Sesen Bio Reports First Quarter 2019 Financial Results and Updated, Preliminary Primary and Additional Secondary Endpoint Data from Phase 3 VISTA Trial for High-Risk Non-Muscle Invasive Bladder Cancer

Company Announces Confirmed Meetings with the U.S. Food and Drug
Administration in May and June 2019 to Review Registration
Strategy for Vicinium^®

Management to Host a Business Update Call Today at 8:00 a.m. EDT

CAMBRIDGE, Mass.–(BUSINESS WIRE)–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 first quarter ended
March 31, 2019. The Company also reported updated, preliminary primary
and additional secondary endpoint data from the Company’s Phase 3 VISTA
trial further supporting the strong benefit-risk profile of Vicinium
for the potential treatment in patients with high-risk, bacillus
Calmette-Guérin (BCG) unresponsive, non-muscle invasive bladder
cancer (NMIBC). The updated preliminary Phase 3 clinical data, along
with the Phase 2 clinical trial data, will serve as the basis for
upcoming meetings with the U.S. Food and Drug Administration (FDA).

“We are very encouraged by the most recent analysis of our 12-month
Phase 3 VISTA trial data, which will be the basis for our meetings with
the FDA in May and June,” said Dr. Thomas Cannell, president and chief
executive officer of Sesen Bio. “We believe these preliminary data,
along with a closely matched Phase 2 data set, support a compelling
benefit-risk profile, and give us confidence in the regulatory
approvability and strong commercial viability of Vicinium. The huge
unmet need for patients with NMIBC is widely acknowledged and has been
exacerbated by the recurring global shortage of BCG. We will continue to
work closely with the FDA in our effort to expeditiously bring a product
to market that has the potential to save and improve the lives of
patients with NMIBC.”

Phase 3 VISTA Trial Progress and Updates

• Updated, Preliminary VISTA Trial Data Reported in BCG-unresponsive
  NMIBC: In March, Sesen Bio announced updated 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 registration 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 (at least two courses of BCG with at least five doses in the
  first course and two doses in the second course). Primary endpoints
  include complete response rate and duration of response for patients
  in Cohort 1. Secondary endpoints include time to disease recurrence
  for patients in Cohort 3, and time to cystectomy, progression-free
  survival, event-free survival, and overall survival for all patients
  across cohorts. As of the March 1, 2019 data cutoff, updated
  preliminary primary and secondary efficacy data for each of the trial
  cohorts were as follows:

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

Patients with Carcinoma in situ with or without papillary disease
that was determined to be refractory or recurred after six months, but
less than 11 months, after their last course of adequate BCG

Patients with Carcinoma in situ with or without papillary disease
that was determined to be refractory or recurred less than 11 months,
after their last course of adequate BCG

