Merrimack Pharmaceuticals to Regain Worldwide Rights to Develop and Commercialize MM-121

Merrimack Pharmaceuticals to Regain Worldwide Rights to Develop and
Commercialize MM-121

Merrimack to Pursue Registration Path for MM-121 and Partnership Opportunities

   Top Line Results Released From Final Phase 2 Study in the Collaboration

CAMBRIDGE, Mass., June 19, 2014 (GLOBE NEWSWIRE) -- Merrimack Pharmaceuticals,
Inc. (Nasdaq:MACK) today announced that it has reached an agreement with
Sanofi to regain worldwide rights to develop and commercialize MM-121, a
monoclonal antibody designed to block ErbB3 (HER3) activation in patients with
heregulin-positive tumors and improve response to standard of care treatments.

In partnership with Sanofi, Merrimack completed an extensive Phase 2 program
for MM-121 which was designed to assess the role of ErbB3 in a number of
cancer indications in both the metastatic and neoadjuvant settings. In
advanced settings of ovarian cancer, ER/PR+ HER2 negative breast cancer and
non-small cell lung cancer, Merrimack was able to identify that heregulin, the
principal ligand that binds to and activates the ErbB3 receptor, is associated
with poor response to standard of care therapy and that adding MM-121 may
restore sensitivity in these most at-risk patients.Consistent across three
metastatic cancer indications with three different standard of care therapies,
patients in these trials with heregulin-positive tumors experienced a
statistically significant reduction in their risk of progression when they
received a combination with MM-121.Heregulin-driven drug resistance pathways
were found to be active in approximately 30-50 percent of patients
tested.Data from these studies were recently presented at the 2014 American
Society of Clinical Oncology (ASCO) Annual Meeting.To view the clinical
posters presented at ASCO 2014, click here.

"We are grateful for Sanofi's support over the last five years and believe
that the data generated through this partnership validate the potential for
MM-121 to help patients most at risk for progression on current
therapies.Regaining MM-121 is an opportunity to capitalize on our leadership
position among the other oncology companies that are pursuing ErbB3," said
Robert Mulroy, President and CEO at Merrimack."With these data and the
feedback we've received from our committed investigators, we believe MM-121
has the potential to be a foundational therapy for use across multiple solid
tumor types and we plan to continue its development through subsequent
strategic partnerships.Our next step is to discuss our Phase 2 data and
potential registration paths with the FDA."

Sanofi will continue to fund the existing MM-121 Phase 2 program for the next
six months. The neoadjuvant cohort of a Phase 2 study testing MM-121 in
combination with paclitaxel in patients with triple negative breast cancer is
the final study to be completed through this collaboration.Top line results
of that study are below.

Results from the Second Cohort of a Phase 2 Study of MM-121 in Combination
with Paclitaxel in Neoadjuvant Breast Cancer

Merrimack and Sanofi completed a randomized (2:1), exploratory Phase 2 study
testing MM-121 in combination with paclitaxel followed by doxorubicin and
cyclophosphamide.The study was conducted in two populations of patients with
either ER/PR positive (ER/PR+) HER2 negative breast cancer (n=101) or triple
negative breast cancer (TNBC) (n=99).There was no formal quantitative
endpoint specified for this study.

In November 2013, Merrimack released initial top line results from the ER/PR+
HER2 negative cohort. Those results demonstrated that patients who received
the combination of MM-121 and paclitaxel achieved a pCR rate of 10.6% (95% CI
[5.2; 20.3]) compared to 3.3% (95% CI [0.6; 16.7]) for the control arm. Top
line results from this second cohort of patients with TNBC showed that
patients who received the combination of MM-121 and paclitaxel achieved a pCR
rate of 42.9% (95% CI 30.8; 55.9) compared to 51.7 (95% CI 34.4; 68.6) for the
control arm. Translational analysis is ongoing for both cohorts and the full
data set will be reported at a future medical conference. Preliminary analysis
of approximately 50 percent of the pretreatment biopsies suggests a link
between heregulin levels and pCR rates.

