RIFAMPIN ON HEPATIC CYP3A ACTIVITY IN HEALTHY VOLUNTEERS

肝脏在人体与其化学环境的相互作用中起着重要作用。人类中这种相互作用的一个主要位点是肝微粒体血红蛋白P450 （CYP3A4）。CYP3A4催化并在许多情况下可能限制超过70%的临床使用药物的代谢，以及许多环境污染物、致癌物和内源性底物如类固醇激素。人类肝脏中的CYP3A酶，最初是由著名生物学家汉斯·塞尔耶（Hans Selye）的开创性努力发现的，他在大鼠肝脏中发现了一种可诱导的活性，这种活性似乎通过生理或药理学手段对处于慢性应激条件下的动物起保护作用。事实上，从来没有发现一个人缺乏CYP3A活性，这一发现表明这种酶在肝脏代谢中起着至关重要的作用。从人类肝脏活检和非侵入性技术中测量CYP3A的数量表明，患者之间的差异很大，约为10倍。地塞米松、利福平等临床相关药物确实可以诱导人肝脏中CYP3A的含量。然而，没有关于正常人群中CYP3A诱导的频率或强度的信息。较早的文献推断，一些人可能缺乏对利福平的反应能力，CYP3A活性增加。我们的假设是，肝脏缺乏CYP3A的诱导性可能导致“应激相关”疾病，因为慢性应激条件下产生的糖皮质激素和其他内源性产物无法通过代谢消除。对其产物与有毒化学物质的激活或失活密切相关的基因家族的遗传多态性信息的迅速发展，强调了开发易感性个体间差异信息的重要性。尽管CYP3A酶在肝脏中的水平表现出广泛的个体间差异，但尚未证实其表达多态性的遗传成分。对于评估该基因的推定调控区域的临床重要性，迫切需要的是对其药物介导的诱导性过高或过低表达的人类进行表型鉴定。在本研究中，我们提出：1)使用无创的肝脏CYP3A活性测量方法，特别是红霉素呼吸试验，测定正常人类志愿者在基础稳定状态和口服利福平诱导后的CYP3A的量；2)通过测量利福平治疗前后一周的半衰期变化来证实这些志愿者中CYP3A的诱导作用。3)分析从CYP3A4活性表型个体中分离的DNA，以便将特定CYP3A4基因型与特定CYP3A4表型联系起来。

The liver plays an important role in the interaction between man and his chemical environment. A major locus of this interaction in human beings is the liver microsomal hemoprotein cytochrome P450 (CYP3A4). CYP3A4 catalyzes and in many instances may limit the metabolism of over 70% of the clinically used drugs as well as numerous environmental pollutants, carcinogens, and such endogenous substrates as steroid hormones. The CYP3A enzyme in human liver, originally was identified from the pioneering efforts of the celebrated biologist, Hans Selye who identified an inducible activity in rat liver that appeared to play a protective role in animals placed under conditions of chronic stress through physiologic or pharmacologic means. Indeed, there has never been a human identified who lacks CYP3A activity, a finding that suggests that this enzyme plays some essential role in liver metabolism. Measurements of the amounts of CYP3A from human liver biopsies as well as from non-invasive techniques indicate a wide variation among patients about ten-fold. The amounts of CYP3A in human liver are indeed inducible by such clinically relevant drugs as dexamethasone and the antibiotic rifampin. However, there is no information available on the frequency or magnitude of induction of CYP3A among populations of normal humans. Older literature contains inferences that some people may lack the ability to respond to rifampin with an increase in CYP3A activity. Our hypothesis is that lack of inducibility of CYP3A in the liver may contribute to "stress-related" diseases due to a failure to metabolically eliminate glucocorticoids and other endogenous products generated under conditions of chronic stress. Rapidly developing information of genetic polymorphisms for families of genes whose products are intimately involved in the activation or inactivation of toxic chemicals emphasize the importance of developing information on interindividual differences in susceptibility. Although the CYP3A enzymes exhibit a wide range of interindividual variability in their levels in liver, no genetic component responsible for polymorphism in their expression has been confirmed. Critically needed for evaluation of the clinical importance of putative regulatory regions of this gene is phenotypic identification of humans who are either over or under expressers with regards to its drug mediated inducibility. In this study we propose to: 1) use a non-invasive measure of liver CYP3A activity, specifically the erythromycin breath test, to determine the amounts of CYP3A in the basal steady state and after induction by orally administered rifampin in normal human volunteers, 2) provide a confirmation of CYP3A induction in these volunteers by measuring changes in rifampin half-life before and after one week of treatment with rifampin and 3) analyze DNA isolated from individuals phenotyped for CYP3A4 activity in order to associate a specific CYP3A4 genotype with a specific CYP3A4 phenotype.