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."