PEPT1-mediated cefixime uptake into human intestinal epithelial cells is increased by Ca2+ channel blockers

Conversion of Olmesartan to Olmesartan Medoxomil, A Prodrug that Improves Intestinal Absorption, Confers Substrate Recognition by OATP2B1

PURPOSE

Olmesartan medoxomil (olmesartan-MX), an ester-type prodrug of the angiotensin II receptor blocker (ARB) olmesartan, is predominantly anionic at intestinal pH. Human organic anion transporting polypeptide 2B1 (OATP2B1) is expressed in the small intestine and is involved in the absorption of various acidic drugs. This study was designed to test the hypothesis that OATP2B1-mediated uptake contributes to the enhanced intestinal absorption of olmesartan-MX, even though olmesartan itself is not a substrate of OATP2B1.

METHODS

Tetracycline-inducible human OATP2B1- and rat Oatp2b1-overexpressing HEK 293 cell lines (hOATP2B1/T-REx-293 and rOatp2b1/T-REx-293, respectively) were established to characterize OATP2B1-mediated uptake. Rat jejunal permeability was measured using Ussing chambers. ARBs were quantified by liquid chromatography-tandem mass spectrometry.

RESULTS

Significant olmesartan-MX uptake was observed in hOATP2B1/T-REx-293 and rOatp2b1/T-REx-293 cells, whereas olmesartan uptake was undetectable or much lower than olmesartan-MX uptake, respectively. Furthermore, olmesartan-MX exhibited several-fold higher uptake in Caco-2 cells and greater permeability in rat jejunum compared to olmesartan. Olmesartan-MX uptake in hOATP2B1/T-REx-293 cells and in Caco-2 cells was significantly decreased by OATP2B1 substrates/inhibitors such as 1 mM estrone-3-sulfate, 100 µM rifamycin SV, and 100 µM fluvastatin. Rat Oatp2b1-mediated uptake and rat jejunal permeability of olmesartan-MX were significantly decreased by 50 µM naringin, an OATP2B1 inhibitor. Oral administration of olmesartan-MX with 50 µM naringin to rats significantly reduced the area under the plasma concentration-time curve of olmesartan to 76.9%.

CONCLUSION

Olmesartan-MX is a substrate for OATP2B1, and the naringin-sensitive transport system contributes to the improved intestinal absorption of olmesartan-MX compared with its parent drug, olmesartan.

Molecular Insights to the Structure-Interaction Relationships of Human Proton-Coupled Oligopeptide Transporters (PepTs)

Human proton-coupled oligopeptide transporters (PepTs) are important membrane influx transporters that facilitate the cellular uptake of many drugs including ACE inhibitors and antibiotics. PepTs mediate the absorption of di- and tri-peptides from dietary proteins or gastrointestinal secretions, facilitate the reabsorption of peptide-bound amino acids in the kidney, and regulate neuropeptide homeostasis in extracellular fluids. PepT1 and PepT2 have been the most intensively investigated of all PepT isoforms. Modulating the interactions of PepTs and their drug substrates could influence treatment outcomes and adverse effects with certain therapies. In recent studies, topology models and protein structures of PepTs have been developed. The aim of this review was to summarise the current knowledge regarding structure-interaction relationships (SIRs) of PepTs and their substrates as well as the potential applications of this information in therapeutic optimisation and drug development. Such information may provide insights into the efficacy of PepT drug substrates in patients, mechanisms of drug-drug/food interactions and the potential role of PepTs targeting in drug design and development strategies.

Ion channels and transporters regulate nutrient absorption in health and disease

Ion channels and transporters are ubiquitously expressed on cell membrane, which involve in a plethora of physiological process such as contraction, neurotransmission, secretion and so on. Ion channels and transporters is of great importance to maintaining membrane potential homeostasis, which is essential to absorption of nutrients in gastrointestinal tract. Most of nutrients are electrogenic and require ion channels and transporters to absorb. This review summarizes the latest research on the role of ion channels and transporters in regulating nutrient uptake such as K+ channels, Ca2+ channels and ion exchangers. Revealing the mechanism of ion channels and transporters associated with nutrient uptake will be helpful to provide new methods to diagnosis and find potential targets for diseases like diabetes, inflammatory bowel diseases, etc. Even though some of study still remain ambiguous and in early stage, we believe that ion channels and transporters will be novel therapeutic targets in the future.

