Species-dependent uptake of glycylsarcosine but not oseltamivir in Pichia pastoris expressing the rat, mouse, and human intestinal peptide transporter PEPT1

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

Rapid and Sensitive Quantification of Intracellular Glycyl-Sarcosine for Semi-High-Throughput Screening for Inhibitors of PEPT-1

The peptide transporter PEPT-1 (SLC15A1) plays a major role in nutritional supply with amino acids by mediating the intestinal influx of dipeptides and tripeptides generated during food digestion. Its role in the uptake of small bioactive peptides and various therapeutics makes it an important target for the investigation of the systemic absorption of small peptide-like active compounds and prodrug strategies of poorly absorbed therapeutics. The dipeptide glycyl-sarcosine (Gly-Sar), which comprises an N-methylated peptide bond that increases stability against enzymatic degradation, is widely utilized for studying PEPT-1-mediated transport. To support experiments on PEPT-1 inhibitor screening to identify potential substrates, we developed a highly sensitive Gly-Sar quantification assay for Caco-2 cell lysates with a dynamic range of 0.1 to 1000 ng/mL (lower limit of quantification 0.68 nM) in 50 µL of cell lysate. The assay was validated following the applicable recommendations for bioanalytic method validation of the FDA and EMA. Sample preparation and quantification were established in 96-well cell culture plates that were also used for the cellular uptake studies, resulting in a rapid and robust screening assay for PEPT-1 inhibitors. This sample preparation principle, combined with the high sensitivity of the UPLC-MS/MS quantification, is suitable for screening assays for PEPT-1 inhibitors and substrates in high-throughput formats and holds the potential for automation. Applicability was demonstrated by IC₅₀ determinations of the known PEPT-1 inhibitor losartan, the known substrates glycyl-proline (Gly-Pro), and valaciclovir, the prodrug of aciclovir, which itself is no substrate of PEPT-1 and consequently showed no inhibition in our assay.

Characterization of the cellular transport mechanisms for the anti-cachexia candidate compound TCMCB07

BACKGROUND

Cachexia is a debilitating, life-threatening condition whose pathology includes reduced food intake accompanied by hypermetabolism, leading to a catabolic state. The hypothalamic melanocortin system is a critical regulator of metabolic rate with effects being mediated through the melanocortin-4 receptor (MC4R). MC4R activation is also critical to the initiation and maintenance of cachexia. A major problem in the design of anti-cachexia drugs has been the need to cross the blood-brain barrier to access the metabolic rate-controlling centres in the hypothalamus. The overwhelming majority of anti-cachexia drugs are only effective when administered intracerebroventricularly. TCMCB07 is a cyclic nonapeptide peptide MC4R antagonist with parenteral anti-cachexia activity in both small and large animal models. This suggests it can cross the blood-brain barrier. The aim of this study was to examine potential transport mechanisms of TCMCB07 furthering its preclinical development for subsequent studies in humans.

METHODS

In vitro studies were performed in transporter-transfected cells to study whether or not TCMCB07 was an inhibitor as well as substrate for OATP1A2, OATP1B1, OATP1B3, OATP2B1, OCT2, OAT1, OAT3, MATE1, MATE2-K, P-gp (MDR1), and BCRP. In vivo mass balance studies were also performed in mice to evaluate the absorption and disposition of TCMCB07 after oral and intravenous bolus administrations.

