8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8–K

CURRENT REPORT

Pursuant to Section 13 OR 15 (d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): January 3, 2019

SESEN BIO, INC.

(Exact name of registrant as specified in its charter)

Delaware	001-36296	26-2025616
(State or other jurisdiction of incorporation)	(Commission File Number)	(I.R.S. Employer Identification No.)
245 First Street, Suite 1800 Cambridge, MA		02142
(Address of principal executive offices)		(Zip Code)

Registrant’s telephone number, including area code: (617) 444-8550

Not Applicable

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8–K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a–12 under the Exchange Act (17 CFR 240.14a–12)

¨ Pre–commencement communications pursuant to Rule 14d–2(b) under the Exchange Act (17 CFR 240.14d–2(b))

¨ Pre–commencement communications pursuant to Rule 13e–4(c) under the Exchange Act (17 CFR 240.13e–4(c))

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company     þ

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.                                 þ

Item 8.01 – Other Events.

On January 3, 2019, Sesen Bio, Inc. (the “Company”) issued a press release announcing six, nine and 12-month data from the Company’s ongoing Phase 3 VISTA Trial, which is evaluating Vicinium^® for the treatment of patients with high-grade non-muscle invasive bladder cancer who have been previously treated with bacillus Calmette-Guérin. A copy of the press release is attached as Exhibit 99.1 to this report and is incorporated herein by reference.

On January 3, 2019, the Company will post an updated corporate presentation on its website www.sesenbio.com. A copy of the presentation is filed herewith as Exhibit 99.2 and is incorporated herein by reference.

Item 9.01 – Financial Statements and Exhibits.

(d) Exhibits.

Exhibit No.	Description
99.1	Press Release dated January 3, 2019
99.2	Sesen Bio, Inc. Corporate Presentation dated January 3, 2019

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: January 3, 2019

Sesen Bio, Inc.
By:	/s/ Richard F. Fitzgerald
	Richard F. Fitzgerald
	Chief Financial Officer, Secretary and Treasurer

EX-99.1

Sesen Bio Announces Positive Preliminary 12-Month Data from Registration Phase 3 VISTA Trial of Vicinium for Non-Muscle Invasive Bladder Cancer Complete Response Rates from All Four Time Points in Carcinoma in Situ Patients In-line with Phase 2 Data Company to Host Conference Call Today, January 3, at 8:30 a.m. ET CAMBRIDGE, Mass., January 3, 2019 – Sesen Bio, Inc. (Nasdaq: SESN), a late-stage clinical company advancing targeted fusion protein therapeutics for the treatment of cancer, today reported positive preliminary efficacy data for the primary endpoint of its ongoing Phase 3 registration trial, the VISTA Trial, of Vicinium® for the treatment of patients with high-grade non-muscle invasive bladder cancer (NMIBC) who have been previously treated with bacillus Calmette-Guérin (BCG) and deemed BCG-unresponsive. The data reported show clinically meaningful complete response rates in evaluable Carcinoma in situ patients at three, six, nine and 12 months of follow-up in the trial consistent with the data in the completed Phase 1 and Phase 2 clinical trials. Importantly, Vicinium continues to be generally well-tolerated in treated patients. “Non-muscle invasive bladder cancer is a very prevalent cancer that can progress to become incurable. The usual treatment for patients who relapse or become refractory to BCG, today’s standard-of-care, is complete bladder removal or radical cystectomy,” said Michael A.S. Jewett, M.D., Professor of Surgery, Division of Urology, University of Toronto. “Removing the bladder is a potentially morbid and complex surgery with potential for side effects that can drastically reduce a patient’s quality of life. In fact, many patients choose not to undergo bladder removal. I am very encouraged by the data generated to-date with intravesical Vicinium as an alternative after BCG failure. Based on the strength of the clinical activity observed, and the consistently favorable safety and tolerability, I believe that Vicinium has the potential to change the treatment outcome for patients.” VISTA Trial Design The Phase 3 VISTA Trial is a single-arm, multi-center clinical trial designed to support the approval of Vicinium for the treatment of patients with high-grade, BCG-unresponsive NMIBC. The trial enrolled a total of 133 patients across three cohorts based on histology and time to disease recurrence after adequate BCG:  Cohort 1 (n=86): Carcinoma in situ patients with or without papillary disease whose cancer was determined to be refractory or recurred within six months of their last course of adequate BCG  Cohort 2 (n=7): Carcinoma in situ patients with or without papillary disease whose cancer was determined to be refractory or recurred after six months, but less than 12 months, after their last course of adequate BCG  Cohort 3 (n=40): patients with papillary disease without Carcinoma in situ whose cancer was determined to be refractory or recurred within six months of their last course of adequate BCG 1

