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): March 4, 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     ý

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.                                 ý

Item 2.02 - Results of Operations and Financial Condition.

On March 4, 2019, Sesen Bio, Inc. (the “Company”) announced its financial results for the quarter and year ended December 31, 2018. The full text of the press release issued in connection with the announcement is furnished as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

The information provided under this Item 2.02 of this Current Report on Form 8-K (including Exhibit 99.1) shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such a filing.

Item 8.01 – Other Events.

On March 4, 2019, the Company posted a 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 March 4, 2019.
99.2	Sesen Bio, Inc. Corporate Presentation dated March 4, 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: March 4, 2019

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

EX-99.1

Sesen Bio Reports Fourth Quarter and Full-Year 2018 Financial Results and Additional Preliminary Data from Phase 3 VISTA Trial Company Announces Additional Key Primary and Secondary Endpoints Further Supporting Vicinium® Treatment Potential in High-Risk Non-Muscle Invasive Bladder Cancer Company on Track to Report Updated 12-Month Data from Phase 3 VISTA Trial of Vicinium in Mid-2019 Management to Host a Business Update Call on March 4, 2019 at 8:00 a.m. EST CAMBRIDGE, Mass., March 1, 2019 – Sesen Bio (Nasdaq: SESN), a late-stage clinical company developing targeted fusion protein therapeutics for the treatment of patients with cancer, today reported operating results for the fourth quarter and full year ended December 31, 2018. The Company also reported new, preliminary analyses from the Company’s Phase 3 VISTA trial further demonstrating the activity of Vicinium treatment in patients with high-risk non-muscle invasive bladder cancer (NMIBC). “2018 was a year of tremendous progress towards our goal of bringing Vicinium to patients with BCG-unresponsive NMIBC, setting us up for a transformational 2019,” said Dr. Thomas Cannell, president and chief executive officer of Sesen Bio. “We are well underway with our Phase 3 VISTA trial designed to support the full approval of Vicinium for patients with NMIBC. The totality of the efficacy and safety data generated to date in our Phase 3 VISTA trial, and the compelling benefit-risk profile, give us confidence in the approvability of Vicinium for this indication. Due to the limited treatment options, once patients become BCG-unresponsive, their choice is to either undergo a life-altering, complicated surgery of complete bladder removal or live with a highly-progressive cancer. We believe Vicinium can have a substantial impact on how patients with NMIBC are treated, translating into a significant commercial opportunity, as we endeavor to achieve our mission of saving and renewing the lives of patients with cancer.” VISTA Trial Progress • Positive Preliminary VISTA Trial Data Reported in BCG-unresponsive NMIBC: In January, Sesen Bio announced positive preliminary data from its ongoing Phase 3 VISTA trial, a single-arm, multi-center clinical trial designed to support the approval of Vicinium for the treatment of patients with high-risk, BCG-unresponsive NMIBC. The trial completed enrollment in the second quarter of 2018, with a total of 133 patients across three cohorts based on histology and time to disease recurrence after adequate BCG treatment. Cohort 1 enrolled 86 patients with Carcinoma in situ with or without papillary disease that was determined to be refractory or recurred within six months of their last course of adequate BCG. Cohort 2 enrolled seven patients with Carcinoma in situ with or without papillary disease that was determined to be refractory or recurred after six months, but less than 12 months, after their last course of adequate BCG. Cohort 3 enrolled 40 patients with high-risk papillary disease without Carcinoma in situ that was determined to be refractory or recurred within six months of their last course of adequate BCG. As of a December 3, 2018 data cutoff data, preliminary efficacy data for each of the trial cohorts were as follows:

Cohort 1 CRRs (n=86) Evaluable Complete Response Rate Time point Patients (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%) Patients with Carcinoma in situ with or without papillary disease that was determined to be refractory or recurred within six months of their last course of adequate BCG Cohort 2 CRRs (n=7) Evaluable Complete Response Rate Time point Patients (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%) Patients with Carcinoma in situ with or without papillary disease that was determined to be refractory or recurred after six months, but less than 12 months, after their last course of adequate BCG Pooled Cohorts 1 and 2 CRRs (n=93) Evaluable Complete Response Rate Time point Patients (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%) Patients with Carcinoma in situ with or without papillary disease that was determined to be refractory or recurred less than 12 months, after their last course of adequate BCG Cohort 3 Recurrence-Free Rate (n=40)* Evaluable Recurrence-Free Rate (95% Confidence Time point Patients Interval) 3-months n=40 68% (51%-81%) 6-months n=39 56% (40%-72%) 9-months n=38 42% (26%-59%) 12-months n=36 36% (21%-54%) Patients with high-risk papillary disease without Carcinoma in situ that was determined to be refractory or recurred within six months of their last course of adequate BCG *As of the December 3, 2018 data cut off, but not previously reported in January 2019 • Additional Preliminary VISTA Analyses Further Support Vicinium Treatment Potential for NMIBC: Since the first preliminary data were reported in January, Sesen Bio conducted additional analyses, including duration of response in patients with Carcinoma in situ with or without papillary disease, time to cystectomy across all patient types with Carcinoma in situ or papillary disease, and time to disease recurrence and

