	Sep. 01, 2022 USD ($) ft²	Jul. 31, 2019 USD ($) ft²	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2022 USD ($)	Oct. 24, 2022 USD ($)
Operating Leased Assets [Line Items]
Lease liabilities										$ 12,603		$ 12,603
Right-of-use assets, net										$ 4,972		$ 4,972		$ 5,535
Operating lease, weighted average discount rate, percent								11.30%		11.90%	11.60%	11.90%	11.60%
Lessee-paid construction costs										$ 1,126	$ 1,189	$ 3,132	$ 2,913
7495 New Horizon Way Leases
Operating Leased Assets [Line Items]
Area of office space leased \| ft²	6,439	2,009	7,842		19,199
Lease term				7 years 2 months 12 days
Number of options to extend lease \| extension_option				1
Lease renewal term (in years)				5 years
Lease agreement, monthly base rent				$ 900
Base rent upward annual adjustment percentage after first year	3.00%	3.00%	3.00%	3.00%
Lease agreement, first month's rent paid upon execution of lease agreement					$ 100
Lease liabilities		$ 300	$ 1,200	$ 3,600
Right-of-use assets, net		300	1,200	3,700
Tenant improvement recognized as reduction in right-of-use asset and lease liability			$ 100	$ 700
Operating lease, weighted average discount rate, percent			14.00%	14.00%
Additional annual base rent for first year	$ 200	$ 100	$ 300
7495 New Horizon Way Leases \| Letter of Credit
Operating Leased Assets [Line Items]
Long-term restricted cash										300		300		300
7495 New Horizon Way Leases \| Minimum
Operating Leased Assets [Line Items]
Operating lease, cost (as a percent)				100.00%
7495 New Horizon Way Leases \| Maximum
Operating Leased Assets [Line Items]
Operating lease, cost (as a percent)				103.00%
65 Grove Street Lease
Operating Leased Assets [Line Items]
Area of office space leased \| ft²							5,600	7,216	25,078
Lease term								5 years 8 months 12 days	8 years
Lease liabilities															$ 3,200	$ 100
Right-of-use assets, net															$ 3,200	$ 100
Operating lease, weighted average discount rate, percent									8.90%							11.90%
Lessee-paid construction costs									$ 800
Operating lease, expense								$ 100
Upfront rental payment							$ 1,000
Asset impairment charges														7,600
Right-of-use asset impairment expense														7,400
Impairment of long-lived assets														200
65 Grove Street Lease \| Letter of Credit
Operating Leased Assets [Line Items]
Long-term restricted cash										$ 1,400		$ 1,400		$ 1,400
704 Quince Orchard Road Leases
Operating Leased Assets [Line Items]
Operating lease, weighted average discount rate, percent						11.50%
Number of acquired operating leases \| operating_lease						2

Equity - Schedule of Warranty Activity (Details)	Sep. 30, 2025 $ / shares shares
Number of Warrants
Number of warrants, outstanding (in shares)	692,523
Weighted-average exercise price
Weighted average exercise price (in dollars per share) \| $ / shares	$ 46.96
Warrants to purchase common stock
Number of Warrants
Number of warrants, outstanding (in shares)	685,712
Warrants to purchase common stock
Number of Warrants
Number of warrants, outstanding (in shares)	6,811

Fair Value Measurements - Schedule of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 143,384	$ 212,610	$ 219,198
Long-term restricted cash	1,735	1,669	1,669
Total cash, cash equivalents, and restricted cash	$ 145,119	$ 214,279	$ 220,867	$ 78,288