RESULTS

TCMCB07 inhibited the uptake of prototypical substrates in cells transfected with OATP1A2 (IC₅₀ 24.0 μM), OATP1B1 (IC₅₀ 6.8 μM), OATP1B3 (IC₅₀ 307 μM), OATP2B1 (IC₅₀ 524 μM), OCT2 (IC₅₀ 1,169 μM), MATE1 (IC₅₀ 8.7 μM), and MATE2-K (IC₅₀ 20.7 μM) but not in cells transfected with OAT1 and OAT3. TCMCB07 did not affect the P-gp (MDR1)-mediated and BCRP-mediated permeability of prototypical substrates in transfected cells. Importantly, direct evidence was shown for the uptake of TCMCB07 in OATP1A2-transfected cells (i.e. V_max 236 pmol/mg, K_m 58.4 μM, and K_d 0.39 μL/mg), demonstrating that the nonapeptide was a substrate for this transporter. Mass balance studies demonstrated that 24.2% of TCMCB07 was absorbed orally in vivo (P = 0.0033) and excreted primarily in the bile after both oral and intravenous administrations.

CONCLUSIONS

OATP1A2 is the transporter responsible for the oral absorption of TCMCB07 in the intestine and for its pharmacologic response in the brain.

Establishment of SLC15A1/PEPT1-Knockout Human-Induced Pluripotent Stem Cell Line for Intestinal Drug Absorption Studies

Because many peptide and peptide-mimetic drugs are substrates of peptide transporter 1, it is important to evaluate the peptide transporter 1-mediated intestinal absorption of drug candidates in the early phase of drug development. Although intestinal cell lines treated with inhibitors of peptide transporter 1 are widely used to examine whether drug candidates are substrates for peptide transporter 1, these inhibitors are not sufficiently specific for peptide transporter 1. In this study, to generate a more precise evaluation model, we established peptide transporter 1-knockout induced pluripotent stem cells (iPSCs) by using a CRISPR-Cas9 system and differentiated the cells into intestinal epithelial-like cells. The permeability value and uptake capacity of glycylsarcosine (substrate of peptide transporter 1) in peptide transporter 1-knockout intestinal epithelial-like cells were significantly lower than those in wild-type intestinal epithelial-like cells, suggesting that peptide transporter 1 was successfully depleted in the epithelial cells. Taken together, our model can be useful in the development of peptide and peptide-mimetic drugs.

Transport Versus Hydrolysis: Reassessing Intestinal Assimilation of Di- and Tripeptides by LC-MS/MS Analysis

SCOPE

The role of PEPT1 in the uptake of intact peptides as compared to hydrolysis prior to uptake of their constituents is unknown. Here, dipeptides, tripeptides, and amino acids are quantified to study the fate of selected peptides in different intestinal models.

METHODS AND RESULTS

An LC-MS/MS-based method is applied for the simultaneous assessment of rates of hydrolysis and transport of a peptide panel in Caco-2 transwell cell culture, in vitro and in vivo in mice expressing or lacking PEPT1, and in hydrolysis studies in vitro using human intestinal samples. It is shown that susceptibility to hydrolysis of peptides at the brush border membrane or within epithelial cells is practically identical in all tested models and strictly structure-dependent. Peptides with high luminal disappearance show substantial hydrolysis and low basolateral appearance, while peptides with low disappearance show strong PEPT1 dependency and high basolateral appearance in intact form in Caco-2 transwell culture.

CONCLUSION

Hydrolysis and transport of intact peptides are highly variable and structure-dependent. For peptides possessing less polar N-terminal residues, hydrolysis usually dominates over transport via PEPT1. For other peptides with high intrinsic hydrolysis resistance, including anserine, carnosine, ɣ-glutamyl-dipeptides, and aminocephalosporins, PEPT1 is the main determinant for appearance in peripheral blood.

In Silico Prediction of the Absorption and Disposition of Cefadroxil in Humans using an Intestinal Permeability Method Scaled from Humanized PepT1 Mice

