DESCRIPTION OF BUSINESS	12 Months Ended
DESCRIPTION OF BUSINESS	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
DESCRIPTION OF BUSINESS	DESCRIPTION OF BUSINESS Sesen Bio, Inc. ("Sesen Bio" or the “Company”), a Delaware corporation formed in February 2008, is a late-stage clinical company that previously focused on advancing targeted fusion protein therapeutics (“TFPTs”) for the treatment of patients with cancer. The Company’s most advanced product candidate, VicineumTM, also known as VB4-845, is a locally administered targeted fusion protein composed of an anti-epithelial cell adhesion molecule ("EpCAM") antibody fragment tethered to a truncated form of Pseudomonas exotoxin A for the treatment of non-muscle invasive bladder cancer (“NMIBC”). On July 15, 2022, the Company made the strategic decision to voluntarily pause further development of Vicineum in the United States. The decision was based on a thorough reassessment of Vicineum following discussions with the United States Food and Drug Administration ("FDA"), which had implications on the size, timeline and costs of an additional Phase 3 clinical trial, which the FDA previously confirmed would be required for a potential resubmission of a biologics license application ("BLA") for Vicineum for the treatment of NMIBC. As a result of this decision, the Company turned its primary focus to consummating a strategic transaction with the goal of maximizing stockholder value. Following an extensive process of evaluating strategic alternatives, including identifying and reviewing potential candidates for a strategic transaction, on September 20, 2022, the Company, Seahawk Merger Sub, Inc., a Delaware corporation and a wholly-owned subsidiary of the Company (“Merger Sub”), and CARISMA Therapeutics Inc. (“Carisma”), entered into the Agreement and Plan of Merger and Reorganization dated as of September 20, 2022, as amended by the First Amendment thereto dated as of December 29, 2022 and the Second Amendment thereto dated as of February 13, 2023 (the “Merger Agreement”), pursuant to which, among other things, and subject to the satisfaction or waiver of certain conditions set forth in the Merger Agreement, Merger Sub will merge with and into Carisma, with Carisma continuing as a wholly-owned subsidiary of the Company and the surviving corporation of the merger (the “Merger”). The Company’s board of directors unanimously approved the Merger Agreement and resolved to recommend that its stockholders approve the proposals described in the Merger Agreement. If the Merger is completed, the business of Carisma will continue as the business of the combined company. The Company continues to believe that Vicineum has benefits for patients and healthcare providers that can be maximized through a company with a larger infrastructure, and as such, it is seeking a partner that can execute further development to realize the full potential of Vicineum. As a result of such decision and the Company’s subsequent decision to enter into the proposed Merger with Carisma, it no longer plans to pursue regulatory approval of Vicineum for NMIBC in the European Union (the “E.U.”) and has started to wind down certain of its manufacturing operations and business development partnerships. Additionally, the Company is seeking a partner for the further development of Vicineum and has initiated a formal process and engaged a financial advisor for the potential sale of Vicineum. If the proposed Merger is consummated, the combined company does not expect to pursue further development of Vicineum. Anticipated Merger with CARISMA Therapeutics Inc. The Merger is expected to be completed during the first quarter of 2023. In connection with the Merger, the Company is seeking the approval of its stockholders to, among other things, (a) issue the shares of its common stock issuable in connection with the Merger pursuant to the rules of The Nasdaq Stock Market LLC (“Nasdaq”), and (b) amend its amended and restated Certificate of Incorporation (the “Certificate of Incorporation”) to effect a reverse stock split of the outstanding shares of its common stock at a ratio of 1-for-20 (clauses (a) and (b), collectively, the “Sesen Bio Voting Proposals”). The special meeting of stockholders in which the Company’s stockholders will be asked to vote on the Sesen Bio Voting Proposals (the “Special Meeting”) will be held on March 2, 2023 at 10:00 a.m. Eastern Time. Sesen Bio’s future operations are highly dependent on the success of the Merger and there can be no assurances that the Merger will be successfully consummated. In the event that Sesen Bio does not complete the Merger with Carisma, Sesen Bio may continue to review and evaluate strategic alternatives, including, without limitation, a dissolution of Sesen Bio. Viventia Acquisition In September 2016, the Company entered into a Share Purchase Agreement with Viventia Bio, Inc., a corporation incorporated under the laws of the Province of Ontario, Canada ("Viventia"), the shareholders of Viventia named therein (the “Selling Shareholders”) and, solely in its capacity as seller representative, Clairmark Investments Ltd., a corporation incorporated under the laws of the Province of Ontario, Canada (“Clairmark”) (the “Share Purchase Agreement”), pursuant to which the Company agreed to and simultaneously completed the acquisition of all of the outstanding capital stock of Viventia from the Selling Shareholders (the “Viventia Acquisition”). In connection with the closing of the Viventia Acquisition, the Company issued 4.0 million shares of its common stock to the Selling Shareholders, which at that time represented approximately 19.9% of the voting power of the Company as of immediately prior to the issuance of such shares. Clairmark is an affiliate of Leslie L. Dan, a