Human dipeptide transporter, hPEPT1, stably transfected into Chinese hamster ovary cells

Elucidating uptake and metabolic fate of dipeptides in CHO cell cultures using 13C labeling experiments and kinetic modeling

The rapidly growing market of biologics including monoclonal antibodies has stimulated the need to improve biomanufacturing processes including mammalian host systems such as Chinese Hamster Ovary (CHO) cells. Cell culture media formulations continue to be enhanced to enable intensified cell culture processes and optimize cell culture performance. Amino acids, major components of cell culture media, are consumed in large amounts by CHO cells. Due to their low solubility and poor stability, certain amino acids including tyrosine, leucine, and phenylalanine can pose major challenges leading to suboptimal bioprocess performance. Dipeptides have the potential to replace amino acids in culture media. However, very little is known about the cleavage, uptake, and utilization kinetics of dipeptides in CHO cell cultures. In this study, replacing amino acids, including leucine and tyrosine by their respective dipeptides including but not limited to Ala-Leu and Gly-Tyr, supported similar cell growth, antibody production, and lactate profiles. Using 13C labeling techniques and spent media studies, dipeptides were shown to undergo both intracellular and extracellular cleavage in cultures. Extracellular cleavage increased with the culture duration, indicating cleavage by host cell proteins that are likely secreted and accumulate in cell culture over time. A kinetic model was built and for the first time, integrated with 13C labeling experiments to estimate dipeptide utilization rates, in CHO cell cultures. Dipeptides with alanine at the N-terminus had a higher utilization rate than dipeptides with alanine at the C-terminus and dipeptides with glycine instead of alanine at N-terminus. Simultaneous supplementation of more than one dipeptide in culture led to reduction in individual dipeptide utilization rates indicating that dipeptides compete for the same cleavage enzymes, transporters, or both. Dipeptide utilization rates in culture and cleavage rates in cell-free experiments appeared to follow Michaelis-Menten kinetics, reaching a maximum at higher dipeptide concentrations. Dipeptide utilization behavior was found to be similar in cell-free and cell culture environments, paving the way for future testing approaches for dipeptides in cell-free environments prior to use in large-scale bioreactors. Thus, this study provides a deeper understanding of the fate of dipeptides in CHO cell cultures through an integration of cell culture, 13C labeling, and kinetic modeling approaches providing insights in how to best use dipeptides in media formulations for robust and optimal mammalian cell culture performance.

Tracking dipeptides at work-uptake and intracellular fate in CHO culture

Market demands for monoclonal antibodies (mAbs) are steadily increasing worldwide. As a result, production processes using Chinese hamster ovary cells (CHO) are in the focus of ongoing intensification studies for maximizing cell-specific and volumetric productivities. This includes the optimization of animal-derived component free (ADCF) cultivation media as part of good cell culture practice. Dipeptides are known to improve CHO culture performance. However, little or even conflicting assumptions exist about their putative import and functionality inside the cells. A set of well-known performance boosters and new dipeptide prospects was evaluated. The present study revealed that dipeptides are indeed imported in the cells, where they are decomposed to the amino acids building blocks. Subsequently, they are metabolized or, unexpectedly, secreted to the medium. Monoclonal antibody production boosting additives like l-alanine-l-glutamine (AQ) or glycyl-l-glutamine (GQ) can be assigned to fast or slow dipeptide uptake, respectively, thus pinpointing to the need to study dipeptide kinetics and to adjust their feeding individually for optimizing mAb production.

Keeping a critical eye on the science and the regulation of oral drug absorption: a review

This review starts with an introduction on the theoretical aspects of biopharmaceutics and developments in this field from mid-1950s to late 1970s. It critically addresses issues related to fundamental processes in oral drug absorption such as the complex interplay between drugs and the gastrointestinal system. Special emphasis is placed on drug dissolution and permeability phenomena as well as on the mathematical modeling of oral drug absorption. The review ends with regulatory aspects of oral drug absorption focusing on bioequivalence studies and the US Food and Drug Administration and European Medicines Agency guidelines dealing with Biopharmaceutics Classification System and Biopharmaceutic Drug Disposition Classification System.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

Evaluation of the effect of ethanol's toxic metabolite acetaldehyde on the gastrointestinal oligopeptide transporter, PEPT1: in vitro and in vivo studies

BACKGROUND

The effects of alcohol consumption and its subsequent metabolism on drug transport, absorption and pharmacokinetics are poorly understood. This study examines the effects of the ethanol metabolite, acetaldehyde, on the clinically relevant drug transporter, PEPT1. The metabolism of ethanol and the following acetaldehyde formation is thought to modulate the uptake capacity of PEPT1 within the gastrointestinal tract for a variety of clinically important peptidomimetic drug compounds.

METHODS

Glycylsarcosine ([(3)H]-GlySar), a nonhydrolysable PEPT1 specific substrate was used in our studies. In vitro uptake studies were performed in the Caco-2 and Chinese hamster ovary (CHO)-hPEPT1 cell models, measuring cellular uptake of labeled compound against increasing levels of unlabeled compound in the presence of acetaldehyde. In vivo absorption of [(3)H]-GlySar was measured in male Sprague-Dawley rats that were treated with oral dose of ethanol/disulfiram (5 g/kg / 100 mg/kg) for 6 days. These results were compared to control rats treated with saline, ethanol alone or disulfiram alone.

RESULTS

In vitro uptake of [(3)H]-GlySar in CHO-hPEPT1 cells treated with 1 mM acetaldehyde was significantly decreased (p < 0.05) as compared to untreated controls. The uptake of [(3)H]-GlySar in Caco-2 cell monolayers treated with 1 mM acetaldehyde was also significantly decreased as compared to the untreated control cells. In vivo absorption of [(3)H]-GlySar in ethanol treated rats, as measured by AUC(0-12 hours) were decreased by approximately 50% versus the control rat group.

CONCLUSION

The effects of acetaldehyde due to consumption of ethanol on the uptake and bioavailability of therapeutic drug compounds transported by the PEPT1 oligopeptide transporter have not been documented. In the present studies, we demonstrate that acetaldehyde significantly modulates PEPT1 function and, thereby, affects drug bioavailability. To our best knowledge, this is the first report on the effects of an ethanol metabolite on substrate absorption in the gastrointestinal tract, rather than interactions in the liver, which is an under-represented area of research in alcohol pathophysiology.

Pharmacophore-based discovery of ligands for drug transporters

The ability to identify ligands for drug transporters is an important step in drug discovery and development. It can both improve accurate profiling of lead pharmacokinetic properties and assist in the discovery of new chemical entities targeting transporters. In silico approaches, especially pharmacophore-based database screening methods have great potential in improving the throughput of current transporter ligand identification assays, leading to a higher hit rate by focusing in vitro testing to the most promising hits. In this review, the potential of different in silico methods in transporter ligand identification studies are compared and summarized with an emphasis on pharmacophore modeling. Various implementations of pharmacophore model generation, database compilation and flexible screening algorithms are also introduced. Recent successful utilization of database searching with pharmacophores to identify novel ligands for the pharmaceutically significant transporters hPepT1, P-gp, BCRP, MRP1 and DAT are reviewed and the challenges encountered with current approaches are discussed.

Primary porcine proximal tubular cells as a model for transepithelial drug transport in human kidney

BACKGROUND

Kidney proximal tubular cells play a major role in the transport of endogenous and exogenous compounds. A multitude of different transporters are expressed starting with multidrug ABC transporters (e.g. abcb1, abcc1-6), slc22a6-8 (organic anion transporters) and slc22a1-3 (organic cation transporters). For transport studies of renal drug transport, cell lines like MDCK and LLC-PK1 are often used to overexpress and study one or two transporters, such as abcb1 or abcc1-6. However, the use is limited since under physiological conditions xenobiotics are transported through different transporters at the same time. Therefore, a primary in vitro model expressing functionally different transporters simultaneously, as it is the case in vivo, would be of great benefit.

METHODS

Primary proximal tubular cells were isolated from porcine kidney. Cells were cultured under selective culturing conditions leading to specific growth of primary proximal tubular cells. Expression of important proximal transporters was checked at mRNA level with RT-PCR, at protein level with immunocytochemistry and functionally by transport and uptake assays.

