Sertraline inhibits the transport of PAT1 substrates in vivo and in vitro

BACKGROUND AND PURPOSE

Intestinal nutrient transporters may mediate the uptake of drugs. The aim of this study was to investigate whether sertraline interacts with the intestinal proton-coupled amino acid transporter 1 PAT1 (SLC36A1).

EXPERIMENTAL APPROACH

In vitro investigations of interactions between sertraline and human (h)PAT1, hSGLT1 (sodium-glucose linked transporter 1) and hPepT1 (proton-coupled di-/tri-peptide transporter 1) were conducted in Caco-2 cells using radiolabelled substrates. In vivo pharmacokinetic investigations were conducted in male Sprague-Dawley rats using gaboxadol (10 mg·kg(-1), p.o.) as a PAT1 substrate and sertraline (0-30.6 mg·kg(-1)). Gaboxadol was quantified by hydrophilic interaction chromatography followed by MS/MS detection.

KEY RESULTS

Sertraline inhibited hPAT1-mediated L-[(3)H]-Pro uptake in Caco-2 cells. This interaction between sertraline and PAT1 appeared to be non-competitive. The uptake of the hSGLT1 substrate [(14)C]-α-methyl-D-glycopyranoside and the hPepT1 substrate [(14)C]-Gly-Sar in Caco-2 cells was also decreased in the presence of 0.3 mM sertraline. In rats, the administration of sertraline (0.1-10 mM, corresponding to 0.3-30.6 mg·kg(-1), p.o.) significantly reduced the maximal gaboxadol plasma concentration and AUC after its administration p.o.

CONCLUSIONS AND IMPLICATIONS

Sertraline is an apparent non-competitive inhibitor of hPAT1-mediated transport in vitro. This inhibitory effect of sertraline is not specific to hPAT1 as substrate transport via hPepT1 and hSGLT1 was also reduced in the presence of sertraline. In vivo, sertraline reduced the amount of gaboxadol absorbed, suggesting that the inhibitory effect of sertraline on PAT1 occurs both in vitro and in vivo. Hence, sertraline could alter the bioavailability of drugs absorbed via PAT1.

Function and expression of the proton-coupled amino acid transporter PAT1 along the rat gastrointestinal tract: implications for intestinal absorption of gaboxadol

BACKGROUND AND PURPOSE

Intestinal absorption via membrane transporters may determine the pharmacokinetics of drug compounds. The hypothesis is that oral absorption of gaboxadol (4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridine-3-ol) in rats occurs via the proton-coupled amino acid transporter, rPAT1 (encoded by the gene rSlc36a1). Consequently, we aimed to elucidate the in vivo role of rPAT1 in the absorption of gaboxadol from various intestinal segments obtained from Sprague-Dawley rats.

EXPERIMENTAL APPROACH

The absorption of gaboxadol was investigated following its administration into four different intestinal segments. The intestinal expression of rSlc36a1 mRNA was measured by quantitative real-time PCR. Furthermore, the hPAT1-/rPAT1-mediated transport of gaboxadol or L-proline was studied in hPAT1-expressing Xenopus laevis oocytes, Caco-2 cell monolayers and excised segments of the rat intestine.

KEY RESULTS

The absorption fraction of gaboxadol was high (81.3-91.3%) following its administration into the stomach, duodenum and jejunum, but low (4.2%) after administration into the colon. The pharmacokinetics of gaboxadol were modified by the co-administration of L-tryptophan (an hPAT1 inhibitor) and L-proline (an hPAT1 substrate). The in vitro carrier-mediated uptake rate of L-proline in the excised intestinal segments was highest in the mid jejunum and lowest in the colon. The in vitro uptake and the in vivo absorption correlated with the expression of rSlc36a1 mRNA along the rat intestine.

CONCLUSIONS AND IMPLICATIONS

These results suggest that PAT1 mediates the intestinal absorption of gaboxadol and therefore determines its oral bioavailability. This has implications for the in vivo role of PAT1 and may have an influence on the design of pharmaceutical formulations of PAT1 substrates.

