EFFECTS OF THE SYNTHETIC THROMBIN INHIBITOR ARGATROBAN ON FIBRIN-INCORPORATED OR CLOT-INCORPORATED THROMBIN - COMPARISON WITH HEPARIN AND RECOMBINANT HIRUDIN

The inhibitory effects of argatroban on clot- or fibrin-bound human th rombin were studied using the thrombin-specific chromogenic substrate S2238 (200 mu M). These effects were compared to those of recombinant hirudin (rHV2 Lys 47) and the heparin/antithrombin III complex. Argatr oban concentration-dependently inhibited the cleavage of S2238 by a th rombin solution, which had been titrated to give the same change in OD 405nm as fibrin-bound thrombin, with an IC50 of 1.1 mu M with 90% inhi bition at 8 mu M rHV2 Lys 47 and heparin had IC50 values of 1.2 nM and 0.003 U/ml respectively under these conditions. However, when the com pounds were tested against fibrin-bound thrombin, argatroban had an IC 50 of 2.8 mu M with 65% inhibiton at 8 mu M, whereas rHV2 Lys 47 had a n IC50 of 23 nM (with only 56% inhibiton at 200 nM), and heparin had a n IC50 of 0.5 +/- 0.38 U/ml (with only 58% inhibition at 5 U/ml); i. e . the two compounds were 19 and 168 times less active against fibrin-b ound thrombin than against thrombin in solution. The differences betwe en the inhibitory effects of the compounds against thrombin bound to a plasma clot were even more striking in that the IC50 of argatroban wa s increased from 1.1 (vs. thrombin in solution) to 2.7 mu M, while, al though rHV2 Lys 47 and heparin had IC50 values of 2.8 nM and 0.004 U/m l against thrombin in solution, they had little (32% inhibition by 4 m u M rHV2 Lys 47) or no effect (even at 5.0 U/ml heparin) against the a midolytic activity of a plasma clot. We conclude that argatroban could present advantages over hirudin and heparin in the treatment of patho logies where the enzymatic activity of clot-bound thrombin may play a significant role.