title: Assessment of the involvement of CYP3A in the vitro metabolism of a new modulator of MDR in cancer chemotherapy, OC144-193, by human liver microsomes.
abstract: The novel substituted imidazole compound, OC144-093 exhibits potent biological activity in vitro and in vivo for reversal of P-glycoprotein (PgP) based resistance to cancer chemotherapy. Its mechanism of action relies upon its inhibitory interaction with the mdr1 gene product, a known mediator of multidrug resistance (MDR). Overlapping substrate specificities and tissue distribution of cytochrome P450 3A (CYP3A) and PgP indicate the potential for drug-drug interactions when modulator and anticancer agent are co-administered. We have examined the metabolism of OC144-093 in vitro using human liver microsomes to determine if CYP3A is involved. Our results show that OC144-093 is converted to one major metabolite (M1) in human liver microsomes which was identified by LCMS to be the O-deethylated derivative. Km and Vmax for O-deethylation were determined as 3.96+/-0.67 microM and 32.08+/-9.73 pmol/mg protein/min, respectively (n=3). Correlation studies conducted in a panel of human livers phenotyped for specific P450 enzyme activity showed a significant relationship between M1 formation and the activity of CYP2C9, CYP2B6, CYP2E1 and CYP3A4. Treatment of microsomes with carbon monoxide gas inhibited M1 formation and diethyldithiocarbamate and ketoconazole (>3 microM), non-specific CYP inhibitors, gave IC50 values of 124.4+/-21.6 microM and 25.3+/-3.2 microM respectively for the inhibition of O-deethylation, also implicating the involvement of CYP enzymes. Specific CYP inhibitors of CYP3A4 were essentially non-inhibitory to M1 formation. We can conclude therefore that OC144-093 is not extensively metabolised in human liver microsomes although conversion to its O-deethylated derivative does occur. Our data indicates that this conversion is not mediated by CYP3A4.
authors.affiliation: Dept of Advanced Therapeutics, BC Cancer Agency, Vancouver, Canada.
authors.forename: E S
authors.initials: ES
authors.lastname: Guns
authors.forename: P L
authors.initials: PL
authors.lastname: Bullock
authors.forename: M L
authors.initials: ML
authors.lastname: Reimer
authors.forename: R
authors.initials: R
authors.lastname: Dixon
authors.forename: M
authors.initials: M
authors.lastname: Bally
authors.forename: L D
authors.initials: LD
authors.lastname: Mayer
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000187:drug effects / Q000378:metabolism; D001189:Aryl Hydrocarbon Hydroxylases; D002851:Chromatography, High Pressure Liquid; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D018432:Drug Resistance, Multiple; D006801:Humans; D007093:Imidazoles / Q000378:metabolism / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000378:metabolism*; D008956:Models, Chemical; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D007093:Imidazoles; C410029:OC 144-093; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating