Intracellular trapping of cycloSal-pronucleotides: modification of prodrugs with amino acid esters.

A new class of d4TMP- cycloSal-pronucleotides bearing enzymatically cleavable amino acid esters is reported. These compounds are designed to trap the pronucleotide inside the cell by a fast conversion of a nonpolar ester group into a charged carboxylate. This should prevent efficient diffusion equilibrium across the cell membrane to the extracellular environment, leading to an intracellular accumulation of the compounds. This initial conversion is followed by a slow release of the nucleoside monophosphate (i.e., d4TMP). The concept is proven by hydrolysis studies in phosphate buffer, cell extracts, and human serum. These investigations revealed a high sensitivity of some amino acid ester modifications to conversion by cellular extracts, resulting in the fast release of a charged intermediate, whereas no cