Merrimack Presents Expanded Analysis from Seribantumab (MM-121) Phase 2 Breast Cancer Study at the AACR Precision Medicine Series

CAMBRIDGE, Mass., May 17, 2016 /PRNewswire/ -- Merrimack Pharmaceuticals, Inc. (NASDAQ: MACK) today presented an expanded analysis of its Phase 2 study of seribantumab (MM-121) in combination with exemestane in HER2-negative, hormone receptor positive metastatic breast cancer.  Top line results from this study were announced in 2014. The final analysis, as well as a poster on Merrimack's investigational companion diagnostic for seribantumab, were presented this week at the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer, in Miami, Florida. 

"This data package underscores the most significant finding from all of our seribantumab Phase 2 studies – the identification of a heregulin-positive cancer cell phenotype that infiltrates approximately 30-50% of tumors and that may directly impact response to current standard-of-care therapies.  This hypothesis is supported by a strong consistent data set that spans three solid tumor types where we saw improved progression free survival when seribantumab was added to each of the standard-of-care regimens," said Akos Czibere, MD, Ph.D., Senior Medical Director and Team Lead for the seribantumab program. "We are currently pursuing a registration study for seribantumab in patients with non-small cell lung cancer whose disease has progressed following immunotherapy."

"These additional analyses from our Phase 2 breast cancer study reinforce the significant opportunity for seribantumab to be the first therapy to treat heregulin-positive disease," said Robert Mulroy, President and CEO of Merrimack.  "We are focused on executing our strategy in NSCLC as our near term path to registration for seribantumab and its heregulin diagnostic."

A randomized trial of exemestane +/- seribantumab (MM-121) in postmenopausal women with locally advanced or metastatic ER/PR+ HER2- breast cancer: Final analysis and extended subgroup analysis

Final analysis and extended subgroup analysis of a Phase 2 study of seribantumab in combination with exemestane in metastatic HER2-negative, ER/PR+ breast cancer was presented.  The study was randomized, double-blinded and placebo-controlled to evaluate whether the combination of seribantumab and exemestane was more effective in prolonging progression free survival (PFS) than exemestane in ER/PR+ metastatic breast cancer (n=118) who had previously failed anti-estrogen therapy. Of the patients with tissue available for heregulin testing, 45% (n=34/76) were heregulin-positive.  Patients in the heregulin-positive group who received seribantumab had a 74% decrease in risk of progression, (HR 0.26; 95% CI [0.11 — 0.63], p=0.003) compared with patients who did not receive seribantumab.  The study did not meet its primary endpoint of PFS though the overall study population trended in favor of the MM-121 arm (HR 0.772; 95% CI [0.496 — 1.201]). Notably, across all treated patients, overall survival data trended in favor of the seribantumab arm with a 59% decrease in risk of death (HR 0.436; 95% CI [0.197 — 0.966]) versus patients on the standard-of-care arm. Click here to view the full data set from the poster.

Identification of heregulin expression as a driver of a difficult-to-treat cancer phenotype and development of a prospective companion diagnostic for the heregulin-ErbB3 targeting drug seribantumab

A poster highlighting the potential importance of a diagnostic in treating heregulin-positive patients across three different solid tumors (breast, lung and ovarian cancers) was also presented.  In three Phase 2 studies, Merrimack's novel heregulin assay was able to identify patients with heregulin-positive tumors where the addition of seribantumab to standard-of-care therapies may provide benefit.  Heregulin was detected at significant levels across multiple solid tumor types, with a prevalence of between 30-60% of patients, potentially defining a critical patient phenotype that has a high-unmet need. Tumor biopsies were measured by RNA-ISH. A fully validated RNA-ISH assay is currently being used to identify heregulin-positive patients in a Phase 2 randomized trial of seribantumab in patients with NSCLC.  Merrimack recently announced a strategic partnership with Leica Biosystems to develop Merrimack's novel heregulin assay for seribantumab into a kit for commercial use.  Click here to view the full data set from the poster.

About Seribantumab

Seribantumab is Merrimack's wholly owned, fully human monoclonal antibody that targets ErbB3, a cell surface receptor that is activated by the ligand heregulin. Heregulin-driven ErbB3 signaling has been implicated as a mechanism of tumor growth and resistance to targeted, cytotoxic and anti-endocrine therapies. When used in the combination setting, seribantumab is designed to block ErbB3 signaling in order to enhance the anti-tumor effect of a combination therapy partner.

While Merrimack is encouraged by the clinical results in breast and ovarian cancers, Merrimack has chosen to initially pursue seribantumab in a potentially registration-enabling Phase 2 study in non-small cell lung cancer (NSCLC). The NSCLC study is an open-label, biomarker-selected randomized study of MM-121 in combination with docetaxel or pemetrexed compared to docetaxel or pemetrexed alone in patients with heregulin-positive, locally advanced or metastatic NSCLC.

About Merrimack

Merrimack is a fully integrated biopharmaceutical company that views cancer as a complex engineering challenge. Through systems biology, which brings together the fields of biology, computing and engineering, Merrimack aims to decrease uncertainty in drug development and clinical validation, and move discovery efforts beyond trial and error. Such an approach has the potential to make individualized treatment of patients a reality. Merrimack's first commercial product, ONIVYDE® (irinotecan liposome injection), was approved by the U.S. FDA in October 2015. With four additional candidates in clinical studies, several in preclinical development and multiple biomarkers designed to support patient selection, Merrimack is building one of the most robust oncology pipelines in the industry. For more information, please visit Merrimack's website at www.merrimack.com or connect on Twitter at @MerrimackPharma.

Forward-Looking Statements

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," "hope" and similar expressions. In this press release, Merrimack's forward-looking statements include statements about the potential effectiveness and safety profile of MM-121 in certain patient populations or subpopulations, the ability to use heregulin as a predictive diagnostic, the initiation of future clinical trials and the ability to translate clinical data into 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, 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 Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission (SEC) on May 2, 2016 and other reports Merrimack files with the SEC.

Contacts: 
Media Contact 
Marianne McMorrow 
774-776-1478 
[email protected]

Investor Contact: 
Geoffrey Grande, CFA 
617-441-7602 
[email protected]

SOURCE Merrimack Pharmaceuticals, Inc.

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