A mathematical model has been developed to simulate the generation of thrombin by the tissue factor pathway. The model gives reasonable predictions of published experimental results without the adjustment of any parameter values. The model also accounts explicitly for the effects of serine protease inhibitors on thrombin generation. Simulations to define the optimum affinity profile of an inhibitor in this system indicate that for an inhibitor simultaneously potent against VIIa, IXa, and Xa, inhibition of thrombin generation decreases dramatically as the affinity for thrombin increases. Additional simulations show that the reason for this behavior is the sequestration of the inhibitor by small amounts of thrombin generated early in the reaction. This model is also useful for predicting the potency of compounds that inhibit thrombosis in rats. We believe that this is the first mathematical model of blood coagulation that considers the effects of exogenous inhibitors. Such a model, or extensions thereof, should be useful for evaluating targets for therapeutic intervention in the processes of blood coagulation.The clotting of blood is an exquisitely complex process. The simultaneous requirements for the free flow of blood under normal conditions and rapid clotting to prevent blood loss in the case of injury require a delicate balance between clot formation and clot lysis. Taken together, these processes involve more than 30 proteins, at least 10 of which are serine proteases. Inhibitors of serine proteases occur as natural anticoagulants, and natural and synthetic inhibitors have been widely studied for therapeutic applications in the prevention of thrombosis (1).When choosing candidate serine protease inhibitors for therapeutic use, the choice of a target enzyme may be less than obvious. Many of the enzymes have multiple activities, there is positive and negative feedback regulation, and there are alternative pathways for activation and inactivation. In addition, serine protease inhibitors (especially synthetic compounds) generally have a spectrum of activities due to the high degrees of homology among the blood coagulation factors. This raises the question of whether the best antithrombotic compound would be specific for a single enzyme or show affinities for several serine proteases (2).Mathematical modeling can help us understand such complex systems, but published models of blood coagulation suffer from the following limitations: consideration of only a small part of the coagulation cascade (3-7), empirical description of interactions for which molecular mechanisms were known (3, 8, 9), determination of some (4, 9, 10) or all (3, 11) of the parameter values from the experimental data to which the model predictions were then compared (curve fitting), and the absence of comparisons of model predictions to experimental data (6,(12)(13)(14). In the best of these studies (4, 5, 7), most or all of the parameter values were independently determined, model predictions were compared to experimental results that had not...

Background and Purpose-Factor Xa (FXa) is a key coagulation protease and target for novel antithrombotic agents for prevention and treatment of diverse thromboembolic disorders. In the present study we describe the effect of a novel, potent, and selective FXa inhibitor, DPC602, on brain damage and neurobehavioral consequence in a rat thromboembolic model of stroke. Methods-Thromboembolic stroke was induced in rats by placement of an autologous clot into the middle cerebral artery. Results-Laser-Doppler monitoring of cerebral blood flow demonstrated that DPC602 (8 mg/kg, single IV/IP bolus pretreatment) markedly improved cerebral blood flow after thromboembolic stroke by 25% to 160% (nϭ6; PϽ0.001) at 1 to 6 hours. DPC602 demonstrated concentration-and time-dependent reductions in infarct size, with maximal effect (89% reduction; nϭ14; PϽ0.001) at the highest dose over controls. Neurological function was also significantly improved in DPC602-treated rats at days 1, 3, and 7 (nϭ13; PϽ0.01). DPC602 treatment did not cause cerebral hemorrhage, assessed by free hemoglobin in the ischemic brain tissues. Conclusions-These data suggest that anticoagulation with a selective FXa inhibitor might ameliorate the extent of ischemic brain damage and neurological deficits after a thromboembolic event. Enhanced clot dissolution and early reperfusion may account for the cerebrovascular-protective effect of the drug.

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Apixaban, previously known as BMS-562247, is a high affinity, highly selective, orally-active, reversible inhibitor of coagulation factor Xa (fXa), in clinical studies as a therapeutic agent for prevention and treatment of thromboembolic diseases. The in vitro characteristics of apixaban were evaluated in purified systems and in human blood from healthy volunteers. Detailed kinetic analysis of apixaban inhibition of human fXa showed that it is a readily reversible, potent and competitive inhibitor versus a synthetic tripeptide substrate with a Ki of 0.08 nM, an association rate of 2 × 107 M−1s−1and a dissociation half life of 3.4 min. Weak affinity (Ki ~3 μM) is observed for thrombin, plasma kallikrein, and chymotrypsin. Affinity for trypsin and all other serine proteases tested is negligible with Ki > 15 μM. Apixaban is an effective inhibitor of free fXa and of prothrombinase, in buffer, platelet poor plasma, and whole blood. The anticoagulant activity of apixaban was determined in platelet-poor human plasma. Apixaban causes concentration dependent prolongation of the fXa mediated clotting assays. The human plasma concentration required to produce a doubling of the clotting time is 3.6 μM for prothrombin time, 7.4 μM for activated partial thromboplastin time and 0.4 μM for HepTest. To support preclinical efficacy and safety studies purified fXa from rabbit, dog and rat plasma was also found to be inhibited by apixaban (0.17, 2.6, and 1.3 nM, respectively). In summary the in vitro properties of apixaban show that it is a highly selective and potentially potent antithrombotic agent for venous and arterial thrombotic diseases.