Synthesis of analogs of l-valacyclovir and determination of their substrate activity for the oligopeptide transporter in Caco-2 cells

l-Valacyclovir, a prodrug of acyclovir, is a substrate for the peptide transporter (PepT1) in the intestinal mucosa, which accounts for its higher than expected oral bioavailability. The substrate activity of l-valacyclovir for PepT1 is surprising, particularly when one considers that the molecule has the structural features of a nucleoside rather than a peptide. In an attempt to better understand the structure–transport relationships (STR) for the interactions of l-valacyclovir with PepT1, analogs of this molecule with structural changes in the guanine moiety were synthesized and their substrate activity for PepT1 in Caco-2 cell monolayers was determined. The analogs synthesized include those that had the guanine moiety of l-valacyclovir substituted with purine, benzimidazole, and 7-azaindole. All three analogs (purine, benzimidazole, and 7-azaindole) exhibited affinity for PepT1 in binding studies, but only the purine analog (as the l-valine ester) showed PepT1-associated transcellular transport across Caco-2 cell monolayers. The benzimidazole and 7-azaindole analogs (as their l-valine esters) were rapidly metabolized by esterase when applied to the apical surface of Caco-2 cells, which probably explains their low penetration as the intact prodrugs via PepT1.