Wang R W, Liu L, Cheng H
Department of Drug Metabolism, Merck Research Laboratories, Rahway, New Jersey 07065, USA.
Drug Metab Dispos. 1996 Jul;24(7):786-91.
Cyclobenzaprine (Flexeril) is a muscle relaxant, possessing a tricyclic structure. Numerous therapeutic agents containing this structure are known to be metabolized by polymorphic cytochrome P4502D6. The aim of this study was to determine if cytochrome P4502D6 and other isoforms are involved in the metabolism of cyclobenzaprine in human liver microsomes. Selective cytochrome P450 inhibitors for CYP1A1/2 (furafylline and 7,8-benzoflavone) and CYP3A4 (troleandomycin, gestodene, and ketoconazole) inhibited the formation of desmethylcyclobenzaprine, a major metabolite of cyclobenzaprine, in human liver microsomes. Antibodies directed against CYP1A1/2 and CYP3A4 inhibited the demethylation reaction whereas anti-human CYP2C9/10, CYP2C19, and CYP2E1 antibodies did not show any inhibitory effects. When a panel of microsomes prepared from human B-lymphoblastoid cells that expressed specific human cytochrome P450 isoforms were used, only microsomes containing cytochromes P4501A2, 2D6, and 3A4 catalyzed N-demethylation. In addition, demethylation catalyzed by these recombinant cytochromes P450 can be completely inhibited with selective inhibitors at concentrations as low as 1 to 20 microM. Interestingly, cyclobenzaprine N-demethylation was significantly correlated with caffeine 3-demethylation (1A2) and testosterone 6 beta-hydroxylation (3A4) but not with dextromethorphan O-demethylation (2D6) in human liver microsomes. To further determine the involvement of cytochrome P4502D6 in cyclobenzaprine metabolism, liver microsomes from a human that lacked CYP2D6 enzyme activities was included in this study. The data showed that cyclobenzaprine N-demethylation still occurred in the incubation with this microsome. These results suggested that cytochrome P4502D6 plays only a minor role in cyclobenzaprine N-demethylation whereas 3A4 and 1A2 are primarily responsible for cyclobenzaprine metabolism in human liver microsomes. Due to the minimum involvement of CYP2D6 in the vitro metabolism of cyclobenzaprine, the polymorphism of cytochrome P4502D6 in man should not be of muci concern in the clinical use of cyclobenzaprine.
环苯扎林（Flexeril）是一种肌肉松弛剂，具有三环结构。已知许多含有这种结构的治疗药物可被多态性细胞色素P4502D6代谢。本研究的目的是确定细胞色素P4502D6和其他同工酶是否参与环苯扎林在人肝微粒体中的代谢。CYP1A1/2（呋拉茶碱和7,8-苯并黄酮）和CYP3A4（醋竹桃霉素、孕二烯酮和酮康唑）的选择性细胞色素P450抑制剂抑制了人肝微粒体中环苯扎林的主要代谢产物去甲基环苯扎林的形成。针对CYP1A1/2和CYP3A4的抗体抑制了去甲基化反应，而抗人CYP2C9/10、CYP2C19和CYP2E1抗体未显示任何抑制作用。当使用一组从表达特定人细胞色素P450同工酶的人B淋巴细胞样细胞制备的微粒体时，只有含有细胞色素P4501A2、2D6和3A4的微粒体催化N-去甲基化。此外，这些重组细胞色素P450催化的去甲基化可以用低至1至20微摩尔浓度的选择性抑制剂完全抑制。有趣的是，在人肝微粒体中，环苯扎林N-去甲基化与咖啡因3-去甲基化（1A2）和睾酮6β-羟基化（3A4）显著相关，但与右美沙芬O-去甲基化（2D6）无关。为了进一步确定细胞色素P4502D6在环苯扎林代谢中的作用，本研究纳入了一名缺乏CYP2D6酶活性的人的肝微粒体。数据显示，在与该微粒体的孵育中，环苯扎林N-去甲基化仍然发生。这些结果表明，细胞色素P4502D6在环苯扎林N-去甲基化中仅起次要作用，而3A4和1A2是人肝微粒体中环苯扎林代谢的主要原因。由于CYP2D6在环苯扎林体外代谢中的参与度最低，细胞色素P4502D6在人体内的多态性在环苯扎林的临床应用中不应引起太多关注。
