Huang Z, Roy P, Waxman D J
Division of Cell Biology, Department of Biology, Boston University, Boston, MA, USA.
Biochem Pharmacol. 2000 Apr 15;59(8):961-72. doi: 10.1016/s0006-2952(99)00410-4.
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.
抗癌烷化剂环磷酰胺（CPA）和异环磷酰胺（IFA）是前体药物，它们经过广泛的细胞色素P450催化代谢，产生活性（4-羟基化）代谢物和治疗上无活性但具有神经毒性的（N-去氯乙基化）代谢物。虽然CPA和IFA 4-羟基化的人肝微粒体细胞色素P450催化剂已得到充分表征，但替代的N-去氯乙基化途径的细胞色素P450酶催化剂却定义不明确。在杆状病毒表达系统（“超微粒体”）中对一组15种人细胞色素P450 cDNA进行分析表明，CYP3A4对CPA和IFA均表现出最高的N-去氯乙基化活性，而CYP2B6对IFA表现出高N-去氯乙基化活性，但对CPA则无此活性。使用最近描述的相对底物活性因子方法计算了每个人细胞色素P450对整体肝微粒体N-去氯乙基化的贡献，发现其与使用CYP3A抑制剂三乙酰竹桃霉素和针对CYP2B6的抑制性单克隆抗体进行的抑制研究结果高度一致。以CPA为底物时，CYP3A4被证明催化≥95%的肝微粒体N-去氯乙基化，而以IFA为底物时，CYP3A4平均催化约70%的肝微粒体N-去氯乙基化（范围为40 - 90%），其余活性由CYP2B6催化（范围为10 - 70%，取决于肝脏中CYP2B6的含量）。由于CYP2B6可对人肝微粒体IFA的N-去氯乙基化产生显著贡献，但对IFA的4-羟基化贡献较小，因此在肝脏CYP2B6含量高的个体中选择性抑制肝脏CYP2B6活性可能是一种有用的方法，可将治疗上无活性但有毒的N-去氯乙基化IFA代谢物的形成降至最低。
