The acquisition of Idenix Pharmaceuticals Inc. will fill a hole in Merck & Co. Inc.'s HCV toolbox and give the pharma a chance to develop an interferon-free regimen that can treat all patients at the same dose regardless of HCV genotype, HIV co-infection or other co-morbidities.

The $3.9 billion acquisition will put Merck in second place behind Gilead Sciences Inc. in the race to develop a pan-genotype regime. But the pharma thinks its combination will have a shorter duration of therapy than the Gilead regimen.