Transport of amino acid aryl amides by the intestinal H+/peptide cotransport system, PEPT1

Transport Characteristics of 5-Aminosalicylic Acid Derivatives Conjugated with Amino Acids via Human H+-Coupled Oligopeptide Transporter PEPT1

5-Aminosalicylic acid (5-ASA) is used as first line therapy for symptom remission and maintenance of inflammatory bowel disease (IBD). Because 5-ASA is well absorbed from the small intestine when orally administered, several 5-ASA formulations for selective delivery to the colon have been developed and used in clinical practice. However, its delivery efficiency to local inflamed colonic sites remains low. Intestinal H⁺-coupled oligopeptide transporter 1 (PEPT1) expression in the colon is low, whereas its expression is induced in the colon under chronic inflammation conditions, such as IBD. Therefore, we considered that PEPT1 would be a target transporter to improve 5-ASA delivery efficiency to local colonic lesions. We evaluated the transport characteristics of dipeptide-like 5-ASA derivatives, which were coupling glycine (Gly), lysine, glutamic acid (Glu), valine (Val) and tyrosine to amino or carboxyl group of 5-ASA, in Caco-2 cells. [³H]Glycylsarcosine (Gly-Sar) uptake into Caco-2 cells was inhibited by all 5-ASA derivatives. In addition, 5-ASA derivatives (Gly-ASA, Glu-ASA and Val-ASA), which were coupled by glycine, glutamic acid and valine to amino group of 5-ASA, were taken up in a pH- and concentration-dependent manner and their uptake was inhibited by excess Gly-Sar. Two-electrode voltage-clamp experiment using human PEPT1 expressing Xenopus oocytes showed that Gly-ASA, Glu-ASA and Val-ASA induced marked currents at pH 6.0. Taken together, these results showed that these 5-ASA derivatives are transportable substrates for PEPT1.

Transporter-Targeted Nano-Sized Vehicles for Enhanced and Site-Specific Drug Delivery

Nano-devices are recognized as increasingly attractive to deliver therapeutics to target cells. The specificity of this approach can be improved by modifying the surface of the delivery vehicles such that they are recognized by the target cells. In the past, cell-surface receptors were exploited for this purpose, but plasma membrane transporters also hold similar potential. Selective transporters are often highly expressed in biological barriers (e.g., intestinal barrier, blood-brain barrier, and blood-retinal barrier) in a site-specific manner, and play a key role in the vectorial transfer of nutrients. Similarly, selective transporters are also overexpressed in the plasma membrane of specific cell types under pathological states to meet the biological needs demanded by such conditions. Nano-drug delivery systems could be strategically modified to make them recognizable by these transporters to enhance the transfer of drugs across the biological barriers or to selectively expose specific cell types to therapeutic drugs. Here, we provide a comprehensive review and detailed evaluation of the recent advances in the field of transporter-targeted nano-drug delivery systems. We specifically focus on areas related to intestinal absorption, transfer across blood-brain barrier, tumor-cell selective targeting, ocular drug delivery, identification of the transporters appropriate for this purpose, and details of the rationale for the approach.

Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease

Mammalian members of the proton-coupled oligopeptide transporter family are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs and couple substrate translocation to the movement of H⁺ , with the transmembrane electrochemical proton gradient providing the driving force. Peptide transporters are responsible for the (re)absorption of dietary and/or bacterial di- and tripeptides in the intestine and kidney and maintaining homeostasis of neuropeptides in the brain. These proteins additionally contribute to absorption of a number of pharmacologically important compounds. In this overview article, we have provided updated information on the structure, function, expression, localization, and activities of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4), and PhT2 (SLC15A3). Peptide transporters, in particular, PepT1 are discussed as drug-delivery systems in addition to their implications in health and disease. Particular emphasis has been placed on the involvement of PepT1 in the physiopathology of the gastrointestinal tract, specifically, its role in inflammatory bowel diseases. © 2018 American Physiological Society. Compr Physiol 8:731-760, 2018.

Accurate Prediction of Ligand Affinities for a Proton-Dependent Oligopeptide Transporter

Membrane transporters are critical modulators of drug pharmacokinetics, efficacy, and safety. One example is the proton-dependent oligopeptide transporter PepT1, also known as SLC15A1, which is responsible for the uptake of the β-lactam antibiotics and various peptide-based prodrugs. In this study, we modeled the binding of various peptides to a bacterial homolog, PepT_St, and evaluated a range of computational methods for predicting the free energy of binding. Our results show that a hybrid approach (endpoint methods to classify peptides into good and poor binders and a theoretically exact method for refinement) is able to accurately predict affinities, which we validated using proteoliposome transport assays. Applying the method to a homology model of PepT1 suggests that the approach requires a high-quality structure to be accurate. Our study provides a blueprint for extending these computational methodologies to other pharmaceutically important transporter families.

