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March 31, 2015 7:00 am ET

WALTHAM, Mass. & KENILWORTH, N.J.–(BUSINESS WIRE)–Syndax Pharmaceuticals, Inc., and Merck (NYSE:MRK), known as MSD outside
the United States and Canada, announced today that they have entered
into a clinical trial collaboration to evaluate the safety and efficacy
of combining Syndax’s entinostat, an investigational epigenetic therapy,
with Merck’s KEYTRUDA^® (pembrolizumab), the first anti-PD-1
therapy approved in the United States. The Phase 1b/2 study will
evaluate this novel combination regimen in patients with either advanced
non-small cell lung cancer (NSCLC) or melanoma. The study is expected to
begin enrolling patients in the second half of 2015.

Entinostat is an oral, highly selective histone deacetylase (HDAC)
inhibitor granted Breakthrough Therapy Designation in combination with
hormone therapy in advanced hormone receptor positive (HR+) breast
cancer and currently in Phase 3 testing in this indication. Entinostat
has been shown in preclinical models to reduce the number and function
of host immune suppressor cells thereby enhancing the anti-tumor
activity of immune checkpoint blockade. KEYTRUDA is a humanized
monoclonal antibody that blocks the interaction between PD-1 (programmed
death receptor-1) and its ligands, PD-L1 and PD-L2.

“We are excited to be working with Merck on this collaboration to
evaluate the potential of these two novel therapies to improve clinical
outcomes for patients,” said Arlene Morris, president and chief
executive officer of Syndax. “We hope that entinostat in combination
with KEYTRUDA may build upon the enormous promise of immunotherapy in
treating multiple forms of cancer.”

“The broad base of clinical data involving our anti-PD-1 therapy,
KEYTRUDA, continues to provide a strong foundation for advancing the
study of different novel combination regimens,” said Dr. Eric Rubin,
vice president and therapeutic area head, oncology early-stage
development, Merck Research Laboratories. “Our collaboration with Syndax
is an important example of this effort and our commitment to further the
study of breakthrough science in the area of immuno-oncology to help
people with cancer.”

The financial terms and additional details of the agreement between
Syndax and Merck, through a subsidiary, were not disclosed. The
agreement includes a provision where the parties may extend the
collaboration to include a potential Phase 3 clinical trial. The planned
Phase 1b/2 multicenter, open-label clinical trial will be conducted in
two parts. The Phase 1b portion is designed to determine the safety and
tolerability of entinostat in combination with KEYTRUDA. The Phase 2
portion is designed to evaluate the efficacy of entinostat combined with
KEYTRUDA in patients with advanced NSCLC and melanoma.

About KEYTRUDA^® (pembrolizumab)

KEYTRUDA (pembrolizumab) is a humanized monoclonal antibody that blocks
the interaction between PD-1 and its ligands, PD-L1 and PD-L2. By
binding to the PD-1 receptor and blocking the interaction with the
receptor ligands, KEYTRUDA releases the PD-1 pathway-mediated inhibition
of the immune response, including the anti-tumor immune response.

KEYTRUDA is indicated in the United States at a dose of 2 mg/kg
administered as an intravenous infusion over 30 minutes every three
weeks for the treatment of patients with unresectable or metastatic
melanoma and disease progression following ipilimumab and, if BRAF V600
mutation positive, a BRAF inhibitor. This indication is approved under
accelerated approval based on tumor response rate and durability of
response. An improvement in survival or disease-related symptoms has not
yet been established. Continued approval for this indication may be
contingent upon verification and description of clinical benefit in the
confirmatory trials.

Merck is advancing a broad and fast-growing clinical development program
for KEYTRUDA with more than 70 clinical trials – across more than 30
tumor types and over 8,000 patients – both as a monotherapy and in
combination with other therapies.

Selected Important Safety Information for KEYTRUDA

Pneumonitis occurred in 12 (2.9%) of 411 patients with advanced melanoma
receiving KEYTRUDA (the approved indication in the United States),
including Grade 2 or 3 cases in 8 (1.9%) and 1 (0.2%) patients,
respectively. Monitor patients for signs and symptoms of pneumonitis.
Evaluate suspected pneumonitis with radiographic imaging. Administer
corticosteroids for Grade 2 or greater pneumonitis. Withhold KEYTRUDA
for Grade 2; permanently discontinue KEYTRUDA for Grade 3 or 4
pneumonitis.