It is difficult to predict the pharmacokinetics and plasma concentration-time profiles of new chemical entities in humans based on animal data. Some pharmacokinetic parameters, such as clearance and volume of distribution, can be scaled allometrically from rodents, mammals, and nonhuman primates with good success. However, it is far more challenging to predict the oral pharmacokinetics of experimental drug candidates. In the present study, we used in situ estimates of intestinal permeability, obtained in silico and from rat, wild-type (WT), and humanized PepT1 (huPepT1) mice, to predict the systemic exposure of cefadroxil, an orally administered model compound, under a variety of conditions. Using the GastroPlus simulation software program (Simulations Plus, Lancaster, CA), we found that the C max and area under the plasma concentration-time curve from time zero to the last measurable concentration of cefadroxil were better predicted using intestinal permeability estimates (both segmental and jejunal) from huPepT1 than from WT mice, and that intestinal permeabilities based on in silico and rat estimates gave worse predictions. We also observed that accurate predictions were possible for cefadroxil during oral dose escalation (i.e., 5, 15, and 30 mg/kg cefadroxil), a drug-drug interaction study (i.e., 5 mg/kg oral cefadroxil plus 45 mg/kg oral cephalexin), and an oral multiple dose study [i.e., 500 mg (6.7 mg/kg) cefadroxil every 6 hours]. Finally, the greatest amount of cefadroxil was absorbed in duodenal and jejunal segments of the small intestine after a 5 mg/kg oral dose. Thus, by combining a humanized mouse model and in silico software, the present study offers a novel strategy for better translating preclinical pharmacokinetic data to oral drug exposure during first-in-human studies.

Effect of biphenyl hydrolase-like (BPHL) gene disruption on the intestinal stability, permeability and absorption of valacyclovir in wildtype and Bphl knockout mice

Biphenyl hydrolase-like protein (BPHL) is a novel human serine hydrolase that was originally cloned from a breast carcinoma cDNA library and shown to convert valacyclovir to acyclovir and valganciclovir to ganciclovir. However, the exclusivity of this process has not been determined and, indeed, it is possible that a number of esterases/proteases may mediate the hydrolysis of valacyclovir and similar prodrugs. The objectives of the present study were to evaluate the in situ intestinal permeability and stability of valacyclovir in wildtype (WT) and Bphl knockout (KO) mice, as well as the in vivo oral absorption and intravenous disposition of valacyclovir and acyclovir in the two mouse genotypes. We found that Bphl knockout mice had no obvious phenotype and that Bphl ablation did not alter the jejunal permeability of valacyclovir during in situ perfusions (i.e., 0.54 × 10^-4 in WT vs. 0.53 × 10^-4 cm/s in KO). Whereas no meaningful changes occurred between genotypes in the gene expression of proton-coupled oligopeptide transporters (i.e., PepT1, PepT2, PhT1, PhT2), enzymatic upregulation of Cyp3a11, Cyp3a16, Abhd14a and Abhd14b was observed in some tissues of Bphl knockout mice. Most importantly, we found that valacyclovir was rapidly and efficiently hydrolyzed to acyclovir in the absence of BPHL, and that hydrolysis was more extensive after the oral vs. intravenous route of administration (for both genotypes). Taken as a whole, BPHL is not obligatory for the conversion of valacyclovir to acyclovir either presystemically or systemically.

Evaluating the intestinal and oral absorption of the prodrug valacyclovir in wildtype and huPepT1 transgenic mice

The purpose of this work was to evaluate the intestinal permeability, oral absorption and disposition of the ester prodrug valacyclovir in wildtype mice and a huPepT1 transgenic mouse model. PepT1 (SLC15A1) is a transporter apically expressed along the lumen of the gastrointestinal tract and is responsible for the absorption of di-/tripeptides, ACE inhibitors, β-lactam antibiotics and numerous prodrugs. Unfortunately, PepT1-mediated substrates that have been extensively studied were shown to exhibit species-dependent absorption and pharmacokinetics. Accordingly, in situ intestinal perfusion studies were conducted and valacyclovir uptake was shown to have a 30-fold lower K_m and 100-fold lower V_max in huPepT1 compared to wildtype mice. Moreover, inhibition studies demonstrated that the huPepT1 transporter alone was responsible for valacyclovir uptake, and segment-dependent studies reported significant reductions in permeability along the length of small intestine in huPepT1 mice. Subsequent oral administration studies revealed that the in vivo rate and extent of valacyclovir absorption were lower in huPepT1 mice, as indicated by 3-fold lower C_max and 3-fold higher T_max values, and an AUC_0-180 that was 80% of that observed in wildtype mice. However, no significant changes in drug disposition were observed between genotypes after intravenous bolus administration of acyclovir. Lastly, mass balance studies established that the bioavailability of acyclovir, after oral dosing of valacyclovir, was 77.5% in wildtype mice and 52.8% in huPepT1 mice, which corroborated values of 51.3% in clinical studies. Thus, it appears the huPepT1 transgenic mice may be a better model to study prodrug absorption and disposition in humans than wildtype mice.

