Apixaban, a direct factor Xa inhibitor, inhibits tissue-factor induced human platelet aggregation in vitro: comparison with direct inhibitors of factor VIIa, XIa and thrombin.

Apixaban is an oral, direct and highly selective factor Xa (FXa) inhibitor in late-stage clinical development. 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. Citrated human platelets-rich plasma (PRP) was treated with 50 mg/ml corn trypsin inhibitor (to block the contact factor pathway) and 3 mM H-Gly-Pro-Arg-Pro-OH-AcOH (to prevent fibrin polymerisation). Human tissue factor (TF) (Innovin; dilution 1:1,000 to 1:1,500) plus 7.5 mM CaCl2 was added to PRP pre-incubated with vehicle or increasing concentrations of inhibitors. The TF-induced platelet aggregation was measured by optical aggregometry. 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 with IC50 (nM) values as follows: 4 +/- 1 (apixaban), 8 +/- 2 (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 microM. These inhibitors at 10 microM had no effect on platelet aggregation induced by ADP and collagen, as expected from their mechanism of action. 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, a FXIa inhibitor did not prevent TF-induced platelet aggregation.

Platelet aggregometry and receptor binding to predict the magnitude of antithrombotic and bleeding time effects of clopidogrel in rabbits.

Target levels of ex vivo inhibition of platelet aggregation (IPA) induced by adenosine diphosphate (ADP) that produce clinically relevant effects of clopidogrel, a P2Y12 antagonist, are unclear. We examined standard and modified IPA and P2Y12 receptor occupancy as predictors of antithrombotic (% thrombus weight reduction) and bleeding time (BT, fold-increase over control) effects of clopidogrel in rabbit models of carotid artery thrombosis and cuticle bleeding, respectively. Standard and modified IPA with 20 microM ADP were measured in the absence and presence of partial P2Y1 blockade, respectively. Clopidogrel maximally produced standard IPA of 57% +/- 5%, antithrombotic effect of 85% +/- 1%, BT increase of 6.0 +/- 0.4-fold and P2Y12 receptor occupancy of 87% +/- 5%. Surprisingly, a clopidogrel dose that produced a low standard IPA of 17% +/- 4% and P2Y12 receptor occupancy of 39% +/- 5% achieved a significant antithrombotic activity of 55% +/- 2% with a moderate increase in BT of 2.0 +/- 0.1-fold. This underestimation of clopidogrel efficacy by standard IPA was improved by measuring either modified IPA or P2Y12 receptor occupancy. These results suggest that in clopidogrel-treated rabbits, low standard IPA is associated with significant antithrombotic effects. Moreover, modified IPA and P2Y12 receptor occupancy appear to better predict the magnitude of clopidogrel's efficacy compared with standard IPA, which may be a better predictor of BT.

Selective inhibition of eosinophil influx into the lung by small molecule CC chemokine receptor 3 antagonists in mouse models of allergic inflammation.

CC chemokine receptor (CCR) 3 is a chemokine receptor implicated in recruiting cells, particularly eosinophils (EPhi), to the lung in episodes of allergic asthma. To investigate the efficacy of selective, small molecule antagonists of CCR3, we developed a murine model of EPhi recruitment to the lung. Murine eotaxin was delivered intranasally to mice that had previously received i.p. injections of ovalbumin (OVA), and the effects were monitored by bronchoalveolar lavage. A selective eosinophilic influx was produced in animals receiving eotaxin but not saline. Furthermore, the number of EPhi was concentration- and time-dependent. Although anti-CCR3 antibody reduced the number of EPhi, the effect of eotaxin in OVA-sensitized mice was not a direct chemotactic stimulus because mast cell deficiency (in WBB6F1-Kitw/Kitw-v mice) significantly reduced the response. Two representative small molecule CCR3 antagonists from our program were characterized as being active at mouse CCR3. They were administered p.o. to wild-type mice and found to reduce eotaxin-elicited EPhi selectively in a dose-dependent manner. Pump infusion of one of the inhibitors to achieve steady-state levels showed that efficacy was not achieved at plasma concentrations equivalent to the in vitro chemotaxis IC90 but only at much higher concentrations. To extend the results from our recruitment model, we tested one of the inhibitors in an allergenic model of airway inflammation, generated by adoptive transfer of OVA-sensitive murine T helper 2 cells and aerosolized OVA challenge of recipient mice, and found that it inhibited EPhi recruitment. We conclude that small molecule CCR3 antagonists reduce pulmonary eosinophilic inflammation elicited by chemokine or allergenic challenge.