recurrence-free rate in patients with papillary disease without Carcinoma in situ as of an assessment date of December 3, 2018. These additional preliminary data, in addition to the data reported in January, support a growing body of evidence demonstrating the anti- tumor activity of Vicinium. • Duration of Response: In addition to the complete response rate in Cohort 1, duration of response in Cohort 1 is a primary endpoint measure. The median duration of complete response for patients in Cohort 1 (n=86) is 227 days (95% CI, 127-516), using the Kaplan-Meier method. Additional ad hoc analysis of pooled data for all patients with Carcinoma in situ (Cohorts 1 and 2, n=93) shows that among patients who achieved a complete response at 3 months, 69% had a complete response of 6 months or longer after starting therapy. • Time to Cystectomy: The U.S. Food and Drug Administration (FDA) guidance states that the goal of therapy in patients with BCG-unresponsive NMIBC is to avoid cystectomy. Therefore, time to cystectomy is a key secondary endpoint in the VISTA trial. Across all 133 patients treated with Vicinium, patients are projected to be cystectomy-free for approximately 519 days (95% CI, 361-523), or 18 months. • Time to Disease Recurrence: High-grade Ta or T1 NMIBC is associated with higher rates of progression and recurrence. Therefore, time to disease recurrence is a key secondary endpoint for patients with high-grade papillary-only (Ta and T1) NMIBC. The median time to disease recurrence for patients in Cohort 3 (n=40) is 270 days (95% CI, 169-452). • Recurrence-free Rate: Sesen Bio conducted an ad hoc analysis on disease recurrence, a standard criterion to evaluate treatment response for patients with high-grade papillary-only (Ta and T1) NMIBC. The analysis showed that for patients in Cohort 3 (n=40), Vicinium treatment demonstrated favorable efficacy with 56% (95% CI, 40%-72%) of patients remaining recurrence-free at 6 months, and 36% (95% CI, 21%-54%) of patients remaining recurrence-free at 12 months. • Vicinium Demonstrated to be Well-tolerated in the VISTA Trial: As of the December 3, 2018 data cut off, in patients across all cohorts (n=133), 78% 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% 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). No patient developed metastatic disease during the Phase 2 clinical trial for Vicinium or the Phase 3 VISTA trial (through the December 3, 2018 data cut off). • Updated VISTA Trial Data on Track to be Reported in Mid-2019: The Phase 3 VISTA trial remains ongoing and the company anticipates reporting updated primary and secondary endpoint data from the VISTA trial in mid-2019. In August 2018, Vicinium was granted Fast Track Designation by the FDA for the treatment of patients with high-

risk NMIBC who have previously received two courses of BCG and whose disease is now BCG-unresponsive. In connection with this designation, the Company is planning to further engage with the FDA in the first half of 2019 to discuss its intended registration strategy for Vicinium for the treatment of high-risk NMIBC. Additional Vicinium Progress • Manufacturing Readiness Underway with FUJIFILM: In October 2018, the Company entered into an agreement for the manufacturing process and technology transfer of Vicinium production with FUJIFILM Diosynth Biotechnologies U.S.A., Inc. (FUJIFILM). Preparations are underway for full-scale GMP manufacturing at FUJIFILM in the second quarter of 2019 to assess FUJIFILM’S ability to produce the bulk drug substance form of Vicinium for commercial purposes if Sesen Bio receives regulatory approval to market Vicinium for the treatment of high-risk NMIBC. • Continued Support of Vicinium Combination Trial in NMIBC: Sesen Bio has continued support of the National Cancer Institute’s ongoing clinical assessment of Vicinium in combination with AstraZeneca’s immune checkpoint inhibitor, durvalumab, for the treatment of patients with high-risk, BCG-unresponsive NMIBC. Sesen Bio believes the Phase 1 trial is based on strong scientific rationale, as well as both clinical and preclinical data, on combining Vicinium with a checkpoint inhibitor. The Company expects biomarker data from this Phase 1 trial to be reported in the second half of 2019. Fourth Quarter and Full-Year 2018 Financial Results • Cash Position: Cash and cash equivalents were $50.4 million as of December 31, 2018, compared to $14.7 million as of December 31, 2017. • Revenue: No revenue was recorded for the three months ended December 31, 2018, nor for the same period in 2017. For the twelve months ended December 31, 2018, Sesen Bio did not record any revenue, compared to $0.4 million in revenue for the same period in 2017. This decrease was due to revenue recognized in 2017 from the License Agreement with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc related to EBI-031 and all other IL-6 antagonist antibody technology, which was a part of the Company’s prior technology platform before the Company acquired Viventia Bio, Inc. in 2016. • R&D Expenses: Research and development (R&D) expenses for the fourth quarter of 2018 were $4.7 million, compared to $3.1 million in R&D expenses for the same period in 2017. For the twelve months ended December 31, 2018, research and development expenses were $14.1 million compared to $12.5 million for the same period in 2017. The fourth quarter and annual increases were both driven by expenses related to the ongoing manufacturing process and technology transfer with FUJIFILM which initiated in mid- 2018. • G&A Expenses: General and administrative expenses for the fourth quarter of 2018 were $3.5 million compared to $2.0 million for the same period in 2017. The increase was due primarily to higher staffing, and consulting costs as well as higher legal and intellectual property related costs in 2018. For the twelve months ended December 31, 2018, general and administrative expenses were $11.6 million and $8.1 million for the same period in

