Efavirenz Primary and Secondary Metabolism In Vitro and In Vivo: Identification of Novel Metabolic Pathways and Cytochrome P450 2A6 as the Principal Catalyst of Efavirenz 7-Hydroxylation

• Published in: Drug metabolism and disposition Vol. 38; no. 7; pp. 1218 - 1229
• Main Authors: Ogburn, Evan T., Jones, David R., Masters, Andrea R., Xu, Cong, Guo, Yingying, Desta, Zeruesenay
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Elsevier Inc 01.07.2010; American Society for Pharmacology and Experimental Therapeutics; The American Society for Pharmacology and Experimental Therapeutics
• Subjects: Alkynes; Aryl Hydrocarbon Hydroxylases - antagonists & inhibitors; Aryl Hydrocarbon Hydroxylases - metabolism; Benzoxazines - pharmacokinetics; Biological and medical sciences; Cyclopropanes; Cytochrome P-450 CYP2A6; Cytochrome P-450 CYP2B6; Enzyme Inhibitors - pharmacology; Humans; Hydroxylation - drug effects; In Vitro Techniques; Inactivation, Metabolic; Kinetics; Medical sciences; Metabolic Networks and Pathways - drug effects; Microsomes, Liver - drug effects; Microsomes, Liver - enzymology; Oxidoreductases, N-Demethylating - antagonists & inhibitors; Oxidoreductases, N-Demethylating - metabolism; Pharmacology. Drug treatments; CLint; AUC0–72; LC/MS/MS; MS; HLM; MRM; P450; HPLC; Antiretroviral agent; Benzoxazine derivatives; RNA-directed DNA polymerase; Acetylenic compound; Hydroxylation; Enzyme; Non nucleoside compound; Transferases; Enzyme inhibitor; Identification; Metabolic pathway; Efavirenz; In vitro; Allosteric inhibitor; Nucleotidyltransferases; Reverse transcriptase inhibitor; Primary; Antiviral; Secondary metabolism
• DOI: 10.1124/dmd.109.031393
• ISSN: 0090-9556; 1521-009X

Efavirenz primary and secondary metabolism was investigated in vitro and in vivo. In human liver microsome (HLM) samples, 7- and 8-hydroxyefavirenz accounted for 22.5 and 77.5% of the overall efavirenz metabolism, respectively. Kinetic, inhibition, and correlation analyses in HLM samples and experiments in expressed cytochrome P450 show that CYP2A6 is the principal catalyst of efavirenz 7-hydroxylation. Although CYP2B6 was the main enzyme catalyzing efavirenz 8-hydroxylation, CYP2A6 also seems to contribute. Both 7- and 8-hydroxyefavirenz were further oxidized to novel dihydroxylated metabolite(s) primarily by CYP2B6. These dihydroxylated metabolite(s) were not the same as 8,14-dihydroxyefavirenz, a metabolite that has been suggested to be directly formed via 14-hydroxylation of 8-hydroxyefavirenz, because 8,14-dihydroxyefavirenz was not detected in vitro when efavirenz, 7-, or 8-hydroxyefavirenz were used as substrates. Efavirenz and its primary and secondary metabolites that were identified in vitro were quantified in plasma samples obtained from subjects taking a single 600-mg oral dose of efavirenz. 8,14-Dihydroxyefavirenz was detected and quantified in these plasma samples, suggesting that the glucuronide or the sulfate of 8-hydroxyefavirenz might undergo 14-hydroxylation in vivo. In conclusion, efavirenz metabolism is complex, involving unique and novel secondary metabolism. Although efavirenz 8-hydroxylation by CYP2B6 remains the major clearance mechanism of efavirenz, CYP2A6-mediated 7-hydroxylation (and to some extent 8-hydroxylation) may also contribute. Efavirenz may be a valuable dual phenotyping tool to study CYP2B6 and CYP2A6, and this should be further tested in vivo.