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A hierarchical computational approach determines ligand affinities to transporters

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Lysine-containing dipeptides proposed to bind vertically like a tripeptide

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Experimental structures are vital for the accurate prediction of affinities

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A model of prodrug interactions to human PepT1 is suggested

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

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.

Bioavailability through PepT1: the role of computer modelling in intelligent drug design

In addition to being responsible for the majority of absorption of dietary nitrogen, the mammalian proton-coupled di- and tri-peptide transporter PepT1 is also recognised as a major route of drug delivery for several important classes of compound, including beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Thus there is considerable interest in the PepT1 protein and especially its substrate binding site. In the absence of a crystal structure, computer modelling has been used to try to understand the relationship between PepT1 3D structure and function. Two basic approaches have been taken: modelling the transporter protein, and modelling the substrate. For the former, computer modelling has evolved from early interpretations of the twelve transmembrane domain structure to more recent homology modelling based on recently crystallised bacterial members of the major facilitator superfamily (MFS). Substrate modelling has involved the proposal of a substrate binding template, to which all substrates must conform and from which the affinity of a substrate can be estimated relatively accurately, and identification of points of potential interaction of the substrate with the protein by developing a pharmacophore model of the substrates. Most recently, these two approaches have moved closer together, with the attempted docking of a substrate library onto a homology model of the human PepT1 protein. This article will review these two approaches in which computers have been applied to peptide transport and suggest how such computer modelling could affect drug design and delivery through PepT1.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Transport of free and peptide-bound pyrraline at intestinal and renal epithelial cells

Pyrraline is a quantitatively dominating glycation compound of the advanced Maillard reaction in foods and can be found in urine after consumption of pyrraline-containing food items. The purpose of this study was to investigate the transport of pyrraline and its dipeptide derivatives alanylpyrraline (Ala-Pyrr) and pyrralylalanine (Pyrr-Ala) at intestinal and renal cell lines. Pyrraline inhibited the l-[(3)H]lysine uptake with IC(50) values of 0.3 mM (Caco-2 cells) and 3.5 mM (OK cells), respectively, but not the uptake of [(14)C]Gly-Sar (Caco-2 and SKPT cells). In contrast, Ala-Pyrr strongly inhibited the uptake of [(14)C]Gly-Sar in Caco-2 and SKPT cells with IC(50) values of 0.19 and 0.017 mM, respectively. Pyrr-Ala inhibited the carrier-mediated uptake of [(14)C]Gly-Sar in Caco-2 and SKPT cells by 50% at concentrations of 0.03 and 0.008 mM, respectively. The transepithelial flux of peptide-bound pyrraline across Caco-2 cell monolayers was up to 15-fold higher compared to the flux of free pyrraline. We conclude that free pyrraline is not a substrate for the intestinal lysine transporter and that the absorption of dietary pyrraline occurs most likely in the form of dipeptides rather than as the free amino acid.

Recognition of 2-aminothiazole-4-acetic acid derivatives by the peptide transporters PEPT1 and PEPT2

The H(+)/peptide cotransporters PEPT1 and PEPT2 have gained considerable interest in pharmaceutical sciences as routes for drug delivery. It is, therefore, of interest to develop uncommon artificial substrates for the two carriers. This study was initiated to investigate the binding affinity of 2-aminothiazole-4-acetic acid (ATAA) conjugates with amino acids to PEPT1 and PEPT2. The 2-aminothiazole-4-acetic acid derivatives have been synthesised and tested for their affinity to PEPT1 and PEPT2. The K(i) values were compared with in silico predicted values from CoMSIA models. C-terminal ATAA-Xaa conjugates proved to be low to medium inhibitors of the [(14)C]Gly-Sar uptake at both carrier systems whereas N-terminal Xaa-ATAA conjugates exhibited medium to high affinity. A promising candidate for further functionalisation is Val-ATAA which shows extraordinary high affinity to PEPT1.

Regulation of the oligopeptide transporter, PEPT-1, in DSS-induced rat colitis

The effect of colitis induced with dextran sodium sulfate (DSS) in rats on the bioavailability of drugs transported by the oligopeptide transporter PepT-1 was analyzed by studying the pharmacokinetics of PepT-1 substrates: cephalexin and valacyclovir, the prodrug of antiviral acyclovir. Western blot, immunohistochemistry, and real-time PCR were used to determine the PepT-1 protein and gene expression. We observed (1) no significant modification of PepT-1 expression in the duodenum and jejunum; (2) a slight decrease in both PepT-1 mRNA (50%) and protein expression (25%) in the ileum following DSS challenge; and (3) ectopic PepT-1 immunostaining in regenerative hyperplasia segments in the distal colon from DSS-treated rats where focal inflammation is localized. However, no modification of pharmacokinetic parameters (C (max), T (max), AUC) of cephalexin or acyclovir was detected. In conclusion, DSS-induced rat colitis did not alter PepT-1 substrate bioavailability despite certain modifications in PepT-1 expression profile.

