Metastatic Breast Cancer Study Reinforces Biomarker Findings From Previously Reported MM-121 Study in Ovarian Cancer Neoadjuvant Breast Cancer Study Shows Favorable Comparison in pCR Rate Between Arms CAMBRIDGE, Mass., Nov. 26, 2013 (GLOBE NEWSWIRE) -- Merrimack Pharmaceuticals, Inc. (Nasdaq:MACK) today announced the results of two Phase 2 studies evaluating MM-121 in the treatment of women with ER/PR+, HER2 negative breast cancer. One study was conducted in metastatic breast cancer (mBC) in combination with exemestane. The second was conducted in the neoadjuvant setting in combination with paclitaxel followed by doxorubicin and cyclophosphamide. MM-121 is a monoclonal antibody designed to target the ErbB3 (HER3) receptor and to interfere with growth factor-mediated resistance to standard-of-care therapies. Results from the Phase 2 Study of MM-121 in Combination with Exemestane in Metastatic ER/PR+, HER2 Negative Breast Cancer This randomized, double-blinded, placebo-controlled study evaluated whether the combination of MM-121 and exemestane was more effective in prolonging progression free survival (PFS) than exemestane in ER/PR+ mBC patients (n=118) who have previously failed anti-estrogen therapy. The estimated hazard ratio (HR) for PFS in the overall study population trended in favor of the MM-121 arm (HR 0.75; 95% CI [0.48 – 1.15]), although the primary endpoint of the study (HR < 0.5) was not met. Overall survival data also trended in favor of the MM-121 arm (HR 0.41; 95% CI [0.19 – 0.90]), but is at this point immature (~25% of patients). Analysis of a pre-specified set of biomarkers mechanistically linked to ErbB3 signaling revealed a subpopulation of patients (31% of the 55 biomarker-evaluable patients) in which the observed HR for PFS was 0.32 (95% CI [0.10-1.0]). The HR for PFS in the biomarker negative population was 0.89 [95% CI 0.44-1.79]. The undisclosed biomarkers identified in this study matched the two biomarkers previously reported for a study combining MM-121 with paclitaxel in women with platinum resistant ovarian cancer.