Revenue Arrangements	9 Months Ended
Revenue Arrangements	Sep. 30, 2025
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements Collaboration and license revenue Astellas Gene Therapies License and Development Agreement In January 2023, the Company entered into the License and Development Agreement, or the Astellas Agreement, with Audentes Therapeutics, Inc., or Astellas. Under the Astellas Agreement, the Company granted Astellas an exclusive license to the Company’s IdeXork technology arising from Xork, to develop and commercialize Xork for use in Pompe disease in combination with an Astellas gene therapy investigational or authorized product. Xork, Genovis’ IgG Protease, was licensed pursuant to an Exclusive License Agreement, or the Genovis Agreement, with Genovis AB (publ.), or Genovis, as described in Note 15, “Collaboration and License Agreements” to these unaudited consolidated financial statements. Astellas paid a $10.0 million upfront payment to the Company upon signing of the Astellas Agreement, and the Company was entitled to receive up to $340.0 million in future additional payments over the course of the partnership that were contingent on the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales where Xork is used as a pre-treatment for an Astellas investigational or authorized product. The Company was also eligible for tiered royalty payments ranging from low to high single digits. Any proceeds received from milestone payments or royalties relating to Xork would have been required to be distributed to holders of CVRs, net of certain deductions. A more detailed description of the Astellas Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14, “Revenue Arrangements” to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. In March 2024, the Company was notified by Astellas of its intention to terminate the Astellas Agreement, which occurred effective June 6, 2024. As of September 30, 2025, there were no unsatisfied performance obligations and no receivable related to the Astellas Agreement. As of December 31, 2024, the Company recorded a receivable of $0.1 million, representing billings for the Xork Development Services (as defined in the Astellas Agreement) that are subject to reimbursement by Astellas. No revenue related to the Astellas Agreement was recognized during the three and nine months ended September 30, 2025 or the three months ended September 30, 2024. During the nine months ended September 30, 2024, revenue of $6.3 million related to the Astellas Agreement was recognized, inclusive of $3.2 million of revenue recognized from performance obligations related to prior periods as a result of the change in transaction price during the nine months ended September 30, 2024. Swedish Orphan Biovitrum AB (publ.) License and Development Agreement In June 2020, the Company and Sobi entered into the Sobi License, which was subsequently amended in October 2023. Pursuant to the Sobi License, the Company agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, upfront payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. A more detailed description of the Sobi License and the Company’s evaluation of this agreement under ASC 606 can be found in Note 14, “Revenue Arrangements” to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. Any proceeds received from milestone payments or royalties relating to the Sobi License would be required to be distributed to holders of CVRs, net of certain deductions. On June 28, 2024, Sobi initiated a rolling biologics license application to the FDA for SEL-212 for the potential treatment of chronic refractory gout which resulted in the achievement of a development milestone and a $30.0 million payment obligation from Sobi to the Company. As a result, the development milestone was no longer constrained and $30.0 million was recognized as revenue during the year ended December 31, 2024 as there were no remaining performance obligations under the Sobi License. The proceeds from the achievement of the development milestone were received from Sobi in July 2024 and were included, net of deductions as specified in the CVR Agreement, in the distribution to holders of the CVRs in March 2025. As of each of September 30, 2025 and December 31, 2024, the Company recorded total outstanding receivables of $0.1 million, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Additionally, as of September 30, 2025 and December 31, 2024, there was no unbilled receivable outstanding. No revenue was recognized during the three and nine months ended September 30, 2025 or the three months ended September 30, 2024. Revenue of $32.8 million, inclusive of the $30.0 million development milestone, related to the Sobi License was recognized during the nine months ended September 30, 2024. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed, or has been partially performed. As of September 30, 2025 and December 31, 2024, there were no unsatisfied performance obligations from contracts with customers. Grant revenue National Institute of Neurological Disorders and Stroke of the National Institutes of Health In June 2024, the Company received funding approval from the National Institute of Neurological Disorders and Stroke of the National Institutes of Health, or NINDS, for an award of $1.5 million granted for the budget period, which ran from June 2024 through May 2025. In June 2025, the Company received funding approval from NINDS for an additional award of $1.5 million granted for the budget period that runs from June 2025 through May 2026. The funding was provided by NINDS to further the Company’s use of RNA-based CAR-T cells to combat autoantibody-associated autoimmune disorders. Grant funding is to be used solely for manufacturing of RNA-based CAR-T cells and analysis of samples to inform mechanism of action. The award period runs through May 31, 2026. The Company will recognize grant revenue when expenses reimbursable under the grant have been incurred. As of September 30, 2025 and December 31, 2024, the Company recorded a receivable of $0.5 million and $0.6 million, respectively, that is subject to reimbursement by NINDS. The Company recognized grant revenue of $0.5 million and $1.4 million during the three and nine months ended September 30, 2025, respectively. The Company recognized grant revenue of $0.4 million and $0.6 million during the three and nine months ended September 30, 2024, respectively.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-9

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-10

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
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-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-15

