Why Merrimack Pharmaceuticals (MACK) Stock is Down in After-Hours Trading Today

NEW YORK (TheStreet) -- Merrimack Pharmaceuticals  (MACK) fell in after-hours trading on Thursday after the company announced it had regained worldwide rights from Sanofi  (SNY) to develop and commercialize MM-121.

MM-121 is "a monoclonal antibody designed to block ErbB3 (HER3) activation in patients with heregulin-positive tumors and improve response to standard of care treatments," according to the company's press release.

The rights acquisition ends Merrimack's five-year partnership with Sanofi, which helped Merrimack complete a Phase 2 program for MM-121. Sanofi will continue to fund the program for the next six months.

Must Read: Warren Buffett's 25 Favorite Stocks

STOCKS TO BUY: TheStreet Quant Ratings has identified a handful of stocks that can potentially TRIPLE in the next 12 months. Learn more.

The stock was down 9.13% to $7.17 at 4:45 p.m.

MACK Price Chart

MACK Price data by YCharts

STOCKS TO BUY: TheStreet Quant Ratings has identified a handful of stocks that can potentially TRIPLE in the next 12 months. Learn more.

Editor's Note: Any reference to TheStreet Ratings and its underlying recommendation does not reflect the opinion of TheStreet, Inc. or any of its contributors including Jim Cramer or Stephanie Link.

More from Markets

Week Ahead: Trade Fears and Stress Tests Signal More Volatility To Come

Week Ahead: Trade Fears and Stress Tests Signal More Volatility To Come

Trump Takes Aim at Auto Imports; Markets End Mixed -- ICYMI

Trump Takes Aim at Auto Imports; Markets End Mixed -- ICYMI

Video: What Oprah's Content Partnership With Apple Means for the Rest of Tech

Video: What Oprah's Content Partnership With Apple Means for the Rest of Tech

REPLAY: Jim Cramer on the Markets, Oil, Starbucks, Tesla, Okta and Red Hat

REPLAY: Jim Cramer on the Markets, Oil, Starbucks, Tesla, Okta and Red Hat

Flashback Friday: The Market Movers

Flashback Friday: The Market Movers