The proton-coupled amino acid transporter, SLC36A1 (hPAT1), transports Gly-Gly, Gly-Sar and other Gly-Gly mimetics

BACKGROUND AND PURPOSE The intestinal proton-coupled amino acid transporter, SLC36A1, transports zwitterionic α-amino acids and drugs such as vigabatrin, gaboxadol and δ-aminolevulinic acid. We hypothesize that SLC36A1 might also transport some dipeptides. The aim of the present study was to investigate SLC36A1-mediated transport of Gly-Gly and Gly-Gly mimetics, and to investigate Gly-Sar transport via SLC36A1 and the proton-coupled dipeptide/tripeptide transporter, SLC15A1 in Caco-2 cells. EXPERIMENTAL APPROACH Transport of a compound via SLC36A1 was determined by its ability to induce an increase in the inward current of two-electrode voltage clamped SLC36A1 cRNA-injected Xenopus laevis oocytes. SLC36A1-mediated L-[³H]Pro uptake in Caco-2 cells was measured in the absence and presence of Gly-Gly or Gly-Sar. In addition, apical [¹⁴C]Gly-Sar uptake was measured in the absence and presence of the SLC36A1 inhibitor 5-hydroxy-L-tryptophan (5-HTP) or the SLC15A1 inhibitor L-4,4'-biphenylalanyl-L-proline (Bip-Pro). KEY RESULTS In SLC36A1-expressing oocytes, an inward current was induced by Gly-Sar, Gly-Gly, δ-aminolevulinic acid, β-aminoethylglycine, δ-aminopentanoic acid, GABA, Gly and Pro, whereas Val, Leu, mannitol, 5-HTP and the dipeptides Gly-Ala, Gly-Pro and Gly-Phe did not evoke currents. In Caco-2 cell monolayers, the apical uptake of 30 mM Gly-Sar was inhibited by 20 and 22% in the presence of 5-HTP or Bip-Pro, respectively, and by 48% in the presence of both. CONCLUSION AND IMPLICATIONS Our results suggest that whereas Gly-Gly amid bond bioisosteres are widely accepted by the hPAT1 carrier, dipeptides in general are not; and therefore, Gly-Sar might structurally define the size limit of dipeptide transport via SLC36A1.

Delta-aminolevulinic acid is a substrate for the amino acid transporter SLC36A1 (hPAT1)

BACKGROUND AND PURPOSE

delta-Aminolevulinic acid (ALA) is used in cancer patients for photodynamic diagnosis or therapy. Oral administration of ALA has been used in patients with prostate and bladder cancer. The present aim was to investigate the mechanism of intestinal absorption of ALA and its transport via the amino acid transporter SLC36A1.

EXPERIMENTAL APPROACH

In vitro investigations of ALA affinity for and uptake via SLC36A1 and SLC15A1 were performed in Caco-2 cell monolayers. Interaction of ALA with SLC15A1 was investigated in MDCK/SLC15A1 cells, whereas interactions with SLC36A1 were investigated in COS-7 cells transiently expressing SLC36A1.

KEY RESULTS

ALA inhibited SLC36A1-mediated L-[(3)H]Pro and SLC15A1-mediated [(14)C]Gly-Sar uptake in Caco-2 cell monolayers with IC(50) values of 11.3 and 2.1 mM respectively. In SLC36A1-expressing COS-7 cells, the uptake of [(14)C]ALA was saturable with a K(m) value of 6.8 +/- 3.0 mM and a V(max) of 96 +/- 13 pmol x cm(-2) x min(-1). Uptake of [(14)C]ALA was pH and concentration dependent, and could be inhibited by glycine, proline and GABA. In a membrane potential assay, translocation of ALA via SLC36A1 was concentration dependent, with a K(m) value of 3.8 +/- 1.0 mM. ALA is thus a substrate for SLC36A1. In Caco-2 cells, apical [(14)C]ALA uptake was pH dependent, but Na(+) independent, and completely inhibited by 5-hydroxy-L-tryptophan and L-4,4'-biphenylalanyl-l-proline. CONCLUSIONS AND IMPLICATIONS. ALA was a substrate for SLC36A1, and the apical absorption in Caco-2 cell was only mediated by SLC36A1 and SLC15A1. This advances our understanding of intestinal absorption mechanisms of ALA, as well as its potential for drug interactions.

Intestinal gaboxadol absorption via PAT1 (SLC36A1): modified absorption in vivo following co-administration of L-tryptophan

BACKGROUND AND PURPOSE

Gaboxadol has been in development for treatment of chronic pain and insomnia. The clinical use of gaboxadol has revealed that adverse effects seem related to peak serum concentrations. The aim of this study was to investigate the mechanism of intestinal absorption of gaboxadol in vitro and in vivo.