Current knowledge of intestinal absorption of bioactive peptides

Peptides have been demonstrated as potentially beneficial compounds against several life-style related diseases such as hypertension, hypercholesterolemia, and atherosclerosis, among others. However, limited research has been carried out on peptide absorption, resulting in a lack of understanding and control of this process. Therefore, this review discusses the recent insights gathered on in vitro and in vivo absorption of peptides across intestinal membranes, into blood circulation. Briefly, some di-/tripeptides permeate through intestinal membranes in their intact forms via peptide transporter systems, while others are vulnerable to protease degradation. Oligopeptides (>tetrapeptides) show a lower transport ability than di-/tripeptides, possibly due to the presence of paracellular tight junctions. The hydrophobicity of peptides (log P) does not seem to influence absorption, while peptide length and degradation of peptides (and peptide sequences) by intestinal proteases may be determinant factors of the absorption process.

Functional Characterization of Human Peptide/Histidine Transporter 1 in Stably Transfected MDCK Cells

The proton-coupled oligopeptide transporter PHT1 (SLC15A4), which facilitates cross-membrane transport of histidine and small peptides from inside the endosomes or lysosomes to cytosol, plays an important role in intracellular peptides homeostasis and innate immune responses. However, it remains a challenge to elucidate functional properties of the PHT1 transporter because of its subcellular localization. The purpose of this study was to resort hPHT1 protein from the subcellular to outer cell membrane of MDCK cells stably transfected with human PHT1 mutants, and to characterize its functional activity in these cells. Using this model, the functional activity of hPHT1 was evaluated by cellular uptake studies with d₃-l-histidine, GlySar, and the bacterial peptidoglycan products MDP and Tri-DAP. We found that the disruption of two dileucine motifs was indispensable for hPHT1 transporter being preferentially targeting to plasma membranes. hPHT1 showed high affinity for d₃-l-histidine and low affinity for GlySar, with K_m values of 16.3 ± 1.9 μM and 1.60 ± 0.30 mM, respectively. Moreover, the bacterial peptidoglycan components MDP and Tri-DAP were shown conclusively to be hPHT1 substrates. The uptake of MDP by hPHT1 was inhibited by di/tripeptides and peptide-like drugs, but not by glycine and acyclovir. The functional activity of hPHT1 was also pH-dependent, with an optimal cellular uptake in buffer pH 6.5. Taken together, we established a novel cell model to evaluate the function of hPHT1 in vitro, and confirmed that MDP and Tri-DAP were substrates of hPHT1. Our findings suggest that PHT1 may serve as a potential target for reducing the immune responses and for drug treatment of inflammatory diseases.

