Laboratory assessment of rivaroxaban: a review.

Research into new anticoagulants for preventing and treating thromboembolic disorders has focused on targeting single enzymes in the coagulation cascade, particularly Factor Xa and thrombin, inhibition of which greatly decreases thrombin generation. Based on the results of phase III clinical trials, rivaroxaban, a direct Factor Xa inhibitor, has been approved in many countries for the management of several thromboembolic disorders. Owing to its predictable pharmacokinetic and pharmacodynamic characteristics, fixed-dose regimens are used without the need for routine coagulation monitoring. In situations where assessment of rivaroxaban exposure may be helpful, anti-Factor Xa chromogenic assays (in tandem with standard calibration curves generated with the use of rivaroxaban calibrators and controls) could be used. It is important to note that test results will be affected by the timing of blood sampling after rivaroxaban intake. In addition, the anti-Factor Xa method measures the drug concentration and not the intensity of the drug's anticoagulant activity, and a higher than expected rivaroxaban plasma level does not necessarily indicate an increased risk of bleeding complications. Therefore, clinicians need to consider test results in relation to the pharmacokinetics of rivaroxaban and other patient risk factors associated with bleeding.

Evaluation of the prothrombin time for measuring rivaroxaban plasma concentrations using calibrators and controls: results of a multicenter field trial.

This study evaluated the prothrombin time (PT) assay for the measurement of plasma concentrations of rivaroxaban using calibrators and controls. The intra- and interlaboratory precision of the measurement was investigated in a field trial involving 21 laboratories. Each laboratory was provided with rivaroxaban calibrators and control plasma samples containing different concentrations of rivaroxaban, and PT reagents. The evaluation was carried out over 2 consecutive weeks using centrally provided and local PT reagents. A calibration curve was produced each day (for inter-run precision), and day-to-day precision was evaluated by testing 3 control plasma samples. A large interlaboratory variation (in seconds) was observed with local PT reagents. The results were less variable when expressed as rivaroxaban concentrations (ng/mL) or when central PT reagent was used (STA Neoplastine CI Plus). The widely available PT assay, in conjunction with rivaroxaban calibrators, may be useful for the measurement of peak plasma levels of rivaroxaban.

Evaluation of the anti-factor Xa chromogenic assay for the measurement of rivaroxaban plasma concentrations using calibrators and controls.

Rivaroxaban is an oral, direct factor Xa inhibitor. Routine coagulation monitoring is not required, but a quantitative determination of rivaroxaban concentrations might be useful in some clinical circumstances. This multicentre study assessed the suitability of the anti-factor Xa chromogenic assay for the measurement of rivaroxaban plasma concentrations (ng/ml) using rivaroxaban calibrators and controls, and the inter-laboratory precision of the measurement. Twenty-four centres in Europe and North America were provided with sets of rivaroxaban calibrators (0, 41, 209 and 422 ng/ml) and a set of rivaroxaban pooled human plasma controls (20, 199 and 662 ng/ml; the concentrations were unknown to the participating laboratories). The evaluation was carried out over 10 days by each laboratory using local anti-factor Xa reagents as well as the centrally provided reagent, a modified STA® Rotachrom® assay. A calibration curve was produced each day, and the day-to-day precision was evaluated by testing three human plasma controls. When using the local anti-factor Xa reagents, the mean rivaroxaban concentrations (measured/actual values) were: 17/20, 205/199 and 668/662 ng/ml, and the coefficient of variance (CV) was 37.0%, 13.7% and 14.1%, respectively. When the modified STA Rotachrom method was used, the measured/actual values were: 18/20, 199/199 and 656/662 ng/ml, and the CV was 19.1%, 10.9% and 10.0%, respectively. The results suggest that, by using rivaroxaban calibrators and controls, the anti-factor Xa chromogenic method is suitable for measuring a wide range of rivaroxaban plasma concentrations (20-660 ng/ml), which covers the expected rivaroxaban plasma levels after therapeutic doses.

Assessment of laboratory assays to measure rivaroxaban--an oral, direct factor Xa inhibitor.

Although there is no need for routine coagulation monitoring with rivaroxaban--an oral, direct factor Xa inhibitor--a haemostasis assay might be valuable to measure its pharmacodynamic effects. This study aimed to find assays, among those commercially available, to measure rivaroxaban pharmacodynamics. Several global conventional clotting tests, as well as clotting or chromogenic assays to measure anti-factor Xa activity, were studied. A thrombin generation test using calibrated automated thrombogram was also done. Tests were performed with the indirect factor Xa inhibitor fondaparinux for comparison. A concentration-dependent prolongation of prothrombin time (PT), dilute PT, and activated partial thromboplastin time was observed with rivaroxaban. The results varied depending on the reagents. This variability cannot be standardised with the international normalised ratio system commonly used for vitamin K antagonists. Using a standard calibration curve, PT test results can be expressed in plasma concentrations of rivaroxaban rather than PT seconds or ratio. Standard methods for HepTest and two-step prothrombinase-induced clotting time (PiCT) resulted in a paradoxical response, with low concentrations of rivaroxaban reducing clotting times. This was not observed with shorter incubation times, or when antithrombin-deficient (immunodepleted) plasma was used. The chromogenic tests found a dose-dependent relationship between anti-factor Xa activity and rivaroxaban concentration. Modified specific factor Xa chromogenic assays are being further investigated. One-step PiCT and HepTest with shortened incubation times, as well as the widely available PT assay (using a rivaroxaban calibrator) could be useful to monitor the pharmacodynamic effects of rivaroxaban accurately. Finally, all clotting and chromogenic assays showed a concentration-dependent effect induced by rivaroxaban.