Phase 2 flop kills Merrimack pancreatic cancer drug, sinks stock

Merrimack Pharmaceuticals has dropped MM-141 after the pancreatic cancer therapy failed to move the needle in a midphase trial. The setback eliminates one of the three drugs Merrimack has focused on since shrinking and restructuring its operation last year.

Investigators enrolled 88 patients with metastatic pancreatic cancer and randomized them to receive placebo or IGF-1R-HER3 bispecific antibody MM-141 on top of standard-of-care chemotherapy. The idea was to use MM-141 to block two pathways Merrimack thinks control the growth of cancer cells, thereby improving progression-free and overall survival.

That idea is now in tatters. Merrimack has yet to share a look at the results but stated MM-141 failed against the primary endpoint—PFS—and secondary endpoints. With MM-141 performing badly in all subgroups, Merrimack is halting investment in MM-141.

The setback wiped 35% off Merrimack’s stock in premarket trading as investors dialled down their expectations for the company. Now that MM-141 is out of the picture, Merrimack’s prospects rest on anti-HER3 antibody MM-121 and EphA2-targeted docetaxel formulation MM-310. Readouts from phase 2 trials of MM-121 and a phase 1 study of MM-310 are due in the second half of the year.

Weak performances in those trials would put Merrimack on the ropes. Merrimack made its name as the developer of a liposomal formulation of irinotecan that won FDA approval in pancreatic cancer in 2015.

Weak sales led Merrimack to sell the drug to Ipsen last year. Merrimack emerged from the sale and accompanying restructuring process with a new CEO, a significantly reduced headcount, no commercial drugs and three key pipeline prospects: MM-141, MM-121 and MM-310.

Merrimack’s willingness to bet big on MM-141 reflects the central role insulin-growth factor (IGF) is thought to play in the advance and spread of pancreatic cancer. That role attracted other companies to IGF before Merrimack, which is far from the first drug developer to go after the target and fall short in the clinic. Amgen took ganitumab as far as phase 3 only to see the anti-IGF-1 antibody fail to improve overall survival. Eli Lilly’s cixutumumab has underwhelmed in the clinic, too.