Analysis of the factors governing the elimination route of therapeutic agents

体内的治疗药物通常通过肝脏和肾脏的新陈代谢和/或排泄来消除。药物的消除途径(肝脏或肾脏)被认为主要取决于药物的物理化学性质。然而，分子生物学的最新进展表明，许多类型的药物转运蛋白在两个器官中都有表达，并参与药物的处置。因此，本研究着眼于这些转运蛋白在体内的功能，以确定药物消除的途径。我们建立了Ntcp和Oatp1的基因转移系统，并分析了它们的底物特异性。许多类型的治疗药物被鉴定为Oatp1的底物，而Ntcp在胆汁酸中具有狭义的底物特异性。通过比较肝细胞和这些转染体之间的转运活性，我们估计了Oatp1对肝细胞对每种底物的总摄取的贡献率。在这项研究中，我们还发现了在肝脏中表达的新克隆OAT3和OAT4。检测肝脏基底膜上表达的MRP3的底物特异性。我们发现，在分离的大鼠肝脏灌流系统中，MRP3接受胆汁酸作为底物，并参与胆汁酸从肝细胞中的外流。胆汁淤积和苯巴比妥治疗导致MRP3的过度表达，这至少部分来自于mRNA水平的增加。由于MRP3接受许多类型的有机阴离子作为底物，这些发现意味着特定的转运系统应该被认为参与了药物从肝脏到血液的外流。

Therapeutic agents administered to the body are generally eliminated by the metabolism and/or excretion both in the liver and kidney. The elimination route (liver or kidney) of the drugs has been believed to mainly depend on the physicochemical properties of the drugs. However, the recent advance in the molecular biology revealed that many types of drug transporters are expressed in both organs and involved in drug disposition. Therefore, the present study focused on the function of such transporters in vivo to determine the drug elimination route. We have established the gene transfectant systems both for Ntcp and Oatp1 and analyzed their substrate specificity. Many types of therapeutic agents were identified as substrates of Oatp1 whereas Ntcp has the narrow substrate specificity within the bile acids. By comparing the transport activity between the hepatocytes and such transfectants, we have estimated the contribution ratio of Oatp1 to the overall uptake of each substrate by hepatocytes. In this study we also identified new clones, Oat3 and Oat4, which are expressed in the liver. Substrate specificity of Mrp3 which is expressed on the basolateral membrane of the liver was examined. We found that Mrp3 accepts bile acids as substrates and is involved in their efflux from the hepatocytes in the isolated rat liver perfusion system. Cholestatic condition as well as phenobarbital treatment results in the overexpression of Mrp3 which come from at least partially the increase in mRNA level. Since Mrp3 accept many types of organic anions as substrates, these findings imply that the specific transport systems should be considered to be involved in the efflux of drugs from the liver to blood.