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Biotechnology startup Spark Therapeutics Inc. said its experimental gene therapy improved vision among patients with hereditary vision impairment in a clinical trial, without the serious safety problems that have dogged the emerging field of gene therapy in the past.

The Philadelphia-based company announced the results in a news release, with few details. Spark says additional data will be presented at a medical meeting in Paris on Oct. 10.

Spark said it plans to seek U.S. Food and Drug Administration approval to market its treatment next year, which could make it the first gene therapy to reach the U.S. market if regulators approve it for sale. Spark's shares closed 21% higher at $53.02 Monday.

Gene therapy involves the injection of genetic material into a person's cells to treat or prevent a disease. The research stalled after some study participants died or developed cancer after receiving gene therapies in the late 1990s and 2000s.

But gene therapy is gaining ground again. In 2012, the European Commission approved the Western world's first gene therapy, UniQure NV's Glybera, for the treatment of patients with a rare enzyme deficiency. The therapy hasn't been approved for sale in the U.S.

Other companies including Bluebird Bio Inc. are developing gene therapies for various diseases.

Spark's therapy, SPK-RPE65, targets mutations in a gene known as RPE65, which can cause visual impairments including loss of night- and peripheral-vision; the conditions can progress toward complete blindness. Spark's therapy contains a copy of a functional RPE65 gene that is encapsulated in a type of virus stripped of its viral DNA. The virus acts as a delivery vessel for the gene, and is injected into the eyes.

The study tested the therapy in 19 people with confirmed RPE65 gene mutations and compared their outcomes with nine patients who didn't receive the therapy.

To track outcomes, researchers had patients walk through indoor obstacle courses with varying levels of lighting, Spark Chief Executive Jeffrey D. Marrazzo said in an interview.

Researchers videotaped the participants to track whether they could navigate the obstacle courses relying on their vision. Spark said patients receiving the therapy had improved "functional vision" in the year following the treatment, compared with the control group.

Patients receiving the therapy also experienced improved sensitivity to light versus the control group, but there was no statistically significant difference in "visual acuity" between patient groups. Visual acuity is measured by the ability to read letters or numbers at a set distance.

Spark said there were no serious side effects attributable to the therapy. There were side effects related to the procedure to inject the therapy, such as eye inflammation and elevated intraocular pressure.

Debra Thompson, professor of ophthalmology and visual sciences at the University of Michigan's Kellogg Eye Center, said the Spark study appears to be significant.

"We've all been hoping gene therapy will be approved for treating these retinal disorders, and to see the first example of that getting very close is exciting," said Dr. Thompson, who has conducted research on hereditary retinal disorders but wasn't involved in Spark's studies.

While Spark announcement didn't quantify the visual improvements among patients, she said "even small gains in visual function can make a huge difference in quality of life."

Spark Therapeutics was spun out of Children's Hospital of Philadelphia in 2013 based on research conducted at the hospital. The company, which now has about 90 employees, had its initial public stock offering in January 2015.

Glybera—the only gene therapy currently on the market in the Western world—costs about $49,000 per vial, or $930,000 total for the amount the average patient would need, according to Chiesi Farmaceutici SpA, UniQuire's marketing partner for the drug. This has raised concerns about the affordability of gene therapy. Chiesi says it is a breakthrough treatment that is given only once and has a long-lasting effect.

Spark hasn't said how much it would charge for its therapy if it makes it to market.

Write to Peter Loftus at peter.loftus@wsj.com

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(END) Dow Jones Newswires

October 05, 2015 18:55 ET (22:55 GMT)

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