PEPT1

PEPT1 (peptide transporter 1)

Aliases: HPECT1, HPEPT1
Gene name: Solute carrier family 15 member 1 (SLC15A1)

Summary

Human peptide transporter 1 (PEPT1) is an uptake transporter with a major role in the absorption of dietary di- and tripeptides from the small intestinal lumen. It is a high capacity, low affinity (K_M of 0.2-10 mM), proton-coupled cotransporter of diverse di- and tripeptides and peptidomimetic substrates, and is primarily expressed on the apical microvilli of enterocytes in the small intestine, with lower expression in epithelial cells in the kidney proximal tubule. In the kidney, PEPT1 reabsorbs peptides from the primary filtrate in the proximal tubule, in conjunction with a similar transporter, PEPT2.

PEPT1 also mediates the active oral absorption of drugs that contain peptide-like structures, most prominently β-lactam antibiotics. This had led to its successful exploitation in improving the systemic exposure of some drugs, notably antivirals and ACE inhibitors; peptide and peptide bond-like prodrugs significantly improved the bioavailability of their active moieties by engaging PEPT1 in the small intestine. As DDIs are not reported or anticipated, there are no recommendations for PEPT1 transporter investigation in either the FDA or EMA guidance.

Localization

PEPT1 is mainly expressed in the apical plasma membrane of enterocytes in the small intestine, with increasing expression from duodenum to ileum; in renal proximal tubular cells of the S1 segment, and in bile duct epithelial cells [1]. PEPT1/2 were also found to be expressed in the mitochondrial membrane of the human cancer cell lines PC3 and U118. There these transporters facilitate the transport of melatonin into the mitochondria [2, 3]. Transport activity similar to PEPT1 was also observed in lysosomal membranes of hepatocytes, and immunoreactivity was observed in the nuclei of vascular smooth muscle cells [4], but the relevance of these findings are largely unexplored. 

Function, physiology, and clinically significant polymorphisms

PEPT1 (most recently reviewed in [1] and [5]) is a high capacity, low affinity, sodium-independent symporter, or cotransporter, which catalyzes the electrogenic uphill transport of L-enantiomers of di- and tri-peptides in a sequence-independent manner. Peptide translocation is coupled with the movement of H⁺, and the transmembrane electrochemical proton gradient provides the driving force. PEPT1 plays a key role in the supply of nitrogen to the body; it absorbs di- and tripeptides released by the digestion of dietary or endogenous proteins from the small intestine. As a high capacity, low affinity transporter of peptides, PEPT1 is not likely to saturate even at the very high substrate concentrations typically encountered in the intestine.
PEPT1-mediated active absorption is implicated in the high bioavailability of orally active peptide-based drugs (e.g. cephalosporins and penicillins) and ester prodrugs (e.g. ACE inhibitors, protease inhibitors, bestatin, valacyclovir, L-DOPA), as well as artificial di- and tripeptides such as Gly-Sar [6-10] [11]. Substances without obvious peptide-like bonds may also be substrates (e.g. w-amino-levulonic acid, w-amino fatty acids) [12]. The large number of known substrate xenobiotics enables structure-based identification of possible substrates with good confidence [13], and this work is further facilitated by the recent structural determination of the transporter-valacyclovir complex using a prokaryotic homolog of mammalian PEPTs [14].
Several inhibitors of PEPT1 have been identified: sulfonylurea antidiabetic drugs such as nateglinide, glibenclamide, tolbutamide and chlorpropamide, sartans, and the ester prodrugs of ACE inhibitors [15-17].
PEPT1 expression is upregulated at the transcriptional level by dietary dipeptides (e.g. Gly-Sar; Gly-Phe; Gly-Gln) that enhance PEPT1 promoter activity. PEPT1 expression can also be regulated by intracellular proteins (e.g. PDZK1; AICAR), hormones (e.g. insulin; EGF; rhGH), and pharmacological agents (e.g. floxuridine; gemcitabine; 5-fluorouracil; enrofloxacin and xylanase), and may change in disease states (e.g., Crohn’s disease, inflammatory bowel disease, cancer) [18]. 

Clinical significance

No reports of clinical DDIs due to PEPT1 inhibition have been posted thus far. Because of this, PEPT1 is not regarded as a likely candidate for DDI liabilities. An in vivo study showed no interaction between a metabotropic glutamate 2/3 receptor agonist prodrug and valacyclovir via intestinal PEPT1 [19]. However, a food-drug interaction between milk and the PEPT1 substrate olsetamavir has been reported [20]. In vitro investigations suggest that quinolone antibiotics such as moxifloxacine may interfere with the absorption of PEPT1 substrates [21], and a mutual inhibition of PEPT1- and OAT1/3-driven absorption may exist between the protease inhibitor bestatin and cefixime [22].
Targeting intestinal uptake transporters such as PEPT1 by chemically modifying poorly absorbed drugs is a successful strategy for improving their bioavailability and therapeutic efficacy [23-25]. Once absorbed, the resulting prodrugs undergo non-specific enzymatic cleavage, releasing the pharmacologically active parent compound. Because PEPT1 has broad substrate specificity for peptides, high capacity, a high level of expression in the intestinal epithelium, and relatively restricted expression elsewhere, it is an attractive target for prodrug strategies. Valacyclovir, the L-valyl ester pro-drug of acyclovir, has 3–5 times higher oral bioavailability than the parent molecule, and plasma ganciclovir concentrations are 10-time higher when dosed as the prodrug valganciclovir [26, 27]. Enalapril, the ester pro-drug of the ACE-inhibitor enalaprilat, is a substrate for PEPT1, and oral bioavailability of enalaprilat is increased from 3–12% to 60–70% on dosing the prodrug [13]. Enhanced permeability over the parent molecule was also observed for dipeptidyl derivatives of L-a-methyl-DOPA. Talaglumetad (LY544344), an L-alanylamide prodrug of LY354740, had improved oral bioavailability, facilitated by PEPT1 [28]. 
Known clinical consequences of PEPT1 genetic variants are scarce, although a number of SNPs have been identified. In a whole-genome analysis study, variation in SLC15A2 was associated with the response of hepatocellular carcinoma to sorafenib treatment: patients with the rs2257212 genotype showed longer progression-free survival [29]. The nonsynonymous SNP F28Y was linked to reduced cephalexin uptake, whereas P586L was connected to impaired transport capacity and decreased PEPT1 protein expression [1].
PEPT1 appears to be upregulated in the colon of inflammatory bowel disease patients compared to healthy individuals. The significance of this finding is not well understood, but a number of studies confirmed PEPT1-mediated transport of peptides of bacterial origin into colonocytes, which may be a pro-inflammatory mechanism [30]. 

