Exhibit 99.1 

Sesen Bio Receives Complete Response Letterfrom FDA for Vicineum™ (oportuzumab monatox-qqrs)

Company to host conference call on Monday, August 16at 8:00 a.m. ET

CAMBRIDGE, Mass., August 13, 2021 – Sesen Bio (Nasdaq:SESN), a late-stage clinical company developing targeted fusion protein therapeutics for the treatmentof patients with cancer, today announced that it received a Complete Response Letter (CRL) from the U.S. Food and Drug Administration(FDA) regarding its Biologics License Application (BLA) for Vicineum™ (oportuzumab monatox-qqrs) for the treatment of BCG-unresponsivenon-muscle invasive bladder cancer (NMIBC).

The FDA has determined that it cannot approve the BLA for Vicineumin its present form and has provided recommendations specific to additional clinical/statistical data and analyses in addition to Chemistry,Manufacturing and Controls (CMC) issues pertaining to a recent pre-approval inspection and product quality.

“We are deeply disappointed by this unexpected result, and itis an unfortunate day for patients suffering from BCG-unresponsive NMIBC,” said Dr. Thomas Cannell, president, and chief executiveofficer of Sesen Bio. “We remain dedicated to our mission to save and improve the lives of patients by bringing new treatment optionsto patients, and we intend to work closely with the FDA to understand next steps.”

The Company plans to request a Type A meeting as soon as possible withthe FDA to discuss the next steps that are needed before the application may be approved.

As of June 30, 2021, the Company had $151.1 million in cash, cashequivalents and restricted cash.

Conference Call and Webcast Information

Members of the Sesen Bio management team willhost a conference call Monday, August 16, 2021, at 8:00 AM ET. To participate in the conference call, please dial (844) 831-3025(domestic) or (315) 625-6887 (international) and refer to conference ID 2772032. The teleconference details can be accessed in the InvestorRelations section of the Company's website at www.sesenbio.com. A replay of the teleconference will be available in the investorsection of the Company’s website at www.sesenbio.com for 60 days following the call.

About Vicineum™

Vicineum,a locally administered fusion protein, is Sesen Bio’s lead product candidate being developed for the treatment of BCG-unresponsivenon-muscle invasive bladder cancer (NMIBC). Vicineum is comprised of a recombinant fusion protein that targets epithelial cell adhesionmolecule (EpCAM) antigens on the surface of tumor cells to deliver a potent protein payload, Pseudomonas Exotoxin A. Vicineum is constructedwith a stable, genetically engineered peptide tether to ensure the payload remains attached to the antibody binding fragment until itis internalized by the cancer cell. This fusion protein design is believed to decrease the risk of toxicity to healthy tissues, therebyimproving its safety. In prior clinical trials conducted by Sesen Bio, EpCAM has been shown to be overexpressed in NMIBC cells with minimalto no EpCAM expression observed on normal bladder cells. Sesen Bio is currently in the follow-up stage of a Phase 3 registration trialin the US for the treatment of BCG-unresponsive NMIBC. In February 2021, the FDA accepted the Company’s BLA file for Vicineumfor the treatment of BCG-unresponsive NMIBC and granted the application Priority Review with a target PDUFA date of August 18, 2021.On August 13, 2021, the Company received a Complete Response Letter (CRL) from the FDA regarding its BLA for Vicineum. Additionally,Sesen Bio believes that cancer cell-killing properties of Vicineum promote an anti-tumor immune response that may potentially combinewell with immuno-oncology drugs, such as checkpoint inhibitors. For this reason, the activity of Vicineum in BCG-unresponsive NMIBC isalso being explored at the US National Cancer Institute in combination with AstraZeneca’s immune checkpoint inhibitor durvalumab.

