Transporter-mediated Drug Interactions

The miRNA Landscape in Crohn's disease: Implications for novel therapeutic approaches and interactions with Existing therapies

MicroRNAs (miRNAs), which are non-coding RNAs consisting of 18-24 nucleotides, play a crucial role in the regulatory pathways of inflammatory diseases. Several recent investigations have examined the potential role of miRNAs in forming Crohn's disease (CD). It has been suggested that miRNAs serve as diagnostics for both fibrosis and inflammation in CD due to their involvement in the mechanisms of CD aggravation and fibrogenesis. More information on CD pathophysiology could be obtained by identifying the miRNAs concerned with CD and their target genes. These findings have prompted several in vitro and in vivo investigations into the putative function of miRNAs in CD treatment. Although there are still many unanswered questions, the growing body of evidence has brought miRNA-based therapy one step closer to clinical practice. This extensive narrative study offers a concise summary of the most current advancements in CD. We go over what is known about the diagnostic and therapeutic benefits of miRNA mimicry and inhibition so far, and we see what additional miRNA family targets could be useful for treating CD-related inflammation and fibrosis.

Biological Distribution of Orally Administered [123I]MIBG for Estimating Gastrointestinal Tract Absorption

Gastrointestinal tract absorption of cationic anticancer drugs and medicines was estimated using whole-body imaging following oral [¹²³I]MIBG administration. [¹²³I]MIBG was added to cultures of human embryonic kidney (HEK)293 cells expressing human organic anion transporting polypeptide (OATP)2B1, carnitine/organic cation transporter (OCTN)1 and OCTN2, and organic cation transporter (OCT)1, OCT2, and OCT3 with and without cimetidine (an OCTN and OCT inhibitor) and L-carnitine (an OCTN inhibitor). Biodistribution analyses and single-photon emission computed tomography (SPECT) imaging in normal and dextran sodium sulfate (DSS)-induced experimental colitis mice were conducted using [¹²³I]MIBG with and without cimetidine. [¹²³I]MIBG uptake was significantly higher in HEK293/OCTN1, 2, and OCT1-3 cells than in mock cells. Uptake via OCTN was inhibited by L-carnitine, whereas OCT-mediated uptake was inhibited by cimetidine. Biodistribution analyses and SPECT imaging studies showed significantly lower accumulation of [¹²³I]MIBG in the blood, heart, liver, and bladder in DSS-induced experimental colitis mice and mice with cimetidine loading compared with normal mice, whereas significantly higher accumulation in the stomach and kidney was observed after [¹²³I]MIBG injection. [¹²³I]MIBG imaging after oral administration can be used to estimate gastrointestinal absorption in the small intestine via OCTN and/or OCT by measuring radioactivity in the heart, liver, and bladder.

Curcumin ameliorates duodenal toxicity of AFB1 in chicken through inducing P-glycoprotein and downregulating cytochrome P450 enzymes

It has been reported that oral intake of aflatoxin B1 (AFB1)-contaminated feed could cause acute, sub-chronic, or chronic toxicity in livestock and poultry. However, the harmful effect of AFB1 on the small intestine is still controversial. Therefore, blocking the entry of AFB1 into the body through the digestive tract is one of the important methods to prevent its toxicity. In the present study, 1-day-old Arbor Acres broilers were randomly divided into 6 groups including control group, curcumin control group (450 mg curcumin/kg feed), curcumin low-, medium-, and high-dose group (150, 300, and 450 mg curcumin/kg feed + 5 mg AFB1/kg feed), and AFB1 group (5 mg AFB1/kg feed). After 28 d, the samples of chickens' duodenums were collected for further analyses. AFB1 caused abnormal functional and morphological changes in the duodenum, including histological lesions, increased the length of the duodenum and depth of crypt, decreased the unit weight of the duodenum, height of villus, and the value of villus height/crypt depth. Meanwhile, AFB1 administration enhanced malonaldehyde activity, 8-HOdG level, and the mRNA expression of cytochrome P450 (CYP450) enzymes, and reduced superoxide dismutase, catalase, adenosine triphosphatase (ATPase) activity and the mRNA expression of Abcb1. Importantly, curcumin supplementation partially ameliorated AFB1-induced abnormal functional and morphological signs of the duodenum, alleviated AFB1-induced oxidative stress, and decreased the mRNA expression of CYP450 enzymes. Furthermore, curcumin ameliorated AFB1-induced decrease in the Abcb1 mRNA expression, P-glycoprotein (P-gp) level, and ATPase activities. It has been suggested from these results that curcumin supplementation in the feed could ameliorate AFB1-induced duodenal toxicity and damage through downregulating CYP450 enzymes, promoting ATPase activities, and inducing P-gp in chickens.

Clinical Profiles of Nalfurafine Hydrochloride for the Treatment of Pruritus Patients

Nalfurafine hydrochloride is a selective kappa-opioid agonist that has antipruritic effects. Here we describe the clinical trials for treatment of uremic pruritus in dialysis patients and on hepatic pruritus in patients with chronic liver disease. Among cytochrome P-450 (CYP) isoforms in humans, CYP3A4 is the major isoform involved in metabolic decyclopropylmethylation of nalfurafine hydrochloride. Nalfurafine hydrochloride was found to be a substrate for P-glycoprotein (P-gp), but had no inhibitory effects on P-gp-mediated transport. The efficacy of oral nalfurafine hydrochloride at 2.5 and 5 μg for refractory pruritus in hemodialysis patients was observed within the first 7 days of treatment, and the effects persisted for the 52-week treatment period. Nalfurafine hydrochloride is also effective in the treatment of conventional refractory pruritus in peritoneal dialysis patients. Moreover, nalfurafine hydrochloride at 2.5 and 5 μg is effective for the treatment of refractory pruritus in chronic liver disease patients within the first 7 days of drug administration. In all the clinical trials, most adverse drug reactions (ADRs) were mild and resolved quickly and there was no clinical safety problem. Following 52 weeks of treatment, hemodialysis patients did not develop physical or psychological dependence, indicating no addiction risks. In summary, nalfurafine hydrochloride administered orally at doses of 2.5 and 5 μg was safe and effective for treatment of refractory pruritus in patients undergoing hemodialysis or peritoneal dialysis and in chronic liver disease patients.

