Merrimack Pharmaceuticals Announces Top-Line Results From A Single Arm Phase 2 Advanced Lung Cancer Study In Patients With

Merrimack Pharmaceuticals Announces Top-Line Results From A Single Arm Phase 2
    Advanced Lung Cancer Study In Patients With Resistance To An Anti-EGFR
                          Tyrosine Kinase Inhibitor

PR Newswire

CAMBRIDGE, Mass., April 4, 2013

CAMBRIDGE, Mass., April 4, 2013 /PRNewswire-USNewswire/ -- Merrimack
Pharmaceuticals, Inc. (NASDAQ:MACK) today announced that one cohort of a Phase
2 non-small cell lung cancer (NSCLC) study did not meet its primary endpoint.
The cohort evaluated MM-121 in combination with erlotinib to treat patients
with NSCLC whose disease progressed on an anti-EGFR tyrosine kinase inhibitor
(EGFR-TKI). MM-121 is being evaluated in two additional NSCLC cohorts as well
as Phase 2 studies for the treatment of advanced ovarian cancer,
hormone-receptor positive breast cancer and HER2 negative breast cancer.

The primary objective of this 50 patient, single arm study was to determine
the potential of MM-121, a novel signaling inhibitor targeting ErbB3, to
modulate or reverse resistance to erlotinib, an EGFR targeted therapy,
commonly used in the treatment of NSCLC, in patients who have acquired
resistance to EGFR-TKI therapy. The primary endpoint of this cohort was to
obtain a 40 percent progression free survival rate at four months of
treatment. Secondary endpoints included biomarker assessment of patients
enrolled in the study.

A secondary objective of the study was to evaluate tissue samples from patient
biopsies to assess whether the biomarker hypothesis behind MM-121 could be
translated into a clinical setting. In order to conduct this analysis,
pre-dose biopsies were required from each patient upon entering the study.

"These are data on the first of three NSCLC patient populations where we are
studying the potentialbenefitof MM-121 in combination witherlotinib. A
strong component of these studies is a focused translational program directed
toward the identificationof a biomarker profileforthe combination in this
difficult to treat cancer,"said Akos Czibere, MD, PhD,SeniorMedical
Director of the MM-121 program at Merrimack."We are disappointedby the
overall clinical results in this population, but are encouraged by our
preliminary biomarker analysis which webelieve will be important for the
identification of a predictive diagnostic when analyzed in the context of the
overall clinical development program."

This is one cohort (Group C) of a larger, ongoing randomized Phase 2 study
evaluating MM-121 to treat other groups of patients with NSCLC. The study also
includes patients with EGFR wild-type tumors who have recurring or progressive
disease following at least one chemotherapy-containing regimen in the
metastatic setting and who have had no prior EGFR-TKI therapy (Group A) and
patients with a known EGFR-activating mutation who have had no prior EGFR-TKI
therapy in the metastatic setting (Group B). MM-121 is also being evaluated in
Phase 2 studies for the treatment of advanced ovarian cancer, hormone-receptor
positive breast cancer and HER2 negative breast cancer.

MM-121 is a fully human monoclonal antibody that targets ErbB3, a cell surface
receptor implicated in tumor growth and survival. By inhibiting ErbB3
signaling, MM-121 is designed to restore sensitivity, delay resistance and
enhance the anti-tumor effect of a combination therapy partner. Sanofi and
Merrimack entered into an exclusive, global license and collaboration
agreement for MM-121 in 2009.

About Merrimack Pharmaceuticals,
Inc.
Merrimack Pharmaceuticals 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 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," and similar expressions. In this press release,
Merrimack's forward-looking statements include statements about the potential
effectiveness of its drug candidates, including the potential benefits of
MM-121 in combination with erlotinib, the safety of its drug candidates, the
ability to develop a predictive diagnostic and the ability to achieve future
clinical success. 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 and conduct 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 theSecurities and Exchange Commission(SEC) onMarch 20,
2013and other reports Merrimack files with theSEC.

SOURCE Merrimack Pharmaceuticals, Inc.