Chemical Modulation of the Human Oligopeptide Transporter 1, hPepT1

In humans, peptides derived from dietary proteins and peptide-like drugs are transported via the proton-dependent oligopeptide transporter hPepT1 (SLC15A1). hPepT1 is located across the apical membranes of the small intestine and kidney, where it serves as a high-capacity low-affinity transporter of a broad range of di- and tripeptides. hPepT1 is also overexpressed in the colon of inflammatory bowel disease (IBD) patients, where it mediates the transport of harmful peptides of bacterial origin. Therefore, hPepT1 is a drug target for prodrug substrates interacting with intracellular proteins or inhibitors blocking the transport of toxic bacterial products. In this study, we construct multiple structural models of hPepT1 representing different conformational states that occur during transport and inhibition. We then identify and characterize five ligands of hPepT1 using computational methods, such as virtual screening and QM-polarized ligand docking (QPLD), and experimental testing with uptake kinetic measurements and electrophysiological assays. Our results improve our understanding of the substrate and inhibitor specificity of hPepT1. Furthermore, the newly discovered ligands exhibit unique chemotypes, providing a framework for developing tool compounds with optimal intestinal absorption as well as future IBD therapeutics against this emerging drug target.

Species Differences in Human and Rodent PEPT2-Mediated Transport of Glycylsarcosine and Cefadroxil in Pichia Pastoris Transformants

The proton-coupled oligopeptide transporter PEPT2 (SLC15A2) plays an important role in the disposition of di/tripeptides and peptide-like drugs in kidney and brain. However, unlike PEPT1 (SLC15A1), there is little information about species differences in the transport of PEPT2-mediated substrates. The purpose of this study was to determine whether PEPT2 exhibited a species-dependent uptake of glycylsarcosine (GlySar) and cefadroxil using yeast Pichia pastoris cells expressing cDNA from human, mouse, and rat. In such a system, the functional activity of PEPT2 was evaluated with [³H]GlySar as a function of time, pH, substrate concentration, and specificity, and with [³H]cefadroxil as a function of concentration. We observed that the uptake of GlySar was pH-dependent with an optimal uptake at pH 6.5 for all three species. Moreover, GlySar showed saturable uptake kinetics, with K_m values in human (150.6 µM) > mouse (42.8 µM) ≈ rat (36.0 µM). The PEPT2-mediated uptake of GlySar in yeast transformants was specific, being inhibited by di/tripeptides and peptide-like drugs, but not by amino acids and nonsubstrate compounds. Cefadroxil also showed a saturable uptake profile in all three species, with K_m values in human (150.8 μM) > mouse (15.6 μM) ≈ rat (11.9 μM). These findings demonstrated that the PEPT2-mediated uptake of GlySar and cefadroxil was specific, species dependent, and saturable. Furthermore, based on the K_m values, mice appeared similar to rats but both were less than optimal as animal models in evaluating the renal reabsorption and pharmacokinetics of peptides and peptide-like drugs in humans.

Species differences in the pharmacokinetics of cefadroxil as determined in wildtype and humanized PepT1 mice

PepT1 (SLC15A1) is a high-capacity low-affinity transporter that is important in the absorption of digested di/tripeptides from dietary protein in the small intestine. PepT1 is also crucial for the intestinal uptake and absorption of therapeutic agents such as the β-lactam aminocephalosporins and antiviral prodrugs. Species differences, however, have been observed in PepT1-mediated intestinal absorption and pharmacokinetics, thereby, making it more difficult to predict systemic drug exposure. In the present study, we evaluated the in situ intestinal permeability of the PepT1 substrate cefadroxil in wildtype and humanized PepT1 (huPepT1) mice, and the in vivo absorption and disposition of drug after escalating oral doses. The in situ perfusions indicated that cefadroxil had a twofold higher affinity (i.e., twofold lower Km) for jejunal PepT1 in huPepT1 mice, lower but substantial permeability in all regions of the small intestine, and low but measureable permeability in the colon as compared to wildtype animals. The in vivo experiments indicated almost superimposable pharmacokinetic profiles between the two genotypes after intravenous bolus dosing of cefadroxil. In contrast, after oral dose escalation, the systemic exposure of cefadroxil was reduced in huPepT1 mice as compared to wildtype animals. Moreover, the AUC and Cmax versus dose relationships were nonlinear for huPepT1 but not wildtype mice, and similar to that observed from human subjects. In conclusion, our findings indicate that huPepT1 mice may provide a valuable tool in the drug discovery process by better predicting the oral pharmacokinetic profiles of PepT1 substrates in humans.