Interspecies comparison of putative ligand binding sites of human, rat and mouse P-glycoprotein

Prior to the clinical phases of testing, safety, efficacy and pharmacokinetic profiles of lead compounds are evaluated in animal studies. These tests are primarily performed in rodents, such as mouse and rats. In order to reduce the number of animal experiments, computational models that predict the outcome of these studies and thus aid in prioritization of preclinical candidates are heavily needed. However, although computational models for human off-target interactions with decent quality are available, they cannot easily be transferred to rodents due to lack of respective data. In this study, we assess the transferability of human P-glycoprotein activity data for development of in silico models to predict in vivo effects in rats and mouse using a structure-based approach. P-glycoprotein (P-gp) is an ATP-dependent efflux transporter that transports xenobiotic compounds such as toxins and drugs out of cells and has a broad substrate and inhibitor specificity. Being mostly expressed at barriers, it influences the bioavailability of drugs and thus contributes also to toxicity. Comparison of the binding site interaction profiles of human, rat and mouse P-gp derived from docking studies with a set of common inhibitors suggests that the inhibitors share potentially similar binding modes. These findings encourage the use of in vitro human P-gp data for predicting in vivo effects in rodents and thus contributes to the 3Rs (Replace, Reduce and Refine) of animal experiments.

Evaluation of Adverse Drug Properties with Cryopreserved Human Hepatocytes and the Integrated Discrete Multiple Organ Co-culture (IdMOC(TM)) System

Human hepatocytes, with complete hepatic metabolizing enzymes, transporters and cofactors, represent the gold standard for in vitro evaluation of drug metabolism, drug-drug interactions, and hepatotoxicity. Successful cryopreservation of human hepatocytes enables this experimental system to be used routinely. The use of human hepatocytes to evaluate two major adverse drug properties: drug-drug interactions and hepatotoxicity, are summarized in this review. The application of human hepatocytes in metabolism-based drug-drug interaction includes metabolite profiling, pathway identification, P450 inhibition, P450 induction, and uptake and efflux transporter inhibition. The application of human hepatocytes in toxicity evaluation includes in vitro hepatotoxicity and metabolism-based drug toxicity determination. A novel system, the Integrated Discrete Multiple Organ Co-culture (IdMOC) which allows the evaluation of nonhepatic toxicity in the presence of hepatic metabolism, is described.

Intriguing possibilities and beneficial aspects of transporter-conscious drug design

It has been revealed that many types of drugs interact with transporter proteins within an organism. Transporter proteins absorb or excrete materials, including drugs and nutrients, across the cell membrane. Some hydrophobic drugs are excreted from the cell as xenobiotics by ATP-binding cassette (ABC) transporters. However, solute carrier (SLC) transporters are tissue-specifically expressed and have substrate specificities. Thus, transporter-conscious drug design is an excellent method of delivering drugs to pharmaceutical target organs and provides advantages in absorption, distribution, excretion, and toxicity of drugs (ADMET) due to transport systems. In fact, based on this strategy, the bioavailability of prodrugs designed as peptide transporter 1 (PEPT1) substrates was better than that of the corresponding parent compounds due to the transport system in the small intestine. Furthermore, in central nervous system (CNS) drug developing, drug delivery into brain across the blood-brain barrier (BBB) is a serious problem. However, this problem can be also solved by the use of the transport systems at the BBB. Therefore, transporter-consciously designed drugs not only may effectively elicit activity but also may control adverse side effects caused by off-targets and drug-drug interactions and, consequently, may show good performance in clinical trials. In this review, I introduce possibilities and advantages of transporter-conscious drug designs.

Identification and functional characterization of novel nonsynonymous variants in the human multidrug and toxin extrusion 2-K

This study was performed to identify genetic polymorphisms in multidrug and toxin extrusion 2-K (MATE2-K, SLC47A2), a proton/organic cation antiporter that plays a role in the transport of organic cations across the apical membrane in kidney epithelial cells into the urine, and to demonstrate their effects on MATE2-K functions in vitro. Four of the thirty single nucleotide polymorphisms (SNPs) we identified in three ethnic groups (Caucasian, African American, and Japanese) were novel [308C>G (P103R), c.487-8C>T, 818A>G (Y273C), and c.1018+14T>C]. The transport activities of the prototypical substrates, tetraethylammonium and metformin, for four nonsynonymous SNPs (P103R, P162L, G211V, and Y273C) were significantly different from those of the wild-type. In particular, transport activity was higher in P103R than in the wild-type, which is the first time elevated transport activity was demonstrated due to these coding SNPs. Kinetic analysis revealed that P103R had a higher Vmax value, whereas Y273C had a lower value than that in the wild-type. Cell surface protein expression levels were higher for P103R than for the wild-type, whereas Y273C expression was decreased. Immunofluorescence analysis revealed that the P103R protein was localized to the plasma membrane, whereas Y273C showed cytoplasmic localization. Therefore, the difference in transport activities between P103R and Y273C variants was suggested to be responsible for the different protein expression levels observed at the plasma membrane. Four nonsynonymous SNPs in this study showed relatively low allelic frequencies (0.5 to 2.1%), but these were associated with markedly reduced or increased MATE2-K function.

MicroRNAs as regulators of drug transporters, drug-metabolizing enzymes, and tight junctions: implication for intestinal barrier function

Drug transporters, drug-metabolizing enzymes, and tight junctions in the small intestine function as an absorption barrier and sometimes as a facilitator of orally administered drugs. The expression of these proteins often fluctuates and thereby causes individual pharmacokinetic variability. MicroRNAs (miRNAs), which are small non-coding RNAs, have recently emerged as a new class of gene regulator. MiRNAs post-transcriptionally regulate gene expression by binding to target mRNA to suppress its translation or regulate its degradation. They have been shown to be key regulators of proteins associated with pharmacokinetics. Moreover, the role of miRNAs on the expression of some proteins expressed in the small intestine has recently been clarified. In this review, we summarize current knowledge regarding the role of miRNAs in the regulation of drug transporters, drug-metabolizing enzymes, and tight junctions as well as its implication for intestinal barrier function. MiRNAs play vital roles in the differentiation, architecture, and barrier function of intestinal epithelial cells, and directly and/or indirectly regulate the expression and function of proteins associated with drug absorption in intestinal epithelial cells. Moreover, the variation of miRNA expression caused by pathological and physiological conditions as well as genetic factors should affect the expression of these proteins. Therefore, miRNAs could be significant factors affecting inter- and intra-individual variations in the pharmacokinetics and intestinal absorption of drugs. Overall, miRNAs could be promising targets for personalized pharmacotherapy or other attractive therapies through intestinal absorption of drugs.