Drug Screening, Oral Bioavailability and Regulatory Aspects: A Need for Human Organoids

The intestinal epithelium critically contributes to oral bioavailability of drugs by constituting an important site for drug absorption and metabolism. In particular, intestinal epithelial cells (IEC) actively serve as gatekeepers of drug and nutrient availability. IECs' transport processes and metabolism are interrelated to the whole-body metabolic state and represent potential points of origin as well as therapeutic targets for a variety of diseases. Human intestinal organoids represent a superior model of the intestinal epithelium, overcoming limitations of currently used in vitro models. Caco-2 cells or rodent explant models face drawbacks such as their cancer and non-human origin, respectively, but are commonly used to study intestinal nutrient absorption, enterocyte metabolism and oral drug bioavailability, despite poorly correlative data. In contrast, intestinal organoids allow investigating distinct aspects of bioavailability including spatial resolution of transport, inter-individual differences and high-throughput screenings. As several countries have already developed strategic roadmaps to phase out animal experiments for regulatory purposes, intestinal organoid culture and organ-on-a-chip technology in combination with in silico approaches are roads to go in the preclinical and regulatory setup and will aid implementing the 3Rs (reduction, refinement and replacement) principle in basic science.

Organoids to Study Intestinal Nutrient Transport, Drug Uptake and Metabolism - Update to the Human Model and Expansion of Applications

Intestinal transport and sensing processes and their interconnection to metabolism are relevant to pathologies such as malabsorption syndromes, inflammatory diseases, obesity and type 2 diabetes. Constituting a highly selective barrier, intestinal epithelial cells absorb, metabolize, and release nutrients into the circulation, hence serving as gatekeeper of nutrient availability and metabolic health for the whole organism. Next to nutrient transport and sensing functions, intestinal transporters including peptide transporter 1 (PEPT1) are involved in the absorption of drugs and prodrugs, including certain inhibitors of angiotensin-converting enzyme, protease inhibitors, antivirals, and peptidomimetics like β-lactam antibiotics. Here, we verify the applicability of 3D organoids for in vitro investigation of intestinal biochemical processes related to transport and metabolism of nutrients and drugs. Establishing a variety of methodologies including illustration of transporter-mediated nutrient and drug uptake and metabolomics approaches, we highlight intestinal organoids as robust and reliable tool in this field of research. Currently used in vitro models to study intestinal nutrient absorption, drug transport and enterocyte metabolism, such as Caco-2 cells or rodent explant models are of limited value due to their cancer and non-human origin, respectively. Particularly species differences result in poorly correlative data and findings obtained in these models cannot be extrapolated reliably to humans, as indicated by high failure rates in drug development pipelines. In contrast, human intestinal organoids represent a superior model of the intestinal epithelium and might help to implement the 3Rs (Reduction, Refinement and Replacement) principle in basic science as well as the preclinical and regulatory setup.

Segment-specific effects of resveratrol on porcine small intestinal dipeptide absorption depend on the mucosal pH and are due to different mechanisms: potential roles of different transport proteins and protein kinases

Numerous beneficial features of the polyphenol resveratrol (RSV) have been demonstrated in several tissues and cell culture models. There is also evidence, that RSV impairs intestinal nutrient transport but the underlying mechanisms are not understood. The aim of the present study was to evaluate whether RSV has also an impact on the H⁺-coupled transport of peptides via the peptide transporter 1 (PepT1) and to characterize RSV mediated changes in the apical abundance of nutrients transport proteins and protein kinases that may be involved. RSV decreased the H⁺-coupled transport of peptides in the porcine small intestines in a pH and location specific manner (jejunum vs ileum) as measured in Ussing chamber experiments. The comparison of the effects of RSV with the effects of the cAMP/PKA-activating agent forskolin indicates that different mechanisms may be responsible in the intestinal segments. Additionally, it seems that the transport of peptides and glucose in the jejunum are inhibited via the same mechanism while there might be two mechanisms involved in the ileum. Functional data and protein expression data indicate, that, besides PepT1, the activity of the Na⁺/H⁺-exchanger 3 (NHE3) may be involved. Protein kinase A (PKA) and AMP-activated kinase (AMPK) are both activated by RSV while the extracellular regulated kinase (ERK) and the serum and glucocorticoid induced kinase (SGK) are widely unaffected. Although PKA and AMPK are activated, AMPK seems not to be related to the effects of RSV. Additionally, both the functional data and the protein expression data reveal some interesting pH- and segment-specific differences.

Calcium-sensing receptor regulates intestinal dipeptide absorption via Ca2+ signaling and IKCa activation

