Three direct thrombin inhibitors (DTIs) differ in their ability to increase the International Normalized Ratio (INR) for a given prolongation of the activated partial thromboplastin time (APTT), as follows: argatroban > bivalirudin > lepirudin. This phenomenon is clinically-relevant, as it can complicate the transition to coumarin therapy (a risk factor for venous limb ischemia in patients with heparin-induced thrombocytopenia and deep-vein thrombosis, i.e., a clinical situation in which a DTI may be given). Since this observed order of INR prolongation by DTIs is the reverse of that expected based on their known relative affinities for human thrombin (see Table), we tested whether these DTIs (as well as the small-molecule thrombin inhibitor, D-Phe-Pro-ArgCH2Cl [PPACK]) also inhibit free and/or prothrombinase-bound factor Xa (FXa). Prothrombinase-bound FXa was assembled on rabbit brain phospholipids supplemented with human plasma factor X-depleted plasma, to which human FXa (0.5 nM, final) was added. We found that argatroban inhibited free human FXa, i.e., FXa in the fluid-phase (IC50 = 5 μM), whereas it inhibited bovine FXa much less effectively (IC50 = 110 μM). In contrast, only supratherapeutic concentrations of bivalirudin inhibited human and bovine FXa, whereas lepirudin had no effect (see Table). Neither therapeutic nor supratherapeutic concentrations of the three DTIs inhibited prothrombinase-bound FXa.
Comparison of DTIs on INR, APTT, IIa, and Inhibition of Human and Bovine Fxa
PPACK Argatroban Bivalirudin Lepirudin * published values Effect on INR N.D. Greatest Intermediate Least [DTI] doubling APTT N.D. 1 μM 0.25 μM 0.06 μM Ki for human thrombin* ~40 nM ~2 nM ~0.0001 nM IC50 human FXa 3 μM 5 μM 1,700 μM >10,000 μM IC50 bovine FXa 5 μM 110 μM 2,900 μM >10,000 μM
CONCLUSION: Compared with the other two DTIs, argatroban effectively inhibits human FXa at concentrations (IC50 = 5 μM) within the order of magnitude of the usual therapeutic drug levels (about 1 to 2 μM). Inhibition of free FXa is a plausible reason why argatroban prolongs the INR more effectively than the other two DTIs.