Roche, the Switzerland-based drugmaker, says it has extended its $4.3 billion offer to buy all shares of Philadelphia-based Spark Therapeutics Inc. from investors past its July 31 target date until Sept. 3, so U.S. and U.K. regulators can finish their longer-than-expected review on the combination.

"The offer was extended to provide additional time for the U.S. Federal Trade Commission and the UK Competition and Markets Authority to complete their previously disclosed reviews” of the deal, Roche and Spark said in this joint statement.

The companies had hoped to combine Roche, which makes the hemophilia treatment Hemlibra among many other drugs, with Spark, one of several biotech companies developing a gene therapy hemophilia cure among its projects, by last spring.

The $4.3 billion sale, first announced in February, was the biggest payout for a gene therapy company since doctors at research hospitals, including the Children’s Hospital of Philadelphia and the University of Pennsylvania, began gene therapy trials in the 1990s.

But Roche and Spark have had to delay the deal closing several times, as U.S. and U.K.regulators consider how the combination will change the companies’ incentives and plans to speed product development, and how that could affect prices and availability of their therapies.

The price remains the same — $114.50 a share. And one-quarter of Spark investors have already cashed in their shares at that sale price, Roche and Spark noted in their statement.

But the delays discouraged traders, and Spark share values slipped below $100 in the last three weeks. The stock rose 79 cents after Wednesday’s news and closed at $100.02, up $0.79.