Interaction and transport characteristics of mycophenolic acid and its glucuronide via human organic anion transporters hOAT1 and hOAT3
Introduction
An immunosuppressive agent, mycophenolate mofetil (MMF) is commonly prescribed with the calcineurin inhibitor tacrolimus or cyclosporin A and corticosteroids for patients who have received a solid organ transplantation. After orally administered and absorbed, MMF is converted to an active metabolite, mycophenolic acid (MPA; Fig. 1) by serum esterases. MPA is mainly excreted into urine after being metabolized to 7-O-MPA-glucuronide (MPAG) by the hepatic uridine diphosphate-glucuronosyltransferases [1], [2]. The genetic variants of uridine diphosphate-glucuronosyltransferases contribute to the extensive variability in the pharmacokinetics of the immunosuppressant [3], [4]. Furthermore, the enterohepatic circulation of MPAG/MPA exists, and tubular secretion as well as glomerular filtration is responsible for the urinary excretion of MPAG [1]. Thus, the process by which MMF is eliminated, is very intricate.
Besides immunosuppressive agents, many drugs are given to recipients of organ transplantations, to prevent or treat infections, gastrointestinal ulcers, thrombus, ascites, pleural fluid, hypertension, diabetes, osteoporosis, gout, bronchitis and so on. Accordingly, side effects caused by drug interactions often occur. In addition to tacrolimus and cyclosporin A, MMF was shown to interact with various drugs. For instance, MMF reduced the renal clearance of acyclovir and ganciclovir, and the pharmacokinetics of MPA or MPAG were affected by cyclosporin A, glucocorticoids and non-steroidal anti-inflammatory drugs [1], [5], [6], [7], [8]. Taking these MPA and/or MPAG-mediated drug interactions into account, together with the complexity of the fate of MMF, it is important to identify the drug-metabolizing enzymes and drug transporters interacting with this immunosuppressive agent for a successful organ transplantation.
The human organic anion transporters (hOATs) mediate transport of clinically important drugs, such as diuretics, antibiotics, antivirals, histamine H2 receptor antagonists, non-steroidal anti-inflammatory drugs and so on [9], [10]. Among the family, hOAT1 (SLC22A6) and hOAT3 (SLC22A8) were shown to be predominantly expressed in the basolateral membrane of the renal proximal tubules [11], suggesting that they play main roles in the renal tubular uptake of organic compounds from blood. Furthermore, as previously reported [12], it is possible that hOAT1 and hOAT3 are targets of the interaction between methotrexate and non-steroidal anti-inflammatory drugs.
This background suggests that the renal organic anion transporters are concerned with the renal excretion of the metabolites of MMF and drug interaction with them, but to our knowledge, no report has examined the interaction of MPA and MPAG with the renal organic anion transporters at a molecular level. In the present study, the inhibitory effects of MPA and MPAG on hOAT1 and hOAT3 were assessed. In addition, the contribution of hOAT1 and hOAT3 to the renal tubular secretion of MPA and MPAG was investigated.
Section snippets
Materials
p-[glycyl-1-14C]Aminohippurate (1.9 GBq/mmol) and [6,7-3H(N)]estrone sulfate, ammonium salt (2.1 TBq/mmol) were obtained from NEN™ Life Science Products Inc. (Boston, MA, USA) and Perkin-Elmer Life Sciences Inc. (Boston, MA, USA), respectively. MPA and its glucuronide were from Wako Pure Chemical Industries (Osaka, Japan) and Analytical Services International Ltd. (London, UK), respectively. Tacrolimus and cyclosporin A were kindly supplied by Fujisawa Pharmaceutical (Newly, Astellas Pharma Inc.,
Inhibitory effects of MPA and MPAG on hOAT1 and hOAT3
First, to examine whether MPA and MPAG interact with hOAT1 and hOAT3, the effects of MPA and MPAG on the time-dependent uptake of p-[14C]aminohippurate by HEK-hOAT1 and of [3H]estrone sulfate by HEK-hOAT3 were investigated. As shown in Fig. 2A, the amount of p-[14C]aminohippurate taken up by HEK-hOAT1 increased linearly for 2 min. MPA at 300 μM completely inhibited the hOAT1-mediated transport of p-[14C]aminohippurate. MPAG at 300 μM also inhibited the uptake of p-[14C]aminohippurate by hOAT1, but
Discussion
In organ transplantation, many drugs, including immunosuppressants, antibiotics, antivirals, antifungals, diuretics, histamine H2 receptor antagonists, proton pump inhibitors, anticoagulants, bronchodilators and hypouricemic agents are administered to recipients. Accordingly, drugs should be prescribed with predictions of drug interactions [14]. To avoid adverse effects via drug interactions, information on the routes of elimination of a drug and its inhibitory effects on drug-metabolizing
Acknowledgements
This work was supported in part by a grant-in-aid for Research on Advanced Medical Technology from the Ministry of Health, Labor and Welfare of Japan, by a Japan Health Science Foundation “Research on Health Sciences Focusing on Drug Innovation”, by a grant-in-aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan and by the 21st Century COE program “Knowledge Information Infrastructure for Genome Science”.
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