RESULTS

A model of primary proximal tubular cells was established expressing the most important transporters: abcb1, abcc1, abcc2, slc22a8, slco1a2, slc15a1, slc5a2 and slc4a4. In freshly isolated cells, slc22a1 and slc22a6 were expressed, but were down-regulated in culture. Abcb1, abcc1, abcc2 and slc4a4 were detected at protein level with immunostaining. Functional activity was confirmed for abcb1, abcc1/2, slc22a8, slc15a1/2 and slc5a1/2. The tightness of the monolayers of this model was better than in previously established in vitro models.

CONCLUSION

This primary cell culture model might be an interesting tool to investigate proximal tubular transport and to predict toxicity and drug interactions since it expresses functionally several transporters simultaneously.

Recovery of functional peptide transporter PepT1 in budded baculovirus fraction

Transporters play a critical role in many physiological and pathological states and expression of the functional transporter protein is essential in exploring its kinetics and developing effective drugs. We describe here the recovery of functional transporter protein in the baculovirus fraction. We introduced a gene encoding human peptide transporter PepT1, important for the absorption of protein hydrolytic products or peptide-mimetic drugs, into a baculovirus vector. After infection, a large amount of PepT1 appeared in the budded virus fraction compared with Sf9 cells. Uptake of [14C]glycylsarcosine was markedly increased in an acidic condition and showed a clear overshoot in PepT1-expressing virus fraction. The apparent Michaelis constant for [14C]glycylsarcosine was 0.55 +/- 0.06 mM. [14C]Glycylsarcosine uptake was inhibited by di- and tripeptides and orally active beta-lactam antibiotics. These results suggest that functional PepT1 recovers efficiently in a budded virus fraction, and, thus, this expression system will be a useful tool for characterization and screening of peptide-mimetic drugs in drug discovery.

Genetic Variants of the Human Dipeptide Transporter PEPT1

We tested whether genetic polymorphisms affect activity of the dipeptide transporter PEPT1, which mediates bioavailability of peptidomimetic drugs. All 23 exons and adjoining intronic sections of PEPT1 (SLC15A1) were sequenced in 247 individuals of various ethnic origins (Coriell collection). Of 38 single nucleotide polymorphisms (SNPs), 21 occurred in intronic and noncoding regions and 17 in exonic coding region, of which nine were nonsynonymous. Eight nonsynonymous variants were cloned into expression vectors and functionally characterized after transient transfection into Cos7 and Chinese hamster ovary cells. None of the variants had altered transport activity for various ligands, supporting previous results, except for the new, low-frequency PEPT1-F28Y. This variant displayed significantly reduced cephalexin uptake attributable to increased K(m). Altered pH dependence of substrate transport suggested a role for F28Y in H(+)-driven translocation. Haplotype analysis revealed significant differences among ethnic populations. To search for cis-acting polymorphisms affecting transcription and mRNA processing, we measured allelic PEPT1 mRNA expression in human intestinal biopsy samples using a frequent-marker SNP in exon 17. Of 24 heterozygous samples, significant differences in allelic mRNA levels of 20 to 30% were observed in seven tissues. However, the small difference suggests that cis-acting regulatory factors have only limited effects on transporter activity. We also measured the relative formation of a splice variant (PEPT1-RF). PEPT1-RF mRNA levels ranged from 2 to 44% of total PEPT1-related mRNA, with potential consequences for drug absorption. Together with previous results, this study reveals a relatively low level of genetic variability in polymorphisms affecting both protein function and gene regulation.

In vitro and pharmacophore-based discovery of novel hPEPT1 inhibitors

PURPOSE

The human proton-coupled small peptide carrier (hPEPT1) is a low-affinity, high-capacity transporter with broad substrate specificity. We have taken an iterative in vitro and in silico approach to the discovery of molecules with hPEPT1 affinity.

METHODS

A pharmacophore-based approach was taken to identifying hPEPT1 inhibitors. The well-characterized and relatively high affinity ligands Gly-Sar, bestatin, and enalapril were used to generate a common features (HIPHOP) pharmacophore. This consisted of two hydrophobic features, a hydrogen bond donor, acceptor, and a negative ionizable feature.