About Sesen Bio

Sesen Bio, Inc.is a late-stage clinical company advancing targeted fusion protein therapeutics for the treatment of patients with cancer. The Company’slead program, Vicineum™, also known as oportuzumab monatox, is currently in the follow-up stage of a Phase 3 registration trialfor the treatment of BCG-unresponsive non-muscle invasive bladder cancer (NMIBC). In February 2021, the FDA accepted the Company’sBLA file for Vicineum for the treatment of BCG-unresponsive NMIBC and granted the application Priority Review with a target PDUFA dateof August 18, 2021. On August 13, 2021, the Company received a Complete Response Letter (CRL) from the FDA regarding its BLAfor Vicineum. Sesen Bio retains worldwide rights to Vicineum with the exception of Greater China, the Middle East and North Africa (MENA)and Turkey, for which the Company has partnered with Qilu Pharmaceutical, Hikma Pharmaceuticals and Eczacibasi Pharmaceuticals Marketing(EIP), respectively, for commercialization. Vicineum is a locally administered targeted fusion protein composed of an anti-EpCAM antibodyfragment tethered to a truncated form of Pseudomonas Exotoxin A, which is being developed for the treatment of BCG-unresponsive NMIBC.For more information, please visit the Company’s website at www.sesenbio.com.

COVID-19Pandemic Potential Impact

Sesen Bio continuesto monitor the rapidly evolving environment regarding the potential impact of the COVID-19 pandemic on the Company. The Company has notyet experienced any disruptions to our operations as a result of COVID-19, however, we are not able to quantify or predict with certaintythe overall scope of potential impacts to our business, including, but not limited to, our ability to raise capital and, if approved,commercialize Vicineum. Sesen Bio remains committed to the health and safety of patients, caregivers and employees.

Cautionary Note onForward-Looking Statements

Any statements in thispress release about future expectations, plans and prospects for the Company, the Company’s strategy, future operations, and otherstatements containing the words “anticipate,” “believe,” “expect,” “intend,” “may,” “plan,” “predict,” “target,” “potential,” “will,” “continue,”and similar expressions, constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995.For example, statements regarding the Company’s expectations regarding potential FDA approval of Vicineum for the treatment of BCG-unresponsiveNMIBC, the Company’s ability to bring new treatment options to patients with cancer, the Company’s intentions to work closelywith the FDA to understand next steps for its BLA for Vicineum for the treatment of BCG-unresponsive NMIBC, the Company’s plansto request a Type A meeting with the FDA to discuss next steps for Vicineum for the treatment of BCG-unresponsive NMIBC, the impact ofCOVID-19 on the Company, including its ability to raise capital, and, if approved, its ability to commercialize Vicineum for the treatmentof BCG-unresponsive NMIBC. Actual results may differ materially from those indicated by such forward-looking statements as a result ofvarious important factors, including: the risk that clinical trials of Vicineum for the treatment of BCG-unresponsive NMIBC may fail todemonstrate safety and efficacy to the satisfaction of the FDA or otherwise produce favorable results, the risk that the FDA may not approvethe BLA for Vicineum, the risk that Vicineum for the treatment of BCG-unresponsive NMIBC may cause undesirable side effects, serious adverseevents or have other properties that could delay or halt clinical trials, delay or prevent its regulatory approval by the FDA, limit thecommercial profile of its labeling, if approved, or result in significant negative consequences following any marketing approval, andother factors discussed in the “Risk Factors” section of the Company’s Annual Report on Form 10-K, Quarterly Reportson Form 10-Q and other reports filed with the Securities and Exchange Commission. In addition, the forward-looking statements includedin this press release represent the Company’s views as of the date hereof. The Company anticipates that subsequent events and developmentswill cause the Company’s views to change. However, while the Company may elect to update these forward-looking statements at somepoint in the future, the Company specifically disclaims any obligation to do so. These forward-looking statements should not be reliedupon as representing the Company’s views as of any date subsequent to the date hereof.

Contacts 

Investor Relations: 

Erin Clark, Vice President, Corporate Strategy &Investor Relations
ir@sesenbio.com

Media: 

Lindsay Rocco, Elixir Health PR 

lrocco@elixirhealthpr.com