Description of the Business	12 Months Ended
Description of the Business	Dec. 31, 2024
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 mRNA cell therapy for the treatment of autoimmune diseases leveraging its proprietary technology and manufacturing platform to introduce one or more mRNA molecules into cells to enhance their function. The Company believes its mRNA cell therapies have the potential to deliver deep, durable clinical benefit to a broad group of patients with autoimmune diseases because they can be administered over a short period of time, in an outpatient setting, and without pre-treatment chemotherapy. 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. 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. For additional information, see Note 4. 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 provides 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. For additional information, see Note 11. 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 6. On March 27, 2024, the Company’s stockholders approved the Conversion Proposal (as defined below). For additional information, see Note 11. 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 Annual Report on Form 10-K relating to shares of the Company’s common stock (such as share amounts, per share amounts, and conversion rates and prices), have been adjusted to 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 Series B Conversion Proposal (as defined below). For additional information, see Note 11. 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. Liquidity and Management’s Plan As of December 31, 2024, the Company had an accumulated deficit of $692.1 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. As of December 31, 2024, the Company’s cash, cash equivalents, and restricted cash were $214.3 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 December 31, 2024 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. 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 flow 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 6. 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.