title: Stereochemical aspects of itraconazole metabolism in vitro and in vivo.
abstract: Itraconazole (ITZ) has three chiral centers and is administered clinically as a mixture of four stereoisomers. This study evaluated stereoselectivity in ITZ metabolism. In vitro experiments were carried out using heterologously expressed CYP3A4. Only (2R,4S,2'R)-ITZ and (2R,4S,2'S)-ITZ were metabolized by CYP3A4 to hydroxy-ITZ, keto-ITZ, and N-desalkyl-ITZ. When (2S,4R,2'R)-ITZ or (2S,4R,2'S)-ITZ was incubated with CYP3A4, neither metabolites nor substrate depletion were detected. Despite these differences in metabolism, all four ITZ stereoisomers induced a type II binding spectrum with CYP3A4, characteristic of coordination of the triazole nitrogen to the heme iron (K(s) 2.2-10.6 nM). All four stereoisomers of ITZ inhibited the CYP3A4-catalyzed hydroxylation of midazolam with high affinity (IC(50) 3.7-14.8 nM). Stereochemical aspects of ITZ pharmacokinetics were evaluated in six healthy volunteers after single and multiple oral doses. In vivo, after a single dose, ITZ disposition was stereoselective, with a 3-fold difference in C(max) and a 9-fold difference in C(min) between the (2R,4S)-ITZ and the (2S,4R)-ITZ pairs of diastereomers, with the latter reaching higher concentrations. Secondary and tertiary ITZ metabolites (keto-ITZ and N-desalkyl-ITZ) detected in plasma were of the (2R,4S) stereochemistry. After multiple doses of ITZ, the difference in C(max) and C(min) decreased to 1.5- and 3.8-fold, respectively. The initial difference between the stereoisomeric pairs was most likely due to stereoselective metabolism by CYP3A4, including stereoselective first-pass metabolism as well as stereoselective elimination. However, stereoselective elimination was diminished after multiple dosing, presumably as a result of CYP3A4 autoinhibition. In conclusion, the metabolism of ITZ is highly stereoselective in vitro and in vivo.
authors.affiliation: Department of Pharmaceutics, H272 Health Sciences Building, Box 357610, University of Washington, Seattle, WA 98195, USA.
authors.forename: Kent L
authors.initials: KL
authors.lastname: Kunze
authors.forename: Wendel L
authors.initials: WL
authors.lastname: Nelson
authors.forename: Evan D
authors.initials: ED
authors.lastname: Kharasch
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Thummel
authors.forename: Nina
authors.initials: N
authors.lastname: Isoherranen
mesh_terms: D000293:Adolescent; D000328:Adult; D000935:Antifungal Agents / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D017964:Itraconazole / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013237:Stereoisomerism
chemical_list: D000935:Antifungal Agents; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D017964:Itraconazole; D003577:Cytochrome P-450 Enzyme System; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human