A charge pair interaction between Arg282 in transmembrane segment 7 and Asp341 in transmembrane segment 8 of hPepT1

PURPOSE

To determine whether R282 in transmembrane segment 7 (TMS7) of hPepT1 forms a salt bridge with D341 in TMS8.

METHODS

Mutated hPepT1 transporters containing point mutations at R282 and/or D341 were transiently transfected into HEK293 cells. Their steady state expression and functional activity were measured using immunoprecipitation and 3H-gly-sar uptake, respectively. Gly-sar uptake by cysteine mutants (R282C and D341C) was also measured in the presence and absence of cysteine-modifying MTS reagents.

RESULTS

The reverse-charge mutants R282D-hPepT1 and D341R-hPepT1 showed significantly reduced gly-sar uptake, but the double mutant (R282D/D341R-hPepT1) has functionality comparable to that of wild-type hPepT1. Gly-sar uptake by R282C-hPepT1 is reduced, but pre-incubation with 1 mM MTSET, a positively charged cysteine-modifying reagent, restored function to wild-type levels. Similarly, pre-incubation of D341C-hPepT1 with 10 mM MTSES, a negatively charged cysteine-modifying reagent, increased gly-sar uptake compared to unmodified D341C-hPepT1. In contrast, MTSET modification of D341C-hPepT1 (giving a positive charge at position 341) resulted in significant reduction in gly-sar uptake, compared to D341C-hPepT1.

CONCLUSION

Our results are consistent with a salt bridge between R282 and D341 in hPepT1, and we use these and other data to propose a role for the R282-D341 charge pair in the hPepT1 translocation mechanism.

Structural requirements for the substrates of the H+/peptide cotransporter PEPT2 determined by three-dimensional quantitative structure-activity relationship analysis

The renal type H(+)/peptide cotransporter PEPT2 has a substantial influence on the in vivo disposition of dipeptides and tripeptides as well as peptide-like drugs within the body, particularly in kidney, lung, and the brain. The comparative molecular similarity indices analysis (CoMSIA) method was applied to identify those regions in the substrate structures that are responsible for recognition and for differences in affinity. We have developed a comprehensive 3D quantitative structure-activity relationship (3D-QSAR) model based on 83 compounds that is able to explain and predict the binding affinities of new PEPT2 substrates. This 3D-QSAR model possesses a high predictive power (q(2) = 0.755; r(2) = 0.893). An additional 3D-QSAR model based on the same compounds was generated and correlated with affinity data of the intestinal H(+)/peptide cotransporter PEPT1. By comparing the CoMSIA contour plots, differences in selectivity between the intestinal and the renal type peptide carrier become evident.

The renal type H+/peptide symporter PEPT2: structure-affinity relationships

The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

Transepithelial flux of early and advanced glycation compounds across Caco-2 cell monolayers and their interaction with intestinal amino acid and peptide transport systems

Maillard products arise from condensation reactions between amino acids or proteins with reducing sugars during food processing. As ubiquitous components of human food, these early or advanced glycation products may be subject to intestinal absorption. The present study was performed to investigate the intestinal uptake of Maillard products and to determine whether they are substrates for peptide and amino acid transporters expressed at the apical membrane of Caco-2 cells. At a concentration of 10 mM, N(epsilon)-(carboxymethyl)-L-lysine, N(alpha)-hippuryl-N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine, N(alpha)-hippuryl-N(epsilon)-(carboxymethyl)-L-lysine and N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine inhibited the [(14)C]glycylsarcosine uptake mediated by the H(+)-peptide co-transporter PEPT1 by 13 to 45%. For N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine, an inhibitory constant of 8.7 mM was determined, reflecting a low affinity to PEPT1 in comparison with natural dipeptides. Uptake of L-[(3)H]lysine was weakly affected by N(epsilon)-(carboxymethyl)-L-lysine, N(alpha)-hippuryl-L-lysine and N(alpha)-hippuryl-N(epsilon)-(carboxymethyl)-L-lysine but strongly inhibited by N(epsilon)-(1-deoxy-D-fructosyl)-L-lysine (81%). None of the Maillard products was able to inhibit the uptake of L-[(3)H]leucine by more than 15%. We also studied the transepithelial flux of Maillard products across Caco-2 cell monolayers cultured on permeable filters. The flux rates of Maillard products ranged from 0.01 to 0.3%/cm(2) per h and were shown to be much lower than those of carrier substrates such as glycylsarcosine, L-proline and the space marker [(14)C]mannitol. We conclude that the Maillard products investigated in the present study are neither transported by PEPT1 nor by carriers for neutral amino acids. The low transepithelial flux measured for these compounds most probably occurs by simple diffusion.

