The kinetics and interplay of thrombin inhibition by 4 plasma proteinase inhibitors

A novel therapeutic approach for restoring hemostasis in hemophilia is to reduce antithrombin (AT) to rebalance reduced thrombin generation. In plasma, multiple inhibitors including AT, heparin cofactor II (HCII), α₂-macroglobulin (A2M), and α₁-proteinase inhibitor (A1PI) play a role in thrombin inhibition. The goal was to study the kinetics of thrombin inhibition and the roles of various inhibitors across a broad range of AT levels. Thrombin inhibition was measured at varied concentrations of AT with and without A2M, HCII, and A1PI. Reducing AT to 0 from plasma levels in the presence HCII, A2M, and A1PI, results in slower thrombin inhibition with the time required to inhibit half the thrombin increasing approximately fourfold. Computational models of thrombin inhibition and thrombin generation in hemophilia were constructed and used to analyze thrombin inhibition and the relative contribution of each inhibitor. In a model of thrombin generation, decreased thrombin inhibition resulted in increased peak thrombin and increased area under the thrombin curve. Even at high concentrations of thrombin, all of the thrombin was inhibited with the relative contribution of other inhibitors increasing as AT was decreased. These studies show that in a system without heparin-like glycosaminoglycans, AT is the dominant inhibitor of thrombin, followed by A2M, HCII, and, finally, A1PI. As AT levels decrease, thrombin inhibition is slower, resulting in higher levels of thrombin in a computational model of thrombin generation. Ultimately, the other inhibitors compensate for AT to maintain a level of thrombin regulation.