Merrimack' MM-121 Shows Positive Signal In 2 Phase2 ER/PR+ Breast Cancer Studies

(RTTNews) - Merrimack Pharmaceuticals Inc. (MACK) 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 or 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.

The company said that the randomized, double-blinded, placebo-controlled study evaluated whether the combination of MM-121 and exemestane was more effective in prolonging progression free survival or 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).

In addition, the company noted that the randomized, open-label Phase 2 study was designed to compare the pathologic complete response (pCR) rate in ER/PR+ breast cancer patients (n=99) treated in the neoadjuvant setting with MM-121 in combination with paclitaxel followed by doxorubicin and cyclophosphamide, with the pCR rate in patients receiving paclitaxel alone followed by doxorubicin and cyclophosphamide. pCR measures the absence of invasive cancer in breast and lymph node tissue following neoadjuvant therapy and is widely viewed as a strong indicator for prolonged disease-free survival.

Patients who received the combination of MM-121 and paclitaxel achieved a pCR rate of 10.8% (95% CI [5.3; 20.6]) compared to 3.3% (95% CI [0.6; 16.7]) for the control arm. There was no formal quantitative endpoint specified for this study. Extensive translational analysis is ongoing based on serial biopsies and will be reported in the future.

In a separate press release, Merrimack Pharmaceuticals and Actavis plc, announced that the companies have entered into a collaboration agreement. As per the agreement, Merrimack will utilize its proprietary nanoliposomal technology platform to develop and manufacture various pharmaceutical products for Actavis to commercialize around the world.

As per the terms of the agreement, Merrimack is eligible to receive up to $15.5 million, including $2.0 million upfront and the remainder in committed near-term funding and development, regulatory and commercial milestone payments related to the first product to come out of the collaboration.

In addition, Merrimack will receive a double-digit share of profits on future global sales of any commercialized products derived from the collaboration. Merrimack will be responsible for manufacturing bulk product at its Cambridge, Mass. nanoliposomal manufacturing facility.