Human PEPT1 Pharmacophore Distinguishes between Dipeptide Transport and Binding

The human intestinal oligopeptide transporter (PEPT1) facilitates the absorption of dipeptides, tripeptides, and many peptidomimetic drugs. In this study, a large number of peptides were selected to investigate the structural features required for PEPT1 transport. Binding affinity was determined in a Gly-Sar uptake inhibition assay, whereas functional transport was ranked in a membrane depolarization assay. Although most of the peptides tested could bind to PEPT1, not all were substrates. As expected, single amino acids and tetrapeptides could not bind to or be transported by PEPT1. Dipeptide transport was influenced by charge, hydrophobicity, size, and side chain flexibility. The extent of transport was variable, and unexpectedly, some dipeptides were not substrates of PEPT1. These included dipeptides with two positive charges or extreme bulk in either position 1 or 2. Our results identify key features required for PEPT1 transport in contrast to most previously described pharmacophores, which are based on the inhibition of transport of a known substrate.

Interaction of intestinal nucleoside transporter hCNT2 with amino acid ester prodrugs of floxuridine and 2-bromo-5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

Amino acid ester prodrugs of antiviral and anticancer nucleoside drugs were developed to improve oral bioavailability or to reduce systemic toxicity. We studied the interaction of human concentrative nucleoside transporter (hCNT2) cloned from intestine with various amino acid ester prodrugs of floxuridine (FUdR) and 5,6-dichloro-2-bromo-1-beta-D-ribofuranosylbenzimidazole (BDCRB). Na(+)-dependent uptakes of [(3)H]-inosine and [(3)H]-adenosine were measured in U251 cells transiently expressing intestinal hCNT2. FUdR significantly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine (60-70% of control), while its amino acid ester prodrugs including Val, Phe, Pro, Asp, and Lys esters exhibited markedly decreased inhibition potency (10-30% of control). On the other hand, BDCRB and its amino acid prodrugs markedly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine. Val, Phe, and Pro ester prodrugs of BDCRB showed similar inhibition capacities as parent compound BDCRB (80-90% for adenosine and 60-80% for inosine). The amino acid site of attachment (3'- and 5'-monoesters) and stereochemistry (L- and D-amino acid esters), did not significantly affect the uptake of [(3)H]-inosine and [(3)H]-adenosine. These results demonstrate that the hCNT2 favorably interacts with BDCRB and its amino acid prodrugs, compared to those of FUdR, and that neutral amino acid esters of BDCRB have a high affinity toward this transporter. Therefore, the intestinal hCNT2 may be a target transporter as a factor for modulating oral pharmacokinetics of BDCRB prodrugs.

Affinity prediction for substrates of the peptide transporter PepT1

A quantitative method has been developed for determining the affinity of substrates for the peptide transporter PepT1, allowing oral availability of drugs via PepT1 to be estimated.

Three-Dimensional Quantitative Structure−Activity Relationship Analyses of β-Lactam Antibiotics and Tripeptides as Substrates of the Mammalian H+/Peptide Cotransporter PEPT1

The utilization of the membrane transport protein PEPT1 as a drug delivery system is a promising strategy to enhance the oral bioavailability of drugs. Since very little is known about the substrate binding site of PEPT1, computational methods are a meaningful tool to gain a more detailed insight into the structural requirements for substrates. Three-dimensional quantitative structure-activity relationship (3D-QSAR) studies using the comparative molecular similarity indices analysis (CoMSIA) method were performed on a training set of 98 compounds. Affinity constants of beta-lactam antibiotics and tripeptides were determined at Caco-2 cells. A statistically reliable model of high predictive power was obtained (q(2) = 0.828, r(2) = 0.937). The results derived from CoMSIA were graphically interpreted using different field contribution maps. We identified those regions which are crucial for the interaction between peptidomimetics and PEPT1. The new 3D-QSAR model was used to design a new druglike compound mimicking a dipeptide. The predicted K(i) value was confirmed experimentally.

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.

Analysis of the transport properties of side chain modified dipeptides at the mammalian peptide transporter PEPT1

This study was initiated to examine systematically the effect of side chain modifications at dipeptides on their transport via PEPT1. We synthesized a series of Xaa(R)-Ala and Ala-Xaa(R) dipeptides with the functional groups of the side chains modified by structurally different blocking groups R. Recognition and transport of these derivatives by PEPT1 was measured in Caco-2 cells, in transgenic Pichia pastoris cells and in Xenopus laevis oocytes expressing PEPT1. The dipeptide derivatives displayed K(i) values between 0.002 and 4 mM. Electrophysiological analyses showed that the Ala-Xaa(R) derivatives were transported by PEPT1. In contrast, most Xaa(R)-Ala derivatives--although recognized--did not show significant transport rates. Substitution of a terminal phenyl residue in the side chain blocking group by a p-nitrophenyl residue enhanced the affinity of several dipeptide derivatives for interaction with PEPT1. However, none of these compounds showed electrogenic transport in oocytes. With a K(i) value of 0.002 mM, Lys[Z(NO(2))]-Val displayed the highest affinity to PEPT1 ever reported. We conclude that the transport of side chain modified dipeptides into enterocytes depends (a) on the position of the modified trifunctional amino acid in the dipeptide, (b) the distance between its alpha-carbon and the side chain blocking group and (c) the hydrophobic character of the side chain modification.

