Molecular interactions between dipeptides, drugs and the human intestinal H+ -oligopeptide cotransporter hPEPT1

Effect of X-ray irradiation on renal excretion of bestatin through down-regulating organic anion transporters via the vitamin D receptor in rats

Pharmacokinetic changes induced by radiation following radiotherapy ("RT-PK" phenomenon) are of great significance to the effectiveness and safety of chemotherapeutic agents in clinical settings. The aims of this study were to clarify the organic anion transporters (Oats) involved in the "RT-PK" phenomenon of bestatin in rats following X-ray irradiation and to elucidate its potential mechanism via vitamin D signalling. Pharmacokinetic studies, uptake assays using rat kidney slices and primary proximal tubule cells, and molecular biological studies were performed. Significantly increased plasma concentrations and systemic exposure to bestatin were observed at 24 and 48 h following abdominal X-ray irradiation, regardless of oral or intravenous administration of the drugs in rats. Reduced renal clearance and cumulative urinary excretion of bestatin were observed at 24 and 48 h post-irradiation in rats following intravenous administration. The uptake of the probe substrates p-aminohippuric acid and oestrone 3-sulfate sodium in vitro and the expression of Oat1 and Oat3 in vivo were reduced in the corresponding models following irradiation. Moreover, the upregulation of the vitamin D receptor (Vdr) in mRNA and protein levels negatively correlated with the expressions and functions of Oat1 and Oat3 following irradiation. Additionally, elevated plasma urea nitrogen levels and histopathological changes were observed in rats after exposure to irradiation. The "RT-PK" phenomenon of bestatin occurs in rats after exposure to irradiation, possibly resulting in the regulation of the expressions and activities of renal Oats via activation of the Vdr signalling pathway.

Structural basis for antibiotic transport and inhibition in PepT2, the mammalian proton-coupled peptide transporter

The uptake and elimination of beta-lactam antibiotics in the human body are facilitated by the proton-coupled peptide transporters PepT1 (SLC15A1) and PepT2 (SLC15A2). The mechanism by which SLC15 family transporters recognize and discriminate between different drug classes and dietary peptides remains unclear, hampering efforts to improve antibiotic pharmacokinetics through targeted drug design and delivery. Here, we present cryo-EM structures of the mammalian proton-coupled peptide transporter, PepT2, in complex with the widely used beta-lactam antibiotics cefadroxil, amoxicillin and cloxacillin. Our structures, combined with pharmacophore mapping, molecular dynamics simulations and biochemical assays, establish the mechanism of antibiotic recognition and the important role of protonation in drug binding and transport.

Unraveling the kinetics and pharmacology of human PepT1 using solid supported membrane-based electrophysiology

The human Peptide Transporter 1 (hPepT1) is known for its broad substrate specificity and its ability to transport (pro-)drugs. Here, we present an in-depth comprehensive study of hPepT1 and its interactions with various substrates via solid supported membrane-based electrophysiology (SSME). Using hPepT1-containing vesicles, we could not identify any peptide induced pre-steady-state currents, indicating that the recorded peak currents reflect steady-state transport. Electrogenic co-transport of H+/glycylglycine (GlyGly) was observed across a pH range of 5.0 to 9.0. The pH dependence is described by a bell-shaped activity curve and two pK values. KM and relative Vmax values of various canonical and non-canonical peptide substrates were contextualized with current mechanistic understandings of hPepT1. Finally, specific inhibition was observed for various inhibitors in a high throughput format, and IC50 values are reported. Taken together, these findings contribute to promoting the design and analysis of pharmacologically relevant substances.

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.

The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research

After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.

PTR2/POT/NPF transporters: what makes them tick?

PTR2/POT/NPF are a family of primarily proton coupled transporters that belong to the major facilitator super family and are found across most kingdoms of life. They are involved in uptake of nutrients, hormones, ions and several orally administered drug molecules. A wealth of structural and functional data is available for this family; the similarity between the protein structural features have been discussed and investigated in detail on several occasions, however there are no reports on the unification of substrate information. In order to fill this gap, we have collected information about substrates across the entire PTR2/POT/NPF family in order to provide key insights into what makes a molecule a substrate and whether there are common features among confirmed substrates. This review will be of particular interest for researchers in the field trying to probe the mechanisms responsible for the different selectivity of these transporters at a molecular resolution, and to design novel substrates.

Physiologically-Based Pharmacokinetic Model on the Oral Drug Absorption in Roux-en-Y Gastric Bypass Bariatric Patients: Amoxicillin Tablet and Suspension

The potential of a physiologically-based pharmacokinetic (PBPK) model to predict oral amoxicillin bioavailability, by considering the physiological changes after "Roux-en-Y gastric bypass" (RYGB) surgery in bariatric patients, was evaluated. A middle-out approach for parameter estimations was undertaken using in vitro, in situ, and in vivo data. The observed versus predicted plasma concentrations and the model sensitivity of the simulated parameters of AUC_0-inf and C_max of amoxicillin (AMX) were used to confirm the reliability of the estimation. The model considers that a drug-transporter (Transp) in the initial segments of the normal intestine plays a significant role in the AMX absorption. A lower fraction absorbed (F_abs) was observed in RYGB patients (54.43% for suspension and 45.21% for tablets) compared to healthy subjects (77.48% capsule). Furthermore, the tablet formulation presented a lower dissolved fraction (F_d) and F_abs compared to the suspension formulation of AMX in RYGB patients (91.70% and 45.21% versus 99.92% and 54.43%, respectively). The AUC_0-inf and C_max were sensitive to changes in R_tintestine, Peff_AMX, and Transp for both healthy and RYGB models. Additionally, AUC_0-inf and C_max were also sensitive to changes in the t_lag parameter for tablet formulation in RYGB patients. The PBPK model showed a reduction in AMX bioavailability as a consequence of reduced intestinal length after RYGB surgery. Additionally, the difference in the predicted F_d and F_abs between suspension and tablet suggests that liquid formulations are preferable in postbariatric patients.

Reduced bioavailability of oral amoxicillin tablets compared to suspensions in Roux-en-Y gastric bypass bariatric subjects

AIMS

To evaluate the relative bioavailability of oral amoxicillin (AMX) tablets in comparison to AMX suspension in Roux-en-Y gastric bypass bariatric subjects.

METHODS

A randomized, double-blind, cross-over study was performed on the bioavailability of oral AMX tablets and suspension in Roux-en-Y gastric bypass subjects operated at least 3 months previously . Doses of 875 mg of the AMX tablet or 800 mg of the AMX suspension were given to all the subjects, allowing a washout of 7 days between the periods. Blood samples were collected at 0, 0.25, 0.5, 1, 1.5, 2, 4, 6 and 8 hours after drug administration and the AMX levels were quantified by liquid chromatography coupled with triple quadrupole tandem mass spectrometry. The pharmacokinetic parameters were calculated by noncompartmental analysis, normalized to an 875 mg dose and the bioavailability of the AMX from the tablets was compared to that from the suspension formulation.

RESULTS

Twenty subjects aged 42.65 ± 7.21 years and with a body mass index of 29.88 ± 4.36 kg/m² were enrolled in the study. The maximum AMX plasma concentration of the tablets and the suspension (normalized to 875 mg) were 7.42 ± 2.99 mg/L and 8.73 ± 3.26 mg/L (90% confidence interval of 70.71-99.11), and the total area under the curve from time zero to infinity were 23.10 ± 7.41 mg.h/L and 27.59 ± 8.32 mg.h/L (90% confidence interval of 71.25-97.32), respectively.

CONCLUSION

The tablets presented a lower bioavailability than the suspension formulation and the total absorbed amount of AMX in these subjects was lower in comparison to the standard AMX absorption rates in nonbariatric subjects, regardless of the formulation.

Meta-Analysis for Correlating Structure of Bioactive Peptides in Foods of Animal Origin with Regard to Effect and Stability

Amino acid (AA) sequences of 807 bioactive peptides from foods of animal origin were examined in order to correlate peptide structure with activity (antihypertensive, antioxidative, immunomodulatory, antimicrobial, hypolipidemic, antithrombotic, and opioid) and stability in vivo. Food sources, such as milk, meat, eggs, and marine products, show different frequencies of bioactive peptides exhibiting specific effects. There is a correlation of peptide structure and effect, depending on type and position of AA. Opioid peptides contain a high percentage of aromatic AA residues, while antimicrobial peptides show an excess of positively charged AAs. AA residue position is significant, with those in the first and penultimate positions having the biggest effects on peptide activity. Peptides that have activity in vivo contain a high percentage (67%) of proline residues, but the positions of proline in the sequence depend on the length of the peptide. We also discuss the influence of processing on activity of these peptides, as well as methods for predicting release from the source protein and activity of peptides.

Antidepressant Effect of an Orally Administered Dipeptide Mimetic of the Brain-Derived Neurotrophic Factor

Involvement of BDNF in the regulation of neuroplasticity and neurogenesis in the hippocampus, impairment of which underlies the pathophysiology of depression, makes this endogenous protein a promising object for the development of new-generation antidepressants with a neurophysiologically based mechanism of action. A low-molecular-weight BDNF mimetic, GSB-106 (a substituted dimeric dipeptide, bis-(N-monosuccinyl- L-seryl-L-lysine) hexamethylenediamide), was designed and synthesized at the Zakusov Institute of Pharmacology. GSB-106 was found to activate BDNF-specific TrkB receptors and their main post-receptor signaling pathways MAPK/ERK and PI3K/AKT. GSB-106 exhibited pronounced antidepressant activity in a rodent test battery at a dose of 0.1 to 1.0 mg/kg administered intraperitoneally. Because oral administration is preferable in the treatment of depression, which is associated with a prolonged duration and outpatient character of pharmacotherapy, we examined the antidepressant properties of GSB-106 administered orally as a pharmaceutical substance (PS) and in tablet dosage form (TDF). In the study, we used the Porsolt forced swim test in rats; a conventional antidepressant, Amitriptyline, was used as a reference drug. The antidepressant activity of GSB-106 was found to retain upon oral administration and to manifest at doses of 0.5-5.0 mg/kg for PS and 0.01-5.0 mg/kg for TDF. The effective dose of TDF was 50-fold lower than that of PS, and the efficacy of tableted GSB-106 exceeded that of Amitriptyline, the "gold standard" in antidepression care. Therefore, GSB-106, both as a substance and as a tablet dosage form, exhibits antidepressant activity when administered orally, which makes it a promising antidepressant agent, the first in the class of BDNF mimetics.

