Roche Holdings AG, the Swiss drug giant, and Spark Therapeutics, the West Philadelphia gene therapy company, have agreed to extend Roche’s takeover offer expiration date from Dec. 31 to April 1, 2020.

The extension is meant to "provide the parties with additional time to satisfy the regulatory conditions (with respect to antitrust laws),” Roche said in a statement filed with the U.S. Securities and Exchange Commission on July 5. Regulatory agencies, including the U.S. Federal Trade Commission, have been looking into how the merger could affect consumers.

In February, Roche offered to pay $4.3 billion (or $114.50 a share), for Spark, expecting to get the deal done as soon as last spring.

The multibillion-dollar offer for a company started by scientists from Children’s Hospital of Philadelphia, and 10 percent owned by the hospital’s foundation, greatly encouraged Philadelphia’s large community of gene and cell therapy scientist-entrepreneurs, as well as the many out-of-town investors who have backed University City start-ups such as Spark, which is publicly traded.

Among its many other products, Roche, through its U.S. subsidiary Genentech, sells Hemlibra, a drug that helps people with hemophilia form blood clots so they don’t easily bleed to death. Spark is working on a therapy that could cure hemophilia if taken once (that’s why Spark’s stock ticker is ONCE).

The federal Food and Drug Administration and antitrust investigators have been reviewing whether the acquisition of innovative, potentially very efficient new treatments by companies with a vested interest in current products could give the buyers too much incentive to charge very high profit margins for the new treatments or delay them from reaching patients.

Roche and Spark have said they are cooperating with government reviews.

The string of delays in closing the deal has discouraged investors, with Spark trading at prices below Roche’s offer. The stock fell 1 percent Tuesday to close at $100.25, a 12 percent discount to Roche’s offer price.