Shares of biotech company Amgen Inc. fell sharply, a drop some analysts linked to a competitive threat to its blockbuster Epogen drug. They said the threat comes from Cambridge, Mass.-based Transkaryotic Therapies Inc., which is developing a product similar to Epogen, the Amgen drug widely used to treat renal failure in kidney dialysis patients. Amgen shares fell $3.875 to close at $63; Transkaryotic rose $1.625 to $23.375. Both trade on Nasdaq. Thousand Oaks-based Amgen in April sued Transkaryotic for patent infringement, but Transkaryotic maintains it has found a way to produce a drug that acts like Epogen without violating Amgen's patent.