Sequence-Dependent Interfacial Adsorption and Permeation of Dipeptides across Phospholipid Membranes

We investigate permeation of three blocked dipeptides with different side chain polarity across a phospholipid membrane and their behavior at the water-membrane interface by way of molecular dynamics simulations. Hydrophilic serine-serine dipeptide is found to desorb from the interface to aqueous phase, whereas hydrophobic phenylalanine-leucine and amphiphilic serine-leucine tend to accumulate at the interface with a free energy minimum of -3 kcal/mol. All three dipeptides exhibit free energy barriers to permeation across the membrane located at the center of the bilayer. The height of the barrier is strongly sequence dependent and increases with the dipeptide polarity. It is equal to 3.5, 6.4, and 10.0 kcal/mol for phenylalanine-leucine, serine-leucine, and serine-serine, respectively. The corresponding permeability coefficients are equal to 4.6 × 10^-3, 4.5 × 10^-5, and 8.7 × 10^-8 cm/s. The apparent insensitivity of membrane permeability to hydrophobicity of dipeptides, found in some experiments, is attributed to neglecting corrections for unstirred water layers near membrane surface, which are significant for hydrophobic species. Different hydrophobicity of the dipeptides also influences their conformations and orientations, both at the interface and inside the membrane. In particular, penetration of hydrophilic serine-serine dipeptide causes the formation of water-filled defects in the bilayer. These results are relevant to the delivery of peptide-based therapeutic agents.

Absorption sites of orally administered drugs in the small intestine

INTRODUCTION

In pharmacotherapy, drugs are mostly taken orally to be absorbed systemically from the small intestine, and some drugs are known to have preferential absorption sites in the small intestine. It would therefore be valuable to know the absorption sites of orally administered drugs and the influencing factors. Areas covered:In this review, the author summarizes the reported absorption sites of orally administered drugs, as well as, influencing factors and experimental techniques. Information on the main absorption sites and influencing factors can help to develop ideal drug delivery systems and more effective pharmacotherapies. Expert opinion: Various factors including: the solubility, lipophilicity, luminal concentration, pKa value, transporter substrate specificity, transporter expression, luminal fluid pH, gastrointestinal transit time, and intestinal metabolism determine the site-dependent intestinal absorption. However, most of the dissolved fraction of orally administered drugs including substrates for ABC and SLC transporters, except for some weakly basic drugs with higher pKa values, are considered to be absorbed sequentially from the proximal small intestine. Securing the solubility and stability of drugs prior to reaching to the main absorption sites and appropriate delivery rates of drugs at absorption sites are important goals for achieving effective pharmacotherapy.

Glycans in the intestinal peptide transporter PEPT1 contribute to function and protect from proteolysis

Despite the fact that many membrane proteins carry extracellular glycans, little is known about whether the glycan chains also affect protein function. We recently demonstrated that the proton-coupled oligopeptide transporter 1 (PEPT1) in the intestine is glycosylated at six asparagine residues (N50, N406, N439, N510, N515, and N532). Mutagenesis-induced disruption of the individual N-glycosylation site N50, which is highly conserved among mammals, was detected to significantly enhance the PEPT1-mediated inward transport of peptides. Here, we show that for the murine protein the inhibition of glycosylation at sequon N50 by substituting N50 with glutamine, lysine, or cysteine or by replacing S52 with alanine equally altered PEPT1 transport kinetics in oocytes. Furthermore, we provide evidence that the uptake of [¹⁴C]glycyl-sarcosine in immortalized murine small intestinal (MODE-K) or colonic epithelial (PTK-6) cells stably expressing the PEPT1 transporter N50Q is also significantly increased relative to the wild-type protein. By using electrophysiological recordings and tracer flux studies, we further demonstrate that the rise in transport velocity observed for PEPT1 N50Q is bidirectional. In line with these findings, we show that attachment of biotin derivatives, comparable in weight with two to four monosaccharides, to the PEPT1 N50C transporter slows down the transport velocity. In addition, our experiments provide strong evidence that glycosylation of PEPT1 confers resistance against proteolytic cleavage by proteinase K, whereas a remarkable intrinsic stability against trypsin, even in the absence of N-linked glycans, was detected.NEW & NOTEWORTHY This study highlights the role of N50-linked glycans in modulating the bidirectional transport activity of the murine peptide transporter PEPT1. Electrophysiological and tracer flux measurements in Xenopus oocytes have shown that removal of the N50 glycans increases the maximal peptide transport rate in the inward and outward directions. This effect could be largely reversed by replacement of N50 glycans with structurally dissimilar biotin derivatives. In addition, N-glycans were detected to stabilize PEPT1 against proteolytic cleavage.

Biowaiver Monograph for Immediate-Release Solid Oral Dosage Forms: Amoxicillin Trihydrate

Literature and experimental data relevant to waiver of in vivo bioequivalence (BE) testing for the approval of immediate-release solid oral dosage forms containing amoxicillin trihydrate are reviewed. Solubility and permeability characteristics according to the Biopharmaceutics Classification System (BCS), therapeutic uses, therapeutic index, excipient interactions, as well as dissolution and BE and bioavailability studies were taken into consideration. Solubility and permeability studies indicate that amoxicillin doses up to 875 mg belong to BCS class I, whereas 1000 mg belongs to BCS class II and doses of more than 1000 mg belong to BCS class IV. Considering all aspects, the biowaiver procedure can be recommended for solid oral products of amoxicillin trihydrate immediate-release preparations containing amoxicillin as the single active pharmaceutical ingredient at dose strengths of 875 mg or less, provided (a) only the excipients listed in this monograph are used, and only in their usual amounts, (b) the biowaiver study is performed according to the World Health Organization-, U.S. Food and Drug Administration-, or European Medicines Agency-recommended method using the innovator as the comparator, and (c) results comply with criteria for "very rapidly dissolving" or "similarly rapidly dissolving." Products containing other excipients and those containing more than 875 mg amoxicillin per unit should be subjected to an in vivo BE study.

