1-Aminobenzotriazole coincubated with (S)-warfarin results in potent inactivation of CYP2C9.

ABSTRACT

1-Aminobenzotriazole (ABT) is a nonselective, mechanism-based inactivator of cytochrome P450 (P450) and a useful tool compound to discern P450- from non-P450-mediated metabolism. ABT effectively inactivates major human P450 isoforms, with the notable exception of CYP2C9. Here we propose that ABT preferentially binds to the warfarin-binding pocket in the CYP2C9 active-site cavity; thus, ABT bioactivation and subsequent inactivation is not favored. Therefore, coincubation with (S)-warfarin would result in displacement of ABT from the warfarin-binding pocket and subsequent binding to the active site, converting ABT into a potent inactivator of CYP2C9. To test this hypothesis, in vitro studies were conducted using various coincubation combinations of ABT and (S)-warfarin or diclofenac to modulate the effectiveness of CYP2C9 inactivation by ABT. Coincubation of ABT with (S)-warfarin (diclofenac probe substrate) resulted in potent inactivation, whereas weak inactivation was observed following coincubation of ABT with diclofenac [(S)-warfarin probe substrate]. The kinetic parameters of time-dependent inhibition of ABT for CYP2C9 in the absence and presence of (S)-warfarin (20 µM) were 0.0826 and 0.273 min(-1) for kinact and 3.49 and 0.157 mM for KI, respectively. In addition, a 73.4-fold shift was observed in the in vitro potency (kinact/KI ratio), with an increase from 23.7 ml/min/mmol (ABT alone) to 1740 ml/min/mmol [ABT with (S)-warfarin (20 µM)]. These findings were supported by in silico structural modeling, which showed ABT preferentially binding to the warfarin-binding pocket and the displacement of ABT to the active site in the presence of (S)-warfarin.