RESULTS

The pharmacophore was used to search the Comprehensive Medicinal Chemistry (CMC) database of more than 8000 drug-like molecules and retrieved 145 virtual hits mapping to the pharmacophore features. The highest scoring compounds within this set were selected and tested in a stably transfected CHO-hPepT1 cell model. The antidiabetic repaglinide and HMG CoA reductase inhibitor fluvastatin were found to inhibit hPEPT1 with sub-millimolar potency (IC(50) 178 +/- 1.0 and 337 +/- 4 microM, respectively). The pharmacophore was also able to identify known hPEPT1 substrates and inhibitors in further database mining of more than 500 commonly prescribed drugs.

CONCLUSIONS

This study demonstrates the potential of combining computational and in vitro approaches to determine the affinity of compounds for hPEPT1 and, in turn, provides insights into key molecular interactions with this transporter.

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

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

Gene expression in the human intestine and correlation with oral valacyclovir pharmacokinetic parameters

The transport of valacyclovir, the l-valyl ester of acyclovir, has been suggested to be mediated by several carrier-mediated pathways in cell culture and animal models. The role and importance of these transporters in modulating valacyclovir absorption in humans has not been determined, however. Recent advances in genomic technology have facilitated the rapid and simultaneous determination of global mRNA expression profiles for thousands of genes in tissue biopsies directly associated with the absorption process, thereby dramatically increasing the value of studies in humans. In this article, we describe correlations of pharmacokinetic parameters following oral valacyclovir or acyclovir administration with expression levels of intestinal genes in humans. Highly positive and significant correlations were observed with 4F2hc, an activator of cation-preferring amino acid transport systems, and human oligopeptide transporter (HPT1), an oligopeptide transporter expressed at higher levels in the human intestine compared with oligopeptide transporter (PEPT1). The validation of HPT1 microarray data with reverse transcription-polymerase chain reaction and the enhanced valacyclovir uptake in HeLa/HPT1 cells suggest that the role of HPT1 in transport of peptides and peptidomimetics drugs needs to be examined in more detail. The interrelation of 4F2hc and HPT1 in transport may be of interest. No significant correlations of valacyclovir pharmacokinetic parameters with PEPT1 and with organic cation or anion transporter expression levels were observed. The highly negative correlations observed with known efflux pumps such as MDR1 (P-glycoprotein) and MRP2 (cMOAT), as well as with the CYP450 IIIA subfamily may indicate that these proteins may regulate the cellular accumulation and metabolism of acyclovir.

Delivery of peptide drugs to the brain by adenovirus-mediated heterologous expression of human oligopeptide transporter at the blood-brain barrier

The feasibility of using adenovirus-mediated human oligopeptide transporter (hPEPT1) gene transfer to achieve peptide drug delivery to the brain across the blood-brain barrier was tested by examining the accumulation of model peptides in a rat brain endothelial cell line (RBEC1) and rat brain after transduction with a recombinant adenovirus encoding hPEPT1-enhanced yellow fluorescent protein fusion gene (AdhPEPT1-EYFP). In vitro uptake of [(3)H]GlySar was determined in RBEC1 transduced with AdhPEPT1-EYFP. In vivo, the accumulation of cefadroxil in rat brain was evaluated after transduction of AdhPEPT1-EYFP. At pH 6.0, the uptake of [(3)H]GlySar by RBEC1 transduced with AdhPEPT1-EYFP was increased 4-fold compared with that of nontransduced cells. At pH 7.4, uptake of [(3)H]GlySar in AdhPEPT1-EYFP transduced RBEC1 was 1.5 times higher than that of nontransduced cells. Unlabeled glycylsarcosine (10 mM) reduced the uptake of [(3)H]GlySar to a level comparable with that of nontransduced cells. At 30 min after intravenous administration of cefadroxil to rats transduced with AdhPEPT1-EYFP at 3.2 x 10(9) plaque-forming units/rat by an in situ brain perfusion method, the brain-to-plasma concentration ratio (Kp) of cefadroxil was increased about 2 times compared with that of nontransduced or AdGFP (control vector)-transduced rats, although this was not statistically significant. In contrast, Kp of [(14)C]inulin, a marker for extracellular fluid space, remained unchanged after adenoviral transduction. In conclusion, our results suggest that adenovirus-mediated heterologous expression of hPEPT1 in vivo could be a useful approach to deliver oligopeptides to the brain.