Although absorption of di- and tripeptides into intestinal epithelial cells occurs via the peptide transporter 1 (PEPT1, also called solute carrier family 15 member 1 (SLC15A1)), the detailed regulatory mechanisms are not fully understood. We examined: (a) whether dipeptide absorption in villous enterocytes is associated with a rise in cytosolic Ca2+ ([Ca2+ ]cyt ), (b) whether the calcium sensing receptor (CaSR) is involved in dipeptide-elicited [Ca2+ ]cyt signaling, and (c) what potential consequences of [Ca2+ ]cyt signaling may enhance enterocyte dipeptide absorption. Dipeptide Gly-Sar and CaSR agonist spermine markedly raised [Ca2+ ]cyt in villous enterocytes, which was abolished by NPS-2143, a selective CaSR antagonist and U73122, an phospholipase C (PLC) inhibitor. Apical application of Gly-Sar induced a jejunal short-circuit current (Isc), which was reduced by NPS-2143. CaSR expression was identified in the lamina propria and on the basal enterocyte membrane of mouse jejunal mucosa in both WT and Slc15a1-/- animals, but Gly-Sar-induced [Ca2+ ]cyt signaling was significantly decreased in Slc15a1-/- villi. Clotrimazole and TRM-34, two selective blockers of the intermediate conductance Ca2+ -activated K+ channel (IKCa ), but not iberiotoxin, a selective blocker of the large-conductance K+ channel (BKCa ) and apamin, a selective blocker of the small-conductance K+ channel (SKCa ), significantly inhibited Gly-Sar-induced Isc in native tissues. We reveal a novel CaSR-PLC-Ca2+ -IKCa pathway in the regulation of small intestinal dipeptide absorption, which may be exploited as a target for future drug development in human nutritional disorders.

Population Pharmacokinetics of Cis-, Trans-, and Total Cefprozil in Healthy Male Koreans

Cefprozil, one of cephalosporin antibiotics, has been used extensively in clinics. However, pharmacokinetic (PK) information on cefprozil is still very limited. There have been no reports of population pharmacokinetics (PPKs). A PPK model for cefprozil will be a great advantage for clinical use. Thus, the aim of this study was to develop a PPK model for cefprozil for healthy male Koreans. Clinical PK and demographic data of healthy Korean males receiving cefprozil at a dose of 1000 mg were analyzed using Phoenix^® NLME™. A one-compartment model with first-order absorption with lag-time was constructed as a base model. The model was extended to include covariates that influenced between-subject variability. Creatinine clearance significantly influenced systemic clearance of cefprozil. The final PPK model for cis-, trans-, and total cefprozil was established and validated. PPK parameter values of cis- and total cefprozil were similar to each other, but different from those of trans-isomer. Herein, we describe the establishment of accurate PPK models of cis-, trans-, and total cefprozil for healthy male Koreans for the first time. It may be useful as a dosing algorithm for the general population. These results might also contribute to the development of stereoisomeric cefprozil.

Effects of quercetin on pharmacokinetics of cefprozil in Chinese-Han male volunteers

1. The primary objective of this study was to evaluate the effects of quercetin on the pharmacokinetics of cefprozil. The secondary objective was to evaluate the safety of the combined use of cefprozil and quercetin. 2. An open-label, two-period, crossover phase I trial among 24 Han Chinese male subjects was conducted. Participants were given 500 mg of quercetin orally once daily for 15 d followed by single dose of cefprozil (500 mg) on day 15. Serum concentrations of cefprozil were then measured in all participants on day 15. A 15-d washout period was then assigned after which a 500 mg dose of cefprozil was administered and measured in the serum on day 36. 3. All subjects completed the trial, and no serious adverse events were reported. We measured mean serum concentrations of cefprozil in the presence and absence of quercetin in all participants. The maximum serum concentration of cefprozil in the presence of quercetin was 8.18 ug/ml (95% CI: 7.55-8.81) versus a maximum cefprozil concentration of 8.35 ug/ml (95% CI: 7.51-9.19) in the absence of quercetin. We conclude that the concurrent use of quercetin has no substantial effect on serum concentrations of orally administered cefprozil. 4. Co-administration of quercetin showed no statistically significant effects on the pharmacokinetics of cefprozil in healthy Chinese subjects.

Intestinal organoids for assessing nutrient transport, sensing and incretin secretion

Intestinal nutrient transport and sensing are of emerging interest in research on obesity and diabetes and as drug targets. Appropriate in vitro models are lacking that allow both, studies on transport processes as well as sensing and subsequent incretin hormone secretion including intracellular signaling. We here demonstrate that murine small-intestinal organoids are the first in vitro model system enabling concurrent investigations of nutrient and drug transport, sensing and incretin hormone secretion as well as fluorescent live-cell imaging of intracellular signaling processes. By generating organoid cultures from wild type mice and animals lacking different nutrient transporters, we show that organoids preserve the main phenotypic features and functional characteristics of the intestine. This turns them into the best in vitro model currently available and opens new avenues for basic as well as medical research.