The data reported build upon preliminary three-month data presented from a subset of patients in May 2018 and are for the primary endpoint of the VISTA Trial, which is the complete response rate and duration of response in patients in Cohort 1. The company also reported data from Cohort 2, separately and pooled with Cohort 1, based on final U.S. Food and Drug Administration guidance on treatment of BCG-unresponsive Carcinoma in situ patients (defined as patients with recurrent Carcinoma in situ within 12 months of adequate BCG therapy)1. The patient population in Cohort 3 represents an opportunity for future label expansion, and the company plans to report efficacy data from this cohort, as well as the secondary endpoints in the VISTA Trial, in mid-2019. Preliminary Efficacy Results in Carcinoma in situ Patients Cohort 1 (n=86) Time point Evaluable Patients Complete Response Rate (95% Confidence Interval) 3-months n=86 37% (27%, 48%) 6-months n=85 25% (16%, 35%) 9-months n=84 18% (10%, 28%) 12-months n=81 14% (7%, 23%) Cohort 2 (n=7) Time point Evaluable Patients Complete Response Rate (95% Confidence Interval) 3-months n=7 57% (18%, 90%) 6-months n=7 57% (18%, 90%) 9-months n=7 43% (10%, 82%) 12-months n=7 14% (0%, 58%) Pooled Cohorts 1 and 2 (n=93) Time point Evaluable Patients Complete Response Rate (95% Confidence Interval) 3-months n=93 39% (29%, 49%) 6-months n=92 27% (18%, 37%) 9-months n=91 20% (12%, 29%) 12-months n=88 14% (7%, 23%) Notably, the interim Phase 3 complete response rates in pooled patients from Cohorts 1 and 2 are in-line with the complete response rates in pooled patients in the completed Phase 2 clinical trial. Preliminary Phase 3 CRR vs Phase 2 CRR Phase 3 Pooled CRR Phase 2 Pooled CRR Time point (95% Confidence Intervals) (95% Confidence Interval) 3-months 39% (29%, 49%) 40% (26%, 56%) 6-months 27% (18%, 37%) 27% (15%, 42%) 1 United States Food and Drug Administration, BCG-Unresponsive Nonmuscle Invasive Bladder Cancer: Developing Drugs and Biologics for Treatment Guidance for Industry, February 2018 2

9-months 20% (12%, 29%) 18% (8%, 32%) 12-months 14% (7%, 23%) 16% (7%, 30%) The company also reported an update on the durability of responses in the VISTA Trial. While the median has not yet been reached, the preliminary data show that Vicinium treatment resulted in a prolonged duration of response in many patients. This is particularly notable given that, in order for patients to remain on study, they have to have achieved a complete response at each assessment time period. These findings suggest that Vicinium has the potential to benefit patients by delaying the time to a radical cystectomy, a secondary endpoint that will be measured and reported in mid-2019. Preliminary Safety Results Vicinium continues to be well-tolerated by patients treated in the VISTA Trial. As of the December 3, 2018 data cut off, in patients across all three cohorts (n=133), 78 percent of adverse events were Grade 1 or 2. The most commonly reported treatment-related adverse events were dysuria (13%), hematuria (12%) and urinary tract infection (11%) – all of which are consistent with the profile of bladder cancer patients and the use of catheterization for treatment delivery. These adverse events were determined to be manageable and reversible, and only five patients discontinued treatment due to an adverse event. Serious adverse events, regardless of treatment attribution, were reported in 14 percent of patients. There were four treatment-related SAEs reported in three patients including acute renal injury (Grade 3), pyrexia (Grade 2), cholestatic hepatitis (Grade 4) and renal failure (Grade 5). “We are very pleased with these preliminary data, which are consistent with the data in our completed Phase 2 clinical trial of Vicinium for the treatment of high-grade NMIBC, and further support our belief that Vicinium has the potential to change how patients are treated after BCG,” said Dr. Thomas Cannell, president and chief executive officer of Sesen Bio. “The design of the VISTA Trial aligns with FDA’s guidance for NMIBC drug development, and the findings are highly encouraging, demonstrating that treatment with Vicinium results in clinically meaningful efficacy and favorable safety and tolerability. Overall, the data reinforce our belief that Vicinium is positioned to provide a valuable benefit to patients by treating their disease with long-term responses and extending their time to face such a decision as removing their bladder. 2019 is set to be a transformational year for Sesen Bio, and we look forward to advancing the VISTA Trial and assessing the full 12-month data from all patients later this year.” The VISTA Trial completed enrollment in the second quarter of 2018, and complete 12-month efficacy data from all patients in the clinical trial are expected to be reported at a medical meeting in mid-2019. Conference Call Information To participate in the conference call, please dial (844) 831-3025 (domestic) or (315) 625-6887 (international) and refer to conference ID 4263106. The webcast can be accessed in the Investor Relations section of the company's website at www.sesenbio.com. The replay of the webcast will be available in the investor section of the company’s website at www.sesenbio.com for 60 days following the call. 3

