title: Metabolism of the human immunodeficiency virus protease inhibitors indinavir and ritonavir by human intestinal microsomes and expressed cytochrome P4503A4/3A5: mechanism-based inactivation of cytochrome P4503A by ritonavir.
abstract: Both ritonavir and indinavir were readily metabolized by human intestinal microsomes. Comparison of the patterns of metabolites in incubations with enterocyte microsomes and expressed cytochrome P450 (CYP) isozymes and immunoinhibition and chemical inhibition studies showed the essential role of the CYP3A subfamily in the metabolism of both protease inhibitors by the small intestine. Ritonavir was similarly biotransformed by microsomes containing expressed CYP3A4 or CYP3A5 isozymes (KM = 0.05-0.07 microM, Vmax = 1-1.4 nmol/min/nmol CYP). In contrast, both the patterns of metabolites and the enzyme kinetic parameters for the metabolism of indinavir by expressed CYP3A5 (KM = 0.21 microM, Vmax = 0.24 nmol/min/nmol CYP) and CYP3A4 (KM = 0.04 microM, Vmax = 0.68 nmol/min/nmol CYP) were different. The biotransformation of both indinavir and ritonavir in human enterocyte microsomes was characterized by very low KM values (0.2-0.4 microM for indinavir and <0.1 microM for ritonavir). The Vmax for indinavir metabolism was greater in enterocyte (163 +/- 35 pmol/min/mg protein) than in liver (68 +/- 44 pmol/min/mg protein) microsomes. The metabolism of ritonavir in liver and enterocyte microsomes was associated with inactivation of CYP3A. The initial Vmax for ritonavir metabolism by enterocyte microsomes was 89 +/- 59 pmol/min/mg protein. The apparent inactivation rate constants for intestinal CYP3A and expressed CYP3A4 were 0.078 and 0.135 min-1, respectively. Metabolic inactivation of CYP3A by ritonavir explains the improved bioavailability and pharmacokinetics of ritonavir and the sustained elevation of blood levels of other, concomitantly administered, substrates of CYP3A.
authors.affiliation: Department of Pharmacology, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: T
authors.initials: T
authors.lastname: Koudriakova
authors.forename: E
authors.initials: E
authors.lastname: Iatsimirskaia
authors.forename: I
authors.initials: I
authors.lastname: Utkin
authors.forename: E
authors.initials: E
authors.lastname: Gangl
authors.forename: P
authors.initials: P
authors.lastname: Vouros
authors.forename: E
authors.initials: E
authors.lastname: Storozhuk
authors.forename: D
authors.initials: D
authors.lastname: Orza
authors.forename: J
authors.initials: J
authors.lastname: Marinina
authors.forename: N
authors.initials: N
authors.lastname: Gerber
mesh_terms: D019380:Anti-HIV Agents / Q000493:pharmacokinetics*; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D017320:HIV Protease Inhibitors / Q000493:pharmacokinetics*; D006801:Humans; D019469:Indinavir / Q000493:pharmacokinetics*; D007413:Intestinal Mucosa / Q000378:metabolism*; D007654:Ketoconazole / Q000494:pharmacology; D007700:Kinetics; D008861:Microsomes / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000502:physiology*; D019438:Ritonavir / Q000493:pharmacokinetics* / Q000494:pharmacology
chemical_list: D019380:Anti-HIV Agents; D065607:Cytochrome P-450 Enzyme Inhibitors; D017320:HIV Protease Inhibitors; D019469:Indinavir; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; C510164:CYP3A5 protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D019438:Ritonavir; D007654:Ketoconazole