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-12

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/606/tableOfContent

+ Details

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders’ Deficit - USD ($) $ in Thousands	Total	Series A Preferred Stock	Series B Preferred Stock	Preferred stock Series A Preferred Stock	Preferred stock Series B Preferred Stock	Common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2023	435,120.513
Beginning balance at Dec. 31, 2023	$ 296,851
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A Preferred Stock in connection with private placement and settlement of related forward contract (in shares)	99,140.326
Issuance of Series A Preferred Stock in connection with private placement and settlement of related forward contract	$ 75,197
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity (in shares)	(534,260.839)
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	$ (372,048)
Ending balance (in shares) at Mar. 31, 2024	0
Ending balance at Mar. 31, 2024	$ 0
Beginning balance at Dec. 31, 2023	3,703
Increase (Decrease) In Temporary Equity, Option, Roll Forward [Roll Forward]
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	(3,703)
Ending balance at Mar. 31, 2024	0
Preferred stock, beginning balance (in shares) at Dec. 31, 2023				0	0
Beginning balance at Dec. 31, 2023	(440,184)			$ 0	$ 0	$ 1	$ 179,062	$ (614,647)	$ (4,600)
Common stock, beginning balance (in shares) at Dec. 31, 2023						5,397,597
Increase (Decrease) in Stockholders' Equity
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity (in shares)				534,260.839
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	375,751						375,751
Issuance of common stock upon exercise of options (in shares)						52,558
Issuance of common stock upon exercise of options	154						154
Issuance of common stock upon exercise of warrants (in shares)						65,681
Issuance of common stock upon exercise of warrants	2,877						2,877
Stock-based compensation expense	1,431						1,431
Currency translation adjustment	(5)								(5)
Net loss	(56,824)							(56,824)
Preferred stock, ending balance (in shares) at Mar. 31, 2024				534,260.839	0
Ending balance at Mar. 31, 2024	$ (116,800)			$ 0	$ 0	$ 1	559,275	(671,471)	(4,605)
Common stock, ending balance (in shares) at Mar. 31, 2024						5,515,836
Beginning balance (in shares) at Dec. 31, 2023	435,120.513
Beginning balance at Dec. 31, 2023	$ 296,851
Ending balance (in shares) at Sep. 30, 2024	0
Ending balance at Sep. 30, 2024	$ 0
Beginning balance at Dec. 31, 2023	3,703
Ending balance at Sep. 30, 2024	0
Preferred stock, beginning balance (in shares) at Dec. 31, 2023				0	0
Beginning balance at Dec. 31, 2023	(440,184)			$ 0	$ 0	$ 1	179,062	(614,647)	(4,600)
Common stock, beginning balance (in shares) at Dec. 31, 2023						5,397,597
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	(6)
Net loss	(67,171)
Preferred stock, ending balance (in shares) at Sep. 30, 2024				166,341.592	437,927
Ending balance at Sep. 30, 2024	$ 752			$ 0	$ 0	$ 2	687,174	(681,818)	(4,606)
Common stock, ending balance (in shares) at Sep. 30, 2024						23,896,525
Beginning balance (in shares) at Mar. 31, 2024	0
Beginning balance at Mar. 31, 2024	$ 0
Ending balance (in shares) at Jun. 30, 2024	0
Ending balance at Jun. 30, 2024	$ 0
Beginning balance at Mar. 31, 2024	0
Ending balance at Jun. 30, 2024	0
Preferred stock, beginning balance (in shares) at Mar. 31, 2024				534,260.839	0
Beginning balance at Mar. 31, 2024	(116,800)			$ 0	$ 0	$ 1	559,275	(671,471)	(4,605)
Common stock, beginning balance (in shares) at Mar. 31, 2024						5,515,836
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)						36,451
Issuance of common stock upon exercise of options	120						120
Equity offering costs	(219)						(219)
Conversion of Preferred Stock to common stock (in shares)				(367,919.247)		12,263,951
Conversion of Preferred Stock to common stock	0					$ 1	(1)
Stock-based compensation expense	1,591						1,591
Currency translation adjustment	14								14
Net loss	13,836							13,836
Preferred stock, ending balance (in shares) at Jun. 30, 2024				166,341.592	0
Ending balance at Jun. 30, 2024	$ (101,458)			$ 0	$ 0	$ 2	560,766	(657,635)	(4,591)
Common stock, ending balance (in shares) at Jun. 30, 2024						17,816,238
Ending balance (in shares) at Sep. 30, 2024	0
Ending balance at Sep. 30, 2024	$ 0
Ending balance at Sep. 30, 2024	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)						17,064
Issuance of common stock upon exercise of options	56						56
Conversion of Preferred Stock to common stock (in shares)					(2,499,976)	2,499,976
Issuance of Series B Preferred Stock and common stock in connection with private placement, net (in shares)					2,937,903	3,563,247
Issuance of Series B Preferred Stock and common stock in connection with private placement, net	124,657						124,657
Stock-based compensation expense	1,695						1,695
Currency translation adjustment	(15)								(15)
Net loss	(24,183)							(24,183)
Preferred stock, ending balance (in shares) at Sep. 30, 2024				166,341.592	437,927
Ending balance at Sep. 30, 2024	$ 752			$ 0	$ 0	$ 2	687,174	(681,818)	(4,606)
Common stock, ending balance (in shares) at Sep. 30, 2024						23,896,525
Preferred stock, beginning balance (in shares) at Dec. 31, 2024	0	120,790.402	437,927	120,790.402	437,927
Beginning balance at Dec. 31, 2024	$ (6,802)			$ 0	$ 0	$ 3	689,887	(692,071)	(4,621)
Common stock, beginning balance (in shares) at Dec. 31, 2024	25,767,369					25,767,369
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)						55,690
Issuance of common stock upon exercise of options	$ 183						183
Issuance of common stock upon vesting of restricted stock units (in shares)						113,042
Stock-based compensation expense	2,508						2,508
Currency translation adjustment	32								32
Net loss	(17,710)							(17,710)
Preferred stock, ending balance (in shares) at Mar. 31, 2025				120,790.402	437,927
Ending balance at Mar. 31, 2025	$ (21,789)			$ 0	$ 0	$ 3	692,578	(709,781)	(4,589)
Common stock, ending balance (in shares) at Mar. 31, 2025						25,936,101
Preferred stock, beginning balance (in shares) at Dec. 31, 2024	0	120,790.402	437,927	120,790.402	437,927
Beginning balance at Dec. 31, 2024	$ (6,802)			$ 0	$ 0	$ 3	689,887	(692,071)	(4,621)
Common stock, beginning balance (in shares) at Dec. 31, 2024	25,767,369					25,767,369
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	$ 34
Net loss	$ (37,726)
Preferred stock, ending balance (in shares) at Sep. 30, 2025	0	120,790.402	437,927
Ending balance at Sep. 30, 2025	$ (35,844)			$ 0	$ 0	$ 3	698,537	(729,797)	(4,587)
Common stock, ending balance (in shares) at Sep. 30, 2025	26,003,606			120,790.402	437,927	26,003,606
Preferred stock, beginning balance (in shares) at Mar. 31, 2025				120,790.402	437,927
Beginning balance at Mar. 31, 2025	$ (21,789)			$ 0	$ 0	$ 3	692,578	(709,781)	(4,589)
Common stock, beginning balance (in shares) at Mar. 31, 2025						25,936,101
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)						25,690
Issuance of common stock upon exercise of options	85						85
Issuance of common stock upon vesting of restricted stock units (in shares)						38,274
Stock-based compensation expense	3,279						3,279
Currency translation adjustment	12								12
Net loss	15,886							15,886
Preferred stock, ending balance (in shares) at Jun. 30, 2025				120,790.402	437,927
Ending balance at Jun. 30, 2025	(2,527)			$ 0	$ 0	$ 3	695,942	(693,895)	(4,577)
Common stock, ending balance (in shares) at Jun. 30, 2025						26,000,065
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)						1,564
Issuance of common stock upon exercise of options	5						5
Issuance of common stock upon vesting of restricted stock units (in shares)						1,977
Stock-based compensation expense	2,590						2,590
Currency translation adjustment	(10)								(10)
Net loss	$ (35,902)							(35,902)
Preferred stock, ending balance (in shares) at Sep. 30, 2025	0	120,790.402	437,927
Ending balance at Sep. 30, 2025	$ (35,844)			$ 0	$ 0	$ 3	$ 698,537	$ (729,797)	$ (4,587)
Common stock, ending balance (in shares) at Sep. 30, 2025	26,003,606			120,790.402	437,927	26,003,606