Biomarkers and human hepatocytes

The accuracy of preclinical safety evaluation to predict human toxicity is hindered by species difference in drug metabolism and toxic mechanism between human and nonhuman animals. In vitro human-based experimental systems allowing the assessment of human-specific drug properties represent a logical and practical approach to provide human-specific information. An advantage of in vitro approaches is that they require only limited amounts of time and resources, and, most importantly, do not invoke harm to human patients. Human hepatocytes, with complete hepatic metabolizing enzymes, transporters and cofactors, represent a practical and useful experimental system to assess drug metabolism. The use of human hepatocytes to evaluate two major adverse drug properties, drug–drug interactions and hepatotoxicity, are reviewed. The application of human hepatocytes in metabolism-based drug–drug interactions includes metabolite profiling, pathway identification, CYP450 inhibition, CYP450 induction, and uptake and efflux transporter inhibition. The application of human hepatocytes in toxicity evaluation includes in vitro hepatotoxicity and metabolism-based drug toxicity determination. Correlation of drug toxicity with proteomics and genomics data may allow the discovery of clinical biomarkers for early detection of liver toxicity.

Strategies of bacterial over expression of membrane transporters relevant in human health: the successful case of the three members of OCTN subfamily

The OCTN subfamily includes OCTN1, 2, and 3 which are structurally and functionally related. These transporters are involved in maintenance of the carnitine homeostasis, which is essential in mammals for fatty acid β-oxidation, VLDL assembly, post-translational modifications, and other essential functions. Indeed, defects of these transporters lead to severe pathologies. OCTN1 and OCTN2 are expressed in many human tissues, while OCTN3 gene has been identified only in mouse and rat. The transporters mediate transport of carnitine and other substrates with different efficiencies and mechanisms. In order to over express the three proteins, a screening of many combinations of E. coli strains with plasmid constructs has been conducted. Only Rosetta(DE3) or Rosettagami2(DE3) gave significant expression. Higher protein amounts were firstly obtained with pET-41a(+) or pGEX-4T1 carrying fusion protein tags which required additional purification passages. Vectors carrying only a 6His tag, suitable for single passage purification, were preferred even though they lead to lower initial expression levels. Expressions were then increased optimizing several critical parameters. hOCTN1 was obtained with pH6EX3 in RosettaGami2(DE3)pLysS. hOCTN2 and mOCTN3 were obtained using pET-21a(+) in Rosetta(DE3). In particular, hOCTN2 was expressed only after codon bias, substituting the second triplet CGG with AAA (R2K mutant). The best growth conditions for hOCTN1 and mOCTN3 were 28 °C and 6 h of induction, while 4 h of induction for hOCTN2R2K. The proteins collected in the insoluble fraction of cell lysates, solubilized with sarkosyl, were purified by Ni-chelating chromatography. Final yield was 2.0, 3.0, or 3.5 mg/l of cell culture for mOCTN3, hOCTN1, or hOCTN2R2K. The data indicated that, in spite of the close evolutionary relations, several factors play different critical roles in bacterial expression of the three proteins, thus general criteria cannot be underlined. However, the strategy of dealing with related proteins revealed to be finally successful for over expressing all the three subfamily members.

Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics

Proteoliposomes represent a suitable and up to date tool for studying membrane transporters which physiologically mediate absorption, excretion, trafficking and reabsorption of nutrients and metabolites. Using recently developed reconstitution strategies, transporters can be inserted in artificial bilayers with the same orientation as in the cell membranes and in the absence of other interfering molecular systems. These methodologies are very suitable for studying kinetic parameters and molecular mechanisms. After the first applications on mitochondrial transporters, in the last decade, proteoliposomes obtained with optimized methodologies have been used for studying plasma membrane transporters and defining their functional and kinetic properties and structure/function relationships. A lot of information has been obtained which has clarified and completed the knowledge on several transporters among which the OCTN sub-family members, transporters for neutral amino acid, B0AT1 and ASCT2, and others. Transporters can mediate absorption of substrate-like derivatives or drugs, improving their bioavailability or can interact with these compounds or other xenobiotics, leading to side/toxic effects. Therefore, proteoliposomes have recently been used for studying the interaction of some plasma membrane and mitochondrial transporters with toxic compounds, such as mercurials, H₂O₂ and some drugs. Several mechanisms have been defined and in some cases the amino acid residues responsible for the interaction have been identified. The data obtained indicate proteoliposomes as a novel and potentially important tool in drug discovery.

Retrospective analysis of P-glycoprotein-mediated drug-drug interactions at the blood-brain barrier in humans

To date, the in vitro-in vivo correlation (IVIVC) of P-glycoprotein (P-gp)-mediated drug-drug interaction (DDI) at the blood-brain barrier (BBB) in rats indicated that the cutoff value to significantly affect the brain penetration of digoxin was [I,unbound/Ki] of 1, where I,unbound is the unbound plasma concentration of P-gp inhibitors. On the basis of the IVIVC in rats, we speculated that clinically used P-gp inhibitors do not cause DDI at the human BBB, because none of the compounds studied was [I,unbound/Ki]>1 at therapeutic doses. Recently, positron emission tomography studies with P-gp substrates, such as [(11)C]verapamil, [(11)C]N-desmethyl loperamide, and [(11)C]loperamide, together with potent P-gp inhibitors, have indicated that increases in the influx rate constant for brain entry were observed in humans. Therefore, we aimed to retrospectively analyze the results of P-gp-mediated DDIs with in vitro P-gp inhibition assays and to confirm the appropriate cutoff value. In vitro P-gp inhibition assays using verapamil, N-desmethyl loperamide, and loperamide as P-gp probe substrates were performed in human multidrug resistance protein 1-expressing LLC-PK1 cells. The efflux ratios decreased in the presence of P-gp inhibitors, and the Ki of tariquidar was 10 nmol/L, regardless of probe substrates. Taking the in vitro Ki and unbound plasma concentrations in clinical DDI studies together, the criterion [I,unbound/Ki] of 1 was an appropriate cutoff limit to observe significant P-gp-mediated DDI at the BBB in humans. On the other hand, no significant DDI was observed in cases in which [I,unbound/Ki] was less than 0.1. This criterion was comparable to the previous IVIVC result in rats.