Di- and tripeptide transport in vertebrates: the contribution of teleost fish models

Solute Carrier 15 (SLC15) family, alias H⁺-coupled oligopeptide cotransporter family, is a group of membrane transporters known for their role in the cellular uptake of di- and tripeptides (di/tripeptides) and peptide-like molecules. Of its members, SLC15A1 (PEPT1) chiefly mediates intestinal absorption of luminal di/tripeptides from dietary protein digestion, while SLC15A2 (PEPT2) mainly allows renal tubular reabsorption of di/tripeptides from ultrafiltration, SLC15A3 (PHT2) and SLC15A4 (PHT1) possibly interact with di/tripeptides and histidine in certain immune cells, and SLC15A5 has unknown function. Our understanding of this family in vertebrates has steadily increased, also due to the surge of genomic-to-functional information from 'non-conventional' animal models, livestock, poultry, and aquaculture fish species. Here, we review the literature on the SLC15 transporters in teleost fish with emphasis on SLC15A1 (PEPT1), one of the solute carriers better studied amongst teleost fish because of its relevance in animal nutrition. We report on the operativity of the transporter, the molecular diversity, and multiplicity of structural-functional solutions of the teleost fish orthologs with respect to higher vertebrates, its relevance at the intersection of the alimentary and osmoregulative functions of the gut, its response under various physiological states and dietary solicitations, and its possible involvement in examples of total body plasticity, such as growth and compensatory growth. By a comparative approach, we also review the few studies in teleost fish on SLC15A2 (PEPT2), SLC15A4 (PHT1), and SLC15A3 (PHT2). By representing the contribution of teleost fish to the knowledge of the physiology of di/tripeptide transport and transporters, we aim to fill the gap between higher and lower vertebrates.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

Neuroprotective effect of novel cognitive enhancer noopept on AD-related cellular model involves the attenuation of apoptosis and tau hyperphosphorylation

Background

Noopept (N-phenyl-acetyl-L-prolylglycine ethyl ester) was constructed as a dipeptide analog of the standard cognition enhancer, piracetam. Our previous experiments have demonstrated the cognition restoring effect of noopept in several animal models of Alzheimer disease (AD). Noopept was also shown to prevent ionic disbalance, excitotoxicity, free radicals and pro-inflammatory cytokines accumulation, and neurotrophine deficit typical for different kinds of brain damages, including AD. In this study, we investigated the neuroprotective action of noopept on cellular model of AD, Aβ_25–35-induced toxicity in PC12 cells and revealed the underlying mechanisms.

Results

The neuroprotective effect of noopept (added to the medium at 10 μM concentration, 72 hours before Аβ_25–35) was studied on Аβ_25–35-induced injury (5 μM for 24 h) in PC12 cells. The ability of drug to protect the impairments of cell viability, calcium homeostasis, ROS level, mitochondrial function, tau phosphorylation and neurite outgrowth caused by Аβ_25–35 were evaluated.

Following the exposure of PC12 cells to Аβ_25–35 an increase of the level of ROS, intracellular calcium, and tau phosphorylation at Ser396 were observed; these changes were accompanied by a decrease in cell viability and an increase of apoptosis. Noopept treatment before the amyloid-beta exposure improved PC12 cells viability, reduced the number of early and late apoptotic cells, the levels of intracellular reactive oxygen species and calcium and enhanced the mitochondrial membrane potential. In addition, pretreatment of PC12 cell with noopept significantly attenuated tau hyperphosphorylation at Ser396 and ameliorated the alterations of neurite outgrowth evoked by Аβ_25–35.

Conclusions

Taken together, these data provide evidence that novel cognitive enhancer noopept protects PC12 cell against deleterious actions of Aβ through inhibiting the oxidative damage and calcium overload as well as suppressing the mitochondrial apoptotic pathway. Moreover, neuroprotective properties of noopept likely include its ability to decrease tau phosphorylation and to restore the altered morphology of PC12 cells. Therefore, this nootropic dipeptide is able to positively affect not only common pathogenic pathways but also disease-specific mechanisms underlying Aβ-related pathology.

Comparative activity of proline-containing dipeptide noopept and inhibitor of dipeptidyl peptidase-4 sitagliptin in a rat model of developing diabetes

Developing diabetes was modeled on adult male Wistar rats by repeated intraperitoneal injections of streptozotocin in a subdiabetogenic dose of 30 mg/kg for 3 days. Proline-containing dipeptide drug Noopept or a standard diabetic drug dipeptidyl peptidase-4 inhibitor sitagliptin was administered per os in a dose of 5 mg/kg before each injection of the toxin and then for 16 days after streptozotocin course. In active control group, spontaneously increase glucose level and reduced tolerance to glucose load (1000 mg/kg intraperitoneally) were observed on the next day after the third administration of toxin. Basal glucose level decreased by day 16, but glucose tolerance remained impaired. Noopept normalized the basal blood glucose level and tolerance to glucose load on the next day after administration of streptozotocin. The effect of Noopept persisted to the end of the experiment. At early terms of the experiment, sitagliptin was somewhat superior to Noopept by the effect on baseline glucose level, but was inferior by the influence on glucose tolerance.. By the end of the experiment, Noopept significantly (by 2 times) surpassed sitagliptin by its effect on glucose tolerance.

