Epizyme Earns $8 Million Milestone Payment from GlaxoSmithKline for Initiation of Clinical Development for First-in-Class PRMT1 Inhibitor

Epizyme, Inc. EPZM, a clinical-stage company developing novel epigenetic therapies, today announced it has earned an $8 million milestone payment from GlaxoSmithKline (GSK). The milestone payment follows GSK's initiation of patient dosing in a Phase 1 clinical trial of GSK3368715, a first-in-class protein arginine methyltransferase1 (PRMT1) inhibitor discovered by Epizyme and licensed to GSK. PRMT1 has been implicated in a number of human cancers.

"We are very pleased to see the progress that GSK has made with its PRMT1 inhibitor program, which marks the second program under our collaboration agreement to enter the clinic," said Robert Bazemore, president and chief executive officer of Epizyme. "This milestone reinforces the promising role that epigenetic medicines may play in treating a variety of diseases, and our leadership in the discovery of novel epigenetic targets. It also validates our strategic business model of establishing collaborations that allow us to focus our internal resources on our lead programs, while facilitating the advancement of novel treatments for people with cancer. We look forward to GSK's continued progress with this clinical program."

This Phase 1 study by GSK will assess the safety, pharmacokinetics, pharmacodynamics and preliminary clinical activity of GSK33368715 in patients with relapsed and/or refractory diffuse large B-cell lymphoma (DLBCL) and selected solid tumors with frequent methyl-thioadenosine phosphorylase (MTAP)-deficiency.

About the Epizyme-GSK Collaboration
Under the terms of its collaboration and license agreement with GSK, Epizyme granted GSK exclusive worldwide license rights to methyltransferase inhibitors directed to three targets. Using its proprietary drug discovery platform, Epizyme discovered and optimized compounds targeting three methyltransferases. During the research term of the collaboration, which has been completed, Epizyme was primarily responsible for preclinical research on such compounds. GSK is responsible for subsequent research, development and commercialization of each program. Epizyme has earned an aggregate of $89 million in up-front, research and milestone payments to date, and may receive up to an additional $375 million from GSK if all remaining milestones are met. Epizyme is eligible to receive up to double-digit royalties on worldwide net sales of collaboration products.

About Epizyme, Inc.
Epizyme, Inc. is a clinical-stage biopharmaceutical company committed to rewriting treatment for cancer and other serious diseases through novel epigenetic medicines. Epizyme is broadly developing its lead product candidate, tazemetostat, a first-in-class EZH2 inhibitor, with studies underway in both solid tumors and hematological malignancies, as a monotherapy and combination therapy in relapsed and front-line disease. The company also is developing a novel G9a program with its next development candidate, EZM8266, which is targeting sickle cell disease. By focusing on the genetic drivers of disease, Epizyme's science seeks to match targeted medicines with the patients who need them. For more information, visit www.epizyme.com.

Cautionary Note on Forward-Looking Statements
Any statements in this press release about future expectations, plans and prospects for Epizyme, Inc. and other statements containing the words "anticipate," "believe," "estimate," "expect," "intend," "may," "plan," "predict," "project," "target," "potential," "will," "would," "could," "should," "continue," and similar expressions, constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by such forward-looking statements as a result of various important factors, including: uncertainties relating to the Company's ability to resume enrollment in its tazemetostat trials and the timing of such resumption, and the impact of the safety finding in the company's pediatric trial on enrollment of patients in ongoing and future trials of tazemetostat following the lifting of the partial clinical hold and the resumption of enrollment; uncertainties inherent in the initiation of future clinical studies and in the availability and timing of data from ongoing clinical studies; whether interim results from a clinical trial will be predictive of the final results of the trial; whether results from preclinical studies or earlier clinical studies will be predictive of the results of future trials; whether results from clinical studies will warrant meetings with regulatory authorities, submissions for regulatory approval or review by governmental authorities under the accelerated approval process; whether Fast Track Designation and Orphan Drug Designations will provide the benefits for which tazemetostat is eligible; expectations for regulatory approvals to conduct trials or to market products; whether the company's cash resources will be sufficient to fund the company's foreseeable and unforeseeable operating expenses and capital expenditure requirements; other matters that could affect the availability or commercial potential of the company's therapeutic candidates; and other factors discussed in the "Risk Factors" section of the company's most recent Form 10-Q filed with the SEC and in the company's other filings from time to time with the SEC. In addition, the forward-looking statements included in this press release represent the company's views as of the date hereof and should not be relied upon as representing the company's views as of any date subsequent to the date hereof. The company anticipates that subsequent events and developments will cause the company's views to change. However, while the company may elect to update these forward-looking statements at some point in the future, the company specifically disclaims any obligation to do so.

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