Property and Equipment (Tables)

9 Months Ended

Sep. 30, 2025

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following (in thousands):


	September 30, 2025		December 31, 2024
Laboratory equipment	$	8,283	$	7,295
Computer equipment and software	417		415
Leasehold improvements	4,177		3,427
Furniture and fixtures	269		268
Office equipment	170		169
Construction in process	3,202		695
Total property and equipment	16,518		12,269
Less: Accumulated depreciation	(4,124)		(2,357)
Property and equipment, net	$	12,394	$	9,912

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Cash flows from operating activities
Net loss	$ (37,726)	$ (67,171)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,351	665
Non-cash lease expense	563	1,766
Loss on disposal of property and equipment	0	2
Stock-based compensation expense	8,377	4,717
Gain on change in fair value of warrant liabilities	(2,988)	(2,803)
(Gain) loss on change in fair value of contingent value right liability	(18,746)	51,900
Loss on change in fair value of forward contract liabilities	0	6,890
Changes in operating assets and liabilities:
Accounts receivable	150	1,041
Unbilled receivable	0	2,001
Prepaid expenses, deposits and other assets	(5,562)	763
Accounts payable	1,999	(2,690)
Deferred revenue	0	(5,849)
Accrued expenses and other liabilities	(4,647)	(7,904)
Net cash used in operating activities	(56,229)	(16,672)
Cash flows from investing activities
Purchases of property and equipment	(4,962)	(8,388)
Net cash used in investing activities	(4,962)	(8,388)
Cash flows from financing activities
Equity offering costs	(522)	0
Proceeds from exercise of common warrants	0	2,877
Proceeds from issuance of Series A Preferred Stock, gross in private placement	0	40,000
Net proceeds from issuance of common stock and Series B Preferred Stock in private placement	0	124,438
Proceeds from exercise of stock options	273	330
Payments of contingent value rights distributions	(7,754)	0
Net cash (used in) provided by financing activities	(8,003)	167,645
Effect of exchange rate changes on cash	34	(6)
Net change in cash, cash equivalents, and restricted cash	(69,160)	142,579
Cash, cash equivalents, and restricted cash at beginning of period	214,279	78,288
Cash, cash equivalents, and restricted cash at end of period	145,119	220,867
Non-cash investing and financing activities
Purchase of property and equipment not yet paid	$ 111	$ 392

Equity

9 Months Ended

Sep. 30, 2025

Equity [Abstract]

Equity

Equity Financings

2024 Private Placement

On July 2, 2024, the Company and certain institutional and accredited investors, or the Purchasers, entered into the 2024 Securities Purchase Agreement for the 2024 Private Placement.

Pursuant to the 2024 Securities Purchase Agreement, the Purchasers agreed to purchase an aggregate of 3,563,247 shares of common stock and 2,937,903 shares of Series B Preferred Stock, inclusive of 2,359,500 shares of Series B Preferred Stock purchased by directors and executive officers of the Company, and related parties thereto, each at a price per share of $20.00.

The 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses.

“At-the-Market” Offering

On December 13, 2024, the Company entered into a Sales Agreement, or the Sales Agreement, with Leerink Partners LLC to sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which Leerink Partners LLC will act as sales agent. The shares of common stock sold pursuant to the Sales Agreement will be issued pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-283803), filed on December 13, 2024 with the SEC, and related prospectus supplement, filed on January 8, 2025 with the SEC, for aggregate gross sales proceeds of up to $100.0 million.

During the nine months ended September 30, 2025 and the year ended December 31, 2024, the Company sold no shares of its common stock pursuant to the Sales Agreement.

Warrants

During the nine months ended September 30, 2025, there were no warrants issued, exercised, or cancelled. The following is a summary of the Company’s warrants as of September 30, 2025:


	Number of Warrants
	Equity classified	Liability classified	Total	Weighted-average exercise price




Outstanding at September 30, 2025	6,811	685,712	692,523	$	46.96

See Note 12, “Equity” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024 for further discussion of the terms related to the Company’s warrants.

