Characterization of a Novel Selective Cytochrome P450 3A5 Substrate

描述（由申请方提供）：人们普遍认为，当其中一种药物干扰参与药物代谢的关键酶（主要是细胞色素P450家族中的酶）的活性时，接受多种药物治疗的患者易发生药代动力学药物相互作用。在过去的十年中，FDA通常要求公司进行临床试验，以确定这种药物相互作用的结果。一些药物如酮康唑已被证明能抑制个别P450酶的活性;而其他药物如利福平或奥美拉唑则能诱导肝脏中P450表达水平升高。这些相互作用可能需要调整剂量，以避免药物浓度升高和潜在的毒性增加或药物水平不足。现有的体外工具可以合理地预测可能导致大多数常见的基于代谢的药物相互作用的化合物。然而，目前还没有工具可用于区分主要的CYP 3A酶，CYP 3A 4和CYP 3A 5。这两种酶参与的药物代谢比其他任何P450都多。CYP 3A 4/5活性的改变可能会产生严重的后果，并且通过该途径代谢的药物通常带有警告：不要与葡萄柚汁，酮康唑，圣约翰沃特和许多其他CYP 3A抑制剂或诱导剂一起服用。CYP 3A 5可能是处方药代谢中个体间和种族间差异的最重要遗传因素。具有至少一个活性CYP 3A 5 *1等位基因的个体表达大量的功能性CYP 3A 5，而CYP 3A 5 *3*3引起剪接变体，导致最小的CYP 3A 5表达。大约60%的非洲裔美国人和30%的高加索人至少有一个 *1等位基因，表达大致相等水平的CYP 3A 5和CYP 3A 4。在生物学相关样本中无法明确区分CYP 3A 4和CYP 3A 5活性。目前，几乎每种获批化合物的CYP 3A抑制数据均使用CYP 3A测定法生成，该测定法通过合并的人肝微粒体测量睾酮羟基化或咪达唑仑羟基化。虽然这两种底物均经CYP 3A 4和CYP 3A 5代谢，但所得抑制常数主要反映CYP 3A 4，因为肝微粒体的多个供体池中含有的CYP 3A 4约为CYP 3A 5的5 - 6倍。我们已经确定了CYP 3A 5的第一个高选择性底物，并建议建立一个选择性CYP 3A 5测定能够定量CYP 3A 5的活性和抑制在肝微粒体孵育。

DESCRIPTION (provided by applicant): It is widely accepted that patients on multiple medications are susceptible to pharmacokinetic drug-drug interactions when one of the drugs interferes with the activity of key enzymes involved in drug metabolism, chiefly enzymes in the cytochrome P450 family. In the past decade the FDA has commonly required companies to conduct clinical trials to determine the result of such drug-drug interactions. Some drugs such as ketoconazole have been shown to inhibit the activity of individual P450 enzymes; while other drugs such as rifampicin or omeprazole induce expression of higher P450 levels in the liver. These interactions can require dose adjustments to avoid having elevated drug concentrations and potentially increasing toxicities or having insufficient drug levels to be efficacious. Existing in vitro tools can reasonably predict compounds that are likely to cause most of the common metabolism-based drug-drug interactions. However, tools are not currently available to distinguish between the major CYP3A enzymes, CYP3A4 and CYP3A5. These two enzymes are implicated in the metabolism of more pharmaceuticals than any other P450s. Alteration of CYP3A4/5 activity can have serious consequences and drugs that are metabolized through this pathway often come with warnings: not to take with grapefruit juice, ketoconazole, St. Johns Wart, and numerous other CYP3A inhibitors or inducers. CYP3A5 may be the most important genetic contributor to interindividual and interracial differences in the metabolism of prescription drugs. Individuals with at least one gene for the active CYP3A5*1 allele express large amounts of functional CYP3A5, whereas CYP3A5*3*3 causes a splice variant that results in minimal CYP3A5 expression. Approximately 60% of African Americans and 30% of Caucasians have at least one *1 allele and express roughly equivalent levels of CYP3A5 and CYP3A4. CYP3A4 and CYP3A5 activity cannot be clearly differentiated in biologically relevant samples. Currently, the data for CYP3A inhibition of almost every approved compound is generated using a CYP3A assay measuring testosterone hydroxylation or midazolam hydroxylation by pooled human liver microsomes. While both of these substrates are metabolized by CYP3A4 and CYP3A5, the resulting inhibition constants primarily reflect CYP3A4 because multiple donor pools of hepatic microsomes contain approximately five to six times more CYP3A4 than CYP3A5. We have identified the first highly selective substrate of CYP3A5 and propose to build a selective CYP3A5 assay capable of quantifying CYP3A5 activity and inhibition in hepatic microsomal incubations.