Transport of the phosphonodipeptide alafosfalin by the H+/peptide cotransporters PEPT1 and PEPT2 in intestinal and renal epithelial cells

The interaction of the antibacterial phosphonodipeptide alafosfalin with mammalian H(+)/peptide cotransporters was studied in Caco-2 cells, expressing the low-affinity intestinal type peptide transporter 1 (PEPT1), and SKPT cells, expressing the high-affinity renal type peptide transporter 2 (PEPT2). Alafosfalin strongly inhibited the uptake of [(14)C]glycylsarcosine with K(i) values of 0.19 +/- 0.01 mm and 0.07 +/- 0.01 mm for PEPT1 and PEPT2, respectively. Saturation kinetic studies revealed that in both cell types alafosfalin affected only the affinity constant (K(t)) but not the maximal velocity (V(max)) of glycylsarcosine (Gly-Sar) uptake. The inhibition constants and the competitive nature of inhibition were confirmed in Dixon-type experiments. Caco-2 cells and SKPT cells were also cultured on permeable filters: apical uptake and transepithelial apical to basolateral flux of [(14)C]Gly-Sar across Caco-2 cell monolayers were reduced by alafosfalin (3 mm) by 73%. In SKPT cells, uptake of [(14)C]Gly-Sar but not flux was inhibited by 61%. We found no evidence for an inhibition of the basolateral to apical uptake or flux of [(14)C]Gly-Sar by alafosfalin. Alafosfalin (3 mm) did not affect the apical to basolateral [(14)C]mannitol flux. Determined in an Ussing-type experiment with Caco-2 cells cultured in Snapwells trade mark, alafosfalin increased the short-circuit current through Caco-2 cell monolayers. We conclude that alafosfalin interacts with both H(+)/peptide symporters and that alafosfalin is actively transported across the intestinal epithelium in a H(+)-symport, explaining its oral availability. The results also demonstrate that dipeptides where the C-terminal carboxyl group is substituted by a phosphonic function represent high-affinity substrates for mammalian H(+)/peptide cotransporters.

A rapid in vitro screening for delivery of peptide-derived peptidase inhibitors as potential drug candidates via epithelial peptide transporters

Targeting drugs or prodrugs to a specific enzyme by simultaneously targeting cell membrane carriers for efficient transport should provide the highest bioavailability along with specificity at the site of action. The peptide transporters PEPT1 and PEPT2 are expressed in a variety of tissues, including the brush-border membranes of epithelial cells of the small intestine and kidney. The transporters accept a wide range of substrates and are therefore good targets for a transporter-mediated drug delivery. Here, we report a screening procedure for peptidomimetic drug candidates combining two independent expression systems: 1) a competition assay in transgenic Pichia pastoris yeast cells expressing either mammalian PEPT1 or PEPT2 for identifying substrate interaction with the transporter binding site; and 2) a Xenopus laevis-based oocyte expression of the peptide transporter for assessing electrogenic transport of drug candidates. Based on the known oral availability and in vivo efficacy of the dipeptidyl peptidase IV (DPIV) inhibitor isoleucine-thiazolidide and its peptide-like structure, we first tested whether this compound is a substrate of epithelial peptide transporters. Additionally, a series of structurally related inhibitors were analyzed for transport. We identified various compounds that serve as substrates of the intestinal peptide transporter PEPT1. In contrast, none of these DPIV inhibitors showed electrogenic transport by PEPT2, although a variety of the compounds displayed good affinities for competition in peptide uptake in PEPT2-expressing cells, suggesting that they may serve as efficient inhibitors. In conclusion, we have applied an in vitro screening system that predicts efficient intestinal absorption of peptide-derived peptidase inhibitors via PEPT1 in vivo.

The intestinal H+/peptide symporter PEPT1: structure–affinity relationships

Peptide transporter 1, PEPT1, of the mammalian enterocyte is presently under intense investigation in many laboratories because of its nutritional importance in the absorption of protein hydrolysis products and because more recent studies have shown that many drugs and prodrugs gain entry into the systemic circulation via PEPT1. Until the exact structural features of the substrate binding site of PEPT1 become available, for example by X-ray crystallography, determination of affinities followed by proof of actual membrane translocation will have to suffice when testing for possible new substrates for PEPT1. Affinity constants reflect the strength of their interaction with the binding site of the transporter. A review of the literature shows a wide range of affinity constants between 2 microM and 30 mM. We consider affinity constants for substrates or inhibitors of PEPT1 lower than 0.5 mM as high affinity, between 0.5 and 5.0 mM as medium affinity and above 5 mM as low affinity. Values above 15 mM we consider with great caution. In this mini-review we discuss affinities and structural determinants which affect affinities of a variety of substrates for PEPT1.