About the VISTA Clinical Trial The VISTA Trial is an open-label, multicenter, single-arm Phase 3 clinical trial evaluating the efficacy and tolerability of Vicinium® in patients with high-grade non-muscle invasive bladder cancer (NMIBC) that is Carcinoma in situ, which is cancer found on the inner lining of the bladder that has not spread into muscle or other tissue and/or papillary, which is cancer that has grown from the bladder lining out into the bladder but has not spread into muscle or other tissue, who have been previously treated with bacillus Calmette-Guérin (BCG). The primary endpoint of the trial is the complete response rate in patients with Carcinoma in situ with or without papillary disease. Patients in the trial receive locally administered Vicinium twice a week for six weeks, followed by once-weekly treatment for another six weeks, then treatment every other week for up to two years. Twelve-month data from all patients in the VISTA Trial are anticipated in mid-2019. To learn more about the Phase 3 VISTA Trial, please visit www.clinicaltrials.gov and search the identifier NCT02449239. About Vicinium® Vicinium®, a locally-administered fusion protein, is Sesen Bio’s lead product candidate being developed for the treatment of high-grade non-muscle invasive bladder cancer (NMIBC). Vicinium is comprised of a recombinant fusion protein that targets epithelial cell adhesion molecule (EpCAM) antigens on the surface of tumor cells to deliver a potent protein payload, Pseudomonas Exotoxin A (ETA). Vicinium is constructed with a stable, genetically engineered peptide tether to ensure the payload remains attached until it is internalized by the cancer cell, which is believed to decrease the risk of toxicity to healthy tissues, thereby improving its safety. In prior clinical trials conducted by Sesen Bio, EpCAM has been shown to be overexpressed in NMIBC cells with minimal to no EpCAM expression observed on normal bladder cells. Sesen Bio is currently conducting the Phase 3 VISTA Trial, designed to support the registration of Vicinium for the treatment of high-grade NMIBC in patients who have previously received two courses of bacillus Calmette-Guérin (BCG) and whose disease is now BCG-unresponsive. Complete twelve-month data from the trial are anticipated in mid-2019. Additionally, Sesen Bio believes that Vicinium’s cancer cell-killing properties promote an anti-tumor immune response that may potentially combine well with immuno-oncology drugs, such as checkpoint inhibitors. The activity of Vicinium in BCG-unresponsive NMIBC is also being explored at the US National Cancer Institute in combination with AstraZeneca’s immune checkpoint inhibitor durvalumab. About Non-Muscle Invasive Bladder Cancer Bladder cancer is the sixth most commonly diagnosed cancer in the United States, and approximately 80 percent of patients have non-muscle invasive bladder cancer (NMIBC). In NMIBC, cancer cells are in the lining of the bladder or have grown into the lumen of the bladder but have not spread into muscle or other tissue. NMIBC primarily affects men and is associated with carcinogen exposure. Initial treatment includes surgical resection; however, there is a high rate of recurrence and more than 60 percent of all patients diagnosed with NMIBC will receive bacillus Calmette-Guérin (BCG) immunotherapy. While BCG is effective in many patients, challenges with tolerability have been observed and many patients will experience recurrence of disease. If BCG is not effective or a patient can longer receive BCG, the recommended option for treatment is radical cystectomy, the complete removal of the bladder. 4

