As disclosed in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2016, on August 3, 2016, Momenta Pharmaceuticals, Inc. (the "Company") discontinued further accrual of its Part B, or Phase 2, portion of its Phase 1/2 clinical trial evaluating necuparanib in combination with nab-paclitaxel (ABRAXANE) and gemcitabine in patients with advanced metastatic pancreatic cancer. The decision to discontinue enrollment was based on the recommendation of the independent Data Safety Monitoring Board for the trial following the outcome of a planned interim futility analysis. On August 22, 2016, after confirming the results of the futility analysis and reviewing the unblinded safety and efficacy data and the results of various sensitivity and subgroup analyses, the Company decided to discontinue the necuparanib program.

On February 29, 2016, the Company filed with the Securities and Exchange Commission (the "Commission") a shelf registration statement on Form S-3 (File No. 333-209813) (the "Registration Statement"), which became immediately effective upon filing.

On September 1, 2016, the Company will be filing with the Commission a prospectus supplement, dated September 1, 2016, to the prospectus included in the Registration Statement in connection with the offer and sale of shares of the Company’s common stock from time to time through Stifel, Nicolaus & Company, Incorporated ("Stifel"), pursuant to an At-the-Market Equity Offering Sales Agreement, dated April 21, 2015, between the Company and Stifel, which was filed with the Commission as Exhibit 10.1 to the Company’s Current Report on Form 8-K on April 21, 2015.

In connection with the filing of the prospectus supplement, the Company is filing as Exhibit 5.1 hereto a copy of an opinion of its counsel, Latham & Watkins LLP, regarding the validity of the securities being registered under the prospectus supplement.