Interactions of the Dipeptide Ester Prodrugs of Acyclovir with the Intestinal Oligopeptide Transporter: Competitive Inhibition of Glycylsarcosine Transport in Human Intestinal Cell Line-Caco-2

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Abstract
The oligopeptide transporter may be exploited to enhance the absorption of drugs by synthesizing their dipeptide ester prodrugs, which may be recognized as its substrates. Various dipeptide esters of acyclovir (ACV), an antiviral nucleoside analog, were synthesized. Enzymatic hydrolysis and affinity of the prodrugs toward the human intestinal peptide transporter hPEPT1 were studied using the human intestinal Caco-2 cell line. Affinity studies were performed by inhibiting the uptake of [3H]glycylsarcosine by the prodrugs. The uptake of glycylsarcosine was found to be saturable at higher concentrations and was competitively inhibited by the prodrugs of ACV. All prodrugs except Tyr-Gly-ACV demonstrated a higher affinity (1.41–4.96 mM) toward hPEPT1 than cephalexin (8.19 ± 2.12 mM), which was used as a positive control. Two prodrugs, Gly-Val-ACV and Val-Val-ACV, showed comparable affinity to Val-ACV, an amino acid prodrug of ACV recognized by PEPT1/PEPT2. The permeability of Gly-Val-ACV (2.99 ± 0.59 × 10−6 cm/s) across Caco-2 was comparable with that of Val-ACV (3.01 ± 0.21 × 10−6 cm/s) and was significantly inhibited (63%) in presence of glycylsarcosine. The transport of GVACV across Caco-2 was saturable at higher concentrations, and the parameters were calculated asKm 3.16 ± 0.31 mM andVmax 0.014 ± 0.00058 nmol cm−2 min−1. Overall, the results suggest that the dipeptide prodrugs of ACV have a high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of ocular and oral herpesvirus infections, because cornea and intestinal epithelia seem to express the oligopeptide transporters.

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