EXPERIMENTAL APPROACH

In vitro transport investigations were performed in Caco-2 cell monolayers. In vivo pharmacokinetic investigations were conducted in beagle dogs. Gaboxadol doses of 2.5 mg.kg(-1) were given either as an intravenous injection (1.0 mL.kg(-1)) or as an oral solution (5.0 mL.kg(-1)).

KEY RESULTS

Gaboxadol may be a substrate of the human proton-coupled amino acid transporter, hPAT1 and it inhibited the hPAT1-mediated L-[(3)H]proline uptake in Caco-2 cell monolayers with an inhibition constant K(i) of 6.6 mmol.L(-1). The transepithelial transport of gaboxadol was polarized in the apical to basolateral direction, and was dependent on gaboxadol concentration and pH of the apical buffer solution. In beagle dogs, the absorption of gaboxadol was almost complete (absolute bioavailability, F(a), of 85.3%) and T(max) was 0.46 h. Oral co-administration with 2.5-150 mg.kg(-1) of the PAT1 inhibitor, L-tryptophan, significantly decreased the absorption rate constant, k(a), and C(max), and increased T(max) of gaboxadol, whereas the area under the curve and clearance of gaboxadol were constant.

CONCLUSIONS AND IMPLICATIONS

The absorption of gaboxadol across the luminal membrane of the small intestinal enterocytes is probably mediated by PAT1. This knowledge is useful for reducing gaboxadol absorption rates in order to decrease peak plasma concentrations.

Di/tri-peptide transporters as drug delivery targets: regulation of transport under physiological and patho-physiological conditions

Two human di/tri-peptide transporters, hPepT1 and hPepT2 have been identified and functionally characterized. In the small intestine hPepT1 is exclusively expressed, whereas both PepT1 and PepT2 are expressed in the proximal tubule. The transport via di/tri-peptide transporters is proton-dependent, and the transporters thus belong to the Proton-dependent Oligopeptide Transporter (POT)-family. The transporters are not drug targets per se, however due to their uniquely broad substrate specificity; they have proved to be relevant drug targets at the level of drug transport. Drug molecules such as oral active beta-lactam antibiotics, bestatin, prodrugs of aciclovir and ganciclovir have oral bioavailabilities, which largely are a result of their interaction with PepT1. In the last few years an increasing number of studies concerned with regulation of di/tri-peptide transporter capacity have appeared. Studies on receptor-mediated regulation has shown that both PepT1 and PepT2 is down-regulated by long-term exposure to epidermal growth factor (EGF) due to a decreased gene transcription. PepT1-mediated transport is up-regulated by certain substrates and in response to fasting and starvation at the level of increased gene transcription. PepT1-mediated transport is up-regulated by short-term exposure to receptor agonists such as EGF, insulin, leptin, and clonidine, and down-regulated by VIP. Overall, the regulation of di/tri-peptide transport may be contributed to 1) changes in apical proton-motive force 2) recruitment of di/tri-peptide transporters from vesicular storages 3) changes in gene transcription/mRNA stability. The aim of the present review is to discuss physiological, patho-physiological and drug-induced regulation of di/tri-peptide transporter mediated transport.

Epidermal growth factor and insulin short-term increase hPepT1-mediated glycylsarcosine uptake in Caco-2 cells

AIMS

Little is known about the physiological regulation of the human intestinal di/tri-peptide transporter, hPepT1. In the present study we evaluated the effects of epidermal growth factor (EGF) and insulin on hPepT1-mediated dipeptide uptake in the intestinal cell line Caco-2.

METHODS

Caco-2 cells were grown on filters for 23-27 days. Apical dipeptide uptake was measured using [14C]glycylsarcosine([14C]Gly-Sar). HPepT1 mRNA levels were investigated using RT-PCR, cytosolic pH was determined using the pH-sensitive fluorescent probe BCECF.