Three-Dimensional Quantitative Structure−Activity Relationship Analyses of Peptide Substrates of the Mammalian H+/Peptide Cotransporter PEPT1

The utilization of the carrier protein PEPT1 for the absorption of peptidomimetic drug molecules is a promising strategy for oral drug administration and increasing bioavailability. In the absence of structural information on the binding mode of substrates to PEPT1, a computational study was conducted to explore the structural requirements for substrates and to derive a predictive model that may be used for the design of novel orally active drugs. A comparative molecular field analysis (CoMFA) and a comparative molecular similarity indices analysis (CoMSIA) were performed on a series of 79 dipeptide-type substrates for which affinity data had been collected in a single test system under the same conditions. These studies produced models with conventional r(2) and cross-validated coefficient (q(2)) values of 0.901 and 0.642 for CoMFA and 0.913 and 0.776 for CoMSIA. The models were validated by an external test set of 19 dipeptides and dipeptide derivatives. CoMSIA contour maps were used to identify the recognition elements that are relevant for the binding of PEPT1 substrates. The 3D QSAR models provide an insight in the interactions between substrates and PEPT1 on the molecular level and allow the prediction of affinity constants of new compounds.

An update on renal peptide transporters

The brush-border membrane of renal epithelial cells contains PEPT1 and PEPT2 proteins that are rheogenic carriers for short-chain peptides. The carrier proteins display a distinct surface expression pattern along the proximal tubule, suggesting that initially di- and tripeptides, either filtered or released by surface-bound hydrolases from larger oligopeptides, are taken up by the low-affinity but high-capacity PEPT1 transporter and then by PEPT2, which possesses a higher affinity but lower transport capacity. Both carriers transport essentially all possible di- and tripeptides and numerous structurally related drugs. A unique feature of the mammalian peptide transporters is the capability of proton-dependent electrogenic cotransport of all substrates, regardless of their charge, that is achieved by variable coupling in proton movement along with the substrate down the transmembrane potential difference. This review focuses on the postcloning research efforts to understand the molecular physiology of peptide transport processes in renal tubules and summarizes available data on the underlying genes, protein structures, and transporter function as derived from studies in heterologous expression systems.

Theoretical predictions of drug absorption in drug discovery and development

The clinical development of new drugs is often terminated because of unfavourable pharmacokinetic properties such as poor intestinal absorption after oral administration. Intestinal permeability and solubility are two of the most important factors that determine the absorption properties of a compound. Efficient and reliable computational models that predict these properties as early as possible in drug discovery and development are therefore desirable. In this review, we first discuss the implementation of predictive models of intestinal drug permeability and solubility in drug discovery and development. Secondly, we discuss the mechanisms of intestinal drug permeability and computational methods that can be used to predict it. We then discuss factors influencing drug solubility and models for predicting it. We finally speculate that once these and other predictive computational models are implemented in drug discovery and development, these processes will become much more effective. Further, an increased fraction of drug candidates that are less likely to fail during clinical development will be selected.

Mammalian peptide transporters as targets for drug delivery

Peptide transporters are integral plasma membrane proteins that mediate the cellular uptake of dipeptides and tripeptides in addition to a variety of peptidomimetics. The carriers, which occur predominantly in the brush-border membranes of epithelial cells of the small intestine, lung, choroid plexus and kidney, contribute to absorption, distribution and elimination of their substrates. The cellular uptake of peptides and peptidomimetics involves the cotransport of protons down an inwardly directed, electrochemical proton gradient that provides the driving force and causes the electrogenicity of the translocation step. Peptide transporters represent excellent targets for the delivery of pharmacologically active compounds because their substrate-binding site can accommodate a wide range of molecules of differing size, hydrophobicity and charge.

Transport of peptidomimetic drugs by the intestinal Di/tri-peptide transporter, PepT1

The apical membrane of small intestinal enterocytes possess an uptake system for di- and tripeptides. The physiological function of the system is to transport small peptides resulting from digestion of dietary protein. Moreover, due to the broad substrate specificity of the system, it is also capable of transporting a number of orally administered peptidomimetic drugs. Absorbed peptides may be hydrolysed in the cells due to the high peptidase activity present in the cytosol. Peptidomimetic drugs may, if resistant to the cellular enzyme activity, pass the basolateral membrane via a basolateral peptide transport mechanism and enter the systemic circulation. As the number of new peptide and peptidomimetic drugs are rapidly increasing, the peptide transport system has gained increasing attention as a possible drug delivery system for small peptides and peptide-like compounds. In this paper we give an updated introduction to the transport system and discuss the substrate characteristics of the di/tri-peptide transporter system with special emphasis on chemically modified substrates and prodrugs.