Species differences of organic anion transporters involved in the renal uptake of 4-amino-3-chlorophenyl hydrogen sulfate, a metabolite of resatorvid, between rats and dogs

Previous studies on the metabolic fate of resatorvid (TAK-242) have shown that species differences in the pharmacokinetics of 4-amino-3-chlorophenyl hydrogen sulfate (M-III), a metabolite of TAK-242, between rats and dogs are mainly attributable to the urinary excretion process. In the present study, the renal uptake mechanism of M-III was investigated using kidney slices and Xenopus laevis oocytes expressing rat organic anion transporter 1 (rOat1; Slc22a6) and rOat3 (Slc22a8). The uptake of p-aminohippuric acid (PAH), a substrate for Oats, by kidney slices from rats and dogs increased at 37 °C and M-III inhibited the uptake. The initial uptake clearance of M-III by rat kidney slices was 0.295 and 0.0114 ml/min/g at 37 °C and 4 °C, respectively. The Eadie-Hofstee plot of M-III uptake at 37 °C revealed two-component transport processes with K(m) values being 6.48 and 724 µmol/l. The uptake was inhibited by probenecid (PBC), PAH and benzylpenicillin (PCG). In contrast, in dog kidney slices, the initial uptake clearance of M-III was 8.70 × 10(-3) and 9.00 × 10(-3) ml/min/g at 37 °C and 4 °C, respectively, and the uptake was not inhibited by PBC. Furthermore, rOat1- and rOat3-expressing oocytes mediated M-III uptake and the uptake was inhibited by PAH and PCG, respectively. These results suggest that rOat1 and rOat3 are responsible for the renal uptake of M-III in rats. Moreover, it is speculated that Oat(s) is unable to transport M-III in dogs and that the difference in the substrate recognition of Oat(s) contributes to the species difference in the pharmacokinetics of M-III between rats and dogs.

Drug discovery and regulatory considerations for improving in silico and in vitro predictions that use Caco-2 as a surrogate for human intestinal permeability measurements

There is a growing need for highly accurate in silico and in vitro predictive models to facilitate drug discovery and development. Results from in vitro permeation studies across the Caco-2 cell monolayer are commonly used for drug permeability screening in industry and are also accepted as a surrogate for human intestinal permeability measurements by the US FDA to support new drug applications. Countless studies carried out in this cell line with published permeability measurements have enabled the development of many in silico prediction models. We identify several common cases that illustrate how using Caco-2 permeability measurements in these in silico and in vitro predictive models will not correlate with human intestinal permeability and will further lead to inaccuracies in these models. We provide guidelines and recommendations for improving these models to more accurately predict clinically relevant information, thereby enhancing the drug discovery, development, and regulatory approval processes.

Expression of intestinal transporter genes in beagle dogs

This study was performed to produce a transcriptional database of the intestinal transporters of beagle dogs. Total RNA was isolated from the duodenum and the expression of various mRNAs was measured using GeneChip(®) oligonucleotide arrays. A total of 124 transporter genes were detected. Genes for fatty acid, peptide, amino acid and glucose and multidrug resistance/multidrug resistance-associated protein (MDR/MRP) transport were expressed at relatively higher levels than the other transporter types. The dogs exhibited abundant mRNA expression of the fatty acid transporters (fatty acid binding proteins, FABPs) FABP1 and FABP2, the ATP-binding cassettes (ABCs) ABCB1A and ABCC2, the amino acid/peptide transporters SLC3A1 and SLC15A1, the glucose transporters SLC5A1, SLC2A2 and SLC2A5, the organic anion transporter SLC22A9 and the phosphate transporters SLC20A1 and SLC37A4. In mice, a similar profile was observed with high expression of the glucose transporters SLC5A1 and SLC2As, the fatty acid transporters FABP1 and FABP2, the MDR/MRP transporters ABCB1A and ABCC2 and the phosphate transporter SLC37A4. However, the overall data reveal diverse transcriptomic profiles of the intestinal transporters of dogs and mice. Therefore, the current database may be useful for comparing the intestinal transport systems of dogs with those of mice to better evaluate xenobiotics.

Interactions of bilastine, a new oral H₁ antihistamine, with human transporter systems

Membrane transporters play a significant role in facilitating transmembrane drug movement. For new pharmacological agents, it is important to evaluate potential interactions (e.g., substrate specificity and/or inhibition) with human transporters that may affect their pharmacokinetics, efficacy, or toxicity. Bilastine is a new nonsedating H₁ antihistamine indicated for the treatment of allergic rhinoconjunctivitis and urticaria. The in vitro inhibitory effects of bilastine were assessed on 12 human transporters: four efflux [multidrug resistance protein 1 (MDR1) or P-glycoprotein, breast cancer resistance protein (BCRP), multidrug resistance associated protein 2 (MRP2), and bile salt export pump) and eight uptake transporters (sodium taurocholate cotransporting polypeptide, organic cation transporter (OCT)1, organic anion transporter (OAT)1, OAT3, OCT2, OATP2B1, OATP1B1, and OATP1B3). Only mild inhibition was found for MDR1-, OCT1-, and OATP2B1-mediated transport of probe substrates at the highest bilastine concentration assayed (300 μM; half-maximal inhibitory concentration: ≥300 μM). Bilastine transport by MDR1, BCRP, OAT1, OAT3, and OCT2 was also investigated in vitro. Only MDR1 active transport of bilastine was relevant, whereas it did not appear to be a substrate of OCT2, OAT1, or OAT3, nor was it transported substantially by BCRP. Drug-drug interactions resulting from bilastine inhibition of drug transporters that would be generally regarded as clinically relevant are unlikely. Additionally, bilastine did not appear to be a substrate of human BCRP, OAT1, OAT3, or OCT2 and thus is not a potential victim of inhibitors of these transporters. On the other hand, based on in vitro evaluation, clinically relevant interactions with MDR1 inhibitors are anticipated.

Gene expression analysis of membrane transport proteins in normal and lipopolysaccharide-inflamed rat dental pulp

INTRODUCTION

Membrane transport proteins (transporters) play a crucial role in the transmembrane uptake and/or efflux of various compounds such as inorganic ions, endogenous bioactive substances such as prostaglandins (PGs), and drugs such as nonsteroidal anti-inflammatory drugs. This study aimed to analyze mRNA expression of selected transporters related to drug disposition and PG transport in normal and lipopolysaccharide (LPS)-inflamed rat incisor pulp.

