Dimension Therapeutics Receives Positive Opinion for European Orphan Drug Designation for DTX301 for the Treatment of Ornithine Transcarbamylase (OTC) Deficiency

CAMBRIDGE, Mass., March 17, 2016 -- Dimension Therapeutics, Inc. (NASDAQ:DMTX), a biopharmaceutical company advancing novel, liver-directed treatments for diverse rare diseases based on an adeno-associated virus (AAV) gene delivery technology, today announced that the European Medicines Agency (EMA) Committee for Orphan Medicinal Products (COMP) has issued a positive opinion recommending DTX301 for designation as an orphan medicinal product for the treatment of Ornithine Transcarbamylase (OTC) Deficiency. Designed to address the underlying genetic defect, DTX301 delivers stable expression and activity of OTC. DTX301 has been shown in preclinical studies to normalize levels of urinary orotic acid, a marker of ammonia metabolism, which if elevated, can cause serious neurological deficiencies in patients. Dimension has completed candidate selection in its DTX301 program and plans to submit an investigational new drug (IND) application with the FDA in the second half of 2016.

“The COMP positive opinion for DX301 in the EU, as well as recent orphan drug designation in the US, underscores the compelling need for new treatments for patients living with OTC deficiency. Current medical options are not curative and unfortunately do not eliminate the risk of metabolic crises from OTC deficiency. The DTX301 approach is designed to address these challenges and offer a new option with improved outcomes for patients," said Annalisa Jenkins, MBBS, MRCP, Chief Executive Officer of Dimension. “We are making significant progress on growing and advancing our unique pipeline of programs addressing the inherited metabolic diseases associated with the liver. The planned submission of the IND for DTX301 later this year will be another important milestone for our company and our goal to transform the care of patients living with these devastating diseases.”

The COMP issues an opinion on the granting of orphan drug designation, after which the opinion is submitted to the European Commission (EC) for endorsement of the opinion. Orphan drug designation by the EC provides regulatory and financial incentives for companies to develop and market therapies that treat a life-threatening or chronically debilitating condition affecting no more than five in 10,000 persons in the European Union (EU), and where no satisfactory treatment is available. In addition to a 10-year period of marketing exclusivity in the EU after product approval, orphan drug designation provides incentives for companies seeking protocol assistance from the EMA during the product development phase, and direct access to the centralized authorization procedure.

About DTX301

Dimension is developing its AAV gene therapy product DTX301 for the treatment of individuals with OTC deficiency. DTX301 is designed to deliver Ornithine Transcarbamylase gene expression in a durable fashion, preventing the complications associated with OTC deficiency. Preclinical studies completed to date indicate DTX301 has the potential to be a well-tolerated, effective therapy for OTC deficiency.

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 a leader in discovering and developing new therapeutic products for people living with devastating rare 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 and GSDIa, 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 progress of Dimension's portfolio and lead programs, including DTX301. 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 lead candidate, DTX101, will not successfully be developed or commercialized; and the risks described under the caption "Risk Factors" in Dimension Therapeutics' Quarterly Report on Form 10-Q for the quarter ended September 30, 2015 (File No. 001-37601), which is on file with the Securities and Exchange Commission, as well as other risks detailed in Dimension Therapeutics’ subsequent 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.

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