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Monitoring intracellular pH changes in response to osmotic stress and membrane transport activity using 5-chloromethylfluorescein

Intracellular free H+ concentration (pHi) responds to numerous extracellular stimuli. The use of fluorescent indicator dyes to measure pHi is strongly influenced by the ability of target cells to retain activated dye within the cytoplasmic compartment. Here, 3 pH-sensitive indicator dye - acetoxymethyl (AM) esters of SNARF-1 and BCECF, and the thiol-reactive 5-chloromethyfluorescein (CMFDA) - were examined for monitoring pHi. The stability of pH measurements was strongly affected by temperature, cell type, indicator dye, and use of transport inhibitors to prevent dye export. Cellular retention of CMFDA, which forms covalent complexes, was sufficient to permit monitoring of transient pHi changes over extended time periods in a multi-well plate assay format. In human embryonic kidney (HEK293) and Chinese hamster ovary (CHO) cells, increasing osmotic pressure caused a significant rise in pHi. In contrast, activation of native or transfected beta-adrenergic, cholinergic, and d and m opioid receptors did not measurably affect pHi in HEK293 cells. Decreases in pHi were observed in CHO cells expressing the human H+/peptide transporter PEPT1 upon addition of dipeptide substrates. The use of CMFDA in multi-well formats should facilitate study of osmotic and transport activity and screening for drugs that affect pHi.

Enhanced delivery of drugs to the liver by adenovirus-mediated heterologous expression of the human oligopeptide transporter PEPT1

To explore the feasibility of drug delivery to the liver by the use of adenovirus-mediated human oligopeptide transporter (hPEPT1) gene transfer, we examined the accumulation of L-[(3)H]carnosine in the hepatoma cell line (HepG2 and WIFB9) and mouse liver. We constructed a recombinant adenovirus encoding hPEPT1-enhanced yellow fluorescent protein (EYFP) fusion gene (AdhPEPT1-EYFP). In vitro uptake of L-[(3)H]carnosine was determined in HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP. In vivo, the accumulation of L-[(3)H]carnosine in mouse liver was evaluated after transduction of AdhPEPT1-EYFP. At pH 6.0, the uptake of L-[(3)H]carnosine by HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP was increased 15- and 2-fold, respectively, compared with the cells without transduction. At pH 7.4, uptake of L-[(3)H]carnosine in AdhPEPT1-EYFP transduced HepG2 cells was 3 times greater than that of nontransduced cells. In the presence of carnosine or glycylsarcosine as an inhibitor at 20 mM, the uptake of L-[(3)H]carnosine was reduced to a level comparable to that of nontransduced cells. At 30 min after intravenous administration of L-[(3)H]carnosine to mice transduced with AdhPEPT1-EYFP at 1 x 10(10) plaque-forming units/mouse, the tissue-to-plasma concentration ratio (K(p)) of L-[(3)H]carnosine in the liver was significantly increased to 7 times that of nontransduced mice. In contrast, the K(p) value of [(14)C]inulin, a marker for extracellular fluid space, remained unchanged after adenoviral transduction suggesting minimal pathological damage of tissues. hPEPT1-EYFP was localized at both the basolateral and apical membranes in HepG2 cells, WIFB9 cells, and mouse liver. In conclusion, our results suggest that delivery of oligopeptide to the liver by adenovirus-mediated heterologous expression of hPEPT1 in vivo is feasible.

Modeling of active transport systems

Transport proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made to characterize the P-glycoprotein efflux pump, the peptide transporter (PepT1) and the apical sodium-dependent transporter (ASBT) which are important not only for their native transporter function but also as drug targets to increase absorption and bioactivity. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of transporter function will be led by integrated in vitro and in silico approaches.

A rapid screening system to determine drug affinities for the intestinal dipeptide transporter 1: system characterisation

PURPOSE

To establish an in vitro system for the rapid assessment of the affinities of potential substrates for the di/tri/oligopeptide transport system (DTS).

METHODS

Monolayers of Caco-2 cells were cultured in plastic wells for 7-9 days and the uptake of Gly-[3H]L-Pro, a specific and relatively stable substrate for the DTS was used as an affinity probe. Gly-[3H]L-Pro (50 nM), together with excess L-Pro (10 mM), to suppress uptake of any [3H]L-Pro produced by degradation of the probe, was incubated with the test compound (usually 1 mM) at pH 6 for 3 min. The uptake of radiolabel was determined by liquid scintillation counting.

