SYNTHESIS AND BIOLOGICAL-ACTIVITY OF KETOMETHYLENE PSEUDOPEPTIDE ANALOGS AS THROMBIN INHIBITORS

Ketomethylene pseudopeptide analogues Aa-Pro-Arg-psi(COCH2)Gly-pip, 1, where Aa are D- or L-amino acids (Dpa, beta,beta-diphenylalanine; alpha-Nal, alpha-naphthylalanine; beta-Nal, beta-naphthylalanine; Fgl, fluorenylglycine) with highly lipophilic side chains and psi(COCH2) is a ketomethylene pseudopeptide bond, have been synthesized through a modified Dakin-West reaction under very mild conditions with a high yield using tripeptide 4 with a labile functional group directly on the side chain. Their enzymatic assay of thrombin inhibition has been carried out. The structure-activity relationship study indicated that a lipophilic side chain on the amino acid in the P3 position is very important for binding to the apolar site of thrombin. Compound 1a with D-Dpa at the P3 position has a K(i) of 0.2-mu-M and it doubles thrombin clotting time at only 3 times higher concentration. These values are about 7 times better than those of the corresponding D-Phe analogues. Furthermore, 1a shows poor inhibitory activity against plasmin, factor Xa, urokinase, and kallikrein. Preliminary in vivo testing (3-4-kg rabbit as the animal model) shows no observable side effect (change of blood pressure and accumulation of blood platelet in lungs) at a dose of 1 mg/kg.