Teleost fish models in membrane transport research: the PEPT1(SLC15A1) H+-oligopeptide transporter as a case study

Human genes for passive, ion-coupled transporters and exchangers are included in the so-called solute carrier (SLC) gene series, to date consisting of 52 families and 398 genes. Teleost fish genes for SLC proteins have also been described in the last two decades, and catalogued in preliminary SLC-like form in 50 families and at least 338 genes after systematic GenBank database mining (December 2010-March 2011). When the kinetic properties of the expressed proteins are studied in detail, teleost fish SLC transporters always reveal extraordinary 'molecular diversity' with respect to the mammalian counterparts, which reflects peculiar adaptation of the protein to the physiology of the species and/or to the environment where the species lives. In the case of the H+ -oligopeptide transporter PEPT1(SLC15A1), comparative analysis of diverse teleost fish orthologs has shown that the protein may exhibit very eccentric properties in terms of pH dependence (e.g., the adaptation of zebrafish PEPT1 to alkaline pH), temperature dependence (e.g., the adaptation of icefish PEPT1 to sub-zero temperatures) and/or substrate specificity (e.g., the species-specificity of PEPT1 for the uptake of l-lysine-containing peptides). The revelation of such peculiarities is providing new contributions to the discussion on PEPT1 in both basic (e.g., molecular structure-function analyses) and applied research (e.g., optimizing diets to enhance growth of commercially valuable fish).

Basolateral glycylsarcosine (Gly-Sar) transport in Caco-2 cell monolayers is pH dependent

OBJECTIVES

Transepithelial di/tripeptide transport in enterocytes occurs via the apical proton-coupled peptide transporter, hPEPT1 (SLC15A1) and a basolateral peptide transporter, which has only been characterized functionally. In this study we examined the pH dependency, substrate uptake kinetics and substrate specificity of the transporter.

METHODS

We studied the uptake of [(14) C]Gly-Sar from basolateral solution into Caco-2 cell monolayers grown for 17-22 days on permeable supports, at a range of basolateral pH values.

KEY FINDINGS

Basolateral Gly-Sar uptake was pH dependent, with a maximal uptake rate at a basolateral pH of 5.5. Uptake of Gly-Sar decreased in the presence of the protonophore nigericin, indicating that the uptake was proton-coupled. The uptake was saturable, with a maximal flux (Vmax ) of 408 ± 71, 307 ± 25 and 188 ± 19 pmol/cm(2) /min (mean ± S.E., n = 3) at basolateral pH 5.0, 6.0 and 7.4, respectively. The compounds Gly-Asp, Glu-Phe-Tyr, Gly-Glu-Gly, Gly-Phe-Gly, lidocaine and, to a smaller degree, para-aminohippuric acid were all shown to inhibit the basolateral uptake of Gly-Sar.

CONCLUSIONS

The study showed that basolateral Gly-Sar transport in the intestinal cell line Caco-2 is proton-coupled. The inhibitor profile indicated that the transporter has broad substrate specificity.

An inverse relationship links temperature and substrate apparent affinity in the ion-coupled cotransporters rGAT1 and KAAT1

The effects of temperature on the operation of two ion-coupled cotransporters of the SLC6A family, namely rat GAT1 (SLC6A1) and KAAT1 (SLC6A19) from Manduca sexta, have been studied by electrophysiological means in Xenopus laevis oocytes expressing these proteins. The maximal transport-associated current (I_max) and the apparent substrate affinity (K₀₅) were measured. In addition to the expected increase in transport rate (Q₁₀ = 3–6), both transporters showed greater K₀₅ values (i.e., a decrease in apparent affinity) at higher temperatures. The transport efficiency, estimated as I_max/K₀₅, increased at negative potentials in both transporters, but did not show statistically significant differences with temperature. The observation that the apparent substrate affinity is inversely related to the transport rate suggests a kinetic regulation of this parameter. Furthermore, the present results indicate that the affinities estimated at room temperature for mammalian cotransporters may not be simply extrapolated to their physiological operating conditions.

Temperature effects on the kinetic properties of the rabbit intestinal oligopeptide cotransporter PepT1

The effects of temperature on the functional properties of the intestinal oligopeptide transporter PepT1 from rabbit have been investigated using electrophysiological methods. The dipeptide Gly-Gln at pH 6.5 or 7.5 was used as substrate. Raising the temperature in the range 20-30 °C causes an increase in the maximal transport-associated current (I (max)) with a Q (10) close to 4. Higher temperatures accelerate the rate of decline of the presteady-state currents observed in the absence of organic substrate. The voltage dependencies of the intramembrane charge movement and of the time constant of decline are both shifted towards more negative potentials by higher temperatures. The shift is due to a stronger action of temperature on the outward rate of charge movement compared to the inward rate, indicating a lower activation energy for the latter process. Consistently, the activation energy for the complete cycle is similar to that of the inward rate of charge movement. Temperature also affects the binding rate of the substrate: the K (0.5) -V curve is shifted to more negative potentials by higher temperatures, resulting in a lower apparent affinity in the physiological range of potentials. The overall efficiency of transport, estimated as the I (max)/K (0.5) ratio is significantly increased at body temperature.

