No major effects of vitamin D3 (1,25 dihydroxyvitamin D3) on absorption and pharmacokinetics of folic acid and fexofenadine in healthy volunteers.

PURPOSE: In Caco-2 cells, folate uptake via the proton-coupled folate transporter (PCFT) increases significantly by a 3-day treatment with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Additionally, mRNA content and protein expression of the transporter OATP1A2 were increased up to ninefold with 1,25(OH)2D3. We investigated whether these in vitro findings can be confirmed in humans in vivo. METHODS: Ten healthy volunteers (six women) received 5 mg folic acid orally once before and once together with the last intake of a 10-day course of 0.5 µg 1,25(OH)2D3 orally. One hundred twenty milligrams fexofenadine, an OATP1A2 substrate, was taken in 1 day before the first folic acid intake, and again on the ninth day of 1,25(OH)2D3 intake. Duodenal biopsies were taken for transporter mRNA assessments once before and once on the ninth or tenth day of the vitamin D3 course. Serum folic acid and fexofenadine concentrations were quantified with a chemiluminescence immunoassay and LC-MS/MS, respectively. Pharmacokinetics were compared between periods with standard bioequivalence approaches. RESULTS: While geometric mean folic acid AUC0-2h, which mainly reflects absorption, was 0.403 and 0.414 mg/L·h before and after the vitamin D3 course (geometric mean ratio (GMR), 1.027; 90 % confidence interval (90 % CI), 0.788-1.340), the geometric mean fexofenadine AUC0-2h was 1.932 and 2.761 mg/L·h, respectively (GMR, 1.429; 90 % CI, 0.890-2.294). PCFT- and OATP1A2-mRNA expressions in duodenal biopsies were essentially unchanged. CONCLUSIONS: No significant changes in folic acid and fexofenadine absorption were observed after a 10-day course of 1,25(OH)2D3 in humans in vivo. This study underlines the importance of confirming in vitro findings in vivo in humans.

ABC-transporters are localized in caveolin-1-positive and reggie-1-negative and reggie-2-negative microdomains of the canalicular membrane in rat hepatocytes.

UNLABELLED: The canalicular plasma membrane is constantly exposed to bile acids acting as detergents. Bile acids are essential to mediate release of biliary lipids from the canalicular membrane. Membrane microdomains (previously called lipid rafts) are biochemically defined by their resistance to detergent solubilization at cold temperature. We aimed to investigate the canalicular plasma membrane for the presence of microdomains, which could protect this membrane against the detergent action of bile acids. Highly purified rat liver canalicular plasma membrane vesicles were extracted with 1% Triton X-100 or 1% Lubrol WX at 4 degrees C and subjected to flotation through sucrose step gradients. Both detergents yielded detergent-resistant membranes containing the microdomain markers alkaline phosphatase and sphingomyelin. However, cholesterol was resistant to Lubrol WX solubilization, whereas it was only marginally resistant to solubilization by Triton X-100. The microdomain marker caveolin-1 was localized to the canalicular plasma membrane domain and was resistant to Lubrol WX, but to a large extent solubilized by Triton X-100. The two additional microdomain markers, reggie-1 and reggie-2, were localized to the basolateral and canalicular plasma membrane and were partially resistant to Lubrol WX but resistant to Triton X-100. The canalicular transporters bile salt export pump, multidrug resistance protein 2, multidrug resistance-associated protein 2, and Abcg5 were largely resistant to Lubrol WX but were solubilized by Triton X-100. CONCLUSION: These results indicate the presence of two different types of microdomains in the canalicular plasma membrane: "Lubrol-microdomains" and "Triton-microdomains". "Lubrol-microdomains" contain the machinery for canalicular bile formation and may be the starting place for canalicular lipid secretion.

Tegaserod inhibits the serotonin transporter SERT.

BACKGROUND: Tegaserod is a novel drug for the treatment of constipation-predominant irritable bowel syndrome. Tegaserod is thought to exert its prokinetic effect as a selective partial agonist of serotonin receptor type 4 (5-HT4) receptors located in the enteric nervous system. It is unknown, however, whether tegaserod interacts with the human serotonin reuptake transporter (hSERT) and the uptake transporters for dopamine (hDAT) and norepinephrine (hNET). Therefore, the aim of the present study was to investigate whether tegaserod inhibits SERT-, DAT-, and NET-mediated transport. METHODS: Tegaserod inhibition of SERT-mediated [3H]5-HT and NET- and DAT-mediated [3H]dopamine uptake was measured in human embryonic kidney (HEK) 293 cells stably expressing hSERT, hDAT, and hNET in comparison with untransfected control HEK293-FT cells. RESULTS: Tegaserod inhibited SERT-, DAT-, and NET-mediated transport with IC50-values of 11.7, 20.7, and 3.2 micromol/l, respectively, while 100 micromol/l estrone-3-sulfate or taurocholic acid, used as negative controls, failed to inhibit hSERT-mediated transport. Using Dixon plot analysis, inhibition kinetics yielded a non-competitive type of inhibition with an apparent inhibition constant (Ki) of 3.1 micromol/l for SERT-mediated 5-HT transport. CONCLUSION: In the present study we propose an additional mechanism of action for tegaserod as a serotonin uptake inhibitor. By inhibiting SERT and increasing local 5-HT concentrations in the gut wall, tegaserod might exert its prokinetic action via a synergism between 5-HT4 agonism and low-affinity SERT inhibition.

Regional distribution of solute carrier mRNA expression along the human intestinal tract.