RESULTS

Basolateral application of EGF increased [14C]Gly-Sar uptake with an ED50 value of 0.77 +/- 0.25 ng mL-1 (n = 3-6) and a maximal stimulation of 33 +/- 2% (n = 3-6). Insulin stimulated [14C]Gly-Sar uptake with an ED50 value of 3.5 +/- 2.0 ng mL-1 (n = 3-6) and a maximal stimulation of approximately 18% (n = 3-6). Gly-Sar uptake followed simple Michaelis-Menten kinetics. Km in control cells was 0.98 +/- 0.11 mM (n = 8) and Vmax was 1.86 +/- 0.07 nmol cm-2 min-1 (n = 8). In monolayers treated with 200 ng mL-1 of EGF, Km was 1.11 +/- 0.05 mM (n = 5) and Vmax was 2.79 +/- 0.05 nmol cm-2 min-1 (n = 5). In monolayers treated with 50 ng mL-1 insulin, Km was 1.03 +/- 0.08 mM and Vmax was 2.19 +/- 0.06 nmol cm-2 min-1 (n = 5). Kinetic data thus indicates an increase in the number of active transporters, following stimulation. The incrased Gly-Sar uptake was not accompanied by changes in hPepT1 mRNA, nor by measurable changes in cytosolic pH.

CONCLUSIONS

Short-term stimulation with EGF and insulin caused an increase in hPepT1-mediated uptake of Gly-Sar in Caco-2 cell monolayers, which could not be accounted for by changes in hPepT1 mRNA or proton-motive driving force.

Dipeptide model prodrugs for the intestinal oligopeptide transporter. Affinity for and transport via hPepT1 in the human intestinal Caco-2 cell line

The human intestinal di/tri-peptide carrier, hPepT1, has been suggested as a drug delivery target via increasing the intestinal transport of low permeability compounds by designing peptidomimetic prodrugs. Model ester prodrugs using the stabilized dipeptides D-Glu-Ala and D-Asp-Ala as pro-moieties for benzyl alcohol have been shown to maintain affinity for hPepT1. The primary aim of the present study was to investigate if modifications of the benzyl alcohol model drug influence the corresponding D-Glu-Ala and D-Asp-Ala model prodrugs' affinity for hPepT1 in Caco-2 cells. A second aim was to investigate the transepithelial transport and hydrolysis parameters for D-Asp(BnO)-Ala and D-Glu(BnO)-Ala across Caco-2 cell monolayers. In the present study, all investigated D-Asp-Ala and D-Glu-Ala model prodrugs retained various degrees of affinity for hPepT1 in Caco-2 cells. These affinities are used to establish a QSAR of our benzyl alcohol modified model prodrugs, aided at elucidating the observed differences in model prodrug affinity for hPepT1; additionally, these data suggest that the hydrophobicity of the side-chain model drug is the major determinant in the compounds affinity for hPepT1. Transepithelial transport studies performed using Caco-2 cells of D-Asp(BnO)-Ala and D-Glu(BnO)-Ala showed that the K(m) for transepithelial transport was not significantly different for the two compounds. The maximal transport rate of the carrier-mediated flux component does not differ between the two model prodrugs either. The transepithelial transport of D-Asp(BnO)-Ala and D-Glu(BnO)-Ala follows simple kinetics, and the release of benzyl alcohol is pH-dependent, but unaffected by 1 mM of the esterase inhibitor Paraoxon in 80% human plasma and Caco-2 cell homogenate.

Model prodrugs designed for the intestinal peptide transporter. A synthetic approach for coupling of hydroxy-containing compounds to dipeptides

The human peptide transporter, hPepT1, situated in the small intestine, may be exploited to increase absorption of drugs or model drugs by attaching them to a dipeptide, which is recognised by hPepT1. A synthetic protocol for this kind of model prodrugs was developed, in which model drugs containing a hydroxy group were attached to enzymatically stable dipeptides by hydrolysable ester linkages. Furthermore, a number of benzyl alcohols with various substituents in the 4-position of the phenyl ring were coupled to D-Asp-Ala and D-Glu-Ala. Ideally, a prodrug should be stable in the upper small intestine and be converted to the parent drug during or after transport into the blood circulation. Therefore, we investigated the influence of the electronegativity of the substituent in the 4-position of the phenyl ring on stability in aqueous solution at pH 6.0 and 7.4, corresponding to pH in jejunum and blood, respectively. In addition, the influence of the electronegativity of the substituent on stability upon storage was examined. Model prodrugs containing electron donating substituents in the 4-position of the phenyl ring decomposed upon storage, while model prodrugs containing no substituents or electron withdrawing substituents in the 4-position were stable. In aqueous solution (pH 6.0 and 7.4), electron withdrawing substituents in the 4-position decreased the half-life of the model prodrug. These data provide important information on stability of this kind of model prodrugs upon storage and under aqueous conditions. The results may be applied in the rational design of oligopeptide ester prodrugs to obtain prodrugs, which are stable upon storage and have an optimal release profile of the drug.