Defining minimal structural features in substrates of the H(+)/peptide cotransporter PEPT2 using novel amino acid and dipeptide derivatives

The peptide transporter PEPT2, expressed in a variety of tissues, including kidney, lung, and the central nervous system, mediates the uphill transport of di- and tripeptides, as well as a variety of peptidomimetic drugs. To identify the essential molecular features of substrates that determine affinity and transport by PEPT2, we synthesized a series of amino acid derivatives as well as modified dipeptides. Kinetic constants for the interaction of test compounds with PEPT2 were obtained in a competition assay using Pichia pastoris yeast cells expressing mammalian PEPT2. The two-electrode voltage-clamp technique in Xenopus laevis oocytes was used to assess the substrate's electrogenic transport properties. Whereas omega bar-amino fatty acids showed no affinity for PEPT2, the introduction of a single carbonyl group into the backbone increased both affinity and transport currents more than 30-fold. omega bar-amino fatty acids, at their amino or carboxyl group coupled to an alanine residue, allowed us to determine the importance of the spatial position of functional groups within the molecule. Affinity and transport function declined by elongating the omega bar-amino acid chain when located in the N-terminal position, whereas the elongation in the carboxyl terminal with an N-terminal alanine caused less pronounced effects. The results clearly establish that a free N terminus, a correctly positioned backbone carbonyl group, and a carboxylic group that is in a suitable distance from the intramolecular carbonyl function and the amino terminal head group are the main features for substrate recognition and transport by PEPT2. This information provides the framework for a rational design of peptidomimetic drugs for delivery via PEPT2.

Modified amino acids and peptides as substrates for the intestinal peptide transporter PepT1

The binding affinities of a number of amino-acid and peptide derivatives by the mammalian intestinal peptide transporter PepT1 were investigated, using the Xenopus laevis expression system. A series of blocked amino acids, namely N-acetyl-Phe (Ac-Phe), phe-amide (Phe-NH2), N-acetyl-Phe-amide (Ac-Phe-NH2) and the parent compound Phe, was compared for efficacy in inhibiting the uptake of the peptide [3H]-D-Phe-L-Gln. In an equivalent set of experiments, the blocked peptides Ac-Phe-Tyr, Phe-Tyr-NH2 and Ac-Phe-Tyr-NH2 were compared with the parent compound Phe-Tyr. Comparing amino acids and derivatives, only Ac-Phe was an effective inhibitor of peptide uptake (Ki = 1.81+/- 0.37 mM). Ac-Phe-NH2 had a very weak interaction with PepT1 (Ki = 16.8+/-5.64 mM); neither Phe nor Phe-NH2 interacted with PepT1 with measurable affinity. With the dipeptide and derivatives, unsurprisingly the highest affinity interaction was with Phe-Tyr (Ki = 0.10+/-0.04 mM). The blocked C-terminal peptide Phe-Tyr-NH2 also interacted with PepT1 with a relatively high affinity (Ki = 0.94+/-0.38 mM). Both Ac-Phe-Tyr and Ac-Phe-Tyr-NH2 interacted weakly with PepT1 (Ki = 8.41+/-0.11 and 9.97+/-4.01 mM, respectively). The results suggest that the N-terminus is the primary binding site for both dipeptides and tripeptides. Additional experiments with four stereoisomers of Ala-Ala-Ala support this conclusion, and lead us to propose that a histidine residue is involved in binding the C-terminus of dipeptides. In addition, a substrate binding model for PepT1 is proposed.

Transport of valganciclovir, a ganciclovir prodrug, via peptide transporters PEPT1 and PEPT2

In clinical trials, valganciclovir, the valyl ester of ganciclovir, has been shown to enhance the bioavailability of ganciclovir when taken orally by patients with cytomegalovirus infection. We investigated the role of the intestinal peptide transporter PEPT1 in this process by comparing the interaction of ganciclovir and valganciclovir with the transporter in different experimental systems. We also studied the interaction of these two compounds with the renal peptide transporter PEPT2. In cell culture model systems using Caco-2 cells for PEPT1 and SKPT cells for PEPT2, valganciclovir inhibited glycylsarcosine transport mediated by PEPT1 and PEPT2 with K(i) values (inhibition constant) of 1.68+/-0.30 and 0.043+/- 0.005 mM, respectively. The inhibition by valganciclovir was competitive in both cases. Ganciclovir did not interact with either transporter. Similar studies done with cloned PEPT1 and PEPT2 in heterologous expression systems yielded comparable results. The transport of valganciclovir via PEPT1 was investigated directly in PEPT1-expressing Xenopus laevis oocytes with an electrophysiological approach. Valganciclovir, but not ganciclovir, induced inward currents in PEPT1-expressing oocytes. These results demonstrate that the increased bioavailability of valganciclovir is related to its recognition as a substrate by the intestinal peptide transporter PEPT1. This prodrug is also recognized by the renal peptide transporter PEPT2 with high affinity.

