OATP1A2 and OATP2B1 Are Interacting with Dopamine-Receptor Agonists and Antagonists.

Interaction with the dopaminergic system in the central nervous system is either therapeutically intended or it is a side effect. In both cases, dopamine-receptor agonists (DRA) like the ergoline derivative bromocriptine and dopamine-receptor antagonists (DRAn) like metoclopramide have to cross the blood-brain barrier (BBB). The organic anion transporting polypeptides (OATP) 1A2 and 2B1 are cellular uptake carriers for a variety of endogenous and xenobiotic compounds. As both transporters are expressed in endothelial cells of the BBB, the aim of the present study was to determine whether the DRA bromocriptine, cabergoline, and pergolide and the DRAn metoclopramide and domperidone are interacting with OATP1A2 and 2B1 and could therefore be candidate genes modifying wanted and unwanted effects of these drugs. Localization of both transporters in the brain was confirmed using LC-MS/MS and immunofluorescence stainings. For the functional studies, MDCKII cells stably expressing OATP1A2 or 2B1 were used. Initial interaction studies with the well-characterized transporter substrate estrone 3-sulfate revealed that all tested compounds except pergolide inhibit the transport function of both proteins with the most potent effect for bromocriptine (IC50 = 2.2 µM (OATP1A2) and IC50 = 2.5 µM (OATP2B1)). Further studies using the indirect competitive counterflow method identified bromocriptine, cabergoline, and domperidone as substrates of both transporters, whereas metoclopramide was only transported by OATP1A2. These findings were verified for domperidone by direct measurements using its tritium-labeled form as a tracer. Moreover, the transporter-mediated uptake of this compound was sensitive to the OATP1A2 and OATP2B1 inhibitor naringin. In conclusion, this study suggests that OATP1A2 and 2B1 may play a role in the uptake of DR agonists and antagonists into the brain.

Expression of Organic Anion Transporting Polypeptide 1A2 in Red Blood Cells and Its Potential Impact on Antimalarial Therapy.

Important antimalarial drugs, including quinolines, act against blood schizonts by interfering with hemoglobin metabolism. To reach their site of action, these compounds have to cross the plasma membrane of red blood cells (RBCs). Organic cation transporters (OCTs) and organic anion transporting polypeptides (OATPs) are important uptake transporters and interesting candidates for local drug transport. We therefore studied their interaction with antimalarial compounds (quinine, chloroquine, mefloquine, pyrimethamine, artemisinin, and artesunate) and characterized the expression of OATP1A2 and OATP2B1 in RBCs. Competition assays using transporter-overexpressing Madin-Darby canine kidney (MDCKII) cells and the model substrate estrone-3-sulfate identified quinine and chloroquine as potent inhibitors of OATP1A2 function (IC50 quinine: 0.7 ± 1.2 µM; chloroquine: 1.0 ± 1.5 µM), but no or only moderate effects were observed for OATP2B1. Subsequently, quinine was identified as a substrate of OATP1A2 (Km 23.4 µM). The OATP1A2-mediated uptake was sensitive to the OATP1A2-specific inhibitor naringin. Both OATPs were expressed in human RBCs, and ex vivo transport studies demonstrated naringin-sensitive accumulation of quinine in these cells (60 pmol versus 38 pmol/5 × 10(5) RBCs). Additional transport studies using OCT1-3 and organic cation transporter novel type 1 (OCTN1) indicated only significant quinine uptake by OCT1, which was not detected in RBCs. In conclusion, our data demonstrate expression of OATP2B1 and OATP1A2 in RBCs as well as OATP1A2-mediated uptake of quinine. Therefore, modulation of OATP1A2 function may affect quinine uptake into erythrocytes.