Common Stock

On April 4, 2024, the Company implemented the Reverse Stock Split. The Reverse Stock Split became effective at 4:30 p.m. Eastern Time on April 4, 2024. On April 5, 2024, the Company’s common stock began trading on The Nasdaq Global Market on a split-adjusted basis under the symbol “RNAC” with a new CUSIP number, 816212302. As a result of the Reverse Stock Split, every 30 shares of common stock outstanding were combined, automatically and without any action on the part of the Company or its stockholders, into one share of common stock. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. The Reverse Stock Split did not change the number of authorized shares or par value of the Company’s common or preferred stock.

Preferred Stock

As of September 30, 2025, the Company had 120,790.402 shares of Series A Preferred Stock and 437,927 shares of Series B Preferred Stock issued and outstanding, respectively, which are convertible into 4,464,273 shares of common stock.

Reserved Shares

The Company has authorized shares of common stock for future issuance as of September 30, 2025 as follows:


			September 30, 2025
Exercise of warrants			692,523
Shares available for future stock incentive awards			4,193,019

Unvested restricted stock units			523,254
Outstanding common stock options			2,433,656
Series A Preferred Stock			4,026,346
Series B Preferred Stock			437,927

Total			12,306,725

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-8A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 235
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-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Revenue Arrangements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
Revenue Arrangements (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Jun. 30, 2024	Jun. 28, 2024	Jan. 31, 2023	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Unsatisfied performance obligation	$ 0		$ 0		$ 0
Accounts receivable	722		722		872
Collaboration and license revenue	0	$ 0	400	$ 39,111
Grant revenue	452	387	1,450	561
National Institute of Neurological Disorders and Stroke of the National Institutes of Health Funding
Disaggregation of Revenue [Line Items]
Government assistance, award amount						$ 1,500
Government assistance, potential additional award amount	1,500		1,500
Government assistance, asset, current	500		500		600
Grant revenue	500	400	1,400	600
Astellas \| License
Disaggregation of Revenue [Line Items]
Contract liability								$ 10,000
Future contingent additional payments								$ 340,000
Unsatisfied performance obligation	0		0
Accounts receivable	0		0		100
Revenue recognized	0		0	6,300
Revenue recognized from performance obligations related to prior periods as a result of change in transaction price				3,200
SOBI
Disaggregation of Revenue [Line Items]
Revenue recognized				30,000
Collaboration and license revenue				$ 32,800
SOBI \| License
Disaggregation of Revenue [Line Items]
Contract liability							$ 30,000		$ 75,000
Future contingent additional payments									$ 630,000
Accounts receivable	100		100		100
Revenue recognized	$ 0	$ 0	$ 0		$ 30,000

Restructuring - Schedule of Change in Accrued Restructuring Balance (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 80	$ 3,896
Charges	0	777
Cash Payments	(80)	(4,586)
Ending Balance	$ 0	$ 87

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References + Details

Stock Incentive Plans - Schedule of Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs)	9 Months Ended
	Sep. 30, 2025 $ / shares shares
Number of shares
Unvested at beginning of period (in shares) \| shares	444,238
Granted (in shares) \| shares	256,790
Vested (in shares) \| shares	(153,293)
Forfeited (in shares) \| shares	(24,481)
Unvested at end of period (in shares) \| shares	523,254
Weighted-average grant date fair value ($)
Unvested at beginning of period (in dollars per share) \| $ / shares	$ 19.86
Granted (in dollars per share) \| $ / shares	16.93
Vested (in dollars per share) \| $ / shares	19.86
Forfeited (in dollars per share) \| $ / shares	17.07
Unvested at end of period (in dollars per share) \| $ / shares	$ 18.55

Leases - Schedule of Additional Information Related to Operating Leases (Details)	Sep. 30, 2025	Sep. 30, 2024	Sep. 01, 2022
Leases [Abstract]
Weighted-average remaining lease term	4 years	4 years 8 months 12 days
Weighted-average discount rate	11.90%	11.60%	11.30%

Basis of Presentation	9 Months Ended
Basis of Presentation	Sep. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, Selecta Biosciences Security Corporation, a Massachusetts securities corporation which the Company dissolved in December 2024, and Cartesian Bio, LLC, a Delaware limited liability company, which is a variable interest entity for which the Company is the primary beneficiary. All significant intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: fair value of the intangible assets acquired in connection with the Merger, fair value of the CVRs, deferred income taxes, revenue recognition, accrued research and development expenses, stock-based compensation expense, fair value of the liability-classified warrants, and impairment of long-lived assets. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information Operating segments are defined as components of an enterprise for which separate and discrete information is available for evaluation by the chief operating decision maker, or the CODM, for the purposes of assessing performance and allocating resources. The Company views its operations and manages its business in one operating segment, which prior to the Merger related to the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases and subsequent to the Merger relates to the research and development of cell therapy product candidates. The Company’s CODM function is fulfilled by its Chief Executive Officer. The CODM function assesses performance for the segment and decides how to allocate resources based on consolidated net income (loss) that is also reported on the consolidated statements of operations and comprehensive income (loss). The CODM function uses net income (loss) to monitor budget versus actual results to assess performance of the segment. Segment assets are the same as total assets on the Company’s consolidated balance sheets. All long-lived assets are located in the United States. Long-lived assets consist of property and equipment, net, and operating lease right-of-use assets.