Intestinal absorption of drugs, nutrients, and other compounds is mediated by uptake transporters expressed at the apical enterocyte membrane. These compounds are returned to the intestinal lumen or released into portal circulation by intestinal efflux transporters expressed at apical or basolateral membranes, respectively. One important transporter superfamily, multiple members of which are intestinally expressed, are the solute carriers (SLCs). SLC expression levels may determine the pharmacokinetics of drugs that are substrates of these transporters. In this study we characterize the distribution of 15 human SLC transporter mRNAs in histologically normal biopsies from five regions of the intestine of 10 patients. The mRNA expression levels of CNT1, CNT2, apical sodium-dependent bile acid transporter (ABST), serotonin transporter (SERT), PEPT1, and OCTN2 exhibit marked differences between different regions of the intestine: the first five are predominantly expressed in the small intestine, whereas OCTN2 exhibits strongest expression in the colon. Two transporter mRNAs studied (OCTN1, OATP2B1) are expressed at similar levels in all gut sections. In addition, ENT2 mRNA is present at low levels across the colon, but not in the small intestine. The other six SLC mRNAs studied are not expressed in the intestine. Quantitative knowledge of transporter expression levels in different regions of the human gastrointestinal tract could be useful for designing intestinal delivery strategies for orally administered drugs. Furthermore, changes in transporter expression that occur in pathological states, such as inflammatory bowel disease, can now be defined more precisely by comparison with the expression levels measured in healthy individuals.

hPepT1 selectively transports muramyl dipeptide but not Nod1-activating muramyl peptides.

Muramyl peptides derived from bacterial peptidoglycan are detected intracellularly by Nod1 and Nod2, 2 members of the newly characterized nod-like receptor (NLR) family of pattern recognition molecules. In the absence of bacterial invasion into the host cytosolic compartment, it remains unclear whether muramyl peptides can cross the plasma membrane and localize into the cytosol. We have recently demonstrated that the plasma membrane transporter, hPepT1, was able to efficiently translocate muramyl dipeptide (MDP), a specific Nod2-activating molecule, into host cells. We aimed to characterize the transport properties of hPepT1 towards a spectrum of muramyl peptides, including Nod1-activating molecules. To do so, we designed an original procedure based on the ectopic expression of hPepT1 in oocytes from Xenopus laevis. Our results demonstrated that hPepT1 transports MDP but no other Nod2-activating molecule. Moreover, we observed that Nod1-stimulating muramyl peptides were not transported by hPepT1. Since hPepT1 expression is strongly associated with intestinal epithelial cells, where Nod1 and Nod2 have been shown to play a key role, these observations suggest a distinct contribution of Nod1 and Nod2 in mucosal homeostasis following the cellular uptake of muramyl peptides by hPepT1.

Localization of organic anion transporting polypeptides in the rat and human ciliary body epithelium.

PURPOSE: To identify and localize the expression of multispecific organic anion transporting polypeptides (Oatps/OATPs) in the ciliary body epithelium and to investigate their possible involvement in the transport of the antiglaucoma agent unoprostone. METHODS: Oatps/OATPs were detected by immunoblot analysis and by immunofluorescence microscopy in homogenized and fixed rat and human ciliary body samples using specific polyclonal antibodies. Transport of 3H-labelled unoprostone was measured in Oatp/OATP expressing Xenopus laevis oocytes. RESULTS: Immunoblots of ciliary body extracts were positive for rat Oatp1a4, Oatp1a5 and Oatp1b2 and for human OATP1A2, OATP1C1, OATP2B1, OATP3A1 and OATP4A1. Confocal immunofluorescence microscopy localized Oatp1a4 and Oatp1b2 as well as all immunoblot positive human OATPs at the basolateral plasma membrane of the non-pigmented rat and human ciliary body epithelium, respectively. However, for human OATPs additional regional differences in expression were found with OATP1A2 and OATP1C1 being expressed only in the pars plana of human ciliary body epithelium. Furthermore, OATP1C1, OATP3A1 and OATP4A1 were also expressed at the basolateral plasma membrane of the pars plana pigmented epithelium. And finally, deesterified unoprostone carboxylate was found to be transported by OATP1A2, OATP2B1 and OATP4A1 with approximate K(m)-values of 93, 91 and 132 microm, respectively. CONCLUSIONS: Several multispecific organic anion transporting polypeptides are expressed at the basolateral plasma membrane of the non-pigmented, and to a lesser extent also of the pigmented, epithelium in rat and human ciliary body. These Oatps/OATPs can account for the previously suggested 'liver-like' transport functions of mammalian ciliary body epithelium.

Interactions of glycyrrhizin with organic anion transporting polypeptides of rat and human liver.

Glycyrrhizin (GL) is used in Japan for the treatment of chronic hepatitis C. Following intravenous administration, GL is eliminated mainly by excretion into bile. Hepatocyte uptake of GL is a carrier-mediated process with characteristics resembling the organic anion transporting polypeptides (OATPs, solute carrier gene family SLC21A). We, therefore, studied whether GL is a potential transport substrate of the OATPs of rat and human liver. Because transport of GL could not be measured directly, GL-mediated cis-inhibition of [3H]estrone-3-sulfate or [35S]bromosulfophthalein uptake was analyzed kinetically in Xenopus laevis oocytes injected with cRNA coding for OATPs. GL inhibited [3H]estrone-3-sulfate uptake by 75-100% in oocytes expressing rat Oatp4, human OATP-C or human OATP8, members of the OATP1B subfamily that are expressed predominantly in hepatocytes. Dixon plots indicated a non-competitive type of inhibition, with Ki values of 6.1, 15.9 and 12.5 µmol/l, respectively. In contrast, GL inhibition of rat Oatp1, Oatp2 and Oatp3 and human OATP-A and OATP-B was only between 0 and 53%. In conclusion, GL is an inhibitor and, therefore, potentially a transport substrate of the liver-specific OATPs in rat and man. The rate at which GL is taken up into the liver may depend upon the function and expression levels of these hepatocellular OATPs.