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UPDATE 2-FDA approves Spark's gene therapy for rare form of blindness

(Adds FDA comment)

Dec 19 (Reuters) - The U.S. Food and Drug Administration on Tuesday approved Spark Therapeutics Inc's treatment for a rare form of blindness, marking the first time the agency has approved a gene therapy for an inherited disease.

The approval is expected to raise new questions about how the United States pays for major medical breakthroughs. The company said it would announce a price for the treatment early next year. Analysts expect it to cost around $1 million.

The drug, Luxturna, treats inherited retinal disease caused by defects in a gene known as RPE65, which tells cells to produce an enzyme critical for normal vision. The condition affects between 1,000 and 2,000 people in the United States.

Luxturna works by delivering 150 billion viral vector particles containing a correct copy of the RPE65 gene to retinal cells, restoring their ability to make the needed enzyme. It is designed to be given just once, though it is unclear how long the benefit will ultimately last.

Clinical trials of Luxturna showed that 93 percent of participants experienced some improvement in their functional vision as measured by their ability to navigate obstacles in poor light after a year.

Gene therapy in general aims to treat disease by manipulating genes at a cellular level. The field is gaining momentum following several high-profile failures in the late 1990s and early 2000s, and the FDA appears eager to smooth the way for further approvals in the field.

"We're at a turning point when it comes to this novel form of therapy, and at the FDA we're focused on establishing the right policy framework to capitalize on this scientific opening." FDA Commissioner Scott Gottlieb said in a statement.

Next year the agency plans to issue guidance documents that will provide instructions for companies developing gene therapies for specific diseases, including "more efficient parameters" than currently exist.

Those parameters could include new, accelerated measures, Gottlieb said. The FDA's accelerated approval program allows for earlier approval of drugs that treat serious conditions based on a marker that is expected to lead to a clinical benefit.

Inherited retinal diseases are a group of rare conditions caused by mutations in one or more than 220 different genes, including the RPE65 gene.

Luxturna was tested in patients with one such condition, Leber congenital aumaurosis, but can be used in any person with a condition mediated by the RPE65 mutation.

Spark said Luxturna is expected to be available in some treatment centers late in the first quarter of 2018. (Reporting by Toni Clarke in Washington; Editing by Susan Thomas and Jonathan Oatis)