(Reuters) - Spark Therapeutics Inc’s experimental gene therapy for a rare inherited form of blindness is effective, though it is unclear whether the benefit lasts over time, according to a preliminary review by the U.S. Food and Drug Administration.

The therapy, Luxturna, or voretigene neparvovec, would be the first-ever gene therapy for any inherited disease to be approved in the United States.

The FDA’s review, posted on Tuesday on the agency’s website, comes two days ahead of a meeting of outside advisers who will discuss the treatment and recommend whether it should be approved.

Michael Yee, an analyst at Jefferies, said the FDA documents “look fairly benign and as expected,” and said he expects the advisory panel is likely to vote in favor of approval.

The FDA noted there is no available long-term data to show whether the effectiveness of the therapy is maintained after a year, so the clinical benefit beyond that “is unclear.”

Inherited retinal diseases are a group of rare conditions caused by mutations in one or more than 220 different genes, including one known as RPE65, which tells cells how to produce a crucial enzyme needed for normal vision.

Spark studied Luxturna in people with one of these conditions, Leber congenital aumaurosis, whose disease was mediated by defects in the RPE65 gene.

Clinical trial results showed 93 percent of participants experienced some improvement in their functional vision as measured by their ability to navigate obstacles in poor light.

In January the FDA granted the company’s request to alter the drug’s proposed patient population to include any person with an inherited retinal disease due to a biallelic RPE65 mutation, meaning that the mutation is found in both copies of the gene.