Pyrazinone thrombin inhibitors

Thrombin is a serine protease present in blood plasma in the form of a precursor, prothrombin. Thrombin plays a central role in the mechanism of blood coagulation by converting the solution plasma protein, fibrinogen, into insoluble fibrin. Compounds of the invention are useful in inhibiting thrombin and associated thrombotic occlusions and are selected from the group consisting of 3-Fluoro-2-pyridylmethyl-3-(2,2-difluoro-2-(2-pyridyl)ethylamino)-6-chloropyrazin-2-one-1-acetamide polymorphic crystalline Anhydrous Form B, 3-Fluoro-2-pyridylmethyl-3-(2,2-difluoro-2-(2-pyridyl)ethylamino)-6-chloropyrazin-2-one-1-acetamide polymorphic crystalline form Type A Monohydrate, and 3-Fluoro-2-pyridylmethyl-3-(2,2-difluoro-2-(2-pyridyl)ethylamino)-6-chloropyrazin-2-one-1-acetamide polymorphic crystalline form Type B monohydrate, or a pharmaceutically acceptable salt thereof.