Three-dimensional culture of human proximal tubular epithelial cells for an in vitro evaluation of drug-induced kidney injury.

Drug-induced kidney injury (DIKI) is the major cause of acute kidney injury (AKI). Renal proximal tubular epithelial cells (RPTECs) are the primary target sites of DIKI and express transporters involved in renal drug disposition. In the present study, we focused on three-dimensionally cultured human RPTECs (3D-RPTECs) with elevated expression of drug transporters to investigate their utility in DIKI evaluation. Intracellular ATP levels in 3D-RPTECs are reduced by tenofovir and cisplatin that are substrates of an organic anion transporter 1 and an organic cation transporter 2, respectively. In addition, 3D-RPTECs were exposed to 17 and 15 drugs that are positive and negative to RPTEC toxicity, respectively, for up to 28 d. The 20% decreasing concentration of drugs for ATP amount (EC20) was obtained, and the ratio of EC20 values and clinical maximum concentration (Cmax) ≤100 were used as cut-off value to evaluate potential of DIKI. The sensitivities of 3D-RPTECs were 82.4% and 88.2% after 7 d and 28 d of drug exposure, respectively, and the specificities were 100% and 93.3%, respectively. The predictive performance of 3D-RPTECs was higher than that of two-dimensional cultured RPTECs and the kidney cell line HK-2. In conclusion, 3D-RPTECs are useful for in vitro evaluation of RPTEC injury by measuring intracellular ATP levels.

Improvement of Protein Expression Profile in Three-Dimensional Renal Proximal Tubular Epithelial Cell Spheroids Selected Based on OAT1 Gene Expression: A Potential In Vitro Tool for Evaluating Human Renal Proximal Tubular Toxicity and Drug Disposition.

The proximal tubule plays an important role in the kidney and is a major site of drug interaction and toxicity. Analysis of kidney toxicity via in vitro assays is challenging, because only a few assays that reflect functions of drug transporters in renal proximal tubular epithelial cells (RPTECs) are available. In this study, we aimed to develop a simple and reproducible method for culturing RPTECs by monitoring organic anion transporter 1 (OAT1) as a selection marker. Culturing RPTECs in spherical cellular aggregates increased OAT1 protein expression, which was low in the conventional two-dimensional (2D) culture, to a level similar to that in human renal cortices. By proteome analysis, it was revealed that the expression of representative two proximal tubule markers was maintained and 3D spheroid culture improved the protein expression of approximately 7% of the 139 transporter proteins detected, and the expression of 2.3% of the 4,800 proteins detected increased by approximately fivefold that in human renal cortices. Furthermore, the expression levels of approximately 4,800 proteins in three-dimensional (3D) RPTEC spheroids (for 12 days) were maintained for over 20 days. Cisplatin and adefovir exhibited transporter-dependent ATP decreases in 3D RPTEC spheroids. These results indicate that the 3D RPTEC spheroids developed by monitoring OAT1 gene expression are a simple and reproducible in vitro experimental system with improved gene and protein expressions compared with 2D RPTECs and were more similar to that in human kidney cortices. Therefore, it can potentially be used for evaluating human renal proximal tubular toxicity and drug disposition. SIGNIFICANCE STATEMENT: This study developed a simple and reproducible spheroidal culture method with acceptable throughput using commercially available RPTECs by monitoring OAT1 gene expression. RPTECs cultured using this new method showed improved mRNA/protein expression profiles to those in 2D RPTECs and were more similar to those of human kidney cortices. This study provides a potential in vitro proximal tubule system for pharmacokinetic and toxicological evaluations during drug development.

Characterization of Aripiprazole Uptake Transporter in the Blood-Brain Barrier Model hCMEC/D3 Cells by Targeted siRNA Screening.

AIM: Identification of blood-brain barrier (BBB) uptake transporters is a major challenge in the research and development of central nervous system (CNS) drugs. However, conventional methods that consider known drug uptake characteristics have failed at identifying the responsible transporter molecule. The present study aimed at identifying aripiprazole uptake transporters in BBB model hCMEC/D3 cells using a knockdown screening study targeting various transporters, including uncharacterized ones. METHODS: We evaluated the effect of 214 types of siRNA targeting transporters on the uptake of aripiprazole, an atypical antipsychotic drug, in hCMEC/D3 cells. Aripiprazole uptake was determined using Xenopus oocytes expressing the candidate genes extracted from the siRNA screening assay. RESULTS: The estimated unbound brain to plasma concentration ratio (Kp,uu,brain) of aripiprazole was estimated as 0.67 in wild-type mice and 1.94 in abcb1a/1b/abcg2 knockout mice, suggesting the involvement of both uptake and efflux transporters in BBB permeation. According to siRNA knockdown screening studies, organic cation/carnitine transporter 2 (OCTN2) and long-chain fatty acid transporter 1 (FATP1) were identified as candidate genes. The uptake of aripiprazole by hCMEC/D3 cells was decreased by OCTN2 inhibitors, but not by FATP1 inhibitors. A partially increased uptake of aripiprazole was observed in OCTN2-expressing Xenopus oocytes. Finally, to evaluate transporter-mediated BBB permeation of drugs, the reported and estimated Kp,uu,brain values were summarized. CONCLUSIONS: A knockdown screening study in combination with Kp,uu,brain values showed that aripiprazole was a potential substrate of OCTN2. The technique described in this study can be applied to identifying novel BBB transporters for CNS drugs.