Factor Xa Inhibitors Can Be Differentiated in Xa, IIa and Microparticle Generation and Other Whole Blood Based Assays.

Currently used oral anticoagulants such as Vitamin K antagonists have drawbacks, which reportedly limit their safety and efficacy. Oral Factor Xa and IIa inhibitors are claimed to overcome these limitations. Factor Xa inhibitors provide more complete suppression of thrombin generation than Factor IIa inhibitors. Four Factor Xa anticoagulant agents, both direct and indirect, namely A,B,C and D with Ki values 6–200 nM, were studied in various assays at equigravimetric final concentration of 10 ug/ml to determine their relative potencies. Apparent differences in their biochemical profiles were noted in thrombin generation, Factor Xa generation and microparticle generation inhibition assays. Furthermore, the anticoagulant potential of these Xa inhibitors was studied in celite activated clotting time (ACT) and modified celite ACT system using different concentrations of tissue factor. Microparticle generation was performed using agent- supplemented whole blood that was incubated for three minutes with varying concentrations of tissue factor. Following three minutes the reaction was stopped using EDTA stop solution. The functional assay for the determination of microparticle procoagulant activity in the plasma was performed using Zymuphen MP-Activity assay kit from Hyphen BioMed (Neuville-sur-Oise, France). While three of the Xa inhibitors studied showed microparticle generation inhibition levels of 31.37 nM, one agent provided a level of 17.87 nM compared to a saline control value of 39.28nM. Supplementation studies were also carried out in whole blood PT, APTT and Heptest assays in the concentration range of 0 to 10 ug/ml. The whole blood prothrombin time assay showed a maximum of 208.8, 61.3, 77.8 and 15.9 seconds at a final concentration of 10 ug/ml for the respective anticoagulant agents when compared to a normal value of 12.3 seconds. Similarly the whole blood APTT assay showed a maximum of 267.9, 161.8, >300 and 71.9 seconds respectively when compared to a control value of 46.8 seconds. The whole blood Heptest assay showed a varying maximum anticoagulant effect from >300, 42.7, >300 and >300 seconds respectively when compared to a control value of 13.8 seconds. In each of these assays Factor Xa inhibitors showed concentration-dependent effects and had varying potencies. In TF-mediated platelet activation assays the different Xa agents produced varying effects which were not proportionate to their Ki values. Differentiation of Factor Xa inhibitors have a clinical impact in dosage selection, dosage adjustment and their monitoring when given in large dosages. Synthetic factor Xa agents exhibit Ki values of 20–200 nM, while pentasaccharide -AT complex has a Ki value of 60 nM. However, these results do not translate into proportionate anticoagulant effects to their Ki values. Large-scale clinical studies are necessary to validate these preliminary results.