The role of membrane transporters in the pharmacokinetics of psychotropic drugs: in vitro studies with special focus on organic cation transporters

The treatment of common psychiatric disorders like major depression, schizophrenia and bipolar disorder is characterized by low efficacy and variability in the case of depression and bipolar disorder, and of undesirable side effects in the case of schizophrenia. One of the explanations is that the drug may not be reaching its site of action, at concentrations that are high enough to provoke a response. On the other hand, poor elimination of the drug from the body may lead to high plasma concentrations, which may cause undesirable side effects. Variations in membrane transport at the blood-brain barrier might affect the concentra- tion of psychotropic drugs at their site of action. In organs such as the liver and kidney, variations in membrane transport may affect drug elimination. Using a parallel artificial membrane assay, 31 commonly used psychotropic drugs were screened for their ability to penetrate cell membranes by passive diffusion. Using custom made TaqMan® low-density gene expression arrays, the mRNA expression of 90 drug transporters was analyzed in organs relevant for drug pharmacokinetics and in human primary brain cells. HEK293 cells overexpressing organic cation transporters were used to study the transporter-mediated cellular uptake of psychotropic drugs. Finally, the immortalized human brain microvascular endothelial cell line, hCMEC/D3, was used as a blood-brain barrier model to study influx transport. In human primary brain microvascular endothelial cells (HBMECs), the expression of organic cation transporters was substantially lower than in other organs like the liver and the kidney. Nonetheless organic cation transporters were detected in HBMECs. OCTN2 was the organic cation transporter with the highest expression, followed by OCTN1, OCT1 and OCT3. Amisulpride, sulpiride, sultopride and tiapride were identified as drugs with low mem- brane permeability, which may require influx transport to reach their site of action in the brain. Amisulpride and sulpiride were identified in vitro as substrates of the organic caiton trasnporters of the SLC22 family and may depend on organic cation mediated transport to cross the blood-brain barrier. The presence of a carrier-mediated trans- port mechanism for the uptake of amisulpride and sulpiride was confirmed in the brain endothelial cell line model hCMEC/D3. ￼Furthermore, absorption and elimination of amisulpride and sulpiride may also depend on organic cation transporters. OCT1 may contribute to the billiary elimination of amisulpride and sulpiride. In addition, the transporters OCT2, MATE1 and MATE2-K may contribute for the renal elimination of amisulpride and sulpiride in the proximal tubule epithelium. Common genetic polymorphisms on the OCT1 gene were found to affect the cellular uptake of amisulpride and sulpiride. The majority of the psychotropic drugs, like amitriptyline, have high membrane perme- ability and may not benefit from drug transporters to permeate cellular barriers in the in vitro models used in this work. However, these drugs can still interact strongly with membrane transporters, like OCT1. Clinical studies, providing in vivo evidence for the interaction of high permeability drugs with membrane transporters, will be needed in the future. Weak basic psychotropic drugs may inhibit the OCT1-mediated uptake of other important drugs, like morphine. The psychotropic drugs amitriptyline, clomipramine, imipramine and fluoxetine, and also irinotecan, ondansetron and verapamil, inhibited the OCT1-mediated uptake of morphine at therapeutically relevant concentrations. Furthermore, the effect of genetic polymorphisms in the OCT1 gene on the OCT1- mediated uptake of the biogenic amine tyramine was studied. In addition, an MDCK II cell line carrying a site for targeted chromosomal gene integration was developed. This model should in the future enable the analysis of the effects of genetic polymorphisms on drug transport by efflux transporters, which are present at the blood-brain barrier. In conclusion, this study demonstrates that influx transporters may mediate the uptake of psychotropic drugs with low membrane permeability like amisulpride and sulpiride, and may influence their pharmacokinetics and distribution to the brain. This work, and the tools which were developed here, can serve as a basis for further work on the role of organic cation transporters at the blood-brain barrier, and to study in more detail the role of organic cation transporters in the pharmacokinetics of amisulpride and sulpiride.