Colitis (including microscopic colitis) occurred in 4 (1%) of 411
patients, including Grade 2 or 3 cases in 1 (0.2%) and 2 (0.5%) patients
respectively, receiving KEYTRUDA. Monitor patients for signs and
symptoms of colitis. Administer corticosteroids for Grade 2 or greater
colitis. Withhold KEYTRUDA for Grade 2 or 3; permanently discontinue
KEYTRUDA for Grade 4 colitis.

Hepatitis (including autoimmune hepatitis) occurred in 2 (0.5%) of 411
patients, including a Grade 4 case in 1 (0.2%) patient, receiving
KEYTRUDA. Monitor patients for changes in liver function. Administer
corticosteroids for Grade 2 or greater hepatitis and, based on severity
of liver enzyme elevations, withhold or discontinue KEYTRUDA.

Hypophysitis occurred in 2 (0.5%) of 411 patients, including a Grade 2
case in 1 and a Grade 4 case in 1 (0.2% each) patient, receiving
KEYTRUDA. Monitor for signs and symptoms of hypophysitis. Administer
corticosteroids for Grade 2 or greater hypophysitis. Withhold KEYTRUDA
for Grade 2; withhold or discontinue for Grade 3; and permanently
discontinue KEYTRUDA for Grade 4 hypophysitis.

Nephritis occurred in 3 (0.7%) patients receiving KEYTRUDA, consisting
of one case of Grade 2 autoimmune nephritis (0.2%) and two cases of
interstitial nephritis with renal failure (0.5%), one Grade 3 and one
Grade 4. Monitor patients for changes in renal function. Administer
corticosteroids for Grade 2 or greater nephritis. Withhold KEYTRUDA for
Grade 2; permanently discontinue KEYTRUDA for Grade 3 or 4 nephritis.

Hyperthyroidism occurred in 5 (1.2%) of 411 patients, including Grade 2
or 3 cases in 2 (0.5%) and 1 (0.2%) patients respectively, receiving
KEYTRUDA. Hypothyroidism occurred in 34 (8.3%) of 411 patients,
including a Grade 3 case in 1 (0.2%) patient, receiving KEYTRUDA.
Thyroid disorders can occur at any time during treatment. Monitor
patients for changes in thyroid function (at the start of treatment,
periodically during treatment, and as indicated based on clinical
evaluation) and for clinical signs and symptoms of thyroid disorders.
Administer corticosteroids for Grade 3 or greater hyperthyroidism.
Withhold KEYTRUDA for Grade 3; permanently discontinue KEYTRUDA for
Grade 4 hyperthyroidism. Isolated hypothyroidism may be managed with
replacement therapy without treatment interruption and without
corticosteroids.

Other clinically important immune-mediated adverse reactions can occur.
The following clinically significant, immune-mediated adverse reactions
occurred in less than 1% of patients treated with KEYTRUDA: exfoliative
dermatitis, uveitis, arthritis, myositis, pancreatitis, hemolytic
anemia, partial seizures arising in a patient with inflammatory foci in
brain parenchyma, adrenal insufficiency, myasthenic syndrome, optic
neuritis, and rhabdomyolysis.

For suspected immune-mediated adverse reactions, ensure adequate
evaluation to confirm etiology or exclude other causes. Based on the
severity of the adverse reaction, withhold KEYTRUDA and administer
corticosteroids. Upon improvement of the adverse reaction to Grade 1 or
less, initiate corticosteroid taper and continue to taper over at least
1 month. Restart KEYTRUDA if the adverse reaction remains at Grade 1 or
less. Permanently discontinue KEYTRUDA for any severe or Grade 3
immune-mediated adverse reaction that recurs and for any
life-threatening immune-mediated adverse reaction.

Based on its mechanism of action, KEYTRUDA may cause fetal harm when
administered to a pregnant woman. If used during pregnancy, or if the
patient becomes pregnant during treatment, apprise the patient of the
potential hazard to a fetus. Advise females of reproductive potential to
use highly effective contraception during treatment and for 4 months
after the last dose of KEYTRUDA.

For the treatment of advanced melanoma, KEYTRUDA was discontinued for
adverse reactions in 6% of 89 patients who received the recommended dose
of 2 mg/kg and 9% of 411 patients across all doses studied. Serious
adverse reactions occurred in 36% of patients receiving KEYTRUDA. The
most frequent serious adverse drug reactions reported in 2% or more of
patients were renal failure, dyspnea, pneumonia, and cellulitis.

