The Importance of Enzyme Inhibition Kinetics for the Effect of Thrombin Inhibitors in a Rat Model of Arterial Thrombosis

Summary

The relation between the antithrombotic effect in vivo, and the inhibition constant (K _i) and the association rate constant (k _on) in vitro was investigated for eight different thrombin inhibitors. The carotid arteries of anaesthetized rats were exposed to FeCl₃ for 1 h, and the thrombus size was determined from the amount of incorporated ¹²⁵I- fibrinogen. The thrombin inhibitors were given intravenously, and complete concentration- and/or dose-response curves were constructed. Despite a 50,000-fold difference between the k _i-values comparable plasma concentrations of hirudin and melagatran were needed (0.14 and 0.12 μmol 1^-1, respectively) to obtain a 50% antithrombotic effect (IC₅₀) in vivo. In contrast, there was a comparable in vitro (k _i-value) and in vivo (IC₅₀) potency ratio for melagatran and inogatran, respectively. These results can be explained by the concentration of thrombin in the thrombus and improved inhibition by the low-molecular-weight compounds. For all eight thrombin inhibitors tested, there was an inverse relationship between k _on-values in vitro and the slope of the dose response curves in vivo. Inhibitors with k _on-values of <1 X 10⁷ M^-1 s^-1 gave steep dose response curves with a Hill coefficient >1. The association time for inhibition of thrombin for slow-binding inhibitors will be too long to give effective antithrombotic effects at low plasma concentrations, but at increasing concentrations the association time will decrease, resulting in a steeper dose-response curve and thereby a more narrow therapeutic interval.

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