Regulation of the human Na+-dependent glucose cotransporter hSGLT2

The human Na(+)-glucose cotransporter SGLT2 is expressed mainly in the kidney proximal convoluted tubule where it is considered to be responsible for the bulk of glucose reabsorption. Phosphorylation profiling has revealed that SGLT2 exists in a phosphorylated state in the rat renal proximal tubule cortex, so we decided to investigate the regulation of human SGLT2 (hSGLT2) by protein kinases. hSGLT2 was expressed in human embryonic kidney (HEK) 293T cells, and the activity of the protein was measured using radiotracer and whole cell patch-clamp electrophysiology assays before and after activation of protein kinases. 8-Bromo-adenosine cAMP (8-Br-cAMP) was used to activate protein kinase A, and sn-1,2-dioctanoylglycerol (DOG) was used to activate protein kinase C (PKC). 8-Br-cAMP stimulated D-[α-methyl-(14)C]glucopyranoside ([(14)C]α-MDG) uptake and Na(+)-glucose currents by 200% and DOG increased [(14)C]α-MDG uptake and Na(+)-glucose currents by 50%. In both cases the increase in SGLT2 activity was marked by an increase in the maximum rate of transport with no change in glucose affinity. These effects were completely negated by mutation of serine 624 to alanine. Insulin induced a 250% increase in Na(+)-glucose transport by wild-type but not S624A SGLT2. Parallel studies confirmed that the activity of hSGLT1 was regulated by PKA and PKC due to changes in the number of transporters in the cell membrane. hSGLT1 was relatively insensitive to insulin. We conclude that hSGLT1 and hSGLT2 are regulated by different mechanisms and suggest that insulin is an SGLT2 agonist in vivo.

Pre-steady-state and reverse transport currents in the GABA transporter GAT1

The role of internal substrates in the biophysical properties of the GABA transporter GAT1 has been investigated electrophysiologically in Xenopus oocytes heterologously expressing the cotransporter. Increments in Cl(-) and/or Na(+) concentrations caused by intracellular injections did not produce significant effects on the pre-steady-state currents, while a positive shift of the charge-voltage (Q-V) and decay time constant (τ)-voltage (τ-V) curves, together with a slowing of τ at positive potentials, was observed following treatments producing cytosolic Cl(-) depletion. Activation of the reverse transport mode by injections of GABA caused a reduction in the displaced charge. In the absence of external Cl(-), a stronger reduction in the displaced charge, together with a significant increase in reverse transport current, was observed. Therefore, complementarity between pre-steady-state and transport currents, observed in the forward mode, is preserved in the reverse mode. All these findings can be qualitatively reproduced by a kinetic scheme in which, in the forward mode, the Cl(-) ion is released first, after the inward charge movement, while the two Na(+) ions can be released only after binding of external GABA. In the reverse mode, internal GABA must bind first to the empty transporter, followed by internal Na(+) and Cl(-).

Functional and structural determinants of reverse operation in the pH-dependent oligopeptide transporter PepT1

The functional and structural basis of reverse operation of PepT1 has been studied in Xenopus oocytes expressing the wild-type and mutated forms of this protein. Using brief pulses from a negative holding potential, wild-type and Arg282 mutants exhibit outward currents in the presence of Gly-Gln. The reversal potential of these currents is affected by both pH and substrate concentration, confirming coupled transport in the wild type and in the mutants as well. Long-lasting voltage and current-clamp experiments show that the outward currents are only temporary, and reflect accumulation and/or depletion effects near the membrane. The ability to operate in reverse mode was confirmed in all isoforms by intracellular injection of substrate. The role of Arg282 and Asp341 in the reverse transport was also investigated using charged substrates. Positive Lys-Gly (but not Gly-Lys) showed enhanced transport currents in the Arg282 mutants. In contrast, negative Gly-Asp and Asp-Gly elicited modest currents in all isoforms.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Residues R282 and D341 act as electrostatic gates in the proton-dependent oligopeptide transporter PepT1

The oligopeptide transporter PepT1 is a protein found in the membrane of the cells of the intestinal walls, and represents the main route through which proteic nutrients are absorbed by the organism. Along the polypeptidic chain of this protein, two oppositely charged amino acids, an arginine in position 282 and an aspartate in position 341 of the sequence, have been hypothesised to form a barrier in the absorption pathway. In this paper we show that appropriate mutations of these amino acids change the properties of PepT1 in a way that confirms that these parts of the protein indeed act as an electrostatic gate in the transport process. The identification of the structural basis of the functional mechanism of this transporter is important because, in addition to its role in nutrient uptake, PepT1 represents a major pathway for the absorption of several therapeutic drugs.

Serine side chain-linked peptidomimetic conjugates of cyclic HPMPC and HPMPA: synthesis and interaction with hPEPT1

Cidofovir (HPMPC), a broad spectrum antiviral agent, cannot be administered orally due to ionization of its phosphonic acid group at physiological pH. One prodrug approach involves conversion to the cyclic form (cHPMPC, 1) and esterification by the side chain hydroxyl group of a peptidomimetic serine. Transport studies in a rat model have shown enhanced levels of total cidofovir species in the plasma after oral dosing with L-Val-L-Ser-OMe cHPMPC, 2a. To explore the possibility that 2a and its three L/D stereoisomers 2b-d undergo active transport mediated by the peptide-specific intestinal transporter PEPT1, we performed radiotracer uptake and electrophysiology experiments applying the two-electrode voltage clamp technique in Xenopus laevis oocytes overexpressing human PEPT1 (hPEPT1, SLC15A1). 2a-d did not induce inward currents, indicating that they are not transported, but the stereoisomers with an L-configuration at the N-terminal valine (2a and 2b) potently inhibited transport of the hPEPT1 substrate glycylsarcosine (Gly-Sar). A "reversed" dipeptide conjugate, L-Ser-L-Ala-OiPr cHPMPC (4), also did not exhibit detectable transport, but completely abolished the Gly-Sar signal, suggesting that affinity of the transporter for these prodrugs is not impaired by a proximate linkage to the drug in the N-terminal amino acid of the dipeptide. Single amino acid conjugates of cHPMPC (3a and 3b) or cHPMPA (5, 6a and 6b) were not transported and only weakly inhibited Gly-Sar transport. The known hPEPT1 prodrug substrate valacyclovir (7) and its L-Val-L-Val dipeptide analogue (8) were used to verify coupled transport by the oocyte model. The results indicate that the previously observed enhanced oral bioavailability of 2a relative to the parent drug is unlikely to be due to active transport by hPEPT1. Syntheses of the novel compounds 2b-d and 3-6 are described, including a convenient solid-phase method to prepare 5, 6a and 6b.