RESULTS

High specific-uptake (> 85%) of Gly-[3H]L-Pro was obtained with cells grown for 7-9 days. Gly-[3H]L-Pro uptake had a substantial active concentration-dependent component (Km of 0.39 +/- 0.02 mM, Vmax of 0.98 +/- 0.04 nmol min(-1) (mg protein)(-1). This process was shown to be specific for the DTS as evidenced by the significant inhibition by compounds reported to be transported by this system and the lack of inhibition by amino acids. The use of low competitor concentrations (1 mM) enabled a range of inhibition values (0-89%) of a series of competitors (amino acids, dipeptides and beta-lactam antibiotics) to be estimated, illustrating that structurally similar compounds can be ranked for affinity to the DTS.

CONCLUSION

A screening system, using Caco-2 cells and the dipeptide Gly-[3H]L-Pro as a displaceable probe, was developed to assess a variety of compounds for recognition by the di/tri/oligopeptide transport system. This fully describes the first system that allows structurally related compounds to be ranked on the basis of their affinity for the DTS recognition site.

Effect of ionization on the variable uptake of valacyclovir via the human intestinal peptide transporter (hPepT1) in CHO cells

Carrier-mediated transport of valacyclovir (vacv), the L-valyl ester prodrug of acyclovir (acv), via the human peptide transporter (hPepT1) has been shown in Xenopus laevis oocytes and in cell lines such as Chinese hamster ovary (CHO) and Caco-2 transfected with the hPepT1 gene. However, significant differences in vacv uptake were observed in those models as extracellular pH varied. The purpose of this work was to characterize the interactions of various ionic species of vacv with the peptide transporter by overexpressing the transporter gene, hPepT1, in CHO cells. Based on the pK(a) values of vacv, it was determined that vacv exists as four different ionic species (di-cationic, cationic, neutral and anionic) with a predominance of cationic and neutral species at physiologically relevant pH conditions. Vacv uptake was shown to increase with increasing pH of the extracellular medium from 5.5 to 7.2. The uptake value was maximal at around pH 7.2 and did not vary for studies done at higher pH. Vacv uptake was concentration dependent and saturable at all pH conditions (5.5, 6.2, 6.8, 7.5 and 7.9) with apparent Michaelis-Menten constants, mean (S.D.), of 7.42(0.32), 6.64(1.20), 5.38(0.88), 2.69(0.23) and 2.23(0.33) mM, respectively. The current results demonstrate that the estimated affinities of the cationic and the neutral species of vacv with hPepT1 are significantly different (7.4 versus 1.2 mM, respectively). Given the axial and radial (microclimate) pH gradients known to exist in the intestine, the greater than six-fold difference in affinity constants suggests that intestinal pH fluctuations may significantly impact upon the variability of vacv uptake.

Overview of membrane transport

Pharmaceutical scientists increasingly utilize transporters for drug delivery and targeting. The biological barriers to drug delivery can basically be divided into epithelial, endothelial, elimination, and target cell barriers. Membrane transporters play an important role in drug entrance and exit from the body. In addition, it is possible to utilize transporters for drug delivery, e.g., improving oral absorption via the peptide transporter. Identification, a better understanding of their transport characteristics, and the regulation of the membrane transporters will allow the development of better drug delivery strategies.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

Evidence for overlapping substrate specificity between large neutral amino acid (LNAA) and dipeptide (hPEPT1) transporters for PD 158473, an NMDA antagonist

PURPOSE

The objective of this research was to investigate the substrate specificity of large neutral amino acid carrier (LNAA) and di/tripeptide (hPEPT1) transporters with respect to PD 158473, an NMDA antagonist.

METHODS

Cellular uptake studies were carried out using two types of Chinese Hamster Ovary (CHO). CHO-K1 cells represent the wild type with inherent large neutral amino acid (LNAA) activity. CHO-PEPT1 cells were generated by stable transfection of hPEPT1 gene into CHO cells. Therefore, these cells possess both LNAA activity and di/tripeptide transporter activities as a result of the transfection. Cellular uptake of PD 158473 was quantified using a HPLC method previously developed in our laboratory.

