Companies frequently wait to file patent infringement suits until competing products are poised to come on the market because an exemption in patent law often allows clinical trials of even infringing products.

Amgen Inc. (AMGN) decided not to wait, filing suit in Boston alleging infringement of its EPO patents in order to prevent Transkaryotic Therapies Inc. (TKTX) and Hoechst Marion Roussel (Kansas City, Mo.) from moving in on AMGN's market for erythropoietin (EPO). The suit comes even before partners TKTX and HMR have entered the clinic with their version of EPO, anticipated in the first half of this year.