ROLE OF ITRACONAZOLE METABOLITES IN CYP3A4 INHIBITION

Itraconazole (ITZ) is a potent inhibitor of CYP3A in vivo. However, unbound plasma concentrations of ITZ are much lower than its reported in vitro K_i, and no clinically significant interactions would be expected based on a reversible mechanism of inhibition. The purpose of this study was to evaluate the reasons for the in vitro-in vivo discrepancy. The metabolism of ITZ by CYP3A4 was studied. Three metabolites were detected: hydroxy-itraconazole (OH-ITZ), a known in vivo metabolite of ITZ, and two new metabolites: keto-itraconazole (keto-ITZ) and N-desalkyl-itraconazole (ND-ITZ). OHITZ and keto-ITZ were also substrates of CYP3A4. Using a substrate depletion kinetic approach for parameter determination, ITZ exhibited an unbound K_m of 3.9 nM and an intrinsic clearance (CL_int) of 69.3 ml·min^-1·nmol CYP3A4^-1. The respective unbound K_m values for OH-ITZ and keto-ITZ were 27 nM and 1.4 nM and the CL_int values were 19.8 and 62.5 ml·min^-1·nmol CYP3A4^-1. Inhibition of CYP3A4 by ITZ, OH-ITZ, keto-ITZ, and ND-ITZ was evaluated using hydroxylation of midazolam as a probe reaction. Both ITZ and OH-ITZ were competitive inhibitors of CYP3A4, with unbound K_i (1.3 nM for ITZ and 14.4 nM for OH-ITZ) close to their respective K_m. ITZ, OH-ITZ, keto-ITZ and ND-ITZ exhibited unbound IC₅₀ values of 6.1 nM, 4.6 nM, 7.0 nM, and 0.4 nM, respectively, when coincubated with human liver microsomes and midazolam (substrate concentration < K_m). These findings demonstrate that ITZ metabolites are as potent as or more potent CYP3A4 inhibitors than ITZ itself, and thus may contribute to the inhibition of CYP3A4 observed in vivo after ITZ dosing.