MACK Merrimack Pharmaceuticals Inc

Merrimack Pharmaceuticals, Inc. (Nasdaq:MACK) today announced that Phase 1 data evaluating safety of the novel agent MM-302 in patients with advanced HER2-positive breast cancer will be presented at the 2013 San Antonio Breast Cancer Symposium (SABCS), December 10-14, 2013 in San Antonio, Texas.

MM-302, an antibody drug conjugated liposomal doxorubicin, specifically targets cancer cells that overexpress the HER2/ErbB2 receptor. This targeted liposomal encapsulation of doxorubicin is designed to allow for the selective uptake of drug into tumor cells while limiting exposure to healthy tissues, such as those of the heart.

The Phase 1 trial for MM-302 enrolled patients with advanced HER2-positive breast cancer who have progressed or recurred on standard therapy. Data to be presented will include safety results for all treated patients in the trial, as well as preliminary activity in patients treated with therapeutically relevant dose levels of MM-302 alone and in combination with trastuzumab.

In addition, results will be presented from a research collaboration with the laboratory of Peter Sorger, Ph.D., from Harvard Medical School. In this collaboration, statistical models were used to generate preclinical response biomarker hypotheses for targeted inhibitors.

Poster Overview:

• Assessment of Safety and Activity in an Expanded Phase 1 Study of MM-302, a HER2-targeted Liposomal Doxorubicin, in Patients with Advanced HER2-positive (HER2+) Breast Cancer (Abstract #P4-12-29)

Poster Session 4: HER2-Targeted Therapy Friday, December 13, 2013 7:30 – 9 a.m. CT, Exhibit Halls A-B

• Basal and Ligand-Induced Receptor Profiles Cluster Cell Lines into Subtypes and Predict Drug Response in a Panel of Breast Cancer Lines (Abstract #P6-05-01)

Poster Session 6: Tumor Cell and Molecular Biology – Biomarkers Saturday, December 14, 2013 7:30 – 9 a.m. CT, Exhibit Hall C

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 its systems biology-based approach to biomedical research throughout the research and development process. Merrimack currently has six targeted therapeutic oncology therapeutics in clinical development. For more information, please visit Merrimack's website at www.merrimackpharma.com.

Forward-Looking Statement

Any statements in this press release about future expectations, plans and prospects for Merrimack constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995, as amended. Actual results may differ materially from those indicated by such forward-looking statements. Merrimack anticipates that subsequent events and developments will cause its views to change. However, while Merrimack may elect to update these forward-looking statements at some point in the future, Merrimack specifically disclaims any obligation to do so.

CONTACT: Media Contacts:
         
         Debbie Tseng, Merrimack
         617-441-7659
         dtseng@merrimackpharma.com
         
         Heather Gitlitz, Spectrum
         202-955-6222
         hgitlitz@spectrumscience.com