A direct thrombin inhibitor suppresses protein C activation and factor Va degradation in human plasma: Possible mechanisms of paradoxical enhancement of thrombin generation

- A direct thrombin inhibitor melagatran enhanced thrombin generation (TG) in plasma.
- Melagatran inhibited protein C (PC) activation at the same concentrations.
- Dabigatran and active site-blocked thrombin showed similar effects to melagatran.
- Melagatran suppressed factor Va degradation and subsequently increased factor Va.
- Inhibition of PC activation/increase in factor Va may cause TG enhancement.

We have demonstrated that antithrombin (AT)-independent thrombin inhibitors paradoxically increase thrombin generation (TG) in human plasma in a thrombomodulin (TM)- and protein C (PC)-dependent manner. We determined the effects of AT-independent thrombin inhibitors on the negative-feedback system, activation of PC and production and degradation of factor Va (FVa), as possible mechanisms underlying the paradoxical enhancement of TG. TG in human plasma containing 10 nM TM was assayed by means of the calibrated automated thrombography. As an index of PC activation, plasma concentration of activated PC-PC inhibitor complex (aPC-PCI) was measured. The amounts of FVa heavy chain and its degradation product (FVa307-506) were examined by western blotting. AT-independent thrombin inhibitors, melagatran and dabigatran (both at 25-600 nM) and 3-30 μg/ml active site-blocked thrombin (IIai), increased peak levels of TG. Melagatran, dabigatran and IIai significantly decreased plasma concentration of aPC-PCI complex at 25 nM or more, 75 nM or more, and 10 and 30 μg/ml, respectively. Melagatran (300 nM) significantly increased FVa and decreased FVa307-506. In contrast, a direct factor Xa inhibitor edoxaban preferentially inhibited thrombin generation (≥ 25 nM), and higher concentrations were required to inhibit PC activation (≥ 150 nM) and FVa degradation (300 nM). The present study suggests that the inhibitions of protein C activation and subsequent degradation of FVa and increase in FVa by antithrombin-independent thrombin inhibitors may contribute to the paradoxical TG enhancement, and edoxaban may inhibit PC activation and FVa degradation as a result of TG suppression.