2017. The increase was due primarily to an increase in professional fees, one-time personnel-related expenses and increased market research consulting costs. • Net Loss: Net loss was $6.8 million, or $0.09 per share, for the fourth quarter of 2018 and $33.7 million, or $0.55 per share, for the twelve months ended December 31, 2018, compared to $6.5 million, or $0.22 per share, for the fourth quarter of 2017 and $29.0 million, or $1.11 per share, for the full year ended December 31, 2017. • Financial Guidance: Based on its current operating plans, Sesen Bio believes it will have capital sufficient to fund its current operating plan into 2020. 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 2179668. 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. 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 as a monotherapy in patients with high-risk, bacillus Calmette-Guérin, or BCG, unresponsive non-muscle invasive bladder cancer (NMIBC). The primary endpoints of the trial are the complete response rate and duration of complete response 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. Updated twelve- month data for 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-risk 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-risk NMIBC in patients who have previously received two courses of bacillus Calmette-Guérin (BCG) and whose disease is now BCG-unresponsive. Updated 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 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’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- risk, BCG un-responsive non-muscle invasive bladder cancer. Updated twelve-month data from the 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 minimizing toxicity to non-cancerous bladder 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 preliminary data of the Phase 3 VISTA trial are not indicative of final clinical results and final clinical trial results 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, whether interim results from a clinical trial will be predictive of the final results of the trial or results of early clinical studies will be indicative of the results of future studies, the adequacy of any clinical models, expectations regarding the manufacturing process and technology transfer with FUJIFILM Diosynth Biotechnologies U.S.A., Inc., expectations regarding regulatory approvals, expectations regarding the adequacy of our existing capital resources to fund our operating plan into 2020 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. Contact: Erin Clark, Executive Director, Strategic Planning & Investor Relations erin.clark@sesenbio.com

SESEN BIO, INC. CONSOLIDATED BALANCE SHEETS (in thousands) December 31, 2018 2017 Assets Current assets: Cash and cash equivalents $ 50,422 $ 14,680 Prepaid expenses and other current assets 1,334 301 Total current assets 51,756 14,981 Property and equipment, net 321 522 Restricted cash 20 10 Intangible assets 46,400 46,400 Goodwill 13,064 13,064 Other assets - 120 Total assets $ 111,561 $ 75,097 Liabilities and stockholders' equity Current liabilities: Accounts payable $ 1,367 $ 907 Accrued expenses 4,746 3,813 Total current liabilities 6,113 4,720 Other liabilities 313 215 Deferred tax liability 12,528 12,528 Contingent consideration 48,400 39,600 Stockholders' equity: Common stock 77 35 Additional paid-in capital 230,154 170,330 Accumulated deficit (186,024) (152,331) Total stockholders' equity 44,207 18,034 Total liabilities and stockholders' equity $ 111,561 $ 75,097

SESEN BIO, INC. CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (in thousands, except per share data) (unaudited) Three Months Ended December 31, Twelve Months Ended December 31, 2018 2017 2018 2017 Total revenue $ - $ - $ - $ 425 Operating expenses: Research and development 4,671 3,108 14,077 12,510 General and administrative 3,495 1,985 11,623 8,070 Loss (gain) from change in fair value of contingent consideration (1,100) 1,500 8,800 9,100 Total operating expenses 7,066 6,593 34,500 29,680 Loss from operations (7,066) (6,593) (34,500) (29,255) Other income, net 309 46 807 226 Net loss before income taxes (6,757) (6,547) (33,693) (29,029) Provision for income taxes - - - - Net loss and comprehensive loss $ (6,757) $ (6,547) $ (33,693) $ (29,029) Net loss per share —basic $ (0.09) $ (0.22) $ (0.55) $ (1.11) Weighted-average number of common shares used in net loss per share —basic 77,345 30,385 61,774 26,105 Net loss per share —diluted $ (0.09) $ (0.22) $ (0.55) $ (1.11) Weighted-average number of common shares used in net loss per share —diluted 77,345 30,385 61,774 26,105