Stock Incentive Plans - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2025	Jun. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Nov. 13, 2023	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)			12,306,725		12,306,725
Stock‑based compensation expense			$ 2,590	$ 1,695	$ 8,377	$ 4,717
Plan modification, stock-based compensation expense					$ 700
Weighted average grant date fair value of stock options (in dollars per share)			$ 9.82	$ 14.23	$ 13.54	$ 15.75
Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)			2,433,656		2,433,656
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term					4 years
Weighted average period					2 years 6 months
Granted (in shares)					256,790
Granted (in dollars per share)					$ 16.93
Estimated forfeitures rate					10.00%
Unrecognized compensation expense			$ 5,900		$ 5,900
Common stock \| Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested employee stock options			$ 13,700		$ 13,700
Weighted average period					2 years 9 months 18 days
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)												40,341
Number of shares authorized, increase (in shares)	1,030,694	3,466,544
Total (in shares)			3,614,406		3,614,406
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)			496,639		496,639
Total (in shares)		360,000					450,000	60,833		66,667	39,166
Old Cartesian Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)			36,179		36,179
Award vesting term					4 years
Award expiration term					10 years
Unrecognized compensation expense related to unvested employee stock options									$ 2,600
Stock‑based compensation expense			$ 100	$ 300	$ 500	$ 1,000
Old Cartesian Plan \| Old Cartestian \| Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)									776,865
Old Cartesian Plan \| Old Cartestian \| Preferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)									14,112.299

Equity (Tables)

9 Months Ended

Sep. 30, 2025

Equity [Abstract]

Schedule of Warrant Activity

The following is a summary of the Company’s warrants as of September 30, 2025:


	Number of Warrants
	Equity classified	Liability classified	Total	Weighted-average exercise price




Outstanding at September 30, 2025	6,811	685,712	692,523	$	46.96

Schedule of Authorized Shares of Common Stock for Future Issuance

The Company has authorized shares of common stock for future issuance as of September 30, 2025 as follows:


			September 30, 2025
Exercise of warrants			692,523
Shares available for future stock incentive awards			4,193,019

Unvested restricted stock units			523,254
Outstanding common stock options			2,433,656
Series A Preferred Stock			4,026,346
Series B Preferred Stock			437,927

Total			12,306,725

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

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+ Details

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Statement of Comprehensive Income [Abstract]
Collaboration and license revenue	$ 0	$ 0	$ 400	$ 39,111
Grant revenue	452	387	1,450	561
Total revenue	452	387	1,850	39,672
Operating expenses:
Research and development	13,802	11,400	43,345	33,799
General and administrative	7,716	6,562	23,271	23,039
Total operating expenses	21,518	17,962	66,616	56,838
Operating loss	(21,066)	(17,575)	(64,766)	(17,166)
Interest income	1,548	2,573	5,311	4,932
Gain on change in fair value of warrant liabilities	516	5,669	2,988	2,803
(Loss) gain on change in fair value of contingent value right liability	(16,900)	(15,100)	18,746	(51,900)
Loss on change in fair value of forward contract liabilities	0	0	0	(6,890)
Other income (expense), net	0	250	(5)	1,050
Net loss	(35,902)	(24,183)	(37,726)	(67,171)
Other comprehensive (loss) income:
Foreign currency translation adjustment	(10)	(15)	34	(6)
Total comprehensive loss	(35,912)	(24,198)	(37,692)	(67,177)
Net loss	$ (35,902)	$ (24,183)	$ (37,726)	$ (67,171)
Net loss per share allocable to common stockholders:
Basic (in dollars per share)	$ (1.38)	$ (1.13)	$ (1.45)	$ (4.61)
Diluted (in dollars per share)	$ (1.38)	$ (1.13)	$ (1.45)	$ (4.61)
Weighted-average common shares outstanding:
Basic (in shares)	26,002,892	21,471,408	25,962,302	14,561,613
Diluted (in shares)	26,002,892	21,471,408	25,962,302	14,561,613

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2025
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes As of September 30, 2025, the Company has not recorded any U.S. federal or state income tax benefits for either the net losses the Company has incurred or its earned research and orphan drug credits, due to the uncertainty of realizing a benefit from those items in the future.

- Definition

The entire disclosure for income tax.

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+ Details

Cover Page - shares	9 Months Ended
Cover Page - shares	Sep. 30, 2025	Oct. 31, 2025
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2025
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Cartesian Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	7495 New Horizon Way
Entity Address, City or Town	Frederick
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	21703
City Area Code	301
Local Phone Number	348-8698
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	RNAC
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		26,003,606
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001453687

Investments	9 Months Ended
Investments	Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract]
Investments	Investments In September 2021, the Company and Cyrus Biotechnology, Inc., or Cyrus, entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share, at a purchase price of $0.8595 per share, for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company has recognized the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. During the nine months ended September 30, 2025 and the year ended December 31, 2024, there were no observable price changes. Therefore, the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheets. The Company’s maximum exposure to loss related to this variable interest entity is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement.

