Monitoring direct FXa-inhibitors and fondaparinux by Prothrombinase-induced Clotting Time (PiCT): relation to FXa-activity and influence of assay modifications.
BACKGROUND
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 microM), with the structurally distinct direct FXa-inhibitor DX 9065a (0 - 18 microM) and with the indirect inhibitor fondaparinux (0 - 0.6 microM). 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.
Pharmazentrum Frankfurt, Institute for Clinical Pharmacology, University Hospital Frankfurt am Main, Germany. harder@em.uni-frankfurt.de
Sebastian
S
Harder
Jeannine
J
Parisius
Bettina
B
Picard-Willems
D000818:Animals; D000925:Anticoagulants / Q000494:pharmacology*; D000991:Antithrombins / Q000494:pharmacology; D001681:Biological Assay; D001780:Blood Coagulation Tests / Q000379:methods*; D002417:Cattle; D004305:Dose-Response Relationship, Drug; D016903:Drug Monitoring / Q000379:methods*; D065427:Factor Xa Inhibitors; D000077425:Fondaparinux; D006801:Humans; D020128:Inhibitory Concentration 50; D010314:Partial Thromboplastin Time; D011134:Polysaccharides / Q000494:pharmacology*; D013925:Thromboplastin / Q000037:antagonists & inhibitors / Q000494:pharmacology
D000925:Anticoagulants; D000991:Antithrombins; D065427:Factor Xa Inhibitors; D011134:Polysaccharides; D013925:Thromboplastin; D000077425:Fondaparinux