BML, One of Sesen Bio’s Top Stockholders, Announces Support for Pending Merger with Carisma Therapeutics Sesen Bio Board Unanimously Recommends All Stockholders to Vote “FOR” the Value Maximizing Merger on the WHITE Proxy Card Today CAMBRIDGE, Mass. — February 15, 2023 — Sesen Bio, Inc. (Nasdaq: SESN) (“Sesen Bio” or the “Company”), today announced that BML Investment Partners, L.P. (“BML”), one of Sesen Bio’s top stockholders, intends to vote all of its shares in support of the Company’s pending merger with Carisma Therapeutics Inc. (“Carisma”). BML owned approximately 4.1% of the Company's outstanding common stock as of the close of business on January 17, 2023, the record date for the Special Meeting of Stockholders. Brad Leonard, Managing Member of BML, said, “Sesen Bio has engaged extensively with stockholders, and we are confident in the Board and management team’s process leading to the selection of Carisma as a merging partner. With a substantial special cash dividend, enhanced CVR and exciting long-term potential of Carisma, we believe this represents the best path forward for Sesen Bio and its stockholders and we are pleased to support the pending merger.” Dr. Jay Duker, Chair of the Sesen Bio Board of Directors, said, “We are thrilled that BML, one of our largest stockholders, recognizes that the pending merger with Carisma maximizes value and is the best available alternative for Sesen Bio stockholders. With the strong support of BML and other top stockholders, including Bradley Radoff and Michael Torok, we look forward to satisfying the remaining conditions to closing and expeditiously completing the transaction.” The support from BML adds to the growing support of the pending merger, including from Bradley L. Radoff and Michael Torok (together with their respective affiliates, the “Investor Group”), which beneficially owns approximately 8.7% of Sesen Bio’s common stock. As previously disclosed, on February 13, 2023, the Investor Group entered into a voting and support agreement and has committed to vote its shares in support of the pending merger. Key benefits of the pending merger include: • Expected special one-time cash dividend of $75 million, $0.36 per share1. • Additional potential cash upside via Contingent Value Right (“CVR”), including from any sale of Sesen Bio’s legacy assets (including Vicineum), which has been extended to March 31, 2027, and from the potential $30 million milestone payment, $0.14 per share2, under the Roche Asset Purchase Agreement. 1 Based on basic outstanding shares including unvested RSUs. 2 Based on basic outstanding shares including unvested RSUs.

• 24.2% stake in the $352 million3 combined company, $0.38 per share4, with potential for long- term upside through Carisma’s proprietary CAR-M platform. The Sesen Bio Board of Directors unanimously recommends that stockholders vote “FOR” each of the proposals listed on the WHITE proxy card enclosed with the previously mailed definitive proxy statement / prospectus today. Shareholders can switch their vote at any time to vote “FOR” the merger. Only the latest-dated proxy card counts. The Special Meeting will be held exclusively via live webcast on March 2, 2023, at 10:00 a.m. Eastern Time. Sesen Bio stockholders of record as of January 17, 2023, will be entitled to vote at the meeting. Sesen Bio’s definitive proxy statement / prospectus and other materials regarding the pending merger can be found at www.SesenBioandCarisma.com. Sesen Bio stockholders who need assistance voting or have questions regarding the Sesen Bio Special Meeting may contact Sesen Bio’s proxy solicitor, MacKenzie Partners, toll-free at 1-800-322-2885 or email at proxy@mackenziepartners.com. SVB Securities is acting as exclusive financial advisor to Sesen Bio for the transaction and Hogan Lovells US LLP is serving as its legal counsel. About Sesen Bio Sesen Bio, Inc. is a late-stage clinical company focused on targeted fusion protein therapeutics for the treatment of patients with cancer. Sesen Bio’s most advanced product candidate, Vicineum™, also known as VB4-845, is a locally-administered targeted fusion protein composed of an anti-epithelial cell adhesion molecule antibody fragment tethered to a truncated form of Pseudomonas exotoxin A for the treatment of non-muscle invasive bladder cancer. On July 15, 2022, Sesen Bio made the strategic decision to voluntarily pause further development of Vicineum in the US. The decision was based on a thorough reassessment of Vicineum, which included the incremental development timeline and associated costs for an additional Phase 3 clinical trial, following Sesen Bio’s discussions with the United States Food and Drug Administration. Sesen Bio has turned its primary focus to assessing potential strategic alternatives with the goal of maximizing shareholder value. Additionally, Sesen