Merrimack Pharmaceuticals Inc (MACK) SEC Filing 10-K Annual report for the fiscal year ending Monday, December 31, 2018

Exhibit 99.1

Merrimack Reports Fourth Quarter and Full Year 2018 Financial Results

Cambridge, Mass., March 6, 2019 – Merrimack Pharmaceuticals, Inc. (Nasdaq: MACK), a clinical-stage oncology company focused on biomarker-defined cancers, today announced its fourth quarter and full year 2018 financial results for the period ended December 31, 2018.

“Last quarter, we engaged external advisors and initiated a process to explore Merrimack’s strategic alternatives. This is an active process that we are working expeditiously to bring to conclusion, with a range of potential outcomes under consideration. We are also committed to preserving the value of the potential milestones that Merrimack remains eligible to receive from Ipsen,” said Richard Peters, M.D., Ph.D., President and Chief Executive Officer. “In light of this ongoing process, we are also prudently advancing our pipeline and are pleased to report today an update from our ongoing Phase 1 study of MM-310 and that six posters have been accepted for presentation at the American Association for Cancer Research (AACR) Annual Meeting, five of which highlight our prioritized preclinical programs, MM-401 and MM-201.”

Program Update:

MM-310:

In November 2018, Merrimack amended its Phase 1 study of MM-310 in patients with solid tumors to extend the dosing interval of MM-310 from every three weeks to every four weeks. To date, three patients have been enrolled in the 360 mg every four weeks dose cohort under the amended protocol, which matches the highest dose level reached during the prior version of the protocol at every three weeks.

As of March 4, 2019, all three patients in the 360 mg every four weeks dose cohort continue to be treated in the study: one patient has completed 98 days of treatment and received four cycles of MM-310, reaching stable disease as a best response to date; the second patient has completed 56 days of treatment and received two cycles of MM-310; and the third patient received the first dose 21 days prior. Importantly, there have been no instances of grade 3 peripheral neuropathy reported in this cohort.

If all three patients in the 360 mg every four weeks dose cohort successfully complete the observation period for dose-limiting toxicities, which is expected to occur in mid-March, Merrimack would plan to begin enrolling the next dose-escalation cohort at 420 mg of MM-310 every four weeks.

Preclinical Programs:

In November 2018, in connection with its corporate restructuring and exploration of strategic alternatives, Merrimack narrowed the scope of its preclinical efforts to its two most promising preclinical programs: MM-401, an agonistic antibody targeting a novel immuno-oncology target, TNFR2; and MM-201, a highly stabilized agonist-Fc fusion protein targeting death receptors 4 and 5.