Merrimack Pharmaceuticals Completes Enrollment of One Cohort in Neoadjuvant Phase 2 Study of MM-121 in HER2-Negative Breast

Merrimack Pharmaceuticals Completes Enrollment of One Cohort in Neoadjuvant
Phase 2 Study of MM-121 in HER2-Negative Breast Cancer

CAMBRIDGE, Mass., April 15, 2013 (GLOBE NEWSWIRE) -- Merrimack
Pharmaceuticals, Inc. (Nasdaq:MACK) announced today that the last patient has
been enrolled in one group of a two-cohort randomized Phase 2 clinical trial
of MM-121 combined with paclitaxel in the neoadjuvant setting of HER2-negative
breast cancer. Developed in collaboration with Sanofi, MM-121 is a fully human
monoclonal antibody that targets ErbB3, a cell surface receptor implicated in
tumor growth and survival.

The Phase 2 study is designed to evaluate whether the combination of MM-121
with paclitaxel is more effective than treatment with paclitaxel alone when
administered as part of a neoadjuvant treatment regimen in two patient groups
of HER2-negative, locally advanced breast cancer patients. The first, fully
enrolled cohort of patients is considered HER2-negative, but hormone sensitive
because either estrogen receptor (ER) and/or progesterone receptor (PR)
markers are positive. A total of 100 patients were enrolled in this patient
group. Final results from this cohort are expected in the second half of 2013.

"This study is very exciting as it gives us an opportunity to see the impact
of MM-121 in patients who have not yet seen prior therapies for their
disease," said Gavin MacBeath, Ph.D., Co-Founder and Vice President of
Translational Research at Merrimack. "It also gives us the opportunity to
assess biomarkers at an early stage in cancer treatment, which we believe will
allow us to better understand biomarker profiles being explored for MM-121."

The second cohort is comprised of patients with triple negative breast cancer
(TNBC), which is diagnosed when a patient's tumor tests negative for HER2, ER
and PR biomarkers. Enrollment is ongoing in the TNBC patient group and results
are expected in 2014.

Following treatment with MM-121 and paclitaxel, or paclitaxel alone, patients
from both groups will receive standard treatment with doxorubicin and
cyclophosphamide and be monitored until surgical resection. The study is being
conducted in the United States.

Sanofi and Merrimack entered into an exclusive, global license and
collaboration agreement for MM-121 in 2009.

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 applies Network Biology, its proprietary
systems biology-based approach to biomedical research, throughout the research
and development process. Merrimack currently has six targeted therapeutic
oncology candidates in clinical development.

Forward-looking statements

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," and similar expressions. In this press release,
Merrimack's forward-looking statements include statements about the potential
for MM-121 to provide clinical benefit, the ability of Merrimack to identify
patients most likely to respond to treatment and the timing of release of
study results. 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, the uncertainties inherent in the
initiation of future clinical trials, 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 Annual Report on
Form 10-K filed with the Securities and Exchange Commission (SEC) on March 20,
2013 and other reports Merrimack files with the SEC.

CONTACT: Media Contacts:
         Kathleen Petrozzelli Gallagher, Merrimack
         617-441-1043
         kgallagher@merrimackpharma.com
        
         Liz Bryan, Spectrum
         202-955-6222
         lbryan@spectrumscience.com