Epidermal growth factor inhibits glycylsarcosine transport and hPepT1 expression in a human intestinal cell line

The human intestinal cell line Caco-2 was used as a model system to study the effects of epidermal growth factor (EGF) on peptide transport. EGF decreased apical-to-basolateral fluxes of [(14)C]glycylsarcosine ([(14)C]Gly-Sar) up to 50.2 +/- 3.6% (n = 6) of control values. Kinetic analysis of the fluxes showed that maximal flux (V(max)) of transepithelial transport decreased from 3.00 +/- 0.17 nmol x cm(-2) x min(-1) in control cells to 0.50 +/- 0.07 nmol x cm(-2) x min(-1) in cells treated with 5 ng/ml EGF (n = 6, P < 0.01). The apparent Michaelis-Menten constant (K(m)) was 2.71 +/- 0.31 mM (n = 6) in control cells and 1.89 +/- 0.28 mM (n = 6, not significantly different from control) in EGF-treated cells. Similarly, apical uptake of [(14)C]Gly-Sar decreased in cells treated with EGF, with an ED(50) value of 0.36 +/- 0.06 ng/ml (n = 6) EGF and a maximal inhibition of 80 +/- 0.02% (n = 6). V(max) decreased from 2.61 +/- 0.4 to 1.06 +/- 0.1 nmol x cm(-2) x min(-1) (n = 3, P < 0.05), whereas K(m) remained constant. Basolateral Gly-Sar uptake showed no changes in V(max) or K(m) after EGF treatment (n = 3). RT-PCR showed a decrease in hPepT1 mRNA (using glucose-6-phosphate dehydrogenase mRNA as control) in cells treated with EGF. Western blotting indicated a decrease in hPepT1 protein in cell lysates. We conclude that EGF treatment decreases Gly-Sar transport in Caco-2 cells by decreasing the number of peptide transporter molecules in the apical membrane.

Model prodrugs for the intestinal oligopeptide transporter: model drug release in aqueous solution and in various biological media

The human intestinal di/tri-peptide carrier, hPepT1, has been suggested as a target for increasing intestinal transport of low permeability compounds by creating prodrugs designed for the transporter. Model ester prodrugs using the stabilized dipeptides D-Glu-Ala and D-Asp-Ala as pro-moieties for benzyl alcohol have been shown to have affinity for hPepT1. Furthermore, in aqueous solution at pH 5.5 to 10, the release of the model drug seems to be controlled by a specific base-catalyzed hydrolysis, indicating that the compounds may remain relatively stable in the upper small intestinal lumen with a pH of approximately 6.0, but still release the model drug at the intercellular and blood pH of approximately 7.4. Even though benzyl alcohol is not a low molecular weight drug molecule, these results indicate that the dipeptide prodrug principle is a promising drug delivery concept. However, the physico-chemical properties such as electronegativity, solubility, and log P of the drug molecule may also have an influence on the potential of these kinds of prodrugs. The purpose of the present study is to investigate whether the model drug electronegativity, estimated as Taft substitution parameter (sigma*) may influence the acid, water or base catalyzed model drug release rates, when released from series of D-Glu-Ala and D-Asp-Ala pro-moieties. Release rates were investigated in both aqueous solutions with varying pH, ionic strength, and buffer concentrations as well as in in vitro biological media. The release rates of all the investigated model drug molecules followed first-order kinetics and were dependent on buffer concentration, pH, ionic strength, and model drug electronegativity. The electronegativity of the model drug influenced acid, water and base catalyzed release from D-Asp-Ala and D-Glu-Ala pro-moieties. The model drug was generally released faster from D-Asp-Ala- than from the D-Glu-Ala pro-moieties. In biological media the release rate was also dependent on the electronegativity of the model drug. These results demonstrate that the model drug electronegativity, estimated as Taft (sigma*) values, has a significant influence on the release rate of the model drug.