EX-99.2

Business Update March 4, 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, the potential advantages or favorability of our product candidates, our presentation, including statements regarding our strategy, future operations, clinical ability to obtain marketing approvals for our product candidates, expectations regarding development of our protein therapies, future financial position, future revenues, our ongoing clinical trials, availability and timing of data from clinical trials, the adequacy projected costs, prospects, plans and objectives of management, are forward-looking of any clinical models, expectations regarding regulatory approvals, our ability to obtain, statements. The words “anticipate,” “believe,” “estimate,” “expect,” “intend,” “may,” “plan,” maintain and protect our intellectual property for our technology and products, other “predict,” “project,” “target,” “potential,” “will,” “would,” “could,” “should,” “continue,” and matters that could affect the financial performance of the Company, other matters that similar expressions are intended to identify forward-looking statements within the could affect the availability or commercial potential of the Company’s product candidates, meaning of the Private Securities Litigation Reform Act of 1995, although not all forward- expectations regarding the adequacy of our existing capital resources to fund our looking statements contain these identifying words. operating plan into 2020, and other factors discussed in the “Risk Factors” section of the Company’s Annual Report on Form 10-K, and other reports on file with the Securities We may not actually achieve the plans, intentions or expectations disclosed in our and Exchange Commission (SEC). The forward-looking statements contained in this forward-looking statements, and you should not place undue reliance on our forward- presentation are made as of the date hereof, and Sesen Bio assumes no obligation to looking statements. Actual results or events could differ materially from the plans, update any forward-looking statements whether as a result of new information, future intentions and expectations disclosed in the forward-looking statements we make as a events, or otherwise except as required by applicable law. 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

AGENDA 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium® may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients and their caregivers, and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 3

BLOOD IN URINE See PCP; prescribed OUR PATH FORWARD STARTS misaligned antibiotics WITH THE PATIENT JOURNEY Still see blood in urine See blood in urine; Patient surveys have shown that the experience of those Confusion and concern on problem try different antibiotic with bladder cancer is one of the poorest when compared Begin tests to other cancers.1 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 1US National Library of Medicine National testing What’s next? Institutes of Health. The patients’ Urologist recommends experience of a bladder cancer diagnosis: a systematic review of the qualitative evidence. Urologist visit bladder removal 2017. 2Ronald de Wit, MD, PhD, group leader “Radical cystectomy is associated with significant morbidity of the Experimental Systematic Therapy of Urogenital Cancers program at and mortality and a negative impact on quality of life, and Erasmus MC Cancer Institute, Rotterdam, 2 The Netherlands, ESMO 2018. many patients actually refuse or are ineligible for cystectomy.” BLADDER LIFE WITH ? ? REMOVAL CANCER 4

BLADDER CANCER Highly Prevalent with th MOST COMMON 6 CANCER1 Tremendous Unmet Medical Need • Median age at diagnosis of 72 years1 • Single most expensive cancer to treat in U.S.2 >80,000 - Average patient cost is ~$150K3 NEW CASES EACH YEAR IN U.S.1 • 75-85% diagnosed as non-muscle invasive bladder cancer4 • Limited treatment options for high-risk NMIBC $4B+ - BCG remains standard-of-care IN ANNUAL COSTS2 - Bladder removal is recommended option after BCG 1National Cancer Institute, SEER Cancer Stat Facts: Bladder Cancer, 2017. 2Mossanen M. Curr Opin Urol. 2014. 3US National Library of Medicine National Institutes of Health. Economic aspects of bladder cancer: what are the benefits and costs? 2009. 4Therapeutic Advances in Urology. Best Practices in the Treatment of Non-muscle Invasive 5 Bladder Cancer. 2012.

