Division of Pharmaceutical Sciences, James L. Winkle College of Pharmacy, University of Cincinnati Medical Center, 3223 Eden Ave., Cincinnati, OH 45267-0004, USA.
Drug Metab Dispos. 2012 Feb;40(2):389-96. doi: 10.1124/dmd.111.039388. Epub 2011 Nov 17.
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 CYP3A53/3 alleles was approximately 3-fold lower than that for HLM expressing CYP3A51/*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.
他莫昔芬是一种用于预防和治疗乳腺癌的抗雌激素药物，广泛被细胞色素 P450 酶代谢。它向 α-羟他莫昔芬（α-OHT）的生物转化，可能具有遗传毒性，和 N-去甲基他莫昔芬（N-DMT），部分羟化为 4-羟基-N-DMT（依西美坦），一种有效的抗雌激素，是由 CYP3A 酶介导的。然而，CYP3A5 的潜在贡献及其低表达变体对这些代谢物形成的影响尚不清楚。因此，我们通过使用重组 CYP3A4 和 CYP3A5 以及 CYP3A5 变体基因分型的人肝微粒体（HLM）评估了 CYP3A4 和 CYP3A5 的贡献，并检查了 CYP3A5 基因型对 α-OHT 和 N-DMT 形成的影响。我们观察到，用重组 CYP3A4 形成 α-OHT 的催化效率（内在清除率[CL(int)）比用重组 CYP3A5 高 5 倍（0.81 比 0.16 nl·min⁻¹·pmol 细胞色素 P450⁻¹）。在三种 CYP3A5 基因分型的 HLM（*1/*1、*1/3 和3/3）之间，CL(int) 值没有显著差异。对于 N-DMT 的形成，用重组 CYP3A4 形成的 CL(int) 仅比用重组 CYP3A5 高 1.7 倍。此外，CYP3A53/3 等位基因的 HLM 形成 N-DMT 的 CL(int) 约为表达 CYP3A51/*1 的 HLM 的 3 倍低。用睾酮 6β-羟化作用对他莫昔芬代谢的回归分析有助于评估 CYP3A5 对两种代谢物形成的贡献。CYP3A5 对 α-OHT 形成的贡献可以忽略不计，而对 N-DMT 形成的贡献范围为 51%至 61%。我们的研究结果表明，多态性 CYP3A5 表达可能影响 N-DMT 的形成，但不影响 α-OHT 的形成。
