Floxuridine amino acid ester prodrugs: Enhancing Caco-2 permeability and resistance to glycosidic bond metabolism

Purpose. The aim of this study was to synthesize amino acid ester prodrugs of 5-fluoro-2′-deoxyuridine (floxuridine) to enhance intestinal absorption and resistance to glycosidic bond metabolism. Methods. Amino acid ester prodrugs were synthesized and examined for their hydrolytic stability in human plasma, in Caco-2 cell homogenates, and in the presence of thymidine phosphorylase. Glycyl-l-sarcosine uptake inhibition and direct uptake studies with HeLa/PEPT1 cells [HeLa cells overexpressing oligopeptide transporter (PEPT1)] were conducted to determine PEPT1-mediated transport and compared with permeability of the prodrugs across Caco-2 monolayers. Results. Isoleucyl prodrugs exhibited the highest chemical and enzymatic stability. The prodrugs enhanced the stability of the glycosidic bond of floxuridine. Thymidine phosphorylase rapidly cleaved floxuridine to 5-fluorouracil, whereas with the prodrugs no detectable glycosidic bond cleavage was observed. The 5′-l-isoleucyl and 5′-l-valyl monoester prodrugs exhibited 8- and 19-fold PEPT1-mediated uptake enhancement in HeLa/PEPT1 cells, respectively. Uptake enhancement in HeLa/PEPT1 cells correlated highly with Caco-2 permeability for all prodrugs tested. Caco-2 permeability of 5′-l-isoleucyl and 5′-l-valyl prodrugs was 8- to 11-fold greater compared with floxuridine. Conclusions. Amino acid ester prodrugs such as isoleucyl floxuridine that exhibit enhanced Caco-2 transport and slower rate of enzymatic activation to parent, and that are highly resistant to metabolism by thymidine phosphorylase may improve oral delivery and therapeutic index of floxuridine.