Homology Medicines Presents Data from PKU Gene Therapy Program Demonstrating Long-Term Durability of Treatment in IND-Enabling Studies

BEDFORD, Mass., May 18, 2018 /PRNewswire/ -- Homology Medicines, Inc. (Nasdaq: FIXX), a genetic medicines company, announced today the presentation of data from its gene therapy and gene editing programs at the 21st Annual Meeting of the American Society of Gene & Cell Therapy (ASGCT). New data presented from Homology's HMI-102 program, a gene therapy for phenylketonuria (PKU) in IND-enabling studies, demonstrate its potential as a one-time treatment designed to restore the normal metabolic pathway in PKU. The Company expects to initiate a Phase 1/2 clinical trial with HMI-102 in adult PKU patients and have initial data in 2019.

Homology scientists also presented data for the first time that show phenotypic correction in a commonly used model of PKU following treatment with its nuclease-free gene editing approach that harnesses the body's natural DNA repair mechanism of homologous recombination. In another presentation, the Company demonstrated that its adeno-associated virus vectors derived from human hematopoietic stem cells (AAVHSCs) have the ability to transfer and express genes in the retina following local injection in a preclinical model.

"The presentations underscore the versatility of our gene editing and gene therapy platform, which enables us to select either approach based on the disease biology, target tissues and unique characteristics of our AAVHSCs," said Albert Seymour, Ph.D., Chief Scientific Officer of Homology Medicines. "Our PKU programs are a great example of how we can use our technology platform to develop potential cures using a gene therapy approach for the adult population and gene editing for the pediatric population. We are continuing to establish the potential of our AAVHSC platform across a range of rare genetic diseases, and our ability to deliver and express genes in the retina with a single local injection provides additional opportunities to help patients with ophthalmic disorders."

The presentation titled, "Sustained Correction of Phenylketonuria by a Single Dose of AAVHSC Packaging a Human Phenylalanine Hydroxylase Transgene," evaluated Homology's HMI-102 PKU gene therapy candidate in an established in vivo model of the disease. The data demonstrate that HMI-102:

• Reduced serum phenylalanine (Phe) levels to normal in mice on a regular diet one-week following a single dose and this was durable for greater than four months. Reduction in Phe is a key biomarker in the development of treatments for PKU.
• Restored the Phe to tyrosine metabolic ratio on a normal diet. HMI-102 restores the normal biochemical pathway of Phe metabolism by targeting the genetic cause of the disease. This pathway is responsible for the production of neurotransmitters and is not addressed by current treatments for PKU.
• Has potential as a one-time gene therapy for PKU patients without dietary restrictions.

An additional presentation titled, "AAVHSC Nuclease-Free Genome Editing Leads to In Vivo Genome Correction and a Significant Reduction in Disease Phenotype in a Mouse Model of Phenylketonuria," evaluated Homology's gene editing approach. The data demonstrate that Homology's AAVHSCs:

• Were capable of in vivo genetic and phenotypic correction by employing the precise DNA repair mechanism of homologous recombination to insert therapeutic genes into the genome.
• Induced efficient and precise gene editing, demonstrating no evidence of insertions or deletions as determined by next-generation sequencing.
• Led to a durable and statistically significant reduction in serum Phe levels from baseline and when compared to untreated controls in a PKU model.

Lastly, Homology presented, "Transduction of Photoreceptor and Pigmented Epithelial Cells Following a Single Subretinal Injection of AAVHSC17 in Minipigs," which evaluated biodistribution of AAVHSC17 following a single local injection to the retina in a preclinical model. The data demonstrate that:

• Subretinal injection was well-tolerated and resulted in biodistribution in all retinal layers.
• AAVHSCs may be useful as therapeutic vectors for treating diseases of the eye.

About Phenylketonuria (PKU)
PKU is a rare, inherited inborn error of metabolism caused by a mutation in the PAH gene. PAH is an enzyme that is normally expressed in the liver and is necessary to metabolize dietary phenylalanine into tyrosine, an amino acid responsible for the production of neurotransmitters. PKU results from mutations in PAH that render its enzymatic activity deficient. The current standard of care is a highly restrictive diet, but it is not always effective. If left untreated, PKU can result in progressive and severe neurological impairment. PKU is estimated to affect approximately 15,000 people in the U.S. and there are no treatments available that address the genetic defect in PKU.

About Homology Medicines, Inc.
Homology Medicines, Inc. is a genetic medicines company dedicated to transforming the lives of patients suffering from rare genetic diseases with significant unmet medical needs by curing the underlying cause of the disease. Homology's proprietary platform is designed to utilize its human hematopoietic stem cell-derived adeno-associated virus vectors (AAVHSCs) to precisely and efficiently deliver genetic medicines in vivo either through a gene therapy or nuclease-free gene editing modality across a broad range of genetic disorders. Homology has a management team with a successful track record of discovering, developing and commercializing therapeutics with a particular focus on rare diseases, and intellectual property covering its suite of 15 AAVHSCs. Homology believes that its compelling preclinical data, scientific expertise, product development strategy, manufacturing capabilities and intellectual property position it as a leader in the development of genetic medicines. For more information, please visit www.homologymedicines.com.

Forward-Looking Statements
This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including statements regarding our goal of improving the lives of patients with rare genetic diseases; the potential of HMI-102 and our AAVHSC platform; the anticipated timing of the release of clinical data; beliefs about preclinical data; our ability to help patients with ophthalmic disorders; and our position as a leader in the development of genetic medicines. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements, including, but not limited to, the following: the fact that we have and expect to continue to incur significant losses; our need for additional funding, which may not be available; failure to identify additional product candidates and develop marketable products; the early stage of our development efforts; our failure or the failure of our collaborators to successfully develop and commercialize drug candidates; risks relating to the regulatory approval process; our product candidates may cause serious adverse side effects; inability to maintain our collaborations, or the failure of these collaborations; our reliance on third parties; the inability to obtain orphan drug exclusivity; failure to obtain international marketing approval; failure to obtain U.S. marketing approval; ongoing regulatory obligations; effects of significant competition; unfavorable pricing regulations, third-party reimbursement practices or healthcare reform initiatives; product liability lawsuits; failure to attract, retain and motivate qualified personnel; the possibility of system failures or security breaches; risks relating to intellectual property; the price of our common stock may be volatile; significant costs as a result of operating as a public company; and any securities class action litigation. These and other important factors discussed under the caption "Risk Factors" in our Quarterly Report on Form 10-Q for the quarter ended March 31, 2018 and our other filings with the SEC could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management's estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, we disclaim any obligation to do so, even if subsequent events cause our views to change.

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SOURCE Homology Medicines, Inc.