Carcinoma in situ: the most difficult form of NMIBC to treat Carcinoma in situ vs. Normal Bladder Lining: Clinical Trial Implications: • Field change disease often involving the entire bladder lining that is very difficult to treat • Failed on two courses of BCG, which is the gold standard for treatment of high-risk NMIBC • Rigorous local and independent central review of all urine cytology and biopsy samples • Complete response definition Normal inner means that the bladder is (Carcinoma in situ) lining completely cancer free at each timepoint Note: Diagram is for illustrative purposes 6

AGENDA 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 7

VICINIUM • Antibody fragment targeting EpCAM tethered to a cytotoxic payload (Pseudomonas exotoxin A) to form a single protein • Highly selective targeting of cancer cells while mostly avoiding healthy cells • Inhibits protein synthesis, and kills both rapidly proliferating and slow- growing cells • Designed to be effective against multi-drug resistant tumors • High potency relative to other available agents 8

VICINIUM Clinical Program Highly Aligned with 2018 FDA Guidance for NMIBC FDA Guidance VICINIUM Clinical Program • Conduct nonclinical studies to assess toxicity in animal models  • Conduct nonclinical studies to demonstrate anti-tumor activity  • Conduct nonclinical studies to determine optimal dose and schedule  • Examine anti-tumor activity and optimal dose schedule in early phase clinical trial  • Papillary cohort endpoint of recurrence-free survival (time to event endpoint)  • CIS studied in single-arm trial with CRR & DoR endpoints  • Papillary cohort not in primary efficacy endpoint  • Prefer intravesical vs. systemic  • Specifically define trial entry criteria  • Definition of BCG-unresponsive disease  • 2004 WHO classification for tumor grading  • Central pathology review of biopsy tissue and urine cytology  • Collect data on patients’ previous anti-cancer therapies  • Enroll patients who reflect clinically relevant patient population  • Optimize risk-benefit balance with dose selection  • Definition of CRR  • Collect time to cystectomy data  • Lower bound of 95% confidence interval rules out clinically unimportant CRR  • Nonclinical studies to determine need for evaluation of systemic toxicity  • Consistent efficacy and safety data across Phase I, II and III studies  FDA Guidance: BCG-Unresponsive Non-muscle Invasive Bladder Cancer: Developing Drugs and Biologics for Treatment Guidance for Industry, February 2018 99

Spectrum of Clinical Trial Endpoints in VISTA Demonstrating Strong Benefit/Risk Profile Efficacy Surrogate Time to Health Endpoints Cystectomy Outcomes Benefit : Risk Safety and Tolerability Favorable profile relative to BCG, Valstar, checkpoint inhibitors Selectively targets cancer cells while generally avoiding healthy cells Intravesical administration FDA Guidance: The approval of a marketing application is based on a favorable risk-benefit assessment. VISTA is an open-label, multicenter, single-arm Phase III registration trial. Preliminary data as of December 3, 2018. 10

Our Phase II and Phase III Clinical Trials are Highly Consistent for Complete Response Rate (CRR) CRR in Carcinoma in situ Patients* 50% Legend: Phase II 40% Phase III 40% 39% 95% CI 30% 27% 27% 20% 20% 18% 16% 10% 14% % of Patients with Complete Complete Response with Patients % of 0% 3-Month3-month CRR 6-Month6-month CRR 9-Month9-month CRR 12-Month12-month CRR Vicinium demonstrates reproducible, strong anti-tumor activity *Note: Data reflect pooled results of patients in cohorts 1&2 (n = 45 and 93 for Phase II and III, respectively) with Carcinoma in situ (with or without papillary) that recurred within ≤11 months of adequate BCG. For cohort-specific complete response rates, including confidence intervals, refer to slides 33-35. 11

VISTA TRIAL: Duration of Response for Patients who Achieved a CR at 3 Months CIS Patients (cohorts 1 & 2) (n=93) Patients with Complete Patients Patients still on treatment in situ Response at 3 months 49% of patients had a CR duration of ≥9 months (or 12 months after starting therapy) Carcinoma Carcinoma *Note: Median of responses for cohorts 1 and 2 are 227 days (95% CI=127-516) and 267 days (95% CI=167-NA), respectively, based on the Kaplan-Meier method. 12 Median duration of response for pooled cohorts 1&2 is 260 days (95% CI=154-516).

VISTA TRIAL: Favorable Efficacy Demonstrated in High-risk Ta or T1 NMIBC Patients Recurrence-Free Rate 80% 70% • Patients deemed to have no visible 60% 68% evidence of disease when starting Free Vicinium treatment - 50% 56% • Disease recurrence remains 40% appropriate response criteria 42% 30% 36% • Time to disease recurrence remains 20% standard clinical endpoint (slide 14) % Patients of Recurrence 10% 0% 3-Month 6-Month 9-Month 12-Month (n=40) (n=39) (n=38) (n=36) FDA Guidance: Sponsors can include patients with completely resected lesions and no evidence of CIS in these single-arm trials but should not include them in the evaluation of the primary efficacy endpoint. Note: Data reflects results of patients in cohort 3 (n = 40) with high-grade Ta or T1 tumors (without Carcinoma in situ) that recurred 13 within 6 months of adequate BCG.

