Aliso Viejo-based Valeant Pharmaceuticals, a drug development firm developing specialty pharmaceuticals, said Thursday that it has signed a definitive agreement with Three Rivers Pharmaceuticals to sell its U.S. and Canadian rights to the hepatitis C drug Infergen. The deal is worth $91M. The firm said the sale would allow the firm to simplify and focus its operations. Infergen was originally developed by Amgen, which sold the rights to Intermune in 2001, which it subsequently sold it to Valeant in 2005.