Monitoring direct FXa-inhibitors and fondaparinux by Prothrombinase-induced Clotting Time (PiCT): Relation to FXa-activity and influence of assay modifications

Abstract Introduction FXa-activity can be measured by the Prothrombinase induced Clotting time (PiCT). The manufactured assay uses bovine FXa as component and employs a incubation period before re-calcification. Its use with new direct FXa-inhibitors is challenged by reports on decreased sensitivity. Methods Blood was incubated with 3 investigational, structurally related (oxazolidinones) direct FXa-inhibitors including the recently approved agent rivaroxaban (0 – 2.0 µM), with the structurally distinct direct FXa-inhibitor DX 9065a (0 – 18 µM) and with the indirect inhibitor fondaparinux (0 – 0.6 µM). We tested modifications of PiCT regarding the source of FXa (bovine or human) and the incubation step (incubation before re-calcification = 2-step, no incubation =1-step), and compared results with inhibition of human or bovine FXa-activity. Results The bovine 2-step PiCT showed a paradoxical decrease with all direct FXa-inhibitors, this effect is surmounted only at high concentrations and is not seen with the bovine 1-step PiCT. The decrease in PiCT is not observed in antithrombin-depleted plasma. The humanized PiCT (1 or 2 step) showed a consistent prolongation under all direct inhibitors. Fondaparinux prolonged PiCT with either assay. The correlation between PiCT and corresponding FXa-activity was significant both for humanized 2-step PiCT or bovine 1 step PiCT (r2 = 0.80), but the 95% prediction interval was large and covered a span of 40% FXa-activity between one agent and another. Conclusions The customary bovine PiCT should only be used to monitor direct FXa-inhibitors when modified as 1-step procedure. PiCT is not suitable to assess similarity of FXa-inhibition when different agents are interchanged.