METHODS

Pulp tissues were subjected to reverse transcription-polymerase chain reaction (PCR) detection for transporter isoforms belonging to organic anion transporting polypeptide (Oatp), organic anion transporter (Oat), organic cation transporter (Oct), multidrug resistance-associated protein (Mrp), and multidrug resistance protein (Mdr) families. The levels of mRNA expression for PG transporters (Oatp1a5, Oatp1b2, Oatp2a1, Oatp2b1, and Oatp3a1) were compared in normal and LPS-inflamed pulps by using real-time PCR.

RESULTS

The pulp tissue expressed mRNAs for various transporters belonging to the Oatp, Oat, Oct, Mrp, and Mdr families. LPS inflammation caused significant up-regulation of Oatp2a1 (P < .01) and significant down-regulation of Oatp1a5, Oatp2b1 (P < .01), and Oatp3a1 (P < .05).

CONCLUSIONS

Rat incisor dental pulp expressed mRNAs for various transporter isoforms. The levels of mRNA expression for PG transporters were significantly up-regulated or down-regulated in LPS-inflamed dental pulp.

Human organic cation transporter 1 is expressed in lymphoma cells and increases susceptibility to irinotecan and paclitaxel

Antineoplastic agents directed at nuclear and cytoplasmic targets in tumor cells represent the current mainstay of treatment for patients with disseminated malignant diseases. Cellular uptake of antineoplastics is a prerequisite for their efficacy. Five of six lymphoma cell lines as well as primary samples from chronic lymphocytic leukemia patients demonstrated significant expression of SLC22A1 mRNA coding for organic cation transporter 1 (OCT1). Functionally, the antineoplastic agents irinotecan, mitoxantrone, and paclitaxel inhibited the uptake of the organic cation [(3)H]1-methyl-4-pyridinium iodide into OCT1-transfected Chinese hamster ovary model cells, with K(i) values of 1.7, 85, and 50 μM, respectively. Correspondingly, OCT1-positive cell lines and transfectants exhibited significantly higher susceptibilities to the cytotoxic effects of irinotecan and paclitaxel compared with those of OCT1-negative controls. We hypothesize that OCT1 can contribute to the susceptibility of cancer cells to selected antineoplastic drugs. In the future, an expression analysis of the transporters and the application of transporter-specific antineoplastic agents could help to tailor cancer therapy.

Quercetin and rutin reduced the bioavailability of cyclosporine from Neoral, an immunosuppressant, through activating P-glycoprotein and CYP 3A4

Quercetin and rutin are popular flavonoids in plant foods, herbs, and dietary supplements. Cyclosporine (CSP), an immunosuppressant with a narrow therapeutic window, is a substrate of P-glycoprotein (P-gp) and cytochrome P-450 3A4 (CYP3A4). This study investigated the effects of quercetin and rutin on CSP pharmacokinetics from Neoral and relevant mechanisms. Rats were orally administered Neoral with and without quercetin or rutin. The blood CSP concentration was assayed by a specific monoclonal fluorescence polarization immunoassay. The results showed that quercetin and rutin significantly decreased the C(max) of CSP by 67.8 and 63.2% and reduced the AUC(0-540) by 43.3 and 57.2%, respectively. The in vitro studies indicated that the quercetin and rutin induced the functions of P-gp and CYP3A4. In conclusion, quercetin and rutin decreased the bioavailability of CSP through activating P-gp and CYP3A. Transplant patients treated with Neoral should avoid concurrent consumption of quercetin or rutin to minimize the risk of allograft rejection.

Intestinal absorption mechanism of tebipenem pivoxil, a novel oral carbapenem: involvement of human OATP family in apical membrane transport

Tebipenem pivoxil (TBPM-PI) is an oral carbapenem antibiotic for treating otolaryngologic and respiratory infections in pediatric patients. This agent is a prodrug to improve intestinal absorption of TBPM, an active form, and an absorption rate of TBPM-PI is higher than those of other prodrug-type β-lactam antibiotics. In the present study, we hypothesized that a certain mechanism other than simple diffusion is involved in the process of improved intestinal absorption of TBPM-PI and examined the mechanism. TBPM-PI uptake by Caco-2 cells was decreased by ATP-depletion and lowering the temperature to 4 °C, suggesting the contribution of carrier-mediated transport mechanisms. This uptake was partially decreased by ACE inhibitors, and the reduction of the absorption by captopril was observed by in vivo study and in situ single-pass intestinal perfusion study in rat, supporting the contribution of influx transporters. Since some ACE inhibitors and β-lactam antibiotics are reported to be substrates of PEPT and OATP families, we measured transporting activity of TBPM-PI by intestinally expressed transporters, PEPT1, OATP1A2, and OATP2B1. As a result, significant transport activities were observed by both OATP1A2 and OATP2B1 but not by PEPT1. Interestingly, pH dependence of TBPM-PI transports was different between OATP1A2 and OATP2B1, showing highest activity by OATP1A2 at pH 6.5, while OATP2B1-mediated uptake was higher at neutral and weak alkaline pH. OATP1A2 exhibited higher affinity for TBPM-PI (K(m) = 41.1 μM) than OATP2B1 (K(m) > 1 mM) for this agent. These results suggested that TBPM-PI has high intestinal apical membrane permeability due to plural intestinal transport routes, including the uptake transporters such as OATP1A2 and OATP2B1 as well as simple diffusion.

Lactation stage influences drug milk-to-serum values and neonatal exposure risk

The purpose of our study was to assess the influence of lactation stage-dependent differences in milk-to-serum (M/S) ratio for an actively transported drug (cefepime) on the calculation of 2 exposure indices, EI(Dose) and EI(Conc). Age-dependent differences in cefepime pharmacokinetics at postnatal days 4 and 10 as well as cefepime M/S values at lactation days 4 and 10 in rats were determined. Significantly higher elimination rate constant and systemic clearance and lower half-life were found in day 10 compared to day 4 pups with no differences in oral bioavailability. The EI(Conc) was quantitatively higher than EI(Dose) at both lactation stages. The approximately 7-fold decrease in cefepime M/S values at lactation day 10 resulted in approximately 7-fold reduction in the EI(Dose) and approximately 13-fold reduction in EI(Conc). Our study confirms the need to evaluate M/S at different lactation stages for actively transported drugs to avoid over- or underestimation of neonatal exposure risk.

