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1 We investigated the effects of grapefruit juice (GFJ) and orange juice (OJ) on drug transport by MDR1 P-glycoprotein (P-gp) and multidrug resistance protein 2 (MRP2), which are efflux transporters expressed in human small intestine. 14 C]saquinavir with MDR1 and MRP2 transfectants showed that VBL and saquinavir are transported by both P-gp and MRP2. GFJ and OJ components inhibited the transport by MRP2 as well as P-gp. However, their inhibitory potencies for P-gp or MRP2 were substrate-dependent. 5 The present study has revealed that GFJ and OJ interact with not only P-gp but also MRP2, both of which are expressed at apical membranes and limit the apical-to-basal transport of VBL and saquinavir in Caco-2 cells.

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The aim of this study is to investigate the placental transport mechanism of cationic compounds by comparison of the uptake of an organic cation into human placental basal membrane vesicles (BLMVs) with that into organic cation transporter 3 (OCT3)-expressing cells. Reverse transcription-polymerase chain reaction analysis demonstrated that OCT3 is the only OCT isoform expressed in the human placenta. The function of OCT3 was investigated by measuring the uptake of 1-methyl-4-phenylpyridinium (MPP ϩ ) into human embryonic kidney (HEK)293 cells stably expressing OCT3 (HEK/OCT3 cells). The OCT3-mediated uptake of MPP ϩ was sodium-and chlorideindependent and saturable, with a Michaelis constant (K m ) of 82.5 M. The OCT3-mediated uptake was inhibited by various cationic drugs in a concentration-dependent manner but not by anionic compounds, such as p-aminohippuric acid and captopril, or a zwitterion, carnitine. Western blotting analysis of membrane vesicles prepared from human term placenta revealed that OCT3 is expressed only in BLMVs but not in microvillous membrane vesicles. The uptake of MPP ϩ into BLMVs was membrane potential-dependent and saturable, with a K m value of 51.8 M, which is similar to that in HEK293/OCT3 cells. The inhibitory spectrum of various compounds on MPP ϩ uptake by BLMVs was also similar to that in HEK293/OCT3 cells. These results suggest that OCT3 is expressed on the basal membrane of human trophoblast cells and plays an important role in the placental transport of cationic compounds.

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1 Nateglinide, a novel oral hypoglycemic agent, rapidly reaches the maximum serum concentration after oral administration, suggesting that it is rapidly absorbed in the gastrointestinal tract. The aim of this work is to clarify the intestinal absorption mechanism of nateglinide by means of in vitro studies. 2 We examined the transcellular transport and the apical uptake of [ 14 C]nateglinide in a human colon carcinoma cell line (Caco-2). We also examined whether nateglinide is transported via monocarboxylate transport-1 (MCT1) by means of an uptake study using MCT1-expressing Xenopus laevis oocytes. 3 In Caco-2 cells, the transcellular transport of [ 14 C]nateglinide from the apical to basolateral side was greater than that in the opposite direction. The uptake of [ 14 C]nateglinide from the apical side was concentration ± dependent, H + -dependent, and Na + -independent. Kinetic analysis revealed that the Kt and Jmax values of the initial uptake rate of [ 14 C]nateglinide were 448 mM and 43.2 nmol mg protein 71 5 min 71 , respectively. Various monocarboxylates, including salicylic acid and valproic acid, and glibenclamide signi®cantly inhibited the uptake of [ 14 C]nateglinide. 4 The uptake study using MCT1-expressing oocytes showed that nateglinide inhibits the MCT1-mediated uptake of [ 14 C]L-lactic acid, though nateglinide itself is not transported by MCT1. 5 Taken together, these results suggest that the uptake of nateglinide from the apical membranes of Caco-2 cells is, at least in part, mediated by a proton-dependent transport system(s) distinct from MCT1.

ABSTRACT:Continuous use of St. John's wort decreases the bioavailabilities of a variety of drugs. This interaction is attributed to the induction of cytochrome P450 3A4 and/or P-glycoprotein. In this study, we aimed to examine the chronic effects of St. John's wort and its constituents, hyperforin and hypericin, on the expression and function of P-glycoprotein in an intestinal cell line, LS 180. We also examined the acute inhibitory effect of St. John's wort on Pglycoprotein by using LLC-GA5-COL150 cells, which overexpress P-glycoprotein. St. John's wort and hyperforin but not hypericin increased the expression of P-glycoprotein in LS 180 cells. Removal of St. John's wort resulted in a restoration of P-glycoprotein level within 48 h. The content of hyperforin in St. John's wort extract was high enough to induce P-glycoprotein, suggesting that the induction of P-glycoprotein by St. John's wort can be almost attributable to hyperforin. The LS 180 cells chronically exposed to St. John's wort or hyperforin exhibited the increase in the function of P-glycoprotein assessed by the efflux of digoxin, and the activities correlated well with P-glycoprotein level. On the other hand, St. John's wort and its two constituents did not show any acute effect on P-glycoprotein-mediated transport of digoxin. St. John's wort induced P-glycoprotein in vitro that functions as a drug efflux pump. Hyperforin is considered to be a primary cause of the inductive effect of St. John's wort. Long-term administration of St. John's wort may cause clinically significant decrease in the plasma concentrations of P-glycoprotein substrates.

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