Design and synthesis of thrombin inhibitors

As part of our programme directed at the development of enzyme inhibitors based on transition-state mimics, we discovered in the early 1980s that P3-P3' fragments of human fibrinogen A alpha, containing the ketomethylene isostere Arg-psi-[COCH2]Gly at P1-P1', were potent inhibitors of thrombin. Such low-molecular-weight inhibitors are expected to be clinically useful as anticoagulant drugs. In our more recent investigations, the P1-P1' moiety has been replaced with various arginine or lysine ketones. The resulting compounds showed the following order of thrombin inhibitory potency: alpha-ketoesters > fluoroketones > alkoxymethylketones > difluoro-beta-ketoamides > beta-ketoesters > alkyl ketones. In contrast to all other lysine/arginine pairs studied previously, the inhibitor based on a lysine alpha-ketoester proved superior to the corresponding arginine analogue. A possible explanation for this finding is discussed. All the highly electrophilic ketones (e.g., fluoroketones) were found to exhibit slow-binding kinetics with thrombin, which is likely to be a disadvantage in clinical use. Alkoxymethyl ketones were devoid of such behaviour and have been developed further to yield nanomolar inhibitors of low molecular weight and good selectivity for thrombin. One of these ketones was found to compare favourably with known thrombin inhibitors in anticoagulant assays. The synthesis of various types of inhibitor mentioned above is described, together with structure-activity correlations for inhibition of thrombin.