PEPT1- and OAT1/3-mediated drug-drug interactions between bestatin and cefixime in vivo and in vitro in rats, and in vitro in human

The purpose of the present study was to elucidate the transporter-mediated pharmacokinetics mechanism of drug-drug interactions (DDIs) between bestatin and cefixime. The plasma concentrations and bioavailabilities of bestatin and cefixime were decreased after oral co-administration in rats. The uptake in rat everted intestinal sacs of bestatin and cefixime were dramatically declined after co-administration of the two drugs. Bestatin and cefixime can mutually competitively inhibit the uptake by hPEPT1-HeLa cells. The plasma concentrations of bestatin and cefixime were increased; however, the cumulative biliary excretion had no significant change, and the cumulative urinary excretion and renal clearance of the two drugs in rats decreased after intravenous coadministration. Moreover, decreased uptake of the two drugs was observed in human kidney slices, rat kidney slices and hOAT1/hOAT3-transfected HEK293 cells when bestatin and cefixime were coadministered. The accumulation of bestatin and cefixime in kidney slices can be inhibited by p-aminohippurate, benzylpenicillin and probenecid, but not by tetraethyl ammonium. The results suggest that intestinal absorption and renal excretion of bestatin and cefixime can be inhibited when the two drugs were co-administered in rats. The pharmacokinetic mechanism indicates that the DDIs between bestatin and cefixime are mainly mediated by Pept1 and Oat1/3 in rats. PEPT1 and OAT1/3 are the target transporters of DDIs between bestatin and cefixime in human kidney slices and human transfected cells, proposing possible drug-drug interaction in humans.

Epithelial transport in inflammatory bowel diseases

The epithelium of the gastrointestinal tract is one of the most versatile tissues in the organism, responsible for providing a tight barrier between dietary and bacterial antigens and the mucosal and systemic immune system while maintaining efficient digestive and absorptive processes to ensure adequate nutrient and energy supply. Inflammatory bowel diseases (Crohn's disease and ulcerative colitis) are associated with a breakdown of both functions, which in some cases are clearly interrelated. In this updated literature review, we focus on the effects of intestinal inflammation and the associated immune mediators on selected aspects of the transepithelial transport of macronutrients and micronutrients. The mechanisms responsible for nutritional deficiencies are not always clear and could be related to decreased intake, malabsorption, and excess losses. We summarize the known causes of nutrient deficiencies and the mechanism of inflammatory bowel disease-associated diarrhea. We also overview the consequences of impaired epithelial transport, which infrequently transcend its primary purpose to affect the gut microbial ecology and epithelial integrity. Although some of those regulatory mechanisms are relatively well established, more work needs to be done to determine how inflammatory cytokines can alter the transport process of nutrients across the gastrointestinal and renal epithelia.

Proton-coupled oligopeptide transporter family SLC15: physiological, pharmacological and pathological implications

Mammalian members of the proton-coupled oligopeptide transporter family (SLC15) are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs. The driving force for uphill electrogenic symport is the chemical gradient and membrane potential which favors proton uptake into the cell along with the peptide/mimetic substrate. The peptide transporters are responsible for the absorption and conservation of dietary protein digestion products in the intestine and kidney, respectively, and in maintaining homeostasis of neuropeptides in the brain. They are also responsible for the absorption and disposition of a number of pharmacologically important compounds including some aminocephalosporins, angiotensin-converting enzyme inhibitors, antiviral prodrugs, and others. In this review, we provide updated information on the structure-function of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4) and PhT2 (SLC15A3), and their expression and localization in key tissues. Moreover, mammalian peptide transporters are discussed in regard to pharmacogenomic and regulatory implications on host pharmacology and disease, and as potential targets for drug delivery. Significant emphasis is placed on the evolving role of these peptide transporters as elucidated by studies using genetically modified animals. Whenever possible, the relevance of drug-drug interactions and regulatory mechanisms are evaluated using in vivo studies.

Effects of amlodipine on the oral bioavailability of cephalexin and cefuroxime axetil in healthy volunteers

In this study, the authors compared the effects of amlodipine (AML) on the bioavailability of cephalexin (LEX) and cefuroxime axetil (CXM). Twenty-four healthy men were randomized to 4 treatments according to a crossover design with a 14-day washout. After an overnight fast, they were administered orally LEX 500 mg alone, LEX 500 mg 2 hours after oral administration of AML 5 mg, CXM 500 mg alone, and CXM 500 mg 2 hours after oral administration of AML 5 mg. All participants completed the whole study without side effects being observed. Pharmacokinetic data were analyzed by noncompartmental modeling with WinNonlin software. The geometric mean (GM) ratios were 1.38 (90% confidence interval [CI], 1.32-1.45) for the area under the concentration-time curve (AUC) for LEX and 1.27 (1.18-1.36) for the maximum concentration of drug in serum (C(max)) for LEX followed by AML versus alone. In contrast, no significant differences were found in the pharmacokinetic parameters of CXM between treatments (P < .05). They authors conclude that AML possesses an enhancement effect in β-lactam antibiotic bioavailability (in this case, LEX), and this interaction may be specific to the peptidomimetic β-lactam antibiotics.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

