Roche will pay $114.50 per share for Spark, more than twice the U.S. company closing price on Feb. 22, the Swiss company said on Monday.

Basel-based Roche is buying Spark and its portfolio of treatments for blindness, haemophilia and neurodegenerative diseases, among other conditions, as rivals including Novartis, also move aggressively into gene therapy.

"Spark Therapeutics' proven expertise in the entire gene therapy value chain may offer important new opportunities for the treatment of serious diseases," Roche Chief Executive Severin Schwan said.

Schwan is counting on new medicines, including gene therapies, to help compensate for patent losses on his $21-billion-per-year trio of cancer medicines Rituxan, Herceptin and Avastin, that are facing competition from cheaper copies.

Roche, the biggest cancer drugs maker, was late to the game in immuno-oncology where Merck's Keytruda is becoming the dominant player and has eclipsed Roche's own product, Tecentriq, that seeks to harness the body's immune system to fight cancer.

Bank Vontobel analysts said the Spark deal would give Roche a proven platform for getting gene therapies to market, but did not come without risks -- including getting beaten to market by rivals with similar treatments in the works.

Roche shares were 0.8 percent lower by 0840 GMT.

Gene therapies use specially engineered viruses, or viral vectors, to deliver genetic material into defective cells, in hopes of improving or potentially even curing an inherited condition.

Spark has an approved gene therapy treatment Luxturna, which is sold in the United States by Spark and elsewhere by Novartis after its approval in 2017.

Luxturna targets a rare genetic disease, Leber's congenital amaurosis, that causes blindness in about 1 in 200,000 people.

Loss-making Spark had $51.6 million in revenue in the first nine months of 2018 from Luxturna and also had income from a deal with Pfizer, which it is partnering on another gene therapy for haemophilia B.

Philadelphia-based Spark's shares are up about 30 percent this year. The U.S.-company's stock tumbled last year after two of 12 patients showed an unfavourable immune response when treated with a higher dose of Spark's haemophilia therapy SPK-8011.

HEAD START

Roche's Spark deal, seen closing in the second quarter, follows Novartis's $8.7 billion purchase of U.S.-based Avexis last year, also to gain a platform of gene therapies for disorders including spinal muscular atrophy.

Novartis has made gene therapy one of its focus areas, giving it a head start on Roche. U.S. approval of Novartis's SMA medicine is slated for coming months.

Among Spark's top drug hopefuls is SPK-8011, for haemophilia A, expected to start Phase 3 trials in 2019. It is also working on treatments for Pompe disease, blindness-causing choroideremia and Huntington's disease.

"The needs of patients and families living with genetic diseases are immediate and their needs vast," Spark Chief Executive Jeffrey Marrazzo said.

"With its worldwide reach and extensive resources, Roche will help us accelerate the development of more gene therapies."

Roche already sells Hemlibra against haemophilia A that helps stop bleeding in patients with the life-threatening genetic disorder that prevents their blood from clotting. Hemlibra, approved in 2017, had 224 million Swiss francs ($224.18 million) in sales last year.

With Spark, Roche enters a crowded haemophilia gene therapy market that could become a big competitor to Hemlibra, as other players - Biomarin Pharma, Uniqure NV and Sangamo Therapeutics - also have gene therapies in the works.

"We view Biomarin likely first to market," Vontobel analyst Stefan Schneider said. "We view the market opportunity big enough to accommodate more than one gene therapy - but a favourable safety profile will likely win gold."

(Reporting by John Miller; editing by Thomas Seythal/Rashmi Aich/Jane Merriman)

By John Miller