Development and characterization of a novel mouse line humanized for the intestinal peptide transporter PEPT1

The proton-coupled oligopeptide transporter PEPT1 (SLC15A1) is abundantly expressed in the small intestine, but not colon, of mammals and found to mediate the uptake of di/tripeptides and peptide-like drugs from the intestinal lumen. However, species differences have been observed in both the expression (and localization) of PEPT1 and its substrate affinity. With this in mind, the objectives of this study were to develop a humanized PEPT1 mouse model (huPEPT1) and to characterize hPEPT1 expression and functional activity in the intestines. Thus, after generating huPEPT1 mice in animals previously nulled for mouse Pept1, phenotypic, PCR, and immunoblot analyses were performed, along with in situ single-pass intestinal perfusion and in vivo oral pharmacokinetic studies with a model dipeptide, glycylsarcosine (GlySar). Overall, the huPEPT1 mice had normal survival rates, fertility, litter size, gender distribution, and body weight. There was no obvious behavioral or pathological phenotype. The mRNA and protein profiles indicated that huPEPT1 mice had substantial PEPT1 expression in all regions of the small intestine (i.e., duodenum, jejunum, and ileum) along with low but measurable expression in both proximal and distal segments of the colon. In agreement with PEPT1 expression, the in situ permeability of GlySar in huPEPT1 mice was similar to but lower than wildtype animals in small intestine, and greater than wildtype mice in colon. However, a species difference existed in the in situ transport kinetics of jejunal PEPT1, in which the maximal flux and Michaelis constant of GlySar were reduced 7-fold and 2- to 4-fold, respectively, in huPEPT1 compared to wildtype mice. Still, the in vivo function of intestinal PEPT1 appeared fully restored (compared to Pept1 knockout mice) as indicated by the nearly identical pharmacokinetics and plasma concentration–time profiles following a 5.0 nmol/g oral dose of GlySar to huPEPT1 and wildtype mice. This study reports, for the first time, the development and characterization of mice humanized for PEPT1. This novel transgenic huPEPT1 mouse model should prove useful in examining the role, relevance, and regulation of PEPT1 in diet and disease, and in the drug discovery process.

Role of the intestinal peptide transporter PEPT1 in oseltamivir absorption: in vitro and in vivo studies

It was reported that oseltamivir (Tamiflu) absorption was mediated by human peptide transporter (hPEPT) 1. Understanding the exact mechanism(s) of absorption is important in the context of drug-drug and diet-drug interactions. Hence, we investigated the mechanism governing the intestinal absorption of oseltamivir and its active metabolite (oseltamivir carboxylate) in wild-type [Chinese hamster ovary (CHO)-K1] and hPEPT1-transfected cells (CHO-PEPT1), in pharmacokinetic studies in juvenile and adult rats, and in healthy volunteers. In vitro cell culture studies showed that the intracellular accumulation of oseltamivir and its carboxylate into CHO-PEPT1 and CHO-K1 was always similar under a variety of experimental conditions, demonstrating that these compounds are not substrates of hPEPT1. Furthermore, neither oseltamivir nor its active metabolite was capable of inhibiting Gly-Sar uptake in CHO-PEPT1 cells. In vivo pharmacokinetic studies in juvenile and adult rats showed that the disposition of oseltamivir and oseltamivir carboxylate, after oral administration of oseltamivir, was sensitive to the feed status but insensitive to the presence of milk and Gly-Sar. Moreover, oseltamivir and oseltamivir carboxylate exhibited significantly higher exposure in rats under fasted conditions than under fed conditions. In humans, oral dosing after a high-fat meal resulted in a statistically significant but moderate lower exposure than after an overnight fasting. This change has no clinical implications. Taken together, the results do not implicate either rat Pept1 or hPEPT1 in the oral absorption of oseltamivir.