Effects of the Novel, Oral, Direct Factor Xa Inhibitor Rivaroxaban on Coagulation Assays

Rivaroxaban is an oral, direct Factor Xa (FXa) inhibitor that has been recommended for approval by the Committee for Medicinal Products for Human Use for the prevention of venous thromboembolism after elective hip and knee replacement, and is in advanced clinical development for the prevention and treatment of thromboembolic disorders. There is no need for routine laboratory monitoring with rivaroxaban. However, a hemostasis assay might be valuable to measure the pharmacodynamic (PD) effects of rivaroxaban in special situations. The aim of this study was to identify a widely available assay (clotting assay, or colorimetric or fluorometric assay) that could be used in clinical practice. Increasing concentrations of rivaroxaban were spiked into citrated pooled human platelet-poor plasma. The global clotting assays prothrombin time (PT), diluted PT (dPT), and activated partial thromboplastin time (aPTT) were compared, as well as the specific clotting assays HepTest^® (heparin test), prothrombinase-induced clotting time (PiCT^®), and diluted Russell’s viper venom time (dRVVT). In addition, the anti-FXa activity-based colorimetric assays STA Rotachrom^® and Stachrom^® and thrombin generation test (TGT) were measured. A concentration-dependent prolongation of PT, dPT, and aPTT was observed with rivaroxaban, although the results varied depending on the reagents used. When testing PT over time using frozen plasma spiked with defined concentrations of rivaroxaban, PT values did not change during storage. Using a standard calibration curve, the results of the PT test can be expressed in rivaroxaban (ng/mL) rather than as PT ratio or international normalized ratio (INR). A concentration-dependent prolongation of the clotting time was also observed with dPT, a test that may be clinically more significant than conventional PT because it is closer to in vivo conditions (i.e. it uses lower tissue factor concentrations). Conventional methods for the HepTest and two-step PiCT (incubation times of 120 seconds and 180 seconds, respectively) resulted in a paradoxical response, with low concentrations of rivaroxaban reducing clotting times. This paradoxical response was not observed with shorter incubation times (0 or 30 seconds for one-step PiCT and 30 seconds for HepTest), or when antithrombin-immunodepleted plasma was used. Rivaroxaban also increased the dRVVT concentration dependently. Rivaroxaban influenced the various parameters of the TGT concentration dependently; peak effect was the most informative parameter. Rivaroxaban concentration-dependently inhibited FXa activity in both the Rotachrom and Stachrom assays– the results were expressed as anti-FXa international unit/mL. PT assays, which are widely used, appear to be valuable assays for monitoring the PD effects of rivaroxaban. One-step PiCT and HepTest with shortened incubation times or anti-FXa assays could also be useful to monitor rivaroxaban, if standardized methods were used.