Kinetics and substrate specificity of membrane-reconstituted peptide transporter DtpT of Lactococcus lactis

The peptide transport protein DtpT of Lactococcus lactis was purified and reconstituted into detergent-destabilized liposomes. The kinetics and substrate specificity of the transporter in the proteoliposomal system were determined, using Pro-[(14)C]Ala as a reporter peptide in the presence of various peptides or peptide mimetics. The DtpT protein appears to be specific for di- and tripeptides, with the highest affinities for peptides with at least one hydrophobic residue. The effect of the hydrophobicity, size, or charge of the amino acid was different for the amino- and carboxyl-terminal positions of dipeptides. Free amino acids, omega-amino fatty acid compounds, or peptides with more than three amino acid residues do not interact with DtpT. For high-affinity interaction with DtpT, the peptides need to have free amino and carboxyl termini, amino acids in the L configuration, and trans-peptide bonds. Comparison of the specificity of DtpT with that of the eukaryotic homologues PepT(1) and PepT(2) shows that the bacterial transporter is more restrictive in its substrate recognition.

Decisive structural determinants for the interaction of proline derivatives with the intestinal H+/peptide symporter

To elucidate the decisive structural factors relevant for dipeptide-carrier interaction, the affinity of short amide and imide derivatives for the intestinal H+/peptide symporter (PEPT1) was investigated by measuring their ability to inhibit Gly-Sar transport in Caco-2 cells. Dipeptides with proline or alanine in the C-terminal position displayed affinity constants (Ki) of 0.15-1.2 mM and 0.08-9.5 mM, respectively. There was no clear relationship between hydrophobicity, size or ionization status of the N-terminal amino acid and the affinity of the dipeptides. However, analyzing the individual peptide bond conformations of Xaa-Pro dipeptides, a striking correlation between the cis/trans ratios (trans contents 24-70%) and the affinity constants was observed. After correcting the Ki values for the incompetent cis isomers, the Ki corr values of most dipeptides were in a small range of 0.1-0.16 mM. This result revealed the decisive role of peptide bond conformation even for a transport protein that is quite promiscuous in substrate translocation. When measuring affinity constants of Xaa-Pro and Xaa-Sar dipeptides, the cis/trans ratios cannot be ignored. Lower affinities of Lys-Pro, Arg-Pro and Pro-Pro indicate that additional molecular factors affect their binding at PEPT1. The Ki values obtained for the corresponding Xaa-Ala dipeptides support this conclusion. Potential substrates or inhibitors of peptide transport were found among Xaa-piperidides and Xaa-thiazolidides. Dipeptides with N-terminal proline displayed a very diverse affinity profile. However, in contrast to current knowledge, several Pro-Xaa dipeptides such as Pro-Leu, Pro-Tyr and Pro-Pro are recognized by PEPT1 with appreciable affinities. Binding seems mainly determined by the hydrophobicity of the C-terminal amino acid and the rigidity of the structure.

Effects of glibenclamide on glycylsarcosine transport by the rat peptide transporters PEPT1 and PEPT2

1 Glibenclamide is a widely used sulphonylurea for the treatment of non-insulin-dependent diabetes mellitus (NIDDM). This agent has been reported to inhibit the activities of various ion channels and transporters. In the present study, we examined the effects of glibenclamide on the function of the H+/peptide cotransporters PEPT1 and PEPT2 by using stable transfectants. 2 Uptake of [14C]-glycylsarcosine, a typical substrate for peptide transporters, by PEPT1- or PEPT2-expressing transfectant was inhibited by glibenclamide as well as other sulphonylureas including tolbutamide. 3 Kinetic analysis revealed that the inhibition by glibenclamide was noncompetitive. Dixon plot analyses showed that the Ki values of this agent were 25 and 7.8 microM for PEPT1 and PEPT2, respectively. 4 Glibenclamide did not inhibit Na+-coupled alanine and alpha-methyl-D-glucoside transport, suggesting that the inhibitory effects of glibenclamide on peptide transporters were not due to nonspecific interactions. 5 There was little uptake of [3H]-glibenclamide by PEPT-expressing transfectants as compared to mock-transfected cells, suggesting that glibenclamide was not a substrate for these peptide transporters. 6 In summary, glibenclamide inhibited the [14C]-glycylsarcosine transport by PEPT1 and PEPT2 in a noncompetitive fashion, although glibenclamide per se was not transported through these transporters. These findings would provide important information for clinical, physiological and biochemical aspects of peptide transporters.