Stock Incentive Plans - Schedule of Stock Option Activity (Details) - 2016 Plan, 2018 Inducement Incentive Award Plan and Old Cartesian Plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2025 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares
Number of common stock options
Beginning balance (in shares) \| shares	1,706,035
Granted (in shares) \| shares	1,055,709
Exercised (in shares) \| shares	(82,944)
Forfeited (in shares) \| shares	(245,144)
Ending balance (in shares) \| shares	2,433,656	1,706,035
Vested (in shares) \| shares	895,577
Vested and expected to vest (in shares) \| shares	2,189,301
Weighted-average exercise price ($)
Beginning balance (in dollars per share) \| $ / shares	$ 11.99
Granted (in dollars per share) \| $ / shares	16.87
Exercised (in dollars per share) \| $ / shares	3.29
Forfeited (in dollars per share) \| $ / shares	17.74
Ending balance (in dollars per share) \| $ / shares	13.83	$ 11.99
Vested (in dollars per share) \| $ / shares	7.76
Vested and expected to vest (in dollars per share) \| $ / shares	$ 13.40
Weighted-average remaining contractual term (in years)
Outstanding, term	7 years 3 months	7 years 7 months 2 days
Vested, term	4 years 9 months 29 days
Vested and expected to vest, term	7 years 2 months 4 days
Aggregate intrinsic value
Outstanding \| $	$ 5,087	$ 12,025
Vested \| $	4,698
Vested and expected to vest \| $	$ 5,085

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2025
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventsThe Company has evaluated subsequent events through the date on which the consolidated financial statements were issued. The Company has concluded that no subsequent events have occurred that require disclosure

Restructuring - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Apr. 30, 2023	Sep. 30, 2025	Sep. 30, 2024
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated, period percent	90.00%
Restructuring charges		$ 0	$ 777
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges			200
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 600

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References + Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

Fair Value Measurements

9 Months Ended

Sep. 30, 2025

Fair Value Disclosures [Abstract]

Fair Value Measurements

The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of September 30, 2025 and December 31, 2024 (in thousands):


	September 30, 2025
	Total		Level 1		Level 2		Level 3
Assets:
Money market funds (included in cash equivalents)	$	40,357	$	40,357	$	—	$	—





Total assets	$	40,357	$	40,357	$	—	$	—

Liabilities:
Warrant liability	$	848	$	—	$	—	$	848
Contingent value right liability	369,000		—		—		369,000
Total liabilities	$	369,848	$	—	$	—	$	369,848


	December 31, 2024
	Total		Level 1		Level 2		Level 3
Assets:
Money market funds (included in cash equivalents)	$	39,088	$	39,088	$	—	$	—





Total assets	$	39,088	$	39,088	$	—	$	—

Liabilities:
Warrant liability	$	3,836	$	—	$	—	$	3,836
Contingent value right liability	395,500		—		—		395,500

Total liabilities	$	399,336	$	—	$	—	$	399,336

There were no transfers within the fair value hierarchy during the nine months ended September 30, 2025 or the year ended December 31, 2024.

Cash, Cash Equivalents, and Restricted Cash

As of September 30, 2025 and December 31, 2024, money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase.

As of September 30, 2025, the Company had restricted cash balances relating to secured letters of credit in connection with its real estate leases. The Company’s consolidated statements of cash flows include the following as of September 30, 2025 and 2024 (in thousands):


	September 30,
	2025		2024
Cash and cash equivalents	$	143,384	$	219,198

Long-term restricted cash	1,735		1,669
Total cash, cash equivalents, and restricted cash	$	145,119	$	220,867

Warrants to Purchase Common Stock

In December 2019, the Company issued warrants to purchase common stock in connection with a private placement, or the 2019 Warrants. The outstanding 2019 Warrants expired on December 23, 2024 in accordance with their terms. Pursuant to the terms of the 2019 Warrants, the Company could have been required to settle the 2019 Warrants in cash in the event of certain acquisitions of the Company and, as a result, the 2019 Warrants were required to be measured at fair value and reported as a liability on the balance sheet. On December 20, 2022, the Company amended the terms of the outstanding 2019 Warrants held by certain members of the Board of Directors, or the Amended 2019 Warrants, to remove the cash settlement provision. As a result, the Amended 2019 Warrants were remeasured at fair value on December 20, 2022 and reclassified from a liability to equity on the balance sheet.

In April 2022, the Company issued warrants in connection with an underwritten offering, or the 2022 Warrants. Pursuant to the terms of the 2022 Warrants, the Company could be required to settle the 2022 Warrants in cash in the event of an acquisition of the Company under certain circumstances and, as a result, the 2022 Warrants are required to be measured at fair value and reported as a liability on the balance sheet.

The Company recorded the fair value of the 2019 Warrants and the 2022 Warrants upon issuance using the Black-Scholes valuation model and is required to revalue the 2019 Warrants and the 2022 Warrants at each reporting date, with any changes in fair value recorded in the statements of operations and comprehensive loss. The valuations of the 2019 Warrants and the 2022 Warrants are classified as Level 3 of the fair value hierarchy due to the need to use assumptions in the valuations that are both significant to the fair value measurement and unobservable, including the stock price volatility and the expected life of the 2019 Warrants and the 2022 Warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement.

The estimated fair values of the 2019 Warrants and the 2022 Warrants were determined using the following inputs to the Black-Scholes simulation valuation:

•Estimated fair value of the underlying stock. The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period.

•Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption.

•Dividend rate. The dividend rate is based on the historical rate, which the Company anticipates will remain at zero.

•Expected life. The expected life of the 2019 Warrants was assumed to be equivalent to their remaining contractual term which expired on December 23, 2024. The expected life of the 2022 Warrants is assumed to be equivalent to their remaining contractual term which expires on April 11, 2027.

•Volatility. The Company estimates stock price volatility based on the Company’s historical volatility for a period of time commensurate with the expected remaining life of the warrants.

