(Adds details, comment from Roche)
ZURICH, Feb 25 (Reuters) - Roche Holding AG said on Monday it will buy Spark Therapeutics in a $4.3 billion deal, as the Swiss drugmaker builds its haemophilia portfolio and seeks to keep pace in gene therapy.
Roche will acquire U.S.-based Spark for $114.50 per share, a premium of about 122 percent to Spark's closing price on Feb. 22, the Swiss company said in a statement.
Roche already sells Hemlibra, a drug against haemophilia A that helps stop bleeding in patients with the life-threatening genetic disorder that prevents their blood from clotting properly.
With Philadelphia-based Spark, Roche jumps into a crowded haemophilia gene therapy market, as other players Biomarin Pharma, Uniqure NV and Sangamo have prospective treatments in the works.
"Spark Therapeutics' haemophilia A programme could become a new therapeutic option for people living with this disease," Roche Chief Executive Severin Schwan said in the statement.
Spark's shares traded last week around $50, well off their July high of more than $95 per share. The U.S.-company's stock tumbled last year after announcing two out of 12 patients showed an unfavourable immune response when treated with a higher dose of Spark's haemophilia therapy SPK-8011.
Spark's $850,000-per patient gene therapy Luxturna, which is sold in the United States by Spark and elsewhere by Novartis after its approval in 2017, is a one-time treatment for a rare genetic disease that causes blindness in about 1 in 200,000 people.
Among Spark's top drug hopefuls in the clinic is SPK-8011, for haemophilia A, that is expected to start Phase 3 trials in 2019. It is also working on a therapy for haemophilia B, as well as treatments for Pompe disease, blindness-causing choroideremia and Huntington's disease, among others. (Reporting by John Miller; editing by Thomas Seythal and Rashmi Aich)