• Duration of Response: The median duration of response for
  patients in Cohort 1 (n=86) is 287 days (95% CI, 127-NA), 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, 54%
  had a complete response for a total of 12 months or longer after
  starting therapy, using the Kaplan-Meier method.
• Time to Disease Recurrence: High-risk papillary (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-risk papillary-only NMIBC. The median time to
  disease recurrence for patients in Cohort 3 (n=40) is 402 days (95%
  CI, 170-NA), using the Kaplan-Meier method.
• Time to Cystectomy: The 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,
  >75% of patients are estimated to remain cystectomy-free at 2.5 years,
  using the Kaplan-Meier method. Additional ad hoc analysis of
  responders and non-responders for all patients shows that responders
  are approximately 15 times more likely to remain cystectomy-free at
  2.5 years compared to non-responders.
• Additional Secondary Endpoint Data from Phase 3 VISTA Trial Support
  a Growing Body of Evidence Demonstrating the Durable Anti-tumor
  Activity of Vicinium: Since the last data update reported in
  March, Sesen Bio has completed preliminary analyses of the remaining
  secondary endpoints in the VISTA trial, including progression-free
  survival, overall survival, and event-free survival, as measured
  across all patient cohorts. Reported data is as of the March 1, 2019
  cutoff:
  • Progression-Free Survival: >85% of all 133 patients treated
    with Vicinium are estimated to remain progression-free at 2 years,
    using the Kaplan-Meier method. Progression-free is defined as the
    time from the date of first dose of study treatment to disease
    progression (e.g. T2 or more advanced disease) or death as a first
    event.
  • Event-Free Survival: 30% of all 133 patients treated with
    Vicinium are estimated to remain event-free at 12 months, using
    the Kaplan-Meier method. Event-free survival is defined as the
    time from the date of first dose of study treatment to disease
    recurrence, progression, or death as a first event.
  • Overall Survival: 91% of all 133 patients treated with
    Vicinium have an overall survival of >2.5 years, using the
    Kaplan-Meier method. Overall survival is defined as the time from
    the date of first dose of study treatment to death from any cause.
• Vicinium Continues to be Well-tolerated by Patients Treated in the
  Phase 3 VISTA Trial: As of the March 1, 2019 data cut off, in
  patients across all cohorts (n=133), 78% 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 by the clinical investigators to be
  manageable and reversible, and only five patients (4%) 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
  kidney injury (Grade 3), pyrexia (Grade 2), cholestatic hepatitis
  (Grade 4) and renal failure (Grade 5). There were no age-related
  increases in adverse events observed in the Phase 3 VISTA trial.
• Positive Data and Safety Monitoring Board (DSMB) Review of Phase 3
  VISTA Trial Data: In March, the independent DSMB completed its
  ninth planned safety review for the Phase 3 VISTA trial and
  recommended the trial continue without modification.
• Completion of Full-Scale GMP Manufacturing Run at FUJIFILM Provides
  Encouraging Preliminary Results, Supporting Analytical Comparability
  Plan to be Reviewed with the FDA: In October 2018, Sesen Bio
  entered into an agreement for the manufacturing process and technology
  transfer of Vicinium production with FUJIFILM Diosynth Biotechnologies
  U.S.A., Inc. (FUJIFILM). In April 2019, the first full,
  commercial-scale GMP run was completed at FUJIFILM. Preliminary
  indicators of success, including the bacterial growth and purification
  profiles, support FUJIFILM’S ability to produce the bulk drug
  substance form of Vicinium for commercial purposes if Sesen Bio
  receives regulatory approval to market Vicinium. Full quality release
  testing is underway, and results are expected to be completed in May
  2019.
• Updated Phase 3 VISTA Trial Data Along with Closely Matched Phase 2
  Clinical Trial Data to Serve as Basis for Upcoming FDA Meetings:
  • Type C CMC Meeting Scheduled for May 20, 2019. In
    conjunction with the technology transfer of Vicinium production
    with FUJIFILM, the Company will seek alignment with the FDA on an
    analytical comparability plan that can be used to assess
    comparability between the supply used in clinical trials and the
    potential commercial supply produced by FUJIFILM.
  • Pre-BLA Meeting Scheduled for June 6, 2019: In concurrence
    with the FDA’s recommendation that the Company schedule a meeting
    in mid-2019, Sesen Bio has confirmed a meeting date with the FDA
    in June to discuss its intended registration strategy for Vicinium
    for the treatment of high-risk NMIBC.

First Quarter 2019 Financial Results

• Cash Position: Cash and cash equivalents were $42.4 million as
  of March 31, 2019, compared to $50.4 million as of December 31, 2018,
  and $19.7 million as of March 31, 2018, the comparable period one year
  ago.
• Revenue: No revenue was recorded for the three months ended
  March 31, 2019, nor for the same period in 2018.
• R&D Expenses: Research and development (R&D) expenses for
  the first quarter of 2019 were $4.7 million compared to $3.3 million
  in R&D expenses for the same period in 2018. The increase was
  primarily due to $1.7 million in costs related to the ongoing
  manufacturing process and technology transfer with FUJIFILM, and
  increased internal and external staffing costs, partially offset by
  reduced expenses related to the Phase 3 VISTA trial.
• G&A Expenses: General and administrative expenses for the
  first quarter of 2019 were $3.1 million compared to $2.0 million for
  the same period in 2018. The increase was primarily due to higher
  legal costs, an increase in professional fees and market research
  costs.
• Net Loss: Net loss was $6.5 million, or $0.08 per share, for
  the first quarter of 2019, compared to $4.0 million, or $0.11 per
  share, for the first quarter of 2018.
• 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
7176228. 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 the
duration of 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. 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. 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 a
minimum of two courses of bacillus Calmette-Guérin (BCG) and whose
disease is now BCG-unresponsive. 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-unresponsive non-muscle invasive bladder
cancer (NMIBC). Vicinium is a locally-administered targeted fusion
protein composed of an anti-EPCAM antibody fragment tethered to a
truncated form of Pseudomonas Exotoxin A for the treatment of
high-risk NMIBC. 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, expectations regarding our upcoming FDA meetings, 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.

Contacts

Erin Clark, Executive Director, Strategic Planning & Investor Relations
[email protected]