The most common adverse reactions (reported in ≥20% of patients) were
fatigue (47%), cough (30%), nausea (30%), pruritus (30%), rash (29%),
decreased appetite (26%), constipation (21%), arthralgia (20%), and
diarrhea (20%).

The recommended dose of KEYTRUDA is 2 mg/kg administered as an
intravenous infusion over 30 minutes every three weeks until disease
progression or unacceptable toxicity. No formal pharmacokinetic drug
interaction studies have been conducted with KEYTRUDA. It is not known
whether KEYTRUDA is excreted in human milk. Because many drugs are
excreted in human milk, instruct women to discontinue nursing during
treatment with KEYTRUDA. Safety and effectiveness of KEYTRUDA have not
been established in pediatric patients.

About Syndax

Syndax is developing entinostat, an oral, highly selective HDAC
inhibitor with epigenetic and immunomodulatory activity that is being
evaluated in multiple combination regimens for the treatment of patients
with therapy-resistant cancers. Entinostat has been designated a
Breakthrough Therapy by the FDA when used in combination with exemestane
in HR+ advanced (locally advanced or metastatic) breast cancer. A
pivotal Phase 3 clinical study, being conducted under a special protocol
assessment (SPA), combining entinostat with exemestane for the treatment
of hormone receptor-positive metastatic breast cancer is currently
enrolling. Syndax is also developing an immuno-oncology platform to
improve responsiveness to checkpoint inhibitors based on entinostat’s
ability to down-regulate immune suppressor cells. Syndax holds rights to
entinostat in most major markets.

Merck’s Focus on Cancer

Our goal is to translate breakthrough science into biomedical
innovations to help people with cancer worldwide. For Merck Oncology,
helping people fight cancer is our passion, supporting accessibility to
our cancer medicines is our commitment, and pursuing research in
immuno-oncology is our focus to potentially bring new hope to people
with cancer. For more information about our oncology clinical trials,
visit www.merck.com/clinicaltrials.

About Merck

Today’s Merck is a global healthcare leader working to help the world be
well. Merck is known as MSD outside of the United States and Canada.
Through our prescription medicines, vaccines, biologic therapies and
animal health products, we work with customers and operate in more than
140 countries to deliver innovative health solutions. We also
demonstrate our commitment to increasing access to healthcare through
far-reaching policies, programs and partnerships. For more information,
visit www.merck.com
and connect with us on Twitter,
Facebook
and YouTube.

Merck Forward-Looking Statement

This news release includes “forward-looking statements” within the
meaning of the safe harbor provisions of the United States Private
Securities Litigation Reform Act of 1995. These statements are based
upon the current beliefs and expectations of Merck’s management and are
subject to significant risks and uncertainties. If underlying
assumptions prove inaccurate or risks or uncertainties materialize,
actual results may differ materially from those set forth in the
forward-looking statements.

Risks and uncertainties include, but are not limited to, general
industry conditions and competition; general economic factors, including
interest rate and currency exchange rate fluctuations; the impact of
pharmaceutical industry regulation and healthcare legislation in the
United States and internationally; global trends toward healthcare cost
containment; technological advances, new products and patents attained
by competitors; challenges inherent in new product development,
including obtaining regulatory approval; Merck’s ability to accurately
predict future market conditions; manufacturing difficulties or delays;
financial instability of international economies and sovereign risk;
dependence on the effectiveness of Merck’s patents and other protections
for innovative products; and the exposure to litigation, including
patent litigation, and/or regulatory actions.

Merck undertakes no obligation to publicly update any forward-looking
statement, whether as a result of new information, future events or
otherwise. Additional factors that could cause results to differ
materially from those described in the forward-looking statements can be
found in Merck’s 2014 Annual Report on Form 10-K and the company’s other
filings with the Securities and Exchange Commission (SEC) available at
the SEC’s Internet site (www.sec.gov).

Please see Prescribing Information for KEYTRUDA (pembrolizumab) at http://www.merck.com/product/usa/pi_circulars/k/keytruda/keytruda_pi.pdf
and the Medication Guide for KEYTRUDA at http://www.merck.com/product/usa/pi_circulars/k/keytruda/keytruda_mg.pdf

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Merck Media Relations:
Pamela Eisele, 267-305-3558
Claire Mulhearn, 908-236-1118
or
Merck Investor Relations:
Justin Holko, 908-740-1879
or
Syndax Corporate:
Bob Goodenow, 781-419-1418
Chief Business Officer
bgoodenow@syndax.com