About Sesen Bio Sesen Bio, Inc. is a late-stage clinical company advancing targeted fusion protein therapeutics for the treatment of cancer. The company’s lead program, Vicinium®, also known as VB4-845, is currently in a Phase 3 registration trial, the VISTA Trial, for the treatment of high-grade, BCG- unresponsive non-muscle invasive bladder cancer. Twelve-month data from all patients in the VISTA Trial are anticipated in mid-2019. Vicinium incorporates a tumor-targeting antibody fragment and a protein cytotoxic payload into a single protein molecule designed to selectively and effectively kill cancer cells while sparing healthy cells. 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 the Company, the Company’s strategy, future operations, and other statements containing the words “anticipate,” “believe,” “estimate,” “expect,” “intend,” “may,” “plan,” “predict,” “project,” “target,” “potential,” “will,” “would,” “could,” “should,” “continue,” and similar expressions, constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by such forward- looking statements as a result of various important factors, including: the uncertainties inherent in the initiation and conduct of clinical trials, the possibility that the available preliminary data of the Phase 3 VISTA Trial are not indicative of final data from all patients in Phase 3 VISTA Trial and final data may not be positive with regard to the safety or efficacy of Vicinium, our ability to successfully develop our product candidates and complete our planned clinical programs, our ability to obtain marketing approvals for our product candidates, expectations regarding our ongoing clinical trials, availability and timing of data from clinical trials, the adequacy of any clinical models, expectations regarding regulatory approvals and other factors discussed in the “Risk Factors” section of the Company’s Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and other reports filed with the Securities and Exchange Commission. In addition, the forward-looking statements included in this press release represent the Company’s views as of the date hereof. The Company anticipates that subsequent events and developments will cause the Company’s views to change. However, while the Company may elect to update these forward-looking statements at some point in the future, the Company specifically disclaims any obligation to do so. These forward-looking statements should not be relied upon as representing the Company’s views as of any date subsequent to the date hereof. Contacts: Monique Allaire, THRUST Strategic Communications monique@thrustsc.com 617-895-9511 Chelcie Lister, THRUST Strategic Communications chelcie@thrustsc.com 910-777-3049 5

EX-99.2

Updated Phase 3 VISTA Trial Data in Patients with BCG-unresponsive Non-muscle Invasive Bladder Cancer January 3, 2019 NASDAQ SESN

FORWARD-LOOKING STATEMENTS This presentation contains forward-looking statements that involve substantial risks and our ability to successfully develop our product candidates and complete our planned uncertainties. All statements, other than statements of historical facts, contained in this clinical programs, our ability to obtain marketing approvals for our product candidates, presentation, including statements regarding our strategy, future operations, clinical expectations regarding our ongoing clinical trials, availability and timing of data from development of our protein therapies, future financial position, future revenues, clinical trials, the adequacy of any clinical models, expectations regarding regulatory projected costs, prospects, plans and objectives of management, are forward-looking approvals, our ability to obtain, maintain and protect our intellectual property for our statements. The words “anticipate,” “believe,” “estimate,” “expect,” “intend,” “may,” “plan,” technology and products, other matters that could affect the financial performance of the “predict,” “project,” “target,” “potential,” “will,” “would,” “could,” “should,” “continue,” and Company, other matters that could affect the availability or commercial potential of the similar expressions are intended to identify forward-looking statements within the Company’s product candidates and other factors discussed in the “Risk Factors” section meaning of the Private Securities Litigation Reform Act of 1995, although not all forward- of the Company’s Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and looking statements contain these identifying words. other reports on file with the Securities and Exchange Commission (SEC). The forward- looking statements contained in this presentation are made as of the date hereof, and We may not actually achieve the plans, intentions or expectations disclosed in our Sesen Bio assumes no obligation to update any forward-looking statements whether as a forward-looking statements, and you should not place undue reliance on our forward- result of new information, future events, or otherwise except as required by applicable looking statements. Actual results or events could differ materially from the plans, law. intentions and expectations disclosed in the forward-looking statements we make as a result of various important factors, including: the uncertainties inherent in the initiation and conduct of clinical trials, the possibility that the available preliminary data of the Phase 3 VISTA Trial are not indicative of final data from all patients in the Phase 3 VISTA Trial and final data may not be positive with regard to the safety or efficacy of Vicinium®, 2