VISTA TRIAL: After Treatment with Vicinium, Patients with High-Risk Ta or T1 NMIBC Remain Disease Free for ~9 Months Vicinium High-risk Ta or T1 Patients (cohort 3, n=40) Patients still on treatment Median time to disease recurrence = 270* (95% CI = 169-452) grade Ta, T1 NMIBC Patients treated with treated T1 NMIBC Patients Ta, grade High - FDA Guidance: Sponsors can include patients with completely resected lesions and no evidence of CIS in these single-arm trials but should not include them in the evaluation of the primary efficacy endpoint. *Median time to disease recurrence is calculated using the Kaplan-Meier method. Note: Data reflects results of patients in cohort 3 (n = 40) with high-grade Ta or T1 tumors (without Carcinoma in situ) that recurred 14 within 6 months of adequate BCG.

VISTA TRIAL: After Treatment with Vicinium, Patients are Projected to be Cystectomy-Free for ~18 Months CIS Patients (cohorts Vicinium 1&2, n=93) High-risk Ta or T1 Patients (cohort 3, n=40) Patient still on trial, no cystectomy as of last visit Patients treated treated with Patients Median cystectomy-free days = 519* (95% CI = 361-523) FDA Guidance: • The goal of therapy in patients with BCG-unresponsive NMIBC is to avoid cystectomy. • Although delay in radical cystectomy is considered a direct patient benefit, the variations in patient and health care provider preferences can confound the interpretation of this endpoint in randomized trials and particularly in single-arm trials. Nevertheless, sponsors should collect these data, which may provide supportive evidence of effectiveness. *Note: The median was estimated using a computer-based, intensive, nonparametric approach commonly used in clinical trials to generate estimates and confidence intervals for data sets that are incomplete. Using the variability within a sample to estimate that sampling distribution empirically, 500 samples with 15 replacement were generated to estimate the median.

Our Phase II and Phase III Clinical Trials are Highly Consistent for Safety and Tolerability No patient developed metastatic bladder cancer while on therapy in Phase II or Phase III trials • Majority of adverse events considered Phase II Phase III mild/moderate in both studies Category (n=46) (n=133) • No patients discontinued due to adverse events in Phase II, and only 4% (n=5) of patients discontinued Any TEAE 43 (94%) 116 (87%) trial treatment in Phase III TEAE Grade 3-5 9 (20%) 29 (22%) • Majority of adverse events observed consistent with patient population and use of catheterization Treatment-related TEAE 30 (65%) 64 (48%) • Treatment-related serious adverse events Treatment-related TEAE 3 (7%) 5 (4%) reported: Grade 3-5 - Phase II Clinical Trial: 3 patients reported 5 events including 2 events of pollakiuria, 2 events of dysuria, Any SAE 6 (13%) 19 (14%) and 1 event of arthralgia. All events resolved without sequelae. Treatment-related SAE 3 (7%) 3 (2%) - Phase III Clinical Trial: 3 patients reported 4 events Discontinuations due to including grade 4 cholestatic hepatitis, grade 5 renal 0 (0%) 5 (4%) failure1, grade 3 acute kidney injury2, and grade 2 AEs pyrexia 1 90-year-old man started the trial Mar. 2016. In May 2016, admitted for renal failure and started dialysis. Two weeks later, patient opted to discontinue dialysis, entered hospice and died in Jun. 2016. Case reported to DSMB, FDA and Health Canada. 2 74-year-old man started the trial Nov. 2016. In Dec. 2016, admitted for acute kidney injury. In 2017, protocol 16 amended to enhance monitoring, and investigators educated. No new serious related renal events since.

AGENDA 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients and their caregivers, and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 17

VICINIUM Dual Mechanism of Action Fusion Protein: Vicinium selectively targets cancer cells (which overexpress T-cell recognizes EpCAM) neoantigen and kills cancer cell MECHANISM 2: Scientific Hypothesis: Vicinium may have an additive or synergistic effect with checkpoint inhibitors enabled by Vicinium’s ability to activate T-cells and the checkpoint inhibitors’ ability to bind to and block the ligand 18

Testing the Scientific Hypothesis: Pre-clinical and Phase I Trial in SCCHN shows evidence of activation of patients’ immune systems Pre-Clinical Evidence Clinical Evidence (as seen in Phase I) After 4wks on Pre- treatment . Immunogenic Cell Death (ICD) treatment • Promotes a pro-inflammatory environment and drives PATIENT A: anti-cancer T cell responses • ICD is is associated with Damage Associated Molecular Patterns (DAMPs) including calreticulin expression, active ATP release and passive release of high mobility Injected Tumor group box 1 protein (HMGB1) • Vicinium killing of cancer cells induces the expression of these key DAMPs PATIENT B: . In a mouse model, local Vicinium treatment of a tumor induced an immune response that, combined with a checkpoint inhibitor, slowed the growth of a 2nd non-injected tumor Non-Injected Tu m o r Injected Tumor Reviewed in Vandenabaele, p et al, Adv. Exp Medical Biology 930:133-49 2016 19 Presented at AACR, 2017