Uptake, transport and regulation of JBP485 by PEPT1 in vitro and in vivo

Cyclo-trans-4-L-hydroxyprolyl-L-serine (JBP485) is a dipeptide with anti-hepatitis activity that has been chemically synthesized. Previous experiments in rats showed that JBP485 was well absorbed by the intestine after oral administration. The human peptide transporter (PEPT1) is expressed in the intestine and recognizes compounds such as dipeptides and tripeptides. The purposes of this study were to determine if JBP485 acted as a substrate for intestinal PEPT1, and to investigate the characteristics of JBP485 uptake and transepithelial transport by PEPT1. The uptake of JBP485 was pH dependent in human intestinal epithelial cells Caco-2. And JBP485 uptake was also significantly inhibited by glycylsarcosine (Gly-Sar, a typical substrate for PEPT1 transporters), JBP923 (a derivative of JBP485), and cephalexin (CEX, a β-lactam antibiotic and a known substrate of PEPT1) in Caco-2 cells. The rate of apical-to-basolateral transepithelial transport of JBP485 was 1.84 times higher than that for basolateral-to-apical transport. JBP485 transport was obviously inhibited by Gly-Sar, JBP923 and CEX in Caco-2 cells. The uptake of JBP485 was increased by verapamil but not by cyclosporin A (CsA) and inhibited by the presence of Zn(2+) or the toxic metabolite of ethanol, acetaldehyde (AcH) in Caco-2 cells. The in vivo uptake of JBP485 was increased by verapamil and decreased by ethanol in vivo, which was consisted with the in vitro study. PEPT1 mRNA levels were enhanced after exposure of the cells to JBP485 for 24h, compared to control. In conclusion, JBP485 was actively transported by the intestinal oligopeptide transporter PEPT1. This mechanism is likely to contribute to the rapid absorption of JBP485 by the gastrointestinal tract after oral administration.

The involvement of intracellular calcium in the MCT-mediated uptake of lactic acid by HeLa cells

The main object of this study was to evaluate the role of intracellular free calcium ion [Ca2+](in) in monocarboxylate transporter (MCT) mediated drug uptake by HeLa cells. It was hypothesized that alterations in the [Ca2+](in) levels affect Na+-H+ exchanger (NHE) regulated pH(in) and thereby produce the proton-motivated driving force for monocarboxylate mediated substrate transport. The changes in intracellular pH (pH(in)) and MCT mediated uptake rates of L-lactic acid by HeLa cells, a human cervical adenocarcinoma cell line, were evaluated under the conditions, whose [Ca2+](in) concentrations were altered by various calcium modulators, such as EGTA-AM (a chelator), nifedipine (a Ca2+ channel antagonist) and A23187 (an ionophore). For the purpose of comparison, the L-lactic acid uptake by HeLa cells was also evaluated under various pH(in) conditions induced by dexamethasone. The effects of the extracellular sodium concentration on the L-lactic acid uptake by HeLa cells were evaluated to determine the involvement of NHE-regulated pH changes in the MCT mediated drug uptake process. The [Ca2+](in) concentrations and pH(in) in HeLa were assessed using fluorescent probes fura-2 and 2',7'-bis[2-carboxyethyl-5-carboxyfluorescein] (BCECF), respectively. The treatment of HeLa cells with A23187 at concentrations of 50 and 100 microM enhanced [Ca2+](in) by 100% and 200% of the control, respectively. EGTA/AM (50 microM) or nifedipine (100 microM) did not cause any significant changes in the [Ca2+](in) levels, whereas EGTA/AM (100 microM) and nifedipine (200 microM) reduced the [Ca2+](in) levels by 30% and 25%, respectively, as compared with the control. A23187 at a concentration of 100 microM in the incubation medium lowered pH(in) (pH 5) and subsequently the uptake rate of lactic acid by 50% (0.47 +/- 0.03 micromol/mg protein/min) of the control. In contrast, nifedipine (200 microM) and EGTA-AM (100 microM), the calcium modulators that lowered the [Ca2+](in) levels and maintained the higher pH(in) (pH > 6) of HeLa cells, enhanced the uptake rate of lactic acid by 60% and 130% of the control, respectively. The results of this study demonstrated that there was a close correlation between the [Ca2+](in) level and pH(in) and that NHEs were involved with the MCT mediated uptake process in HeLa cells. An understanding of the role of [Ca2+](in) in the MCT mediated transport process could provide an efficient strategy to improve the systemic delivery of monocarboxylate substrates through the cervical mucosa.

Peptide transporters and their roles in physiological processes and drug disposition

1. The peptide transporters belong to the peptide transporter (PTR) family and serve as integral membrane proteins for the cellular uptake of di- and tripeptides in the organism. By their ability also to transport peptidomimetics and other substrates with therapeutic activities or precursors of pharmacologically active agents, they are of considerable importance in pharmacology. 2. PEPT1 is the low-affinity, high-capacity transporter and is mainly expressed in the small intestine, whereas PEPT2 is the high-affinity, low-capacity transporter and has a broader distribution in the organism. 3. Targeted mouse models have revealed PEPT2 to be the dominant transporter for the reabsorption of di- and tripeptides and its pharmacological substrates in the organism, and for the removal of these substrates from the cerebrospinal fluid. Moreover, the peptide transporters undergo physiological and pharmacological regulation and, of great interest, are present in disease states where PEPT1 exhibits ectopic expression in colonic inflammation. 4. The paper reviews the structural characteristics of the peptide transporters, the structural requirements for substrates, the distribution of the peptide transporters in the organism, and finally their regulation in the organism in healthy and pathological situations.

