Following discussions with the Food and Drug Administration, the hold was placed on the company's development program due to the "overall risk/benefit profile observed to date in clinical testing," Idenix said in a statement issued Friday.

"We are disappointed with the FDA's perspective on the program and are working with Novartis to evaluate our options for valopicitabine," said Jean-Pierre Sommadossi, chairman and chief executive officer of Idenix. "We remain committed to building a leading antiviral franchise and will continue to focus on ensuring a successful launch of Tyzeka/Sebivo and on advancing our pipeline."

Idenix said it has an ongoing phase 1 trial of a non-nucleoside reverse transcriptase inhibitor for the treatment of HIV, and another HCV treatment, a second-generation nucleoside polymerase inhibitor, currently in animal studies.

The company said, as of June 30, it had roughly $160 million of cash, cash equivalents and marketable securities on hand. "Our balance sheet is strong and we believe that we have enough cash to fund early clinical development of the pipeline," said Ronald Renaud, chief financial officer of Idenix.