title: Assessment of the impact of CYP3A polymorphisms on the formation of α-hydroxytamoxifen and N-desmethyltamoxifen in human liver microsomes.
abstract: Tamoxifen, an antiestrogen used in the prevention and treatment of breast cancer, is extensively metabolized by cytochrome P450 enzymes. Its biotransformation to α-hydroxytamoxifen (α-OHT), which may be genotoxic, and to N-desmethyltamoxifen (N-DMT), which is partially hydroxylated to 4-hydroxy-N-DMT (endoxifen), a potent antiestrogen, is mediated by CYP3A enzymes. However, the potential contribution of CYP3A5 and the impact of its low-expression variants on the formation of these metabolites are not clear. Therefore, we assessed the contributions of CYP3A4 and CYP3A5 and examined the impact of CYP3A5 genotypes on the formation of α-OHT and N-DMT, by using recombinant CYP3A4 and CYP3A5 and human liver microsomes (HLM) genotyped for CYP3A5 variants. We observed that the catalytic efficiency [intrinsic clearance (CL(int))] for α-OHT formation with recombinant CYP3A4 was 5-fold higher than that with recombinant CYP3A5 (0.81 versus 0.16 nl · min⁻¹ · pmol cytochrome P450⁻¹). There was no significant difference in CL(int) values between the three CYP3A5-genotyped HLM (*1/*1, *1/*3, and *3/*3). For N-DMT formation, the CL(int) with recombinant CYP3A4 was only 1.7-fold higher, relative to that with recombinant CYP3A5. In addition, the CL(int) for N-DMT formation by HLM with CYP3A5*3/*3 alleles was approximately 3-fold lower than that for HLM expressing CYP3A5*1/*1. Regression analyses of tamoxifen metabolism with respect to testosterone 6β-hydroxylation facilitated assessment of CYP3A5 contributions to the formation of the two metabolites. The CYP3A5 contributions to α-OHT formation were negligible, whereas the contributions to N-DMT formation ranged from 51 to 61%. Our findings suggest that polymorphic CYP3A5 expression may affect the formation of N-DMT but not that of α-OHT.
authors.affiliation: Division of Pharmaceutical Sciences, James L. Winkle College of Pharmacy, University of Cincinnati Medical Center, 3223 Eden Ave., Cincinnati, OH 45267-0004, USA.
authors.forename: Ganesh M
authors.initials: GM
authors.lastname: Mugundu
authors.forename: Larry
authors.initials: L
authors.lastname: Sallans
authors.forename: Yingying
authors.initials: Y
authors.lastname: Guo
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Shaughnessy
authors.forename: Pankaj B
authors.initials: PB
authors.lastname: Desai
mesh_terms: D000483:Alleles; D018931:Antineoplastic Agents, Hormonal / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D006900:Hydroxylation; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D011110:Polymorphism, Genetic; D011994:Recombinant Proteins / Q000378:metabolism; D020845:Selective Estrogen Receptor Modulators / Q000378:metabolism; D013379:Substrate Specificity; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D018931:Antineoplastic Agents, Hormonal; D011994:Recombinant Proteins; D020845:Selective Estrogen Receptor Modulators; C110902:alpha-hydroxytamoxifen; D013629:Tamoxifen; C510164:CYP3A5 protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; C028787:N-desmethyltamoxifen