RESULTS

The utility of the CHO-PEPT1 cell model was demonstrated by determining the uptake kinetics of Gly-Sar, a prototypical dipeptide transporter substrate. Uptake kinetics of PD 158473 displayed two carrier-mediated transport components in CHO-PEPT1 cells, while in CHO-K1 cells the relationship was consistent with classic one component Michaelis-Menten kinetics. These results confirmed the affinity of PD 158473 for both LNAA and di/tripeptide transporters. Further, results from inhibition experiments using these two cell types indicate that the high affinity-low capacity system was the LNAA carrier and the low affinity-high capacity carrier was the di/tripeptide transporter.

CONCLUSIONS

This study demonstrates overlapping substrate specificity between LNAA carrier and di/tripeptide transporter (hPEPT1) for PD 158473, an amino acid analog. Establishing Structure Transport Relationship (STR) for this overlap will aid in a design strategy for increasing oral absorption or targeting specific drugs to selected tissues.

CHO/hPEPT1 cells overexpressing the human peptide transporter (hPEPT1) as an alternative in vitro model for peptidomimetic drugs

The present study characterized Chinese hamster ovary cells overexpressing a human intestinal peptide transporter, CHO/hPEPT1 cells, as an in vitro model for peptidomimetic drugs. The kinetic parameters of Gly-Sar uptake were determined in three different cell culture systems such as untransfected CHO cells (CHO-K1), transfected CHO cells (CHO/hPEPT1) and Caco-2 cells. Vmax in CHO/hPEPT1 cells was approximately 3-fold higher than those in Caco-2 cells and CHO-K1 cells, while Km values were similar in all cases. The uptake of beta-lactam antibiotics in CHO/hPEPT1 cells was three to twelve fold higher than that in CHO-K1 cells, indicating that CHO/hPEPT1 cells significantly enhanced the peptide transport activity. However, amino acid drugs also exhibited high cellular uptake in both CHO-K1 and CHO/hPEPT1 cells due to the high background level of amino acid transporters. Thus, cellular uptake study in CHO/hPEPT1 cells is not sensitive enough to distinguish the peptidyl drugs from amino acid drugs. The potential of CHO/hPEPT1 cells as an in vitro model for peptidomimetic drugs was also examined through the inhibition study on Gly-Sar uptake. Peptidomimetic drugs such as beta-lactam antibiotics and enalapril significantly inhibited Gly-Sar uptake whereas the nonpeptidyl compounds, L-dopa and alpha-methyldopa, did not compete with Gly-Sar for cellular uptake within the therapeutic concentrations. In conclusion, the present study demonstrates the further characterization of CHO/hPEPT1 cells as an uptake model as well as inhibition study and suggests their utility as an alternative in vitro model for drug candidates targeting the hPEPT1 transporter.

Direct evidence for peptide transporter (PepT1)-mediated uptake of a nonpeptide prodrug, valacyclovir

Xenopus laevis oocytes were used as a gene expression system to characterize the carrier-mediated transport of valacyclovir (vacv), the L-valine ester prodrug of the acyclic nucleoside acyclovir (acv). A significant increase in the uptake of [3H]vacv by Xenopus laevis oocytes injected with human intestinal peptide transporter (hPepT1) cRNA compared to the uptake by water injected oocytes indicated that vacv was translocated by hPepT1. Vacv uptake was found to be concentration dependent, saturable (K(m) = 5.94 +/- 1.91 mM and Jmax = 1.68 +/- 0.25 nmoles/hr/oocyte), pH dependent, and inhibited by various known substrates of hPepT1 but not by acv, valine or pentaglycine. Vacv also inhibited the uptake of 14C-glycylsarcosine, a known substrate of hPepT1, in a concentration-dependent manner (Ki = 4.08 +/- 1.02 mM). These results demonstrate that human intestinal peptide transporter hPepT1 has broad specificity since it recognizes vacv as a substrate even though it lacks a typical peptide bond.

Cellular uptake mechanism of amino acid ester prodrugs in Caco-2/hPEPT1 cells overexpressing a human peptide transporter

PURPOSE

This study characterized the cellular uptake mechanism and hydrolysis of the amino acid ester prodrugs of nucleoside antiviral drugs in the transiently transfected Caco-2 cells overexpressing a human intestinal peptide transporter, hPEPT1 (Caco-2/hPEPT1 cells).

