Abstract

Peptide transporters (PEPT1 and PEPT2) in epithelia play an important role in the absorption of small peptides and peptide-like drugs. Recently, it was demonstrated that various nonpeptidic compounds can be transported by these transporters. In the present study, we focused on the l-amino acid ester compounds and examined the mechanisms of their interaction with rat PEPTs (rPEPTs) using stable transfectants. Valacyclovir, the l-valyl ester prodrug of the antiherpetic agent acyclovir, competitively inhibited [¹⁴C]glycylsarcosine uptake in the rPEPT1- or rPEPT2-expressing cells. Dixon plot analyses showed that the inhibition constant (K_i) values of valacyclovir were 2.7 and 0.22 mM for rPEPT1 and rPEPT2, respectively, suggesting that rPEPT2 had higher affinity for this agent. Various l-valine alkyl esters significantly inhibited [¹⁴C]glycylsarcosine uptake. l-Valine methyl ester (Val-OMe) competitively inhibited [¹⁴C]glycylsarcosine uptake withK_i values of 3.6 and 0.83 mM for rPEPT1 and rPEPT2, respectively, indicating that Val-OMe is also a high-affinity substrate for rPEPT2. Val-OMe had a trans-stimulation effect on [¹⁴C]glycylsarcosine efflux from both transfectants, suggesting the translocation of l-valine methyl ester via rPEPTs. Val-OMe showed the most potent inhibitory effect among the several l-amino acid methyl esters examined. We conclude that Val-OMe, as well as valacyclovir, could be recognized and transported by rPEPT1 and rPEPT2 and that thesel-valyl esters showed higher affinity for rPEPT2 as do most substrates of these transporters. Our results suggest thatl-valine is a desirable l-amino acid for the esterification of poorly permeable drugs to enhance their oral bioavailability targeting intestinal PEPT1.