In vitro evaluation of P-glycoprotein (P-gp) inhibitory potential is now a regulatory issue during drug development, in order to predict clinical inhibition of P-gp and subsequent drug–drug interactions. Assays for this purpose, commonly based on P-gp-expressing cell lines and digoxin as a reference P-gp substrate probe, unfortunately exhibit high variability, raising thus the question of developing alternative or complementary tests for measuring inhibition of P-gp activity. In this context, the present study was designed to investigate the use of the fluorescent dye rhodamine 123 as a reference P-gp substrate probe for characterizing P-gp inhibitory potential of 16 structurally-unrelated drugs known to interact with P-gp. 14/16 of these P-gp inhibitors were found to increase rhodamine 123 accumulation in P-gp-overexpressing MCF7R cells, thus allowing the determination of their P-gp inhibitory potential, i.e., their half maximal inhibitor concentration (IC₅₀) value towards P-gp-mediated transport of the dye. These IC₅₀ values were in the range of variability of previously reported IC₅₀ for P-gp and can be used for the prediction of clinical P-gp inhibition according to Food and Drug Administration (FDA) criteria, with notable sensitivity (80%). Therefore, the data demonstrated the feasibility of the use of rhodamine 123 for evaluating the P-gp inhibitory potential of drugs.

Primary hepatocyte cultures are considered as a useful in vitro system for pharmacological/toxicological studies. Although expression of drug-metabolizing enzymes and canalicular drug transporters has been well documented in this cellular model, less information is available about sinusoidal drug transporter activities. This has led us to investigate functional expression of the major sinusoidal transporters in primary human and rat hepatocytes. Using radiolabeled substrates and chemical transporter inhibitors, activities of organic cation transporter 1, organic anion-transporting polypeptides, organic anion transporter 2, and Na⁺-taurocholate cotransporter were detected in cultured human and rat hepatocytes. In parallel, mRNA expression of these transporters was demonstrated using reverse transcriptase-quantitative polymerase chain reaction assays. Functional expression of sinusoidal transport proteins markedly decreased with time in primary rat hepatocyte cultures; by contrast, it remained relatively constant in primary human hepatocytes all along the culture, illustrating the fact that liver-specific functions, including drug-detoxifying pathways, are usually better preserved in cultured human hepatocytes than in their rodent counterparts. Primary hepatocytes, especially human hepatocytes, thus exhibit a pattern of sinusoidal transporter expression close to that found in vivo, highlighting the interest of hepatocyte cultures for drug detoxification studies.

Background: Sinuosidal and canalicular hepatic drug transporters, involved in drug uptake in the liver and drug secretion in the bile, respectively, play a major role in liver drug clearance. Inflammation is well-known to impair expression of these transporters in rodents; data about this topic have been more recently reported in human hepatocytes. Objective: The present review is designed to summarise the effects of pro-inflammatory cytokines such as IL-1β, TNF-α and IL-6 toward human hepatic drug and bile salt transporters. Methods: Recent studies aimed at analyzing transporter expression and activity in cytokines-exposed primary human hepatocytes and well-differentiated human hepatoma cells are resumed and discussed. Results/conclusion: Exposure to IL-1β, TNF-α or IL-6 markedly alters expression profile of human hepatic transporters. Bile salt transporters as well as sinusoidal solute carrier transporters are usually repressed, whereas ATP-binding cassette drug efflux pumps remain unchanged or are either downregulated or upregulated. These changes are observed at mRNA levels, but also, for some of the transporters, at protein and activity levels. They are likely to contribute to alterations of drug pharmacokinetics, impairment of bile salt secretion and cholestasis caused by inflammation in humans.