IMPORTANCE OF EVALUATING CONTRIBUTIONS OF TRANSPORTERS IN DRUG DEVELOPMENT: AN APPROACH USING SELECTIVE INHIBITORS

Abstract

Recently, molecular cloning methodologies have enabled us to clone the cDNA of transporters, and characterize the transport activity using cDNA transfectants. It is necessary to examine in future studies the contribution of each transporter to the overall disposition of some drugs. The contribution of a transporter can be evaluated using selective inhibitors. In this study, we focused on organic anion transporters such as organic anion transporting polypeptides (OATPs; rOatp1 ?? 3) and organic anion transporters (OATs; rOat1 ?? 3). Stable cDNA transfectants have been established using LLC-PK1 cells as host and inhibition studies for the uptake of their typical ligands were carried out. Digoxin is a specific inhibitor for rOatp2, while cimetidine and benzylpenicillin (PCG) are selective inhibitors for rOat3. Taurocholate has similar affinity for rOatp1 ?? 3, but quite low affinity for rOat1 ?? 3. p-Aminohippurate (PAH) is a inhibitor for rOat1 and rOat3, but not for rOat2 (Ki>1 mM) and rOatp1 ?? 3. Using these inhibitors, we have evaluated the contribution of rOatp2 on the blood brain barrier, and that of rOat3 to the uptake of PCG and PAH, and pravastatin into the choroid plexus and kidney, respectively, has been evaluated. Estradiol 17β glucuronide is rapidly eliminated from the brain (elimination half life is approximately 20 min). By examining inhibitory effect of digoxin, the contribution of rOatp2 has been estimated at most 40% and additional uptake system is present on the blood-brain barrier. The uptake of PCG and PAH, and pravastatin into the choroid plexus and kidney is mainly accounted for by rOat3.