Effects of the Synthetic Thrombin Inhibitor Argatroban on Fibrin- or Clot-Incorporated Thrombin: Comparison with Heparin and Recombinant Hirudin

Summary

The inhibitory effects of argatroban on clot- or fibrin-bound human thrombin were studied using the thrombin-specific chromogenic substrate S2238 (200 μM). These effects were compared to those of recombinant hirudin (rHV2 Lys 47) and the heparin/antithrombin III complex. Argatroban concentration-dependently inhibited the cleavage of S2238 by a thrombin solution, which had been titrated to give the same change in OD405 nm as fibrin-bound thrombin, with an IC₅₀ of 1.1 μM with 90% inhibition at 8 μM. rHV2 Lys 47 and heparin had IC₅₀ values of 1.2 nM and 0.003 U/ml respectively under these conditions. However, when the compounds were tested against fibrin-bound thrombin, argatroban had an IC₅₀ of 2.8 μM with 65% inhibiton at 8 μM, whereas rHV2 Lys 47 had an IC₅₀ of 23 nM (with only 56% inhibition at 200 nM), and heparin had an IC₅₀ of 0.5 α 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-bound thrombin than against thrombin in solution. The differences between the inhibitory effects of the compounds against thrombin bound to a plasma clot were even more striking in that the IC₅₀ of argatroban was increased from 1.1 (vs. thrombin in solution) to 2.7 μM, while, although rHV2 Lys 47 and heparin had IC₅₀ values of 2.8 nM and 0.004 U/ml against thrombin in solution, they had little (32% inhibition by 4 pM rHV2 Lys 47) or no effect (even at 5.0 U/ml heparin) against the amidolytic activity of a plasma clot. We conclude that argatroban could present advantages over hirudin and heparin in the treatment of pathologies where the enzymatic activity of clot-bound thrombin may play a significant role.

• References

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