OUR MISSION IS TO SAVE AND RENEW THE LIVES OF PEOPLE WITH CANCER sesen: an ancient Egyptian word for the giant lotus flower that first rose out of the watery chaos at the beginning of time lotus: a symbol of purity, rebirth and renewal bio: from Greek bios meaning life 3

2018 PROGRESS SETS UP FOR TRANSFORMATIONAL 2019  Completed enrollment in VISTA Trial - Phase 3 registration trial of Vicinium for high-grade non-muscle invasive bladder cancer (NMIBC)  Presented positive preliminary 3-month efficacy data at American Urological Association Annual Meeting  Granted Fast Track Designation for Vicinium for treatment of high-grade NMIBC  Initiated enrollment in NCI-sponsored combination study of Vicinium + durvalumab in NMIBC  Established agreement with FujiFilm for Vicinium manufacturing  Reported new preclinical data on early-stage deBouganin program supporting broad potential  Enhanced leadership team  Strengthened financial position1 1$57.9M in cash and cash equivalents at end of 3Q18; operating runway into 2020 4

BLADDER CANCER Highly Prevalent with th MOST COMMON 6 CANCER1 Tremendous Unmet Medical Need • Median age at diagnosis of 72 years1 >80,000 • Single most expensive cancer to treat in U.S.2 NEW CASES EACH YEAR IN U.S.1 • 80% diagnosed as non-muscle invasive bladder cancer • Limited treatment options for high-grade NMIBC $4B+ - BCG remains standard-of-care 2 IN ANNUAL COSTS - Bladder removal is recommended option after BCG 1National Cancer Institute, SEER Cancer Stat Facts: Bladder Cancer, 2017. 2Mossanen M. Curr Opin Urol. 2014. 5

PROGRESSIVE NATURE OF BLADDER CANCER CIS • 60%-70% will recur1 • 20%-30% will progress to muscle invasive bladder cancer1 1Aldousari, S. Can Urol Assoc J. 2010 Feb. Image adapted from Knowles MA and CD Hurst, 2015. Nature Rev Cancer, 8 | Company Presentation 15: 25-41 6

BLOOD IN URINE See PCP; prescribed OUR PATH FORWARD STARTS misaligned antibiotics WITH THE PATIENT JOURNEY Still see urine in blood See blood in urine; try different antibiotic Confusion and concern on problem Begin tests Referred to urologist Shock and st CT Scan emotional Preparing for 1 TURBT struggle cancer treatment Cytology Bladder BCG BCG again Cystoscopy MRI Fear, anxiety Testing Fear cancer Testing is progressing Hope treatment DIAGNOSIS is working BCG TUMOR HAS RECURRED BCG again BCG has failed More testing What’s next? Urologist recommends Urologist visit 1Ronald de Wit, MD, PhD, group leader bladder removal of the Experimental Systematic Therapy of Urogenital Cancers program at Erasmus MC Cancer Institute, Rotterdam, The Netherlands, ESMO 2018 “Radical cystectomy is associated with significant morbidity and mortality and a negative impact on quality of life, and BLADDER LIFE WITH ? ? many patients actually refuse or are ineligible for cystectomy.”1 REMOVAL CANCER 7

VICINIUM HAS POTENTIAL TO PROVIDE VICINIUM CONTINUITY OF CARE FOR PATIENTS WITH NMIBC VICINIUM BCG TREATMENT TREATMENT 2-hour infusion • Antibody fragment tethered to a cytotoxic Administration through urinary catheter payload to form a single protein • Simple and cost-efficient manufacturing 2-hour “hold” for treatment • Inhibits protein synthesis Treated by urologist (same urologist) • Designed to be effective against multi-drug resistant tumors Medical support team throughout care (same team) • High potency relative to other available agents • Designed to kill both rapidly proliferating Response assessment every 3 months and slow-growing cells 8