Effects of Kampo medicines on P-glycoprotein

The Kampo medicines are more and more often used in recent years, usually together with the western drugs. The need for the investigation of drug interactions between Kampo medicines and western drugs are, therefore, widely recognized. Among the various possible causes for the drug-drug interactions, those related to pharmacokinetics such as drug metabolism and transport are regarded as most frequent and clinically important. In the present study, the effects of Kampo medicines on the P-glycoprotein (P-gp), one of the major drug transporters, were investigated in in vitro studies using human P-gp membranes. The P-gp activity in the presence and absence of commonly used 50 Kampo medicines was evaluated by the ATPase assay detecting the inorganic phosphate produced by the ATP hydrolysis. The ATPase activity was inhibited by most of the Kampo medicines studied, indicating the possibility of their inhibiting the P-gp. The degree of inhibition in the presence of verapamil, a P-gp substrate, showed a significant correlation with that in the absence of verapamil. Furthermore, the inhibitory effect of the Kampo medicines on the ATPase activity correlated with their licorice root (kanzo) content, suggesting the contribution of licorice root in the P-gp inhibition. Because licorice root is one of the most common ingredients in the Kampo medicines and is also often used in the food as a sweetener, it might be necessary to pay attention on the interaction between the licorice root-containing drug/food and the number of drugs transported by P-gp.

Renal excretion of clofarabine: assessment of dose-linearity and role of renal transport systems on drug excretion

The renal excretion of clofarabine was studied in vitro in the isolated perfused rat kidney (IPK) model and in vivo in rats. Clofarabine excretion was studied at four doses (160, 800, 2000 and 4000microg) in the IPK, targeting perfusate levels of 2, 10, 25, 50microg/mL, respectively. Clofarabine (2microg/mL) was also co-perfused with known inhibitors of the, organic cation (cimetidine, ranitidine and tyramine) and organic anion (probenecid, ellagic acid) transport systems. Additionally, the effect of medications including, itraconazole, digoxin, fludarabine and cytarabine (Ara-C) on clofarabine excretion was, evaluated. Based on IPK results, in vivo studies were performed to assess the plasma, pharmacokinetics and urinary recovery of clofarabine (6.5mg/kg, IV) pretreatment, with cimetidine (250mg/kg, IV). Clofarabine clearance was non-linear in the IPK, although at concentrations that were approximately 10- to 100-fold higher than maximum concentrations observed in humans. Excretion ratio data indicated a shift from net, secretion (XR=1.2+/-0.33) to net reabsorption (XR=0.78+/-0.40) at the highest dose, tested. Clofarabine clearance was significantly reduced upon co-administration with, cimetidine (0.83+/-0.22-->0.32+/-0.058mL/min). In vivo data correlated with IPK results as clofarabine clearance decreased 61% (20.2-7.80mL/min/kg), upon co-administration with cimetidine in rats. The results suggest that clofarabine is a substrate for a cimetidine-sensitive organic cation transporter system in the kidney, presumably OCT2. The magnitude of the cimetidine-clofarabine interaction was similar, in IPK and in vivo, demonstrating the utility of the IPK model in characterizing renal, drug excretion and assessing potential drug-drug interactions.

Comparative gene expression of intestinal metabolizing enzymes

The purpose of this study was to compare the expression profiles of drug-metabolizing enzymes in the intestine of mouse, rat and human. Total RNA was isolated from the duodenum and the mRNA expression was measured using Affymetrix GeneChip oligonucleotide arrays. Detected genes from the intestine of mouse, rat and human were ca. 60% of 22690 sequences, 40% of 8739 and 47% of 12559, respectively. Total genes of metabolizing enzymes subjected in this study were 95, 33 and 68 genes in mouse, rat and human, respectively. Of phase I enzymes, the mouse exhibited abundant gene expressions for Cyp3a25, Cyp4v3, Cyp2d26, followed by Cyp2b20, Cyp2c65 and Cyp4f14, whereas, the rat showed higher expression profiles of Cyp3a9, Cyp2b19, Cyp4f1, Cyp17a1, Cyp2d18, Cyp27a1 and Cyp4f6. However, the highly expressed P450 enzymes were CYP3A4, CYP3A5, CYP4F3, CYP2C18, CYP2C9, CYP2D6, CYP3A7, CYP11B1 and CYP2B6 in the human. For phase II enzymes, glucuronosyltransferase Ugt1a6, glutathione S-transferases Gstp1, Gstm3 and Gsta2, sulfotransferase Sult1b1 and acyltransferase Dgat1 were highly expressed in the mouse. The rat revealed predominant expression of glucuronosyltransferases Ugt1a1 and Ugt1a7, sulfotransferase Sult1b1, acetyltransferase Dlat and acyltransferase Dgat1. On the other hand, in human, glucuronosyltransferases UGT2B15 and UGT2B17, glutathione S-transferases MGST3, GSTP1, GSTA2 and GSTM4, sulfotransferases ST1A3 and SULT1A2, acetyltransferases SAT1 and CRAT, and acyltransferase AGPAT2 were dominantly detected. Therefore, current data indicated substantial interspecies differences in the pattern of intestinal gene expression both for P450 enzymes and phase II drug-metabolizing enzymes. This genomic database is expected to improve our understanding of interspecies variations in estimating intestinal prehepatic clearance of oral drugs.

Drug metabolism and pharmacokinetics

In this article, aspects of absorption, distribution, metabolism, and excretion have been described bearing in mind the pathogenesis of allergic diseases and their possible therapeutic opportunities. The importance of the routes of administration of the different therapeutic groups has been emphasized. The classical aspects of drug metabolism and disposition related to oral administration have been reviewed, but special emphasis has been given to intranasal, cutaneous, transdermal, and ocular administration as well as to the absorption and the subsequent bioavailability of drugs. Drug-metabolizing enzymes and transporters present in extrahepatic tissues, such as nasal mucosa and the respiratory tract, have been particularly discussed. As marketed antiallergic drugs include both racemates and enantiomers, aspects of stereoselective absorption, distribution, metabolism, and excretion have been discussed. Finally, a new and promising methodology, microdosing, has been presented, although it has not yet been applied to drugs used in the treatment of allergic diseases.