An energy supply network of nutrient absorption coordinated by calcium and T1R taste receptors in rat small intestine

T1R taste receptors are present throughout the gastrointestinal tract. Glucose absorption comprises active absorption via SGLT1 and facilitated absorption via GLUT2 in the apical membrane. Trafficking of apical GLUT2 is rapidly up-regulated by glucose and artificial sweeteners, which act through T1R2 + T1R3/alpha-gustducin to activate PLC beta2 and PKC betaII. We therefore investigated whether non-sugar nutrients are regulated by taste receptors using perfused rat jejunum in vivo. Under different conditions, we observed a Ca(2+)-dependent reciprocal relationship between the H(+)/oligopeptide transporter PepT1 and apical GLUT2, reflecting the fact that trafficking of PepT1 and GLUT2 to the apical membrane is inhibited and activated by PKC betaII, respectively. Addition of L-glutamate or sucralose to a perfusate containing low glucose (20 mM) each activated PKC betaII and decreased apical PepT1 levels and absorption of the hydrolysis-resistant dipeptide L-Phe(PsiS)-L-Ala (1 mM), while increasing apical GLUT2 and glucose absorption within minutes. Switching perfusion from mannitol to glucose (75 mM) exerted similar effects. c-glutamate induced rapid GPCR internalization of T1R1, T1R3 and transducin, whereas sucralose internalized T1R2, T1R3 and alpha-gustducin. We conclude that L-glutamate acts via amino acid and glucose via sweet taste receptors to coordinate regulation of PepT1 and apical GLUT2 reciprocally through a common enterocytic pool of PKC betaII. These data suggest the existence of a wider Ca(2+) and taste receptor-coordinated transport network incorporating other nutrients and/or other stimuli capable of activating PKC betaII and additional transporters, such as the aspartate/glutamate transporter, EAAC1, whose level was doubled by L-glutamate. The network may control energy supply.

The Caco-2 cell monolayer: usefulness and limitations

BACKGROUND

The Caco-2 monolayer has been used extensively for the high-throughput screening of drug permeability and identification of substrates, inhibitors, and inducers of intestinal transporters, especially P-glycoprotein (P-gp). Traditionally, the Caco-2 monolayer is viewed as a single barrier rather than a polarized cell monolayer consisting of metabolic enzymes that are sandwiched between two membrane barriers with distinctly different transporters.

OBJECTIVE

This review addressed the usefulness and limitations of the Caco-2 cell monolayer in drug discovery and mechanistic studies.

METHODS

This mini-review covered applications of the Caco-2 monolayer, clarified misconceptions, and critically addressed issues on data interpretation.

CONCLUSION

The catenary model extends the usefulness of Caco-2 monolayer and provides proper mechanistic insight and data interpretation.

P-glycoprotein and cytochrome P450 3A4 involvement in risperidone transport using an in vitro Caco-2/TC7 model and an in vivo model

The possible involvement of P-glycoprotein (P-gp) and cytochrome P450 (CYP) 3A4 in risperidone transport was investigated using in vitro and in vivo models. Firstly, uptake studies were performed on a Caco-2/TC7 cell monolayer; the effects of 1 microg ml(-1) risperidone on apparent permeability were determined for secretory and absorptive directions, in the presence or absence of various P-gp and CYP3A4 inhibitors (verapamil, ketoconazole, erythromycin), and of an associated multidrug-resistant protein inhibitor (indomethacin). Secondly, on a conscious rat model, risperidone pharmacokinetic parameters, notably absorption parameters, were determined using compartmental and deconvolution methods. Three groups of seven rats received respectively an IV risperidone dose, an oral risperidone dose (PO group) and the same oral risperidone dose after verapamil administration (POV group). No formation of 9-hydroxyrisperidone was observed on Caco-2 cells after risperidone administration; there was no evidence that intestinal CYP3A4 is involved in risperidone metabolising. Risperidone secretory permeation was higher than absorptive permeation. Verapamil increased risperidone absorption permeation and decreased its secretory permeation. Indomethacin did not modify these permeation values. In rats, verapamil led to an increase in both risperidone and 9-hydroxyrisperidone plasmatic concentrations. The fraction absorbed in the verapamil group was 3.18 times higher than in the oral group (65.9% and 20.7% for POV group and PO group). The absorption rate constant was lower in the verapamil group. Our results indicate that P-gp decreases the intestinal absorption of risperidone and that intestinal CYP3A4 is not involved in risperidone metabolism.