Testing the Scientific Hypothesis: NCI Trial of Vicinium and Checkpoint Inhibitor TRIAL ASSESSING: • Expression of ligands (e.g., PD-L1 and PD-L2) and DATA EXPECTED TO interaction with checkpoints (e.g., PD-1 and CD-80) GUIDE ADDITIONAL • Biomarkers • Complete response rate and duration of response COMBINATION STUDIES • Safety and tolerability Measuring ability of Vicinium to enhance the patient’s immune system via T-cell activation with AstraZeneca’s Imfinzi (anti-PD-L1) VICINIUM 20

AGENDA 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients, and their caregivers and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 21

Physician and Key Opinion Leader Commercial Insights Based on New Clinical Data(6 High- Prescribing Urologists and 5 KOLs) After reviewing the data, these high- After reviewing the data, these prescribing Urologists state they would KOLs would recommend prescribe Vicinium to Vicinium to Would Would Prescribe Recommend Would Not Prescribe Would Not ~60% ~70% Recommend of their patients of their colleagues Key Feedback from these Physicians: . Huge unmet need and patient quality of life are at the forefront in treatment decision making for physicians . All high-prescribing Urologists rated the dual MOA of Vicinium 10/10 for relevance and importance for Vicinium . Duration of response and time to cystectomy may have potential for significant impact for responding patients . Safety and tolerability profile favorable vs. current treatments such as Valstar and checkpoint inhibitors . Favorable safety and tolerability profile enhances efficacy by providing a positive benefit-risk assessment 22 Source: Sesen Bio Qualitative market research, Urologist/KOL IDIs February 2018 n = 11.

Addressable Market (US Only) Annual Volume Price Reference (Annual Cost) ~80K1 $200K Patients diagnosed with bladder cancer 75% - 85% of Average Checkpoint bladder cancer healthcare cost diagnoses are Inhibitors6 NMBIC2 per patient Patients diagnosed with NMIBC with bladder 40% - 50% of cancer7 NMBICs are Patients with high-risk NMIBC: high-risk3 CIS Ta / T1 $100K ~80%2 ~20%2 50% - 90% of BCG treatments fail or disease recurs4 Patients where BCG has failed or disease recurred 30% - 60% of 6 patients would Valstar be treated Patients treated with with Vicinium5 Vicinium $0 Sources: 1National Cancer Institute, SEER Cancer Stat Facts: Bladder Cancer, 2017. 2Therapeutic Advances in Urology. Best Practices in the Treatment of Non-muscle Invasive Bladder Cancer. 2012. 3Aldousari, S. et al (2010). Update on the management of non-muscle invasive bladder cancer. Can Urol Assoc J, 4(1), 56-64. 4Memorial Sloan Kettering Cancer Center. Bladder Cancer Management After BCG Failure. 2014. 5Sesen Bio Qualitative Market Research February 2018 n = 12. 6Center for Medicare and Medicaid Services ASP Price List. 7US National Library of Medicine National Institutes of Health. Economic aspects of bladder cancer: what are the benefits and costs? 2009. 23

VICINIUM has Potential to Provide Continuity of Care for Patients with NMIBC VICINIUM BCG TREATMENT TREATMENT 2-hour infusion, hold, and rotation “I have been struck by the high patient tolerability of Vicinium which is instilled just Administration through urinary catheter like intravesical chemotherapy or BCG, making it easy for the treating Urologist.” Treated by Urologist (same Urologist) - Dr. Michael Jewett, Key Opinion Leader and Phase II clinical trial lead investigator Medical support team throughout care (same team) Response assessment every 3 months 24

Vicinium Go-to-Market Strategy Focuses on 3 Key Customer Segments Customer Segment Key Leverage Points Desired Behavior Clinical trial data Prescribing Vicinium Physician Clinical experience before cystectomy Patient Quality of Life Ask doctor for (Families/Caregivers) Health Outcomes Vicinium Payer Economic Data Reimburse and advocate for Value-based Contracts appropriate use of Vicinium Sources: Internal market research: Patient Journey Insights, Blue Print qualitative study May 2018, n=24; Sesen Market Opportunity, Monitor Deloitte qualitative and quantitative (n=34) study October 2018; Community Urologist in-depth interviews (IDIs), October 2018, n=5; Urologist/KOL IDIs February 2019, n=11. 25

