Companies to explore potential of Clearside’s microinjector technology for use in delivering gene therapeutics to the retina and choroid through the suprachoroidal space

PHILADELPHIA, Penn., April28, 2015 – Spark Therapeutics, Inc. (NASDAQ: ONCE) and Clearside Biomedical, Inc. announced today that they have entered into an option agreement under which Spark acquired exclusive rights to license Clearside’s microinjector technology to deliver gene therapies to the back of the eye. Under the agreement, the companies will explore the feasibility of using Clearside’s microinjector technology to deliver viral vectors to the choroid and the retina through the suprachoroidal space (SCS).

If successful, this technique could create a differentiated alternative to sub-retinal or intravenous administration for future gene therapy applications, potentially broadening the range of conditions treatable through gene therapy.

“We believe gene therapy has tremendous potential to provide one-time, life-altering treatments to patients with debilitating, monogenic blinding conditions, and development of an alternative delivery approach that broadly covers the retina would be an important step in further unlocking this potential,” said Jeffrey D. Marrazzo, co-founder and CEO of Spark. “As we build a pipeline of gene therapies to treat inherited retinal dystrophies, we believe Clearside’s proprietary technology and intellectual property estate for delivery of therapeutics to the SCS may allow us to target new diseases. If successful, the microinjector technology may provide an alternative to current routes of administration, provide ease of delivery for physicians and further expand on our leadership in the ocular gene therapy field.”

Using a proprietary microinjector, Clearside’s technology delivers therapeutic agents to the retina and choroid through the SCS and without substantial diffusion to the vitreous of the eye. Clearside is currently conducting a Phase 2 clinical trial in macular edema associated with uveitis and a Phase 2 clinical trial in macular edema associated with retinal vein occlusion (RVO). In a recently concluded Phase 1/2 trial, subjects with non-infectious uveitis treated with a single suprachoroidal injection of a commercially available formulation of triamcinolone acetonide, using Clearside’s proprietary microinjector, showed improvement in best corrected visual acuity ranging between one and five lines (or up to 25 letters). The procedure was generally well tolerated in this trial and no meaningful intraocular pressure (IOP) increases were observed in any subjects through the six month duration of the trial.

“Clearside is delighted to be working with Spark, a clear leader in applying gene therapy to treat blinding diseases,” said Daniel White, CEO and president of Clearside. “We believe this agreement highlights the important potential of precision delivery to propel the field of ocular gene therapy forward. By reaching ocular tissues that we would otherwise not reach, we are creating the potential to treat a range of debilitating ocular conditions.”

If Spark exercises its option, in return for exclusive, worldwide rights to use Clearside’s microinjection technology and related intellectual property in the field of gene therapy, Spark will pay to Clearside an upfront licensing fee, development related milestones and commercial royalties on sales of Spark’s products covered by the licensed technology.

About Spark Therapeutics

Spark is a gene therapy leader seeking to transform the lives of patients suffering from debilitating genetic diseases by developing one-time, life-altering treatments. Spark’s initial focus is on treating orphan diseases where no, or only palliative, therapies exist. Spark’s most advanced product candidate, SPK-RPE65, which has received both breakthrough therapy and orphan product designation, is in a fully enrolled pivotal Phase 3 clinical trial for the treatment of rare blinding conditions. Spark is leveraging the experience and technology utilized in the development of SPK-RPE65 to address a broad spectrum of blinding conditions, starting with the development of SPK-CHMfor the potential treatment of choroideremia, currently in a Phase 1/2 clinical trial.  Spark also is establishing a pipeline of gene therapy candidates to treat hematologic disorders and neurodegenerative diseases, including through a global collaboration with Pfizer Inc. around the development and commercialization of its SPK-FIXprogram for the treatment of hemophilia B. Spark’s integrated gene therapy platform builds on two decades of research, development and manufacturing at The Children’s Hospital of Philadelphia, including human trials conducted across diverse therapeutic areas and routes of administration. To learn more, please visit www.sparktx.com.

About Clearside Biomedical

Clearside Biomedical, Inc., headquartered in Alpharetta, GA, is a clinical-stage biopharmaceutical company developing first-in-class drug therapies to treat chronic, blinding diseases of the eye.

Clearside’s product candidates focus on diseases affecting the retina and the choroid, especially diseases associated with macular edema. Visit www.clearsidebio.com for more information.

Cautionary Note on Forward-looking Statements

This release contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995, including statements regarding the Spark’s and Clearside’s plans with regard to future product candidates, and the expected uses and potential benefits of alternate delivery vehicles to the eye, including Clearside’s microinjector. Any 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. For a discussion of Spark’s other risks and uncertainties, and other important factors, any of which could cause Spark’s actual results to differ from those contained in the forward-looking statements, see the “Risk Factors” section, as well as discussions of potential risks, uncertainties and other important factors, in Spark’s most recent Annual Report on Form 10-K filed with the Securities and Exchange Commission. All information in this press release is based on information available to Spark and Clearside as of the date of this press release, and Spark and Clearside undertake no duty to update this information unless required by law.

Contacts

Spark Therapeutics, Inc. Stephen W. Webster 888-772-7560 [email protected]

Clearside Biomedical, Inc.

Charles Deignan 678-270-4005 [email protected]