Roche Holding AG again extended its $4.3 billion takeover offer for Spark Therapeutics Inc. to give additional time to U.S. and U.K. regulators to examine the deal.

In March, Roche launched a buyout offer to acquire all outstanding shares of the Philadelphia-based gene-therapy maker for $114.50 apiece. Roche has already extended the offer period several times from the initial April 3 deadline.

The Swiss drugmaker said the offer will now expire at 5 p.m. ET on Dec. 16, unless further extended.