AGENDA 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients and their caregivers, and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 26

Vicinium: A single cGMP Manufacturing Process Vicinium: simple (linear) cGMP manufacturing process producing a single fusion protein QA QA Purification Release Fill/Finish Release Bioreactor (Generates Drug (Generates Drug Market Substance) Product) Single cGMP process ADCs: complex (branched) cGMP manufacturing - multiple cGMP processes involving process intermediates QA QA Bioreactor Purification Release Modification Conjugation Purification Release Fill/Finish (Generates Drug (Generates Drug (mAb production) (mAb intermediate) Reaction Reaction Substance) Product) QA QA Release cGMP process #1 Release Linker Design Linker Synthesis cGMP process #2 Source Cytotoxic Cytotoxic Payload cGMP process #3 Market Payload Preparation cGMP process #4 27

2019 Outlook December 31, 2018 December 31, 2017 Cash and Cash $50.4M $14.7M Equivalents Outstanding Shares 77,456,180 34,702,565 . Cash and cash equivalents sufficient to fund operations into 2020, with no outstanding debt . Potential value inflection points by mid-2019: • Ongoing FDA interaction and feedback • 12-month updated data from VISTA Trial • NCI biomarker data 28

2019 COMPANY HIGHLIGHTS 1. Bladder cancer is an area of significant unmet need, which results in strong stakeholder support for new treatment options 2. Confident in the probability of regulatory approval with a clear path forward 3. Dual MOA suggests that Vicinium may be a preferred choice for combination therapy with checkpoint inhibitors in the future 4. Strong commercial viability with the potential for advocacy for Vicinium from physicians, patients, and their caregivers, and payers 5. Microbial expression system has the potential to significantly reduce COGs while simplifying manufacturing and supply chain 29

Appendix 30

VISTA TRIAL: PATIENT DEMOGRAPHICS COHORT 1 COHORT 2 COHORT 3 Papillary tumors (without CHARACTERISTICS CIS that recurred >6 CIS that recurred within 6 CIS) that recurred within months but ≤11 months of months of adequate BCG 6 months of adequate adequate BCG BCG Total patients enrolled 86 7 40 Evaluable patients at 3-months 86 7 40 Evaluable patients at 6-months 85 7 40 Evaluable patients at 9-months 84 7 39 Evaluable patients at 12-months 81 7 38 Median age (years) 73 67 75 Males/Females 63/23 6/1 34/6 Median prior treatment for NMIBC BCG cycles 3 (range 1-14) 3 (range 1-10) Intravesical chemotherapy 0 (range 0-23) 0 (range 0-6) TURBT 3 (range 0-28) 3 (range 0-10) TURBT: transurethral resection of bladder tumor 31

VICINIUM® PATENT LIFE Pending Applications Dosing Strategies for Targeting EPCAM positive bladder cancer. If allowed, Stablilized Chimeric Immunoglobulins would expire in 2036 or later. (Oct 2021- Jan 2022) US: 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 Methods of Treating Cancer Using an Immunotoxin (Jun 2025) Potential for 12 years of biologics marketing exclusivity against biosimilars from date (TBD) of approval* Pending Applications Dosing Strategies for Targeting EPCAM positive bladder cancer. If allowed, would expire in 2036 or later. Stablilized Chimeric Immunoglobulins (Apr 2020) OUS: 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 Methods of Treating Cancer Using an Immunotoxin (Apr 2024) Note: Patent life assessment reflects independent analysis by Hogan Lovells US LLP. * Data exclusivity granted by FDA under the Biologics Price Competition and Innovation Act of 2009 (codified at 42 U.S.C. § 262(k)) 32

VISTA TRIAL: Data Tables, Complete Response Rate 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%) 33

VISTA TRIAL: Data Tables, Complete Response Rate Pooled cohorts 1 & 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%) 34

VISTA TRIAL: Data Tables, Complete Response Rate Preliminary Phase III CRR vs. Phase II CRR Time Point Phase 3 Pooled CRR (95% Confidence Interval) Phase 2 Pooled CRR (95% Confidence Interval) 3-months 39% (29%, 49%) 40% (26%-56%) 6-months 27% (18%, 37%) 27% (15%-42%) 9-months 20% (12%, 29%) 18% (8%-32%) 12-months 14% (7%, 23%) 16% (7%-30%) Dosing: Phase II: 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 Phase III: 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 35

VISTA TRIAL: Data Tables, Disease Recurrence Cohort 3 (n=40) Recurrence-Free Rate (95% Confidence Time Point Evaluable Patients Interval) 3-months n=40 68% (51%-81%) 6-months n=40 56% (40%-72%) 9-months n=39 42% (26%-59%) 12-months n=38 36% (21%-54%) 36