Bio intends to seek a partner for the further development of Vicineum. For more information, please visit the Company’s website at www.sesenbio.com. Cautionary Note on Forward-Looking Statements Any statements in this press release about future expectations, plans and prospects for Sesen Bio, Inc. (Sesen Bio), CARISMA Therapeutics Inc. (Carisma) or the combined company, Sesen Bio’s, Carisma’s or the combined company’s strategy or future operations, and other statements containing the words “anticipate,” “believe,” “contemplate,” “expect,” “intend,” “may,” “plan,” “predict,” “target,” “potential,” “possible,” “will,” “would,” “could,” “should,” “continue,” and similar expressions, 3 Reflects pro forma company value including estimated equity value of Carisma (~$196.0 million), implied value of Sesen Bio (~$85.0 million assuming $70.0 million cash available at closing), aggregate proceeds from Carisma pre-closing financing (~ $30.6 million) and fully diluted value of the Moderna convertible note (~ $40.1 million). 4 Based on pro forma fully diluted post-close combined company shares outstanding.

constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. For example, statements concerning the proposed transaction, the concurrent financing, the contingent value rights and other matters, including without limitation: statements relating to the satisfaction of the conditions to and consummation of the proposed transaction, the expected timing of the consummation of the proposed transaction, the expected ownership percentages of the combined company, Sesen Bio’s and Carisma’s respective businesses, the strategy of the combined company, future operations, advancement of the combined company’s product candidates and product pipeline, clinical development of the combined company’s product candidates, including expectations regarding timing of initiation and results of clinical trials of the combined company, the ability of Sesen Bio to remain listed on the Nasdaq Stock Market, the completion of the concurrent financing, the receipt of any payments under the contingent value rights, and the amount and timing of distributions to be made to Sesen Bio stockholders, if any, in connection with any potential dissolution or liquidation scenario are forward-looking statements. Actual results may differ materially from those indicated by such forward- looking statements as a result of various important factors, including without limitation: (i) the risk that the conditions to the closing of the proposed transaction are not satisfied, including the failure to obtain stockholder approval of matters related to the proposed transaction in a timely manner or at all; (ii) uncertainties as to the timing of the consummation of the proposed transaction and the ability of each of Sesen Bio and Carisma to consummate the proposed transaction, including completing the concurrent financing; (iii) risks related to Sesen Bio’s ability to correctly estimate its expected net cash at closing and Sesen Bio’s and Carisma’s ability to correctly estimate and manage their respective operating expenses and expenses associated with the proposed transaction; (iv) risks related to Sesen Bio’s continued listing on the Nasdaq Stock Market until closing of the proposed transaction; (v) the risk that as a result of adjustments to the exchange ratio, Sesen Bio stockholders or Carisma stockholders could own less of the combined company than is currently anticipated; (vi) the risk that the conditions to payment under the contingent value rights will not be met and that the contingent value rights may otherwise never deliver any value to Sesen Bio stockholders; (vii) risks associated with the possible failure to realize certain anticipated benefits of the proposed transaction, including with respect to future financial and operating results; (viii) uncertainties regarding the impact any delay in the closing would have on the anticipated cash resources of the combined company upon closing and other events and unanticipated spending and costs that could reduce the combined company’s cash resources; (ix) the effect of uncertainties related to the actions of activist stockholders, which could make it more difficult to obtain the approval of Sesen Bio stockholders with respect to the transaction related proposals and result in Sesen Bio incurring significant fees and other expenses, including for third-party advisors; (x) the occurrence of any event, change or other circumstance or condition that could give rise to the termination of the merger agreement, as amended; (xi) the effect of the announcement, pendency or completion of the merger on Sesen Bio’s or Carisma’s business relationships, operating results and business generally; (xii) costs related to the merger; (xiii) the outcome of any legal proceedings instituted against