Modulation of Oral Drug Bioavailability: From Preclinical Mechanism to Therapeutic Application

Currently, more than one fourth of all anticancer drugs are developed as oral formulations, and it is expected that this number will increase substantially in the near future. To enable oral drug therapy, adequate oral bioavailability must be achieved. Factors that have proved to be important in limiting the oral bioavailability are the presence of ATP-binding cassette drug transporters (ABC transporters) and the cytochrome P450 enzymes. We discuss the tissues distribution and physiological function of the ABC transporters in the human body, their expression in tumors, currently known polymorphisms and drugs that are able to inhibit their function as transporter. Furthermore, the role of the ABC transporters and drug-metabolizing enzymes as mechanisms to modulate the pharmacokinetics of anticancer agents, will be reviewed. Finally, some clinical examples of oral drug modulation are discussed. Among these examples are the coadministration of paclitaxel with CsA, a CYP3A4 substrate with P-glycoprotein (P-gp) modulating activity, and topotecan combined with the BCRP/P-gp transport inhibitor elacridar. Both are good examples of improvement of oral drug bioavailability by temporary inhibition of drug transporters in the gut epithelium.

Simulations of the nonlinear dose dependence for substrates of influx and efflux transporters in the human intestine

The purpose of this study was to develop simulation and modeling methods for the evaluation of pharmacokinetics when intestinal influx and efflux transporters are involved in gastrointestinal absorption. The advanced compartmental absorption and transit (ACAT) model as part of the computer program GastroPlus was used to simulate the absorption and pharmacokinetics of valacyclovir, gabapentin, and talinolol. Each of these drugs is a substrate for an influx or efflux transporter and all show nonlinear dose dependence within the normal therapeutic range. These simulations incorporated the experimentally derived gastrointestinal distributions of transporter expression levels for oligopeptide transporters PepT1 and HPT1 (valacyclovir); System L-amino acid transporter LAT2 and organic cation transporter OCTN1 (gabapentin); and organic anion transporter (OATP1A2) and P-glycoprotein (talinolol). By assuming a uniform distribution of oligopeptide transporter and by application of the in vitro K(m) value for valacyclovir, the simulations accurately reproduced the experimental nonlinear dose dependence. For gabapentin, LAT2 distribution produced simulation results that were much more accurate than OCTN1 distributions. For talinolol, an influx transporter distribution for OATP1A2 and the efflux transporter P-glycoprotein distributed with increasing expression in the distal small intestine produced the best results. The physiological characteristics of the small and large intestines used in the ACAT model were able to accurately account for the positional and temporal changes in concentration and carrier-mediated transport of the three drugs included in this study. The ACAT model reproduced the nonlinear dose dependence for each of these drugs.

L-type fatty acid binding protein transgenic mouse as a novel tool to explore cytotoxicity to renal proximal tubules

A novel biomarker of renal dysfunction, liver-type fatty acid binding protein (L-FABP), which is expressed in human proximal tubules, binds to lipid peroxidation products during renal injury and is excreted into the urine. Here, we examined the usefulness of human L-FABP transgenic (Tg) mice as a tool to explore nephrotoxicity, employing two model drugs, cephaloridine and cisplatin, which are taken up by renal tubules via organic anion and cation transporters, respectively. Urinary excretion of L-FABP increased after administration of cephaloridine in most of the Tg mice, whereas glomerular filtration markers such as blood-urea-nitrogen (BUN) and plasma creatinine (CRE) were almost unchanged. Thus, L-FABP is a highly sensitive detector of the nephrotoxicity of cephaloridine. Urinary excretion of L-FABP in the Tg mice also increased after administration of cisplatin, and this increase was reduced by coadministration of cimetidine. Both BUN and CRE also increased after the cisplatin treatment, but these parameters were minimally affected by coadministration of cimetidine, suggesting that cimetidine reduces cisplatin-induced renal tubular toxicity with only a minimal effect on the glomerulus. These results indicate that the L-FABP Tg mouse should be a useful drug screening system to evaluate specifically the toxicity of transporter substrates to renal tubules.

P-glycoprotein (Abcb1) is involved in absorptive drug transport in skin

The purpose of the present study was to investigate the role of P-glycoprotein (P-gp) in drug disposition in skin. The distribution of P-gp substrates (rhodamine 123 and itraconazole) to the skin after administration from the epidermal side was lower in P-gp gene knockout (mdr1a/1b(-/-)) mice than that in wild-type mice. Coadministration of propranolol, a P-gp inhibitor, decreased the distribution of itraconazole to the skin in wild-type mice, but not in mdr1a/1b(-/-) mice. These results suggest that P-gp contributes to the influx (from the epidermal side) of its substrates into skin, although P-gp is generally involved in efflux of drugs from various tissues. This finding was supported by the lower vectorial transport of rhodamine 123 from the epidermal to the hypodermal side in mdr1a/1b(-/-) mice in Ussing-type chamber experiments and by the immunohistochemical localization of P-gp throughout the dermal layer. Distribution of itraconazole after intravenous administration, on the other hand, was higher in mdr1a/1b(-/-) mice than that in wild-type mice, suggesting that P-gp transports this drug from the skin to the circulation. The present findings are the first to demonstrate involvement of P-gp in dermal drug disposition.

Marked impact of P-glycoprotein on the absorption of TAK-427 in rats

The role of P-glycoprotein (P-gp, ABCB1) on the absorption process was investigated by drug-drug interaction studies of TAK-427 with P-gp inhibitors (erythromycin, ketoconazole or quinidine) in rats and by transport studies using rat multidrug resistance (MDR1) stably expressing cells and rat small intestine mounted in a Ussing-type chamber. TAK-427 showed high efflux activity with low permeability in rat MDR1a and MDR1b stably expressing cells and was revealed to be a typical substrate for P-gps. Although TAK-427 was mainly absorbed from the small intestine in rats, a large part of the dosed compound remained in the gastrointestinal tract. Orally co-administered P-gp inhibitors (50 mg/kg) increased the AUC of TAK-427 after a 5 mg/kg oral dose 5.4- to 18.3-fold, whereas orally administered P-gp inhibitors had a minor effect on the increase in the AUC of TAK-427 (1.3- to 2.2-fold) after a 0.5 mg/kg intravenous dose. Thus, the bioavailability of TAK-427 after oral administration in rats (7.3%) markedly increased when co-administered with P-gp inhibitors (28.6-57.6%). Moreover, the transport of TAK-427 was predominantly secretory throughout the rat small intestine and was inhibited by P-gp inhibitors. In conclusion, P-gp can markedly reduce the absorption of a typical P-gp substrate by its efflux activity throughout the absorption site.

