Sesen Bio and Carisma Therapeutics Announce Increased Special Cash Dividend and Stockholder Support for Pending Merger Amend Merger Agreement to Increase Expected Special Cash Dividend to $75 Millio

Sesen Bio and Carisma Therapeutics Announce

Increased Special Cash Dividend and Stockholder Support for Pending Merger

Amend Merger Agreement to Increase Expected

Special Cash Dividend to $75 Million and Extend Contingent Value Right for Vicineum to March 31, 2027

Bradley L. Radoff and Michael Torok to Vote

Shares in Support of Transaction

Michael Torok to Join Carisma Board of Directors

as Only Sesen Bio Representative

Special Meeting of Stockholders to Approve Merger

to be Held on March 2, 2023

Mass. & PHILADELPHIA - February 14, 2023 - Sesen Bio, Inc. (Nasdaq: SESN) and Carisma Therapeutics

Inc. (Carisma), a privately-held, clinical stage biopharmaceutical company focused on discovering and developing innovative immunotherapies,

today announced that the companies have reached a voting and support agreement (the "Support Agreement") with Bradley L. Radoff

and Michael Torok (together with their respective affiliates, the "Investor Group"). The Investor Group, which beneficially

owns approximately 8.7% of Sesen Bio's outstanding common stock, has committed to vote its shares in support of the pending merger.

connection with the Support Agreement, Sesen Bio and Carisma have further amended the previously amended merger agreement announced on

December 29, 2022, which has been unanimously approved by the Boards of Directors of both companies:

Dr. Jay Duker, Chair of the Sesen Bio Board of Directors, said,

"We appreciate Mr. Radoff's and Mr. Torok's constructive engagement. Our discussions with our stockholders

over the last several weeks have enabled valuable changes to the merger agreement that benefit all stockholders, including the significantly

increased expected special cash dividend of $75 million and an enhanced CVR. We are confident that the merger with Carisma maximizes value

for all Sesen Bio stockholders, and we are pleased Mr. Radoff and Mr. Torok will support the pending transaction. We look forward

to expeditiously closing the merger by the end of the quarter."

Based on basic outstanding shares including unvested RSUs.

Steven Kelly, President and Chief Executive Officer of Carisma, added,

"We are excited to reach this agreement with Sesen Bio and Messrs. Radoff and Torok. The merger provides significant funding

for Carisma's R&D pipeline to advance our revolutionary proprietary cell therapy platform. We are confident the combined company will

be well positioned to create significant value for stockholders."

Mr. Torok, on behalf of the Investor Group, said, "We are

pleased that our extensive engagement with Sesen Bio and its representatives has helped unlock additional value for Sesen Bio's

stockholders. We believe the amended merger terms represent the most value maximizing path for stockholders. We are excited about the

long-term value potential of the Carisma platform, and I look forward to joining the Carisma Board."

Carisma's previously announced approximately $30 million financing

remains committed and is expected to close concurrently with the pending merger.

The issuance of the special cash dividend and CVR remain contingent

on the closing of the pending transaction, which is expected to occur in the first quarter of 2023, subject to approval by Sesen Bio stockholders

and other customary closing conditions.

Following completion of the incremental financing from Carisma's

key investors and subsequent completion of the merger, Sesen Bio stockholders are expected to own 24.2% of the pro forma company consistent

with the exchange ratio formula set forth in the original merger agreement, dated September 20, 2022.

The Sesen Bio Board 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. As previously

disclosed, the special meeting of stockholders 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.

The complete Support Agreement, which includes other customary provisions, will be filed as an exhibit to a Form 8-K to be filed

by the Company with the U.S. Securities and Exchange Commission.

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. Evercore is serving as lead financial advisor to

Carisma for the transaction and BofA Securities, Inc. is also serving as financial advisor to Carisma for the transaction.

Wilmer Cutler Pickering Hale and Dorr LLP is serving as legal counsel to Carisma. BofA Securities, Inc. and Evercore are

serving as co-placement agents for Carisma's concurrent financing and Shearman & Sterling LLP is serving as the

placement agents' legal counsel. Olshan Frome Wolosky LLP is serving as legal counsel to the Investor Group.

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 stockholder value. Additionally, Sesen Bio is

seeking a partner for the further development of Vicineum. For more information, please visit the Company's website at www.sesenbio.com.

About Carisma Therapeutics

Carisma Therapeutics Inc. is a biopharmaceutical company dedicated

to developing a differentiated and proprietary cell therapy platform focused on engineered macrophages, cells that play a crucial role

in both the innate and adaptive immune response. The first applications of the platform, developed in collaboration with the University

of Pennsylvania*, are autologous chimeric antigen receptor (CAR)-macrophages for the treatment of solid tumors. Carisma is headquartered

in Philadelphia, PA. For more information, please visit www.carismatx.com *Carisma has licensed certain Penn-owned intellectual property

from the University of Pennsylvania, and Penn's Perelman School of Medicine receives sponsored research and clinical trial funding from

Carisma. Penn and certain of its faculty members, including Dr. Gill, are current equity holders in Carisma and have received and

may be entitled to receive future financial consideration from Carisma from the development and commercialization of products based on

licensed Penn intellectual property.

Cautionary Note on Forward-Looking Statements

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, 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 and 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

and the receipt of any payments under the contingent value rights, 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