A quantitative structure-activity relationship for translocation of tripeptides via the human proton-coupled peptide transporter, hPEPT1 (SLC15A1)

The human intestinal proton-coupled peptide transporter, hPEPT1 (SLC15A1), has been identified as an absorptive transporter for both drug substances and prodrugs. An understanding of the prerequisites for transport has so far been obtained from models based on competition experiments. These models have limited value for predicting substrate translocation via hPEPT1. The aim of the present study was to investigate the requirements for translocation via hPEPT1. A set of 55 tripeptides was selected from a principal component analysis based on VolSurf descriptors using a statistical design. The majority of theses tripeptides have not previously been investigated. Translocation of the tripeptides via hPEPT1 was determined in a MDCK/hPEPT1 cell-based translocation assay measuring substrate-induced changes in fluorescence of a membrane potential-sensitive probe. Affinities for hPEPT1 of relevant tripeptides were determined by competition studies with [14C]Gly-Sar in MDCK/hPEPT1 cells. Forty tripeptides were found to be substrates for hPEPT1, having K(m)(app) values in the range 0.4-28 mM. Eight tripeptides were not able to cause a substrate-induced change in fluorescence in the translocation assay and seven tripeptides interacted with the probe itself. The conformationally restricted tripeptide Met-Pro-Pro was identified as a novel high-affinity inhibitor of hPEPT1. We also discovered the first tripeptide (Asp-Ile-Arg) that was neither a substrate nor an inhibitor of hPEPT1. To rationalise the requirements for transport, a quantitative structure-activity relationship model correlating K(m)(app) values with VolSurf descriptors was constructed. This is, to our knowledge, the first predictive model for the translocation of tripeptides via hPEPT1.

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.

Variations in amoxicillin pharmacokinetic/pharmacodynamic parameters may explain treatment failures in acute otitis media

Pharmacokinetic/pharmacodynamic (PK/PD) modeling and Monte Carlo simulations suggest that amoxicillin should rarely fail as therapy for Streptococcus pneumoniae and Haemophilus influenzae acute otitis media (AOM) infections except when the S. pneumoniae are highly penicillin resistant or the H. influenzae are beta-lactamase producing. However, important and not infrequent exceptions to this expectation have been described. The objective of this review was to define the biologic variations in amoxicillin PK/PD parameters for the treatment of AOM in children and assess whether these variations could explain why the commonly employed amoxicillin PK/PD model is imperfect in predicting outcome for every patient in this clinical setting. To this end, a literature search of MEDLINE (1966-2006) and EMBASE (1974-2006) was conducted to identify studies that evaluated ampicillin or amoxicillin intestinal absorption, serum concentrations, and/or middle ear fluid (MEF) concentrations. Analysis of studies identified for review showed that the intestinal bioavailability of amoxicillin depends on passive diffusion and a saturable 'pump' mechanism that produces variable serum concentrations of the antibacterial agent. Indeed, substantial differences from patient to patient in serum (5- to 30-fold) and MEF (up to 20-fold) concentrations of amoxicillin occur following oral administration, and 15-35% of children have no detectable amoxicillin in MEF. These findings suggest that variability in PK/PD parameters may impact amoxicillin concentrations in serum and MEF, possibly explaining some AOM treatment failures.

Functional expression of the oligopeptide transporter PepT1 from the sea bass (Dicentrarchus labrax)

Complementary RNA, derived from the intestine of the sea bass Dicentrarchus labrax and putatively coding for a pH-dependent oligopeptide transporter PepT1 (SLC15 family), was injected in Xenopus oocytes that were subsequently tested with electrophysiological techniques. Transport-associated currents were observed when various di- or tripeptides were applied at concentrations ranging between 0.1 and 10 mM. No currents were generated by histidine nor by other single amino acids. Sea bass PepT1 also exhibited presteady-state currents in the absence of substrates. Acidic pH slowed down the relaxation time constant of these currents and shifted both Q/V and tau/V relationships toward more positive voltages. Michaelis-Menten analysis of the transport currents showed an increase in apparent substrate affinity at acidic pH, which was very similar to that exhibited by the related transporter from zebrafish (Danio rerio), but in contrast, did not demonstrate a significant effect of pH on the maximal transport current.

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.