director of the Company until his retirement in July 2019. In addition, under the Share Purchase Agreement, the Company is obligated to pay to the Selling Shareholders certain post-closing contingent cash payments upon the achievement of specified milestones and based upon net sales, in each case subject to the terms and conditions set forth in the Share Purchase Agreement, including: (i) a one-time milestone payment of $12.5 million payable upon the first sale of Vicineum (the “Purchased Product”), in the United States; (ii) a one-time milestone payment of $7.0 million payable upon the first sale of the Purchased Product in any one of certain specified European countries; (iii) a one-time milestone payment of $3.0 million payable upon the first sale of the Purchased Product in Japan; and (iv) quarterly earn-out payments equal to 2% of net sales of the Purchased Product during specified earn-out periods. Such earn-out payments are payable with respect to net sales in a country beginning on the date of the first sale in such country and ending on the earlier of (i) December 31, 2033, and (ii) fifteen years after the date of such sale, subject to early termination in certain circumstances if a biosimilar product is on the market in the applicable country. Under the Share Purchase Agreement, the Company, its affiliates, licensees and subcontractors are required to use commercially reasonable efforts, for the first seven years following the closing of the Viventia Acquisition, to achieve marketing authorizations throughout the world and, during the applicable earn-out period, to commercialize the Purchased Product in the United States, France, Germany, Italy, Spain, United Kingdom, Japan, China and Canada. Certain of these payments are payable to individuals or affiliates of individuals that became employees or members of the Company's board of directors. However, as of December 31, 2022, none of these individuals are active employees or members of the Company's board of directors. Liquidity and Capital Resources As of December 31, 2022, the Company had cash, cash equivalents, and marketable securities of $166.9 million and an accumulated deficit of $336.1 million. The Company incurred positive cash flows from operating activities of $24.9 million for the year ended December 31, 2022. The Company incurred negative cash flows from operating activities $68.9 million and $30.8 million for the years ended December 31, 2021 and 2020, respectively. Since the Company’s inception, it has received no revenue from sales of its products, and the Company anticipates that operating losses will continue for the foreseeable future as it seeks to close the Merger with Carisma. The Company has financed its operations to date primarily through private placements of its common stock, preferred stock, common stock warrants and convertible bridge notes, venture debt borrowings, its initial public offering ("IPO"), follow-on public offerings, sales effected in "at-the-market" ("ATM") offerings, commercialization partnership, out-license agreements and an asset purchase agreement. See “Note 13. Stockholders’ Equity (Deficit)” below for information regarding the Company’s recently completed equity financings. Management believes that the Company’s cash and cash equivalents as of December 31, 2022 will be sufficient to fund the Company's operations for at least the next twelve months from the date these consolidated financial statements were issued. Funding Requirements The Company's future funding requirements will depend on the outcome of the proposed Merger with Carisma. The Company is subject to a number of risks similar to other clinical companies that have determined to focus primarily on pursuing a strategic transaction, including, but not limited to, those which are described under Part I Item 1A. Risk Factors of this Annual Report on Form 10-K. The Company will incur substantial expenses if and as it: •addresses its ongoing litigation related to the Merger; •maintains and protects its intellectual property portfolio; •reduces its personnel and incurs related severance and employee-related costs; •explores, evaluates and pursues any strategic alternatives if the Merger is not completed. The Company's future capital requirements will depend on many factors, including: •the outcome and the timing of the proposed Merger with Carisma; •the outcome and timing of any pending or future litigation involving the Company or its business; •the costs and timing of maintaining and enforcing the Company’s intellectual property rights and defending any intellectual property-related claims; and •its obligation to make milestone, royalty and other payments to third-party licensors under its licensing agreements. Until such time, if ever, as the Company can generate substantial revenues, the Company expects to finance the cash needs through a combination of equity offerings, debt financings, or government or other third-party funding. To the extent that the Company raises additional capital through the sale of equity or convertible debt securities, the ownership interests of existing stockholders will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect the rights of existing stockholders. Debt financing, if available, may involve agreements that include liens or other restrictive covenants limiting the Company’s ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends. If the Company is unable to raise additional funds when needed, it may be required to delay, limit, reduce or terminate its assessment of strategic alternatives. If the Company does not successfully consummate the proposed Merger with Carisma, its board of directors may decide to explore other strategic alternatives, including, without limitation, a dissolution of the company.