Comparative Effects of the Direct Factor Xa Inhibitor, Apixaban, and the Direct Inhibitors of Factor VIIa, XIa and Thrombin on Tissue Factor- Induced Human Platelet Aggregation.

Background: Apixaban is an oral, direct and highly selective factor Xa (FXa) inhibitor in late-stage clinical development for the prevention and treatment of venous thromboembolism, stroke prevention in patients with atrial fibrillation, and secondary prevention in patients with acute coronary syndrome. By inhibiting FXa, apixaban has the potential to reduce thrombin generation and indirectly decrease thrombin-mediated platelet aggregation. This study evaluated the in vitro effect of apixaban on human platelet aggregation induced by thrombin derived via the extrinsic pathway. Direct inhibitors of FXa (rivaroxaban), FVIIa (BMS-593214), thrombin (dabigatran, argatroban) and FXIa (BMS-262084) were included for comparison.

Methods: Citrated human blood, mixed with corn trypsin inhibitor (50 μg/ml) to block the contact factor pathway, was centrifuged to obtain platelets-rich plasma (PRP). The latter were incubated for 2 min at 37°C with 3 mM H-Gly-Pro-Arg-Pro-OH-AcOH (to prevent fibrin polymerization) and with vehicle or increasing concentrations of inhibitors. Human tissue factor (TF) (Innovin^®; dilution 1:900 to 1:1000) plus 7.5 mM CaCl₂ was added to PRP to trigger platelet aggregation. Platelet aggregation response, measured turbidometrically with a platelet aggregometer, was expressed as AUC occurring 5 min after TF stimulation.

Results: TF produced 85±3% aggregation of human platelets in the vehicle-treated group (n=10). Apixaban and other factor inhibitors, except the FXIa inhibitor, inhibited TF-induced platelet aggregation in a concentration-dependent manner with IC₅₀ (nM) values as follows: 3.6±1.3 (apixaban), 7.5±1.5 (rivaroxaban), 13±1 (BMS-593214), 46±1 (dabigatran) and 79±1 (argatroban). BMS-262084 (IC₅₀ = 2.8 nM vs. human FXIa) had no effect on TF-induced platelet aggregation at 10 μM. These inhibitors at 10 μM had no effect on platelet aggregation induced by ADP and collagen, as expected from their mechanism of action.

Conclusions: This study demonstrates that inhibition of thrombin generation by blocking upstream proteases (FVIIa and FXa) in the blood coagulation cascade is as effective as direct thrombin inhibition in preventing TF-induced platelet aggregation. Under these experimental conditions, FXIa did not play a role in platelet aggregation. Further, the potent indirect antiplatelet effect of FXa inhibitors together with their anticoagulant effects suggest that apixaban may have therapeutic potential in patients with arterial thrombosis. Accordingly, apixaban is in development for secondary prevention in patients with acute coronary syndrome.