METHODS

Amino acid ester prodrugs of acyclovir and AZT were synthesized and their apical membrane permeability and hydrolysis were evaluated in Caco-2/hPEPT1 cells. The cellular uptake mechanism of prodrugs was investigated through the competitive inhibition study in Caco-2/hPEPT1 cells.

RESULTS

L-Valyl ester of acyclovir (L-Val-ACV) was approximately ten fold more permeable across the apical membrane than acyclovir and four times more permeable than D-valyl ester of acyclovir (D-Val-ACV). Correspondingly, L-valyl ester of AZT (L- Val-AZT) exhibited three fold higher cellular uptake than AZT. Therefore, amino acid ester prodrugs significantly increased the cellular uptake of the parent drugs and exhibited the D,L-stereoselectivity. Furthermore, prodrugs were rapidly hydrolyzed to the parent drugs by the intracellular hydrolysis, following the apical membrane transport. In the inhibition studies, cephalexin and small dipeptides strongly inhibited the cellular uptake of L-Val-ACV while L-valine had no effect, indicating that the peptide transporter is primarily responsible for the apical membrane transport of L-Val-ACV. In addition, the cellular uptake of L-Val-ACV was five times higher in Caco-2/hPEPT1 cells than the uptake in the untransfected Caco-2 cells, implying the cellular uptake of L-Val-ACV was related to the enhancement of the peptide transport activity in Caco-2/hPEPT1 cells.

CONCLUSIONS

Caco-2/hPEPT1 system is an efficient in vitro model for the uptake study of peptidyl derivatives. Amino acid ester prodrugs significantly improved the cellular uptake of the parent drugs via peptide transport mechanism and were rapidly converted to the active parent drugs by the intracellular hydrolysis.

First-pass effect: significance of the intestine for absorption and metabolism

The occurrence of low systemic availability due to significant metabolism or poor absorption of orally administered drugs has been well recognized. Three rate controlling factors affecting the oral absorption: unstirred water layer, membrane limitation, or flow limitation, have been identified. These are much affected by the physicochemical properties of the drug: pKA, water/lipid solubility, structural mimicry to endogenous substrates for transport proteins, and the physiology of the GI tract. Drug metabolizing enzymes are found to be present in the intestine, albeit the content is lower than that found in liver. The presence of pre-absorptive versus post-absorptive intestinal metabolism is presently discussed in experimental sets of data with luminal and systemic administration of the drugs in the vascularly perfused rat small intestine preparation. The effect of the anterior anatomical placement of the intestine and its contribution to metabolism, in relation to that for the liver, has been examined in our laboratory by the perfused intestine-liver preparation. The effect of concentration and flow have been studied and general principles governing drug absorption and metabolism in the intestine and the subsequent effects on the liver have been discussed.

Sequence alignments of the H(+)-dependent oligopeptide transporter family PTR: inferences on structure and function of the intestinal PET1 transporter

PURPOSE

To study the structure and function of the intestinal H+/ peptide transporter PET1, we compared its amino acid sequence with those of related transporters belonging to the oligopeptide transporter family PTR, and with more distant transporter families.

METHODS

We have developed a new approach to the sequence analysis of proteins with multiple transmembrane domains (TMDs) which takes into account the repeated TMD-loop topology. In addition to conventional analyses of the entire sequence, each TMD and its adjacent loop residues (= TMD segments) were analyzed separately as independent structural units. In combination with hydropathy analysis, this approach reveals any changes in the order of the TMD segments in the primary structure and permits TMD alignments among divergent structures even if rearrangements of the order of TMD segments have occurred in the course of evolution.

RESULTS

Alignments of TMD segments indicate that the TMD order in PTR transporters may have changed in the process of evolution. Consideration of such changes permits the alignment of homologous TMD segments from PTR transporters belonging to distant akaryotic and eukaryotic phyla. Multiple alignments of TMDs reveal several highly conserved regions that may play a role in transporter function. In comparing the PTR transporters with other transporter gene families, alignment scores using the entire primary structure are too low to support a finding of probable homology. However, statistically significant alignments were observed among individual TMD segments if one disregards the order in which they occur in the primary structure.

CONCLUSIONS

Our results support the hypothesis that the PTR transporters may have evolved by rearrangement, duplication, or insertions and deletions of TMD segments as independent modules. This modular structure suggests new alignment strategies for determining functional domains and testing relationship among distant transporter families.