Merrimack Pharmaceuticals Completes Enrollment of Second Cohort in Phase 2 Study of MM-121 in Non-Small Cell Lung Cancer

Merrimack Pharmaceuticals Completes Enrollment of Second Cohort in Phase 2
Study of MM-121 in Non-Small Cell Lung Cancer

CAMBRIDGE, Mass., April 29, 2013 (GLOBE NEWSWIRE) -- Merrimack
Pharmaceuticals, Inc. (Nasdaq:MACK) announced today that the last patient has
been enrolled in the second cohort of a randomized, three-cohort Phase 2
clinical trial of MM-121 in combination with erlotinib for the treatment of
non-small cell lung cancer (NSCLC) patients. 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.

This cohort (Group A), which focuses on patients with EGFR wild-type tumors,
is part of a larger, randomized Phase 2 study evaluating MM-121 in the
treatment of three populations of NSCLC patients. The cohort was designed to
evaluate if MM-121 in combination with erlotinib is more effective than
treatment with erlotinib alone. Patients in Group A have EGFR wild-type NSCLC
tumors with recurring or progressive disease following at least one
chemotherapy-containing regimen and have not received prior EGFR tyrosine
kinase inhibitor therapy. A total of 133 patients were enrolled and final
results from this patient group are expected in the second half of 2013.

"We are very pleased with the support this study has received from patients,
their families and the lung cancer community, which resulted in rapid patient
accrual," said Akos Czibere, MD, PhD, Senior Medical Director of the MM-121
program at Merrimack. "We are hopeful that our translational work will help us
gain a better understanding of the biomarker profile of NSCLC EGFR wild-type
tumors. This could help characterize the role of ErbB3 in this patient
population and identify those patients who are likely to benefit most from the
addition of MM-121 to erlotinib."

A second cohort (Group B) includes patients whose tumors harbor an EGFR
mutation and have not received prior EGFR tyrosine kinase inhibitor therapy.
The enrollment for the second cohort is ongoing.

The third cohort (Group C) included patients who were previously treated with
an EGFR tyrosine kinase inhibitor and progressed on that therapy. This third
cohort is completed and top line results were reported in April 2013.

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
         Liz Bryan, Spectrum
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