The 2019 Warrants expired on December 23, 2024 and therefore, there were no 2019 Warrants outstanding as of December 31, 2024 or September 30, 2025.

A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows:


	September 30, 2025		December 31, 2024
Risk-free interest rate	3.60	%	4.25	%
Dividend yield	—		—
Expected life (in years)	1.53		2.28
Expected volatility	97.44	%	92.92	%


	Warrant liability
Fair value as of December 31, 2024	$	3,836


Change in fair value	(2,988)
Fair value as of September 30, 2025	$	848

Contingent Value Right

On December 6, 2023, as contemplated by the Merger Agreement, the Company entered into a contingent value rights agreement, or the CVR Agreement, pursuant to which each holder of common stock or a 2022 Warrant as of December 4, 2023 was distributed a CVR, issued by the Company for each share of common stock held directly or underlying a 2022 Warrant held by such holder as of December 4, 2023. Holders of warrants other than the 2022 Warrants will be entitled to receive, upon exercise of such warrants and in accordance with the terms of the warrants, 30 CVRs per each share of common stock underlying such warrants.

Each CVR entitles its holder to distributions of the following, pro-rated on a per-CVR basis, during the period ending on the date on which the Royalty Term (as defined in the Sobi License) ends, or the Termination Date:

•100% of all milestone payments, royalties and other amounts paid to the Company or its controlled affiliates, or the Company Entities, under the Sobi License or, following certain terminations of the Sobi License, any agreement a Company Entity enters into that provides for the development and commercialization of SEL-212; and

•100% of all cash consideration and the actual liquidation value of any and all non-cash consideration of any kind that is paid to or is actually received by any Company Entity prior to the Termination Date pursuant to an agreement relating to a sale, license, transfer or other disposition of any transferable asset of the Company existing as of immediately prior to the Merger, other than those exclusively licensed under the Sobi License or which the Company Entities are required to continue to own in order to comply with the Sobi License.

The distributions in respect of the CVRs will be made on a semi-annual basis, and will be subject to a number of deductions, subject to certain exceptions or limitations, including for (i) certain taxes payable on the proceeds subject to the CVR distribution, (ii) certain out of pocket costs incurred by the Company Entities, including audit and accounting fees incurred in connection with reporting obligations relating to the CVRs and other expenses incurred in the performance of their obligations and other actions under the CVR Agreement, (iii) a fixed semi-annual amount of $0.75 million for general and administrative overhead, (iv) payments made and remaining obligations on lease liabilities of Selecta immediately prior to the Merger and (v) amounts paid and remaining obligations with regard to the Xork product candidate. Each of the deductions described in (iv) and (v) will be made only if certain milestone payments under the Sobi License are made and are also subject to certain adjustments as contemplated in the CVR Agreement. Upon the achievement of a development milestone in June 2024, Sobi became obligated to make a $30.0 million payment to the Company and made such payment in July 2024. The proceeds from this payment, net of deductions specified in the CVR Agreement, were included in the scheduled distribution to the holders of the CVR in March 2025.

The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825, Financial Instruments, or ASC 825. Under the fair value option election, the CVRs are initially measured at the aggregate estimated fair value of the CVRs and will be subsequently remeasured at estimated fair value on a recurring basis at each reporting period date. The liability was recorded at the date of approval, November 13, 2023, as a dividend. The estimated fair value of the CVR liability was determined using a Monte Carlo simulation model to estimate future cash flows associated with the legacy assets, including the expected milestone and royalty payments under the Sobi License, net of deductions. Changes in fair value of the CVR liability are presented in the consolidated statements of operations and comprehensive loss. The liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, and expected volatility of future revenues, which represent a Level 3 measurement within the fair value hierarchy. The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows:


	September 30, 2025		December 31, 2024
Estimated cash flow dates	2025 - 2038		2025 - 2038
Estimated probability of success	95.0% - 100.0%		95.0% - 100.0%
Expected volatility of future revenues	22.5	%	22.0	%

The following table reflects a roll-forward of fair value for the Company’s Level 3 CVR liability for the nine months ended September 30, 2025 (in thousands):


	CVR liability
Fair value as of December 31, 2024	$	395,500

Distributions	(7,754)
Change in fair value	(18,746)
Fair value as of September 30, 2025	$	369,000

Forward Contract Liabilities

The Company entered into a contract for the issuance of 149,330.115 shares of Series A Preferred Stock as part of the 2023 Private Placement which was settled in multiple tranches. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Timothy A. Springer, a member of the Company’s Board of Directors, and TAS Partners

LLC, an affiliate of Dr. Springer, represented a forward contract. See Note 11, “Convertible Preferred Stock” to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024. The initial fair value of the forward contract liability on November 13, 2023 was insignificant as the fair value of the underlying Series A Preferred Stock was equal to the purchase price of the Series A Preferred Stock as agreed upon in the 2023 Private Placement. Subsequent measurement of the fair value of the forward contract liability was based on the market price of the Company’s common stock, which represented the redemption and conversion value of the Series A Preferred Stock, less the purchase price, on an as-converted basis. The non-cash settlement of a portion of the liability occurred on December 13, 2023 with the issuance of the first tranche of the Series A Preferred Stock for $14.8 million. The non-cash settlement of the remaining second and third tranches occurred on January 12, 2024 and February 11, 2024, respectively, for a total of $35.2 million

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References

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+ Details