Acute administration of cefepime lowers L-carnitine concentrations in early lactation stage rat milk

Our study investigated the potential for important in vivo drug-nutrient transport interactions at the lactating mammary gland using the L-carnitine transporter substrates, cefepime and L-carnitine, as proof-of-concept. On d 4 (n = 6/treatment) and d 10 (n = 6/treatment) of lactation, rats were administered cefepime (250 mg/h) or saline by continuous i.v. infusion (4 h). Serum and milk L-carnitine and cefepime concentrations were quantified by HPLC-UV. In whole mammary gland, organic cation/carnitine transporter (OCTN)1, OCTN2, OCTN3, amino acid transporter B(0,+) (ATB(0,+)), and L-carnitine transporter 2 expression were determined by quantitative RT-PCR and by western blot and immunohistochemistry when possible. Cefepime caused a 56% decrease in milk L-carnitine concentrations on lactation d 4 (P = 0.0048) but did not affect milk L-carnitine at lactation d 10 or serum L-carnitine concentrations at either time. The mean L-carnitine and cefepime milk:serum ratios (M/S) decreased from 9.1 +/- 0.4 to 4.9 +/- 0.6 (P < 0.0001) and 0.89 +/- 0.3 to 0.12 +/- 0.02 (P = 0.0473), respectively, between d 4 and d 10 of lactation. In both groups, OCTN2 (P < 0.0001), OCTN3 (P = 0.0039), and ATB(0,+) (P = 0.004) mRNA expression and OCTN2 protein (P < 0.0001) were higher in mammary glands at d 4 of lactation compared with d 10. Immunohistochemistry revealed OCTN1 and OCTN2 localization in the mammary alveolar epithelium and OCTN3 expression in the interstitial space and blood vessel endothelium. In conclusion, cefepime significantly decreased milk L-carnitine concentrations only at d 4 of lactation. Relative to d 10, enhanced expression of OCTN2 and ATB(0,+) in mammary glands at d 4 of lactation and higher M/S (L-carnitine and cefepime) suggests cefepime competes with L-carnitine for L-carnitine transporters expressed in the lactating mammary gland to adversely affect L-carnitine milk concentrations and these effects depend upon lactation stage.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Involvement of organic anion transport system in transdermal absorption of flurbiprofen

We previously demonstrated that transdermal permeation of flurbiprofen is mediated by a nonlinear transport mechanism(s). Here, we aimed to characterize this transport mechanism by employing an Ussing-type chamber method with tape-stripped hairless mouse skin. Transdermal permeation of [(3)H]flurbipofen was vectorial, saturable and energy-dependent, suggesting the involvement of a carrier-mediated transport system. Transdermal permeation and uptake from the epidermal side of [(3)H]flurbiprofen were inhibited by various nonsteroidal anti-inflammatory drugs (NSAIDs). The inhibitory potency did not correlate well with lipophilicity; anionic NSAIDs tended to be more potent inhibitors than non-anionic NSAIDs. The inhibition profile of both [(3)H]flurbiprofen permeation and uptake, and the Michaelis constants, were similar for a given anionic compound. These results suggest that an organic anion transport system is involved in flurbiprofen uptake from the epidermal side during the process of transdermal absorption. Efflux of [(3)H]flurbiprofen from the skin to the epidermal side, but not to the hypodermal side, increased in the presence of flurbiprofen or several anionic compounds. Such trans-stimulation may suggest the involvement of an organic anion exchanger system. Organic anion transporter 2 (OAT2) is a candidate for the exchanger involved in uptake and/or efflux of flurbiprofen in the skin.

Species Differences of Inhibitory Effects on P‐glycoprotein‐mediateD Drug Transport

Previously, we clarified the species differences in P-glycoprotein (P-gp)-mediated drug transport activity using human MDR1, monkey MDR1, canine MDR1, rat MDR1a, rat MDR1b, mouse mdr1a, and mouse mdr1b transfected LLC-PK(1) cell lines. However, the species differences in the inhibitory effects on P-gp-mediated drug transport have not been clarified yet. The purpose of the present study was to evaluate the species differences in the inhibitory effects of typical P-gp inhibitors, quinidine and verapamil, on P-gp-mediated drug transport using MDR1 transfected cell lines. The transcellular transport of [(3)H]daunorubicin, [(3)H]digoxin, and [mebmt-beta-(3)H]cyclosporin A across monolayers of the MDR1 transfected cells were measured in the presence or absence of P-gp inhibitors. On daunorubicin transport, the relative IC(50) value (quinidine IC(50)/verapamil IC(50)) of human P-gp was 5.25 and those from other species ranged from 0.89 to 10.70. The transport of digoxin and cyclosporin A also showed different relative IC(50) values among human, monkey, canine, rat, and mouse P-gps. The present study revealed that species differences in the inhibitory effects on P-gp-mediated drug transport should not be disregarded among human, monkey, canine, rat, and mouse. This study will provide useful information for predicting drug interactions mediated by P-gp.

Functional expression of stereoselective metabolism of cephalexin by exogenous transfection of oligopeptide transporter PEPT1

Gastrointestinal absorption of the beta-lactam antibiotic cephalexin (CEX) is highly stereoselective: l- and d-CEX are both taken up by intestinal epithelial cells through the brush-border membrane, most likely via oligopeptide transporter PEPT1, but l-CEX is not found in serum or urine after administration p.o. because of its rapid intestinal metabolism, whereas d-CEX is well absorbed in the unchanged form. We examined the contribution of PEPT1 to the stereoselective uptake and metabolism of CEX. We observed stereoselective metabolism of CEX after exogenous transfection of PEPT1 alone into mammalian cell lines: l-CEX, but not d-CEX, was metabolized to 7-aminodesacetoxycephalosporanic acid (7-ADCA) in HeLa and human embryonic kidney 293 cells stably and transiently expressing human PEPT1, respectively, whereas such metabolism was minor in cells expressing the vector alone. The formation rate of 7-ADCA depended on the amount of PEPT1 cDNA transfected. l-CEX metabolism was rapid because only 7-ADCA was found inside and outside the cells during incubation with l-CEX. The characteristics of PEPT1-mediated metabolism of l-CEX were similar, but not identical, to those of PEPT1-mediated transport. PEPT1-mediated metabolism was also observed in permeabilized cells expressing PEPT1, in which PEPT1-mediated intracellular substrate accumulation was negligible, suggesting that the increase in l-CEX metabolism by PEPT1 transfection cannot be fully explained by an increase in uptake and subsequent exposure to intracellular hydrolases. The present findings show that stereoselectivity in CEX absorption can be fully explained in terms of PEPT1, implying that the l-CEX hydrolase is PEPT1 itself or is induced by PEPT1.