FILE PHOTO: The logo of Swiss drugmaker Roche is seen at its headquarters in Basel, Switzerland February 1, 2018. REUTERS/Arnd Wiegmann/File Photo

ZURICH (Reuters) - Roche ROG.S and Spark Therapeutics ONCE.O agreed to extend the deadline for the Swiss drugmaker's $4.3 billion takeover bid for the U.S. gene therapy specialist, Roche said, adding it remains "fully committed" to a deal it sees closing this year.

The new deadline for both companies to extend the merger agreement is now April 30, 2020, rather than Jan. 31, 2020, Roche said in a U.S. regulatory filing on Monday, saying the change provides “additional time to clear the transaction”.

Roche’s March 5 bid to add Spark’s gene therapy technology is dragging on, with completion pushed back several times as the Swiss company blames time-consuming U.S. Federal Trade Commission scrutiny of any competition issues. Moreover, three U.S. lawsuits challenging the deal are pending, alleging disclosure violations associated with the offer.

“Although closing this deal is taking longer than I had hoped, my excitement about our anticipated partnership has only increased,” Roche Chief Executive Severin Schwan wrote in a note to Spark employees that was published by the U.S. Securities and Exchange Commission.

“While we still expect this transaction to close in 2019, we want to ensure that we proactively identify and remove any potential future obstacles to achieving this outcome.”

Roche is buying Spark to get its one approved medicine, the $850,000 per patient hereditary blindness therapy Luxturna; another investigational gene therapy for haemophilia A, as well as the Philadelphia-based company’s development platform that Roche hopes to leverage into additional disease areas.