Small, low nanomolar, noncovalent thrombin inhibitors lacking a group to fill the 'distal binding pocket'.

Use of a chlorophenoxyacetamide P1 group with a pyridinone acetamide P2/P3 gave an exceptionally potent thrombin inhibitor (K(i)=40 pM). Truncation of the molecule at the N-terminus gave unique, low nanomolar, non-covalent thrombin inhibitors which do not have a group to fill thrombin's 'distal binding pocket'. A co-crystal structure indicates the importance of an intramolecular hydrogen bond between the P1 side chain and P1/P2 amide link in this series.