title: Role of human liver microsomal CYP3A4 and CYP2B6 in catalyzing N-dechloroethylation of cyclophosphamide and ifosfamide.
abstract: The anticancer alkylating agents cyclophosphamide (CPA) and ifosfamide (IFA) are prodrugs that undergo extensive P450-catalyzed metabolism to yield both active (4-hydroxylated) and therapeutically inactive but neurotoxic (N-dechloroethylated) metabolites. Whereas the human liver microsomal P450 catalysts of CPA and IFA 4-hydroxylation are well characterized, the P450 enzyme catalysts of the alternative N-dechloroethylation pathway are poorly defined. Analysis of a panel of fifteen human P450 cDNAs in the baculovirus expression system ('Supersomes') demonstrated that CYP3A4 exhibited the highest N-dechloroethylation activity toward both CPA and IFA, whereas CYP2B6 displayed high N-dechloroethylation activity toward IFA, but not CPA. The contributions of each human P450 to overall liver microsomal N-dechloroethylation were calculated using a recently described relative substrate-activity factor method, and were found to be in excellent agreement with the results of inhibition studies using the CYP3A inhibitor troleandomycin and an inhibitory monoclonal antibody to CYP2B6. With CPA as substrate, CYP3A4 was shown to catalyze >/=95% of liver microsomal N-dechloroethylation, whereas with IFA as substrate, CYP3A4 catalyzed an average of approximately 70% of liver microsomal N-dechloroethylation (range = 40-90%), with the balance of this activity catalyzed by CYP2B6 (range = 10-70%, dependent on the CYP2B6 content of the liver). Because CYP2B6 can make a significant contribution to human liver microsomal IFA N-dechloroethylation, but only a minor contribution to IFA 4-hydroxylation, the selective inhibition of hepatic CYP2B6 activity in individuals with a high hepatic CYP2B6 content may provide a useful approach to minimize the formation of therapeutically inactive but toxic N-dechloroethylated IFA metabolites.
authors.affiliation: Division of Cell Biology, Department of Biology, Boston University, Boston, MA, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Huang
authors.forename: P
authors.initials: P
authors.lastname: Roy
authors.forename: D J
authors.initials: DJ
authors.lastname: Waxman
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D018906:Antineoplastic Agents, Alkylating / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases; D001667:Binding, Competitive; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003520:Cyclophosphamide / Q000378:metabolism*; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics; D005470:Fluorometry / Q000379:methods; D006801:Humans; D007069:Ifosfamide / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014217:Troleandomycin / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D000911:Antibodies, Monoclonal; D018906:Antineoplastic Agents, Alkylating; D018076:DNA, Complementary; D011994:Recombinant Proteins; D003520:Cyclophosphamide; D003577:Cytochrome P-450 Enzyme System; D014217:Troleandomycin; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C104464:CYP3A protein, human; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D007069:Ifosfamide