Effects of Capsaicin on Intestinal Cephalexin Absorption in Rats

The effects of capsaicin on intestinal cephalexin absorption were investigated by means of in situ single pass perfusion in rats to clarify whether this pungent compound present in spice is a potential factor altering the intestinal drug absorption processes. Under the control condition, cephalexin was absorbed at a rate of 1.16+/-0.08 and 0.90+/-0.06 nmol/min/cm in the jejunum and ileum, respectively. The intestinal cephalexin absorption rate was decreased when capsaicin was dissolved in the perfusate at a concentration of 400 microM, being 0.54+/-0.07 and 0.46+/-0.10 nmol/min/cm in the jejunum and ileum, respectively. The inhibitive effect of capsaicin on intestinal cephalexin absorption was diminished when ruthenium red, a non-selective inhibitor of the transient receptor potential (TRP) cation channels, was intravenously infused into the rat during the experiment. Moreover, when we evaluated the paracellular permeability of cephalexin by utilizing a competitive inhibitor, glycylsarcosine, it was demonstrated that glycylsarcosine-insensitive intestinal cephalexin absorption in the jejunum was increased by 4.5 times in the presence of 400 microM capsaicin. These findings indicate that capsaicin affects both transcellular and paracellular pathways of intestinal cephalexin absorption by interacting with the TRP cation channels in intestinal tissues, in which capsaicin seems to change the transport activity of H+/peptide co-transporter 1 (PEPT1), and to a lesser degree, it seems to alter the paracellular permeability of the intestinal epithelia.

Intestinal absorption of drugs mediated by drug transporters: mechanisms and regulation

The absorption of drugs from the gastrointestinal tract is one of the important determinants for oral bioavailability. Development of in vitro experimental techniques such as isolated membrane vesicles and cell culture systems has allowed us to elucidate the transport mechanisms of various drugs across the plasma membrane. Recent introduction of molecular biological techniques resulted in the successful identification of drug transporters responsible for the intestinal absorption of a wide variety of drugs. Each transporter exhibits its own substrate specificity, though it usually shows broad substrate specificity. In this review, we first summarize the recent advances in the characterization of drug transporters in the small intestine, classified into peptide transporters, organic cation transporters and organic anion transporters. In particular, peptide transporter (PEPT1) is the best-characterized drug transporter in the small intestine, and therefore its utilization to improve the oral absorption of poorly absorbed drugs is briefly described. In addition, regulation of the activity and expression levels of drug transporters seems to be an important aspect, because alterations in the functional characteristics and/or expression levels of drug transporters in the small intestine could be responsible for the intra- and interindividual variability of oral bioavailability of drugs. As an example, regulation of the activity and expression of PEPT1 is summarized.

pCLCA1 lacks inherent chloride channel activity in an epithelial colon carcinoma cell line

The effects of CLCA protein expression on the regulation of Cl(-) conductance by intracellular Ca(2+) and cAMP have been studied previously in nonepithelial cell lines chosen for low backgrounds of endogenous Cl(-) conductance. However, CLCA proteins have been cloned from, and normally function in, differentiated epithelial cells. In this study, we examine the effects of differentiation of the Caco-2 epithelial colon carcinoma cell line on modulation of Cl(-) conductance by pCLCA1 protein expression. Cl(-) transport was measured as (36)Cl(-) efflux, as transepithelial short-circuit currents, and as whole cell patch-clamp current-voltage relations. The rate of (36)Cl(-) efflux and amplitude of currents in patch-clamp studies after the addition of the Ca(2+) ionophore A-23187 were increased significantly by pCLCA1 expression in freshly passaged Caco-2 cells. However, neither endogenous nor pCLCA1-dependent Ca(2+)-sensitive Cl(-) conductance could be detected in 14-day-postpassage cells. In contrast to Ca(2+)-sensitive Cl(-) conductance, endogenous cAMP-dependent Cl(-) conductance does not disappear on Caco-2 differentiation. cAMP-dependent Cl(-) conductance was modulated by pCLCA1 expression in Caco-2 cells, and this modulation was observed in freshly passaged and in mature 14-day-postpassage Caco-2 cultures. pCLCA1 mRNA expression, antigenic pCLCA1 protein epitope expression, and pCLCA1 function as a modulator of cAMP-dependent Cl(-) conductance were retained through differentiation in Caco-2 cells, whereas Ca(2+)-dependent Cl(-) conductance disappeared. We conclude that pCLCA1 expression may increase the sensitivity of preexisting endogenous Cl(-) channels to Ca(2+) and cAMP agonists but apparently lacks inherent Cl(-) channel activity under growth conditions where endogenous channels are not expressed.

