Dimension Therapeutics Announces FDA Acceptance of Investigational New Drug Application for DTX301 to Treat OTC Deficiency

On Track to Initiate Phase 1/2 Global, Multi-Center Clinical Trial by Year End 2016

Most Advanced of Five Inherited Metabolic Disease Programs Progressing Towards Clinical Testing

CAMBRIDGE, Mass., Dec. 13, 2016 -- Dimension Therapeutics, Inc. (NASDAQ:DMTX), a biopharmaceutical company advancing novel, adeno-associated virus (AAV) gene therapies targeting the liver, a key organ for human metabolism, today announced that the U.S. Food and Drug Administration (FDA) accepted Dimension’s investigational new drug (IND) application for DTX301 for the treatment of Ornithine Transcarbamylase (OTC) Deficiency. Dimension anticipates commencing a global, multi-center Phase 1/2 clinical trial to evaluate DTX301 in patients with OTC Deficiency by the end of 2016.

DTX301 is designed to deliver stable expression and activity of OTC following a single intravenous infusion and has been shown in academic preclinical studies in relevant mouse models to normalize levels of urinary orotic acid, a marker of ammonia metabolism. In the late onset form of the disease, elevated ammonia can lead to significant medical issues for patients who are in need of better disease-modifying therapies.

“FDA’s acceptance of DTX301’s IND for the treatment of OTC Deficiency is a key pipeline milestone for Dimension. We are pleased to continue the momentum and advancement of our leading pipeline that addresses important inherited metabolic disorders of the liver,” said Dr. Annalisa Jenkins, MBBS, FRCP, Chief Executive Officer of Dimension. ”For individuals with OTC deficiency, current medical options are not curative and unfortunately do not eliminate the risk of metabolic crises. We believe DTX301 holds great promise as an AAV-based approach for patients, and we look forward to partnering with leading physician thought leaders and the patient community across multiple global clinical sites.”

Background on OTC Deficiency

OTC deficiency, the most common urea cycle disorder, is caused by a genetic defect in a liver enzyme responsible for detoxification of ammonia. Individuals with OTC deficiency can build up excessive levels of ammonia in their blood, potentially resulting in neurological deficits and other toxicities. It is estimated that more than 10,000 patients are affected by OTC deficiency worldwide, of which approximately 80% are classified as late-onset, Dimension’s target population. The greatest percentage of patients, including males and females, experience late-onset disease, representing a clinical spectrum of disease severity. Neonatal onset disease occurs in males, presents as severe disease, and can be fatal at an early age. Approved therapies, which must be taken multiple times a day for the patient's entire life, do not eliminate the risk of future metabolic crises. Currently, the only curative approach is liver transplantation.

About Dimension Therapeutics, Inc.

Dimension Therapeutics, Inc. (NASDAQ:DMTX) is the leader in discovering and developing new therapeutic products for people living with devastating rare and metabolic diseases associated with the liver, based on the most advanced, mammalian adeno-associated virus (AAV) gene delivery technology. Dimension is actively progressing its broad pipeline, which features programs addressing unmet needs for patients suffering from inherited metabolic diseases, including OTC deficiency, GSDIa, citrullinemia type 1, PKU, Wilson disease, a collaboration with Bayer in hemophilia A, and a wholly owned clinical program in hemophilia B. The company targets diseases with readily identifiable patient populations, highly predictive preclinical models, and well-described, and often clinically validated, biomarkers. Founded in 2013, Dimension maintains headquarters in Cambridge, Massachusetts.

For more information, please visit www.dimensiontx.com.

Cautionary Note Regarding Forward-Looking Statements

This press release contains "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding the development, preclinical results, and the continued progress of Dimension's portfolio and programs, including the initiation, timing, scope, or likelihood of regulatory filings and approvals, and our ability to develop and advance product candidates into, and successfully complete, clinical studies. All such forward-looking statements are based on management's current expectations of future events and are subject to a number of risks and uncertainties that could cause actual results to differ materially and adversely from those set forth in or implied by such forward-looking statements. These risks and uncertainties include the risks that Dimension’s product candidates, including its candidate, DTX301, will not successfully be developed or commercialized in the times indicated or at all; and the risks described under the caption "Risk Factors" in Dimension Therapeutics’ Quarterly Report on Form 10-Q for the quarter ended September 30, 2016, which is on file with the Securities and Exchange Commission, as well as other risks detailed in Dimension Therapeutics’ additional filings with the Securities and Exchange Commission. All information in this press release is as of the date of the release, and Dimension Therapeutics undertakes no duty to update this information unless required by law.

Jean Franchi
Chief Financial Officer
Dimension Therapeutics
617-714-0709
[email protected]

Burns McClellan, on behalf of Dimension Therapeutics
Media: Justin Jackson
212-213-0006, ext.327
[email protected]