CHR-2797: an antiproliferative aminopeptidase inhibitor that leads to amino acid deprivation in human leukemic cells

CHR-2797 is a novel metalloenzyme inhibitor that is converted into a pharmacologically active acid product (CHR-79888) inside cells. CHR-79888 is a potent inhibitor of a number of intracellular aminopeptidases, including leucine aminopeptidase. CHR-2797 exerts antiproliferative effects against a range of tumor cell lines in vitro and in vivo and shows selectivity for transformed over nontransformed cells. Its antiproliferative effects are at least 300 times more potent than the prototypical aminopeptidase inhibitor, bestatin. However, the mechanism by which inhibition of these enzymes leads to proliferative changes is not understood. Gene expression microarrays were used to profile changes in mRNA expression levels in the human promyelocytic leukemia cell line HL-60 treated with CHR-2797. This analysis showed that CHR-2797 treatment induced a transcriptional response indicative of amino acid depletion, the amino acid deprivation response, which involves up-regulation of amino acid synthetic genes, transporters, and tRNA synthetases. These changes were confirmed in other leukemic cell lines sensitive to the antiproliferative effects of CHR-2797. Furthermore, CHR-2797 treatment inhibited phosphorylation of mTOR substrates and reduced protein synthesis in HL-60 cells, both also indicative of amino acid depletion. Treatment with CHR-2797 led to an increase in the concentration of intracellular small peptides, the substrates of aminopeptidases. It is suggested that aminopeptidase inhibitors, such as CHR-2797 and bestatin, deplete sensitive tumor cells of amino acids by blocking protein recycling, and this generates an antiproliferative effect. CHR-2797 is orally bioavailable and currently undergoing phase II clinical investigation in the treatment of myeloid leukemia.

Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) transporters in the intestine, liver and kidney

The role of carrier-mediated transport in determining the pharmacokinetics of drugs has become increasingly evident with the discovery of genetic variants that affect expression and/or function of a given drug transporter. Drug transporters are expressed at numerous epithelial barriers, such as intestinal epithelial cells, hepatocytes, renal tubular cells and at the blood-brain barrier. Several recent studies have associated alterations in substrate uptake with the presence of SNPs. Here, we summarize the current knowledge on the functional and phenotypic consequences of genetic variation in intestinally, hepatically and renally expressed members of the organic anion-transporting polypeptide family (OATPs; SLC21/SLCO family), the organic anion and organic cation transporters (OATs/OCTs; SLC22 family) and the peptide transporter-1 (PEPT1; SLC15 family).

Substrate-induced changes in the density of peptide transporter PEPT1 expressed in Xenopus oocytes

The adaptation of the capacity of the intestinal peptide transporter PEPT1 to varying substrate concentrations may be important with respect to its role in providing bulk quantities of amino acids for growth, development, and other nutritional needs. In the present study, we describe a novel phenomenon of the regulation of PEPT1 in the Xenopus oocyte system. Using electrophysiological and immunofluorescence methods, we demonstrate that a prolonged substrate exposure of rabbit PEPT1 (rPEPT1) caused a retrieval of transporters from the membrane. Capacitance as a measure of membrane surface area was increased in parallel with the increase in rPEPT1-mediated transport currents with a slope of approximately 5% of basal surface per 100 nA. Exposure of oocytes to the model peptide Gly-l-Gln for 2 h resulted in a decrease in maximal transport currents with no change of membrane capacitance. However, exposure to substrate for 5 h decreased transport currents but also, in parallel, surface area by endocytotic removal of transporter proteins from the surface. The reduction of the surface expression of rPEPT1 was confirmed by presteady-state current measurements and immunofluorescent labeling of rPEPT1. A similar simultaneous decrease of current and surface area was also observed when endocytosis was stimulated by the activation of PKC. Cytochalasin D inhibited all changes evoked by either dipeptide or PKC stimulation, whereas the PKC-selective inhibitor bisindolylmaleimide only affected PKC-stimulated endocytotic processes but not substrate-dependent retrieval of rPEPT1. Coexpression experiments with human Na(+)-glucose transporter 1 (hSGLT1) revealed that substrate exposure selectively affected PEPT1 but not the activity of hSGLT1.

How drugs interact with transporters: SGLT1 as a model

Drugs are transported by cotransporters with widely different turnover rates. We have examined the underlying mechanism using, as a model system, glucose and indican (indoxyl-beta-D-glucopyranoside) transport by human Na+/glucose cotransporter (hSGLT1). Indican is transported by hSGLT1 at 10% of the rate for glucose but with a fivefold higher apparent affinity. We expressed wild-type hSGLT1 and mutant G507C in Xenopus oocytes and used electrical and optical methods to measure the kinetics of glucose (using nonmetabolized glucose analogue alpha-methyl-D-glucopyranoside, alphaMDG) and indican transport, alone and together. Indican behaved as a competitive inhibitor of alphaMDG transport. To examine protein conformations, we recorded SGLT1 capacitive currents (charge movements) and fluorescence changes in response to step jumps in membrane voltage, in the presence and absence of indican and/or alphaMDG. In the absence of sugar, voltage jumps elicited capacitive SGLT currents that decayed to steady state with time constants (tau) of 3-20 ms. These transient currents were abolished in saturating alphaMDG but only slightly reduced (10%) in saturating indican. SGLT1 G507C rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages. Maximal fluorescence increased approximately 150% in saturating indican but decreased approximately 50% in saturating alphaMDG. Modeling indicated that the rate-limiting step for indican transport is sugar translocation, whereas for alphaMDG it is dissociation of Na+ from the internal binding sites. The inhibitory effects of indican on alphaMDG transport are due to its higher affinity and a 100-fold lower translocation rate. Our results indicate that competition between substrates and drugs should be taken into consideration when targeting transporters as drug delivery systems.