VICINIUM Dual Mechanism of Action MECHANISM 2: 9

VISTA TRIAL: Phase 3 Trial to Support Approval of Vicinium for BCG-unresponsive NMIBC • Single-arm, open label, multi-center phase 3 trial of patients with BCG-unresponsive NMIBC - BCG-unresponsive defined as BCG-refractory or disease relapse after receiving adequate BCG (having completed 2+ courses (≥ 5 and ≥ 2 treatments resp.) of full dose BCG starting within 12 months of each other) • 3 cohorts by histology and time to recurrence after adequate BCG - Cohort 1: Carcinoma in situ with or without papillary tumors that recurred within 6 months of adequate BCG - Cohort 2: Carcinoma in situ with or without papillary tumors that recurred >6 months but ≤11 months of adequate BCG - Cohort 3: Papillary tumors (without Carcinoma in situ) that recurred within 6 months of adequate BCG • Dosing: intravesical instillation of 30 mg Vicinium in 50 ml buffered saline held for 2 hours - Induction: biweekly for 6 weeks → weekly for 6 weeks - Patients with complete response proceed to maintenance of every other week for 2 years total 10

VISTA TRIAL: Assessment of Response • PRIMARY ENDPOINT – Complete response rate and duration of response in patients with Carcinoma in situ enrolled in Cohort 1* • KEY SECONDARY ENDPOINTS – Event-free survival, time to disease recurrence, time to cystectomy, progression-free survival, overall survival,n=86 n=7 n=85 n=7 n=84 n=7 n=81 n=7 safety and tolerability Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 *Complete response is defined as non-positive urinary cytology and either normal cystoscopy or abnormal cystoscopy with negative biopsy 11

VISTA Phase 3 Trial Design Aligns with FDA Final Guidance for NMIBC Treatment Development “In BCG- unresponsive NMIBC, a single- arm clinical trial with complete response rate and duration of response as the primary endpoint can provide primary evidence of effectiveness to support a marketing application.” “The goal of therapy in patients with BCG-unresponsive NMIBC is to avoid cystectomy.” BCG-unresponsive CIS defined as persistent or recurrent CIS with or without recurrent Ta/T1 disease within 12 months of completion of adequate BCG therapy BCG-Unresponsive Nonmuscle Invasive Bladder Cancer: Developing Drugs and Biologics for Treatment Guidance for Industry, February 2018 12

VISTA TRIAL: PATIENT DEMOGRAPHICS COHORT 1 & 2 COHORT 1 COHORT 2 COMBINED* CHARACTERISTICS CIS that recurred >6 months CIS that recurred within 6 CIS that recurred within but ≤11 months of adequate months of adequate BCG <11 months of adequate BCG BCG Total patients enrolled 86 7 93 Evaluable patients at 3-months 86 7 93 Evaluable patients at 6-months 85 7 92 Evaluable patients at 9-months 84 7 91 Evaluable patients at 12-months 81 7 88 Median age (years) 73 67 73 Males/Females 63/23 6/1 69/24 Median prior treatment for NMIBC BCG cycles 3 (range 1-14) Intravesical chemotherapy 0 (range 0-23) TURBT 3 (range 0-28) * FDA Guidance from February 2018 Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 TURBT: transurethral resection of bladder tumor 13

Cohort 1 100% Cohort 2 90% VISTA TRIAL: 80% 70% Positive Complete 60% Response Rate in 50% 57% 57% Carcinoma in Situ 40% 43% Patients in 30% 37% Cohorts 1 and 2 20% 25% 18% 10% 14% 14% n=86 n=7 n=85 n=7 n=84 n=7 n=81 n=7 0% 3-Month CRR 6-Month CRR 9-Month CRR 12-Month CRR Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 14

Complete Response Rate in Pooled Cohorts 1 & 2 60% 50% VISTA TRIAL: 40% Positive Efficacy Data 39% in BCG-unresponsive 30% Carcinoma in situ 20% 27% Patients within 12 20% months of Last BCG 10% 14% Treatment* 0% 3-Month CRR 6-Month CRR 9-Month CRR 12-Month CRR • Complete 12-month CRR to be between (n=93) (n=92) (n=91) (n=88) 13% - 18% once all patients are evaluable Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 *FDA Guidance from February 2018 15