Molecular and Integrative Physiology of Intestinal Peptide Transport

Intestinal protein digestion generates a huge variety and quantity of short chain peptides that are absorbed into intestinal epithelial cells by the PEPT1 transporter in the apical membrane of enterocytes. PEPT1 operates as an electrogenic proton/peptide symporter with the ability to transport essentially every possible di- and tripeptide. Transport is enantio-selective and involves a variable proton-to-substrate stoichiometry for uptake of neutral and mono- or polyvalently charged peptides. Neither free amino acids nor peptides containing four or more amino acids are accepted as substrates. The structural similarity of a variety of drugs with the basic structure of di- or tripeptides explains the transport of aminocephalosporins and aminopenicillins, selected angiotensin-converting inhibitors, and amino acid-conjugated nucleoside-based antiviral agents by PEPT1. The high transport capacity of PEPT1 allows fast and efficient intestinal uptake of the drugs but also of amino acid nitrogen even in states of impaired mucosal functions. Transcriptional and post-transcriptional regulation of PEPT1 occurs in response to alterations in the nutritional status and in disease states, suggesting a prime role of this transporter in amino acid absorption.

The proton oligopeptide cotransporter family SLC15 in physiology and pharmacology

Mammalian members of the SLC15 family are electrogenic transporters that utilize the proton-motive force for uphill transport of short chain peptides and peptido-mimetics into a variety of cells. The prototype transporters of this family are PEPT1 (SLC15A1) and PEPT2 (SLC15A2), which mediate the uptake of peptide substrates into intestinal and renal epithelial cells. More recently, other sites of functional expression of the two proteins have been identified such as bile duct epithelium (PEPT1), glia cells and epithelia of the choroid plexus, lung and mammary gland (PEPT2). Both proteins can transport essentially every possible di- and tripeptide regardless of the substrate's net charge, but operate stereoselectively. Based on peptide-like structures, various drugs and prodrugs are transported as well, allowing efficient intestinal absorption of the compounds via PEPT1. In kidney tubules both peptide transporters can mediate the renal reabsorption of the filtered compounds thus affecting their pharmacokinetics. Recently, two new peptide transporters, PHT1 (SLC15A4) and PHT2 (SLC15A3), were identified in mammals. They possess an overall amino acid identity with the PEPT-series of 20% to 25%. PHT1 and PHT2 were shown to transport free histidine and certain di- and tripeptides, but it is not yet clear whether they are located on the plasma membrane or represent lysosomal transporters for the proton-dependent export of histidine and dipeptides from lysosomal protein degradation into the cytosol.

Intestinal membrane transport of drugs and nutrients: genomics of membrane transporters using expression microarrays

Carrier-mediated transport across membranes plays an important role in drug and nutrient absorption. However, relevant transporters remain largely unknown for most substrates. Their identification requires global analysis of expressed mRNAs in intestinal tissues. Microarray technologies capable of measuring mRNA profiles have proven useful in detecting the expression of genes encoding transporters and ion channels in intestines and Caco-2 cells. This colon carcinoma cell line with characteristics of absorptive enterocytes serves as a common model for drug absorption studies. Gene expression patterns of membrane transporters and channels define the cell's overall transport capacity. Moreover, transporter mRNA profiles provide a basis for assessing drug-drug and drug-food interactions in intestinal absorption. To determine relevant transporters for any given substrate, chemogenomic methods have emerged correlating mRNA expression in multiple tissues to drug transport or response. The resultant drug-transporter databases permit the search for transporter-drug relationships at a genomic scale.

Current perspectives on established and putative mammalian oligopeptide transporters

Peptides and peptide-based drugs are increasingly being utilized as therapeutic agents for the treatment of numerous disorders. The increasing development of peptide-based therapeutic agents is largely due to technological advances including the advent of combinatorial peptide libraries, peptide synthesis strategies, and peptidomimetic design. Peptides and peptide-based agents have a broad range of potential clinical applications in the treatment of many disorders including AIDS, hypertension, and cancer. Peptides are generally hydrophilic and often exhibit poor passive transcellular diffusion across biological barriers. Insights into strategies for increasing their intestinal absorption have been derived from the numerous studies demonstrating that the absorption of protein digestion products occurs primarily in the form of small di- and tripeptides. The characterization of the pathways of intestinal, transepithelial transport of peptides and peptide-based drugs have demonstrated that a significant degree of absorption occurs through the role of proteins within the proton-coupled, oligopeptide transporter (POT) family. Considerable focus has been traditionally placed on Peptide Transporter 1 (PepT1) as the main mammalian POT member regulating intestinal peptide absorption. Recently, several new POT members, including Peptide/Histidine Transporter 1 (PHT1) and Peptide/Histidine Transporter 2 (PHT2) and their splice variants have been identified. This has led to an increased need for new experimental methods enabling better characterization of the biophysical and biochemical barriers and the role of these POT isoforms in mediating peptide-based drug transport.