Sesen Bio, Carisma or any of their respective directors or officers related to the merger agreement or the transactions contemplated thereby; (xiv) the ability of Sesen Bio or Carisma to protect their respective intellectual property rights; (xv) competitive responses to the proposed transaction and changes in expected or existing competition; (xvi) the success and timing of regulatory submissions and pre-clinical and clinical trials; (xvii) regulatory requirements or developments; (xviii) changes to clinical trial designs and regulatory pathways; (xix) changes in capital resource requirements; (xx) risks related to the inability of the combined company to obtain sufficient additional capital to continue to advance its product candidates and its preclinical programs; (xxi) legislative, regulatory, political and economic developments; and (xxii) other factors discussed in the “Risk Factors” section of Sesen Bio’s Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and other reports filed with the Securities Exchange Commission (SEC). In addition, the forward-looking

statements included in this press release represent Sesen Bio’s and Carisma’s views as of the date hereof. Sesen Bio and Carisma anticipate that subsequent events and developments will cause the respective company’s views to change. However, while Sesen Bio may elect to update these forward- looking statements at some point in the future, Sesen Bio specifically disclaims any obligation to do so, except as required under applicable law. These forward-looking statements should not be relied upon as representing Sesen Bio’s views as of any date subsequent to the date hereof. Important Additional Information In connection with the proposed transaction between Carisma and Sesen Bio, Sesen Bio filed with the SEC a registration statement on Form S-4 (as amended, the registration statement) that includes a proxy statement of Sesen Bio and also constitutes a prospectus of Sesen Bio with respect to shares of Sesen Bio common stock to be issued in the proposed transaction (proxy statement/prospectus). The definitive proxy statement/prospectus was first mailed to Sesen Bio stockholders on or about January 24, 2023. Sesen Bio may also file other relevant documents regarding the proposed transaction with the SEC. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THESE MATERIALS, INCLUDING THE REGISTRATION STATEMENT, THE DEFINITIVE PROXY STATEMENT/PROSPECTUS, AND ALL OTHER RELEVANT DOCUMENTS THAT ARE OR WILL BE FILED WITH THE SEC IN CONNECTION WITH THE PROPOSED TRANSACTION, INCLUDING ANY AMENDMENTS OR SUPPLEMENTS TO THESE MATERIALS, BECAUSE THEY CONTAIN OR WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION AND THE PARTIES TO THE PROPOSED TRANSACTION. Investors and security holders are able to obtain the definitive proxy statement/prospectus and other documents that are filed or will be filed by Sesen Bio with the SEC free of charge from the SEC’s website at www.sec.gov or from Sesen Bio at the SEC Filings section of www.sesenbio.com. No Offer or Solicitation This press release shall not constitute an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended. Subject to certain exceptions to be approved by the relevant regulators or certain facts to be ascertained, a public offer will not be made directly or indirectly, in or into any jurisdiction where to do so would constitute a violation of the laws of such jurisdiction, or by use of the mails or by any means or instrumentality (including without limitation, facsimile transmission, telephone or internet) of interstate or foreign commerce, or any facility of a national securities exchange, of any such jurisdiction. Participants in the Solicitation Sesen Bio and Carisma and their respective directors, executive officers and other members of management may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information about Sesen Bio’s directors and executive officers is available in Sesen Bio’s Annual Report on Form 10-K for the fiscal year ended December 31, 2021, its definitive proxy statement dated April 28, 2022 for its 2022 Annual Meeting of Stockholders and its Current Report on Form 8-K filed with the SEC on August 31, 2022. Other information regarding the participants in the proxy solicitation and a description of their interests in the proposed transaction, by security holdings or otherwise, is included in the definitive proxy statement/prospectus and other relevant materials that

are or will be filed with the SEC regarding the proposed transaction. Investors should read the definitive proxy statement/prospectus carefully before making any voting or investment decisions. You may obtain free copies of these documents from Sesen Bio or the SEC’s website as indicated above. Investors: Erin Clark, Vice President, Corporate Strategy & Investor Relations ir@sesenbio.com