Complete Response Rate in Pooled Cohorts from 60% Phase 2 and Phase 3 Clinical Trials 50% Phase 2 Vicinium Trial (n = 45)1 VICINIUM Phase 3 VISTA Trial (n = 93)2 40% EFFICACY: 40% 39% CRR Data in 30% Phase 2 and 27% 27% 20% 20% Ongoing Phase 3 18% 16% Clinical Trials 10% 14% are Consistent 0% 3-Month CRR 6-Month CRR 9-Month CRR 12-Month CRR 1Kowalski M The Journal of Urology 2012; Phase 2 cohort 1 dose: 6 weekly induction doses, 6 weeks off, 3 maintenance doses every 3 months for 9 months; cohort 2 dose: 12 weekly induction doses, 3 maintenance doses every 3 months for 9 months 2Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 16 VISTA Trial dosing: biweekly induction doses for 6 weeks followed by weekly dosing for 6 weeks; if a CR achieved, proceed to maintenance of every other week dosing for 2 years total

VISTA TRIAL: Extended Durations of Response Cohort 1 Cohort 2 Patients still on treatment Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 17

Patients (n=133) with: Treatment-Related All TEAEs TEAEs Treatment-Emergent Adverse Event1 All Grades Grade ≥3 All Grades Grade ≥3 VISTA TRIAL: Any TEAE 116 (87%) 29 (22%) 64 (48%) 5 (4%) Vicinium Treatment Urinary tract infection 41 (31%) 5 (4%) 15 (11%) 2 (2%) Results in Low Rate Dysuria 33 (25%) 0 (0%) 17 (13%) 0 (0%) of Discontinuations Hematuria 31 (23%) 2 (2%) 16 (12%) 0 (0%) Pollakiuria (frequency of 21 (16%) 0 (0%) 14 (11%) 0 (0%) • Majority of adverse events (78%) considered urination) mild/moderate Fatigue 17 (13%) 0 (0%) 10 (8%) 0 (0%) • Only 4% (n=5) patients discontinued trial Diarrhea 16 (12%) 0 (0%) 3 (2%) 0 (0%) treatment due to adverse event Micturition urgency 16 (12%) 0 (0%) 12 (9%) 0 (0%) • Adverse events observed consistent with patient population and use of catheterization Nausea 13 (10%) 1 (<1%) 3 (2%) 0 (0%) Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 1 Includes named TEAE occurring in more than 10% of patients regardless of treatment relationship 18

Treatment- Treatment- Patients (n=133) VISTA TRIAL: Emergent SAEs1 Related SAEs Vicinium Continues Any Serious AE 19 (14%) 3 (2%) Acute kidney injury to be Generally 4 (3%) 2 (2%) or renal failure Well-tolerated Small intestinal 3 (2%) 0 (0%) obstruction • 4 treatment-related serious adverse events reported in 3 patients Hematuria 2 (2%) 0 (0%) - 1 patient had grade 4 cholestatic hepatitis and Urinary Tract grade 5 renal failure 2 (2%) 0 (0%) infection - 1 patient had grade 3 acute kidney injury - 1 patient had grade 2 pyrexia Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 1 SAEs reported in 2 or more patients 19

PRELIMINARY VISTA TRIAL DATA SUPPORT VICINIUM TREATMENT POTENTIAL FOR PATIENTS WITH CARCINOMA IN SITU • Encouraging preliminary efficacy data support planned Vicinium BLA submission • Prolonged duration of response observed in many patients • Favorable safety and tolerability with low rate of discontinuations due to AEs • Phase 3 preliminary data in-line with Phase 1 & 2 clinical trials • Convenient treatment administration, consistent with BCG • Opportunity to fulfill unmet medical need for BCG-unresponsive NMIBC patients; otherwise all patients recommended for bladder removal after BCG Preliminary data as of December 3, 2018; Efficacy data from cohorts 1&2; Safety data from cohorts 1, 2 &3 20

STRATEGIC PRIORITIES FOR A SUCCESSUL SESEN BIO FUTURE Bring Vicinium, a targeted fusion protein, to market for Carcinoma in situ patients with NMIBC Expand Vicinium utility as a combination treatment with immuno-oncology agents Explore partnerships to expand Vicinium into additional indications and geographies Leverage proprietary research capabilities to advance new fusion proteins for cancers 21

THANK YOU TO THE PATIENTS, FAMILIIES, CAREGIVERS, PHYSICIANS, NURSES AND MEDICAL STAFFS WHO SUPPORT AND HAVE PARTICIPATED IN VICINIUM CLINICAL TRIALS 22