Cytochrome P450 mediated metabolism is the rate-limiting step of elimination for many drugs. CYP3A4 is the most abundant hepatic isoform and CYP3A4/5 metabolize the largest fraction of drugs. Pharmacogenetic studies have not been able to characterize population variability in CYP3A4 activity because few variant alleles associated with aberrant enzyme activity have been found. Substrate probes such as midazolam and testosterone have been utilized in-vivo and in-vitro to determine catalytic activity of these enzymes, but they suffer from several limitations. Eplerenone, an aldosterone antagonist, is also metabolized by CYP3A enzymes, and it has the potential to be an excellent substrate probe for CYP3A4/5. Eplerenone's primary metabolite, 6 beta-hydroxyeplerenone is formed preferentially via CYP3A4, however, the relative contribution of CYP3A5 to the 21-hydroxyeplerenone metabolite formation is unknown. Through in-vitro microsomal incubations with recombinant CYP3A4 and CYP3A5 enzymes, we identified their relative contributions to 21-hydroxyeplerenone metabolism. The 21-hydroxy metabolite is formed preferentially via CYP3A5 Vmax/KM (3.3) versus CYP3A4 Vmax/KM (1.9). Based on these findings, eplerenone has the potential to serve as an in-vivo substrate probe for CYP3A4 by monitoring 6-beta-hydroxy metabolite formation as well as CYP3A4/5 by monitoring 21-hydroxy metabolite formation.

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CYP3A4 和 CYP3A5 在依普利酮代谢中的相对作用

细胞色素 P450 介导的代谢是许多药物消除的限速步骤。CYP3A4 是最丰富的肝脏亚型，CYP3A4/5 代谢大部分药物。药物遗传学研究无法表征 CYP3A4 活性的群体变异性，因为已发现与异常酶活性相关的变异等位基因很少。咪达唑仑和睾酮等底物探针已在体内和体外用于确定这些酶的催化活性，但它们有几个局限性。依普利酮是一种醛固酮拮抗剂，也由 CYP3A 酶代谢，它有可能成为 CYP3A4/5 的极好底物探针。依普利酮的主要代谢产物 6 β-羟基依普利酮优先通过 CYP3A4 形成，然而，CYP3A5 对 21-羟基依普利酮代谢物形成的相对贡献尚不清楚。通过体外微粒体与重组 CYP3A4 和 CYP3A5 酶的孵育，我们确定了它们对 21-羟基依普利酮代谢的相对贡献。与 CYP3A4 Vmax/KM (1.9) 相比，21-羟基代谢物优先通过 CYP3A5 Vmax/KM (3.3) 形成。基于这些发现，依普利酮有可能通过监测 6-β-羟基代谢物的形成以及 CYP3A4/5 的 21-羟基代谢物的形成，作为 CYP3A4 的体内底物探针。与 CYP3A4 Vmax/KM (1.9) 相比，21-羟基代谢物优先通过 CYP3A5 Vmax/KM (3.3) 形成。基于这些发现，依普利酮有可能通过监测 6-β-羟基代谢物的形成以及 CYP3A4/5 的 21-羟基代谢物的形成，作为 CYP3A4 的体内底物探针。与 CYP3A4 Vmax/KM (1.9) 相比，21-羟基代谢物优先通过 CYP3A5 Vmax/KM (3.3) 形成。基于这些发现，依普利酮有可能通过监测 6-β-羟基代谢物的形成以及 CYP3A4/5 的 21-羟基代谢物的形成，作为 CYP3A4 的体内底物探针。