Regulatory requirements

As DDIs are not reported or anticipated, there are no recommendations for PEPT1 transporter investigation in either the FDA or EMA guidance.  

References

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2.    Mayo, J.C., et al., Melatonin transport into mitochondria. Cell Mol Life Sci, 2017. 74(21): p. 3927-3940.
3.    Huo, X., et al., Human transporters, PEPT1/2, facilitate melatonin transportation into mitochondria of cancer cells: An implication of the therapeutic potential. J Pineal Res, 2017. 62(4).
4.    Liang, R., et al., Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization. J Biol Chem, 1995. 270(12): p. 6456-63.
5.    Viennois, E., et al., Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease. Compr Physiol, 2018. 8(2): p. 731-760.
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13.    Zhang, E.Y., et al., Modeling of active transport systems. Adv Drug Deliv Rev, 2002. 54(3): p. 329-54.
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15.    Faria, T.N., et al., A novel high-throughput pepT1 transporter assay differentiates between substrates and antagonists. Mol Pharm, 2004. 1(1): p. 67-76.
16.    Knutter, I., et al., Transport of angiotensin-converting enzyme inhibitors by H+/peptide transporters revisited. J Pharmacol Exp Ther, 2008. 327(2): p. 432-41.
17.    Knutter, I., et al., High-affinity interaction of sartans with H+/peptide transporters. Drug Metab Dispos, 2009. 37(1): p. 143-9.
18.    Wang, C.Y., et al., Regulation profile of the intestinal peptide transporter 1 (PepT1). Drug Des Devel Ther, 2017. 11: p. 3511-3517.
19.    Pak, Y.A., et al., In Vitro and Clinical Evaluations of the Drug-Drug Interaction Potential of a Metabotropic Glutamate 2/3 Receptor Agonist Prodrug with Intestinal Peptide Transporter 1. Drug Metab Dispos, 2017. 45(2): p. 137-144.
20.    Morimoto, K., et al., Effect of milk on the pharmacokinetics of oseltamivir in healthy volunteers. J Pharm Sci, 2011. 100(9): p. 3854-61.
21.    Arakawa, H., et al., Possible interaction of quinolone antibiotics with peptide transporter 1 in oral absorption of peptide-mimetic drugs. Biopharm Drug Dispos, 2016. 37(1): p. 39-45.
22.    Wang, L., et al., PEPT1- and OAT1/3-mediated drug-drug interactions between bestatin and cefixime in vivo and in vitro in rats, and in vitro in human. Eur J Pharm Sci, 2014. 63: p. 77-86.
23.    Cundy, K.C., et al., XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl] aminomethyl)-1-cyclohexane acetic acid], a novel gabapentin prodrug: I. Design, synthesis, enzymatic conversion to gabapentin, and transport by intestinal solute transporters. J Pharmacol Exp Ther, 2004. 311(1): p. 315-23.
24.    Cundy, K.C., et al., XP13512 [(+/-)-1-([(alpha-isobutanoyloxyethoxy)carbonyl] aminomethyl)-1-cyclohexane acetic acid], a novel gabapentin prodrug: II. Improved oral bioavailability, dose proportionality, and colonic absorption compared with gabapentin in rats and monkeys. J Pharmacol Exp Ther, 2004. 311(1): p. 324-33.
25.    Rautio, J., et al., The expanding role of prodrugs in contemporary drug design and development. Nat Rev Drug Discov, 2018. 17(8): p. 559-587.
26.    Steffansen, B., C.U. Nielsen, and S. Frokjaer, Delivery aspects of small peptides and substrates for peptide transporters. Eur J Pharm Biopharm, 2005. 60(2): p. 241-5.
27.    Sugawara, M., et al., Transport of valganciclovir, a ganciclovir prodrug, via peptide transporters PEPT1 and PEPT2. J Pharm Sci, 2000. 89(6): p. 781-9.
28.    Bueno, A.B., et al., Dipeptides as effective prodrugs of the unnatural amino acid (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), a selective group II metabotropic glutamate receptor agonist. J Med Chem, 2005. 48(16): p. 5305-20.
29.    Lee, Y.S., et al., SLC15A2 genomic variation is associated with the extraordinary response of sorafenib treatment: whole-genome analysis in patients with hepatocellular carcinoma. Oncotarget, 2015. 6(18): p. 16449-60.
30.    Ingersoll, S.A., et al., The role and pathophysiological relevance of membrane transporter PepT1 in intestinal inflammation and inflammatory bowel disease. Am J Physiol Gastrointest Liver Physiol, 2012. 302(5): p. G484-92.