Strategies and progress towards the ideal orally active thrombin inhibitor

Thrombin plays a key role in the control of thrombus formation, for which reason its inhibition has become a target for new antithrombotics. Important issues in the profile of the ideal thrombin inhibitor are: potency, selectivity, oral bioavailability, half-life in the circulatory system and safety. Although many potent direct inhibitors of thrombin have been discovered, most of these inhibitors lack sufficient oral bioavailability. This is often associated with the presence of highly basic functionalities such as guanidine or amidine. These basic functionalities in the P1 moiety are preferred by thrombin and are present in the first generation of thrombin inhibitors. Recently, several orally active direct thrombin inhibitors have been disclosed. Most of these inhibitors originate from leads of the first generation. Two major optimization strategies could be identified to further improve these leads: A: maintain the highly basic P1 moiety and compensate its negative effects, and B: reduce the basicity of the P1 moiety and compensate for the decrease in inhibitory activity. The progress made using these strategies is evaluated. In addition, screening large sets of compounds yielded new structures that provide useful starting points for optimization. The optimization strategy used to convert leads from screening into potent orally active thrombin inhibitors is also be evaluated.