For the TNBC cohort, rates for serious adverse events were 28.1% vs. 15.6%,
and rates for grade 3 or higher adverse events were 50.0% vs. 31.3% between
the treatment and control arms, respectively. As reported previously, in the
ER/PR+ HER2 negative cohort, similar frequencies were reported for adverse
events of any grade between the treatment and control arms, with no unexpected
toxicities experienced and no increase in serious adverse events. Grade 3 or
worse adverse events in the ER/PR+ HER2 negative cohort were 55.2% on
treatment arm versus 53.1% on control arm.

About MM-121

MM-121 is a fully human monoclonal antibody that targets ErbB3, a cell surface
receptor that is activated by the ligand heregulin. Heregulin-driven ErbB3
signaling has been implicated as a mechanism of tumor growth and resistance to
targeted, cytotoxic and anti-endocrine therapies. When used in the combination
setting, MM-121 is designed to block ErbB3 signaling in order to restore or
enhance the anti-tumor effect of a combination therapy partner.

MM-121 has been investigated in four Phase 2 and six Phase 1 clinical studies
covering a broad spectrum of patient populations and drug combinations. An
extensive translational component of the MM-121 clinical program was designed
to establish clinically useful biomarkers that were initially identified using
Merrimack's systems biology approach and confirmed in preclinical studies.

About Merrimack

Merrimack is a biopharmaceutical company discovering, developing and preparing
to commercialize innovative medicines paired with companion diagnostics for
the treatment of cancer. Merrimack seeks to gain a deeper understanding of
underlying cancer biology through its systems biology-based approach and
develop new insights, therapeutics and diagnostics to improve outcomes for
cancer patients. Merrimack currently has six oncology therapeutics in clinical
development and three additional candidates in late stage preclinical
development. Merrimack's lead program, MM-398, recently completed a Phase 3
trial in post-gemcitabine pancreatic cancer.Based on the results of this
study, Merrimack intends to file a New Drug Application for MM-398 in 2014.
For more information, please visit Merrimack's website at
www.merrimackpharma.com.

Forward-Looking Statement

To the extent that statements contained in this press release are not
descriptions of historical facts, they are forward-looking statements
reflecting the current beliefs and expectations of management made pursuant to
the safe harbor provisions of the Private Securities Litigation Reform Act of
1995, as amended. Forward-looking statements include any statements about
Merrimack's strategy, future operations, future financial position and future
expectations and plans and prospects for Merrimack, and any other statements
containing the words "anticipate," "believe," "estimate," "expect," "intend,"
"may," "plan," "predict," "project," "target," "potential," "will," "would,"
"could," "should," "continue," "hope" and similar expressions. In this press
release, Merrimack's forward-looking statements include statements about the
potential effectiveness and tolerability of MM-121 and its other therapeutics
in certain patient populations or subpopulations, its ability to develop a
predictive diagnostic, the initiation of future clinical studies, its ability
to translate clinical data into future clinical success, discussions with and
applications to regulatory authorities, and its ability to establish and
maintain additional collaborations.Such forward-looking statements involve
substantial risks and uncertainties that could cause Merrimack's clinical
development programs, future results, performance or achievements to differ
significantly from those expressed or implied by the forward-looking
statements. Such risks and uncertainties include, among others, availability
of data from ongoing clinical trials, expectations for regulatory approvals,
development progress of Merrimack's companion diagnostics and other matters
that could affect the availability or commercial potential of Merrimack's drug
candidates or companion diagnostics. Merrimack undertakes no obligation to
update or revise any forward-looking statements. Forward-looking statements
should not be relied upon as representing Merrimack's views as of any date
subsequent to the date hereof. For a further description of the risks and
uncertainties that could cause actual results to differ from those expressed
in these forward-looking statements, as well as risks relating to Merrimack's
business in general, see the "Risk Factors" section of Merrimack's Quarterly
Report on Form 10-Q filed with the Securities andExchange Commission (SEC) on
May 6, 2014 and other reports Merrimack files with the SEC.

CONTACT: Investor Contact:
         Geoffrey Grande, CFA
         Merrimack Pharmaceuticals
         617-441-7602
         ggrande@merrimackpharma.com
        
         Media Contacts:
         Kathleen Gallagher
         Merrimack Pharmaceuticals
         617-441-1043
         kgallagher@merrimackpharma.com
        
         Heather Bonsiero
         Spectrum
         202-955-6222
         hbonsiero@spectrumscience.com

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