Synthesis and Structure−Activity Relationships of Potent Thrombin Inhibitors:  Piperazides of 3-Amidinophenylalanine

Thrombin is the key enzyme in the blood coagulation system, and inhibitors of its proteolytic activity are of therapeutic interest since they are potential anticoagulants. The most potent inhibitor of the benzamidine type is N^α-[(2-naphthylsulfonyl)glycyl]-4-amidinophenylalanylpiperidide (NAPAP). However, NAPAP and other benzamidine derivatives do not show favorable pharmacological properties; above all, they have very low systemic bioavailability after oral administration. The goal of designing new compounds was to obtain potent inhibitors with improved pharmacokinetic properties. Piperazide derivatives of 3-amidinophenylalanine as the key building block were synthesized. The piperazine moiety opened the possibility to introduce quite different substituents on the second nitrogen using common synthetic procedures. Some of the newly synthesized compounds are potent inhibitors of thrombin and offer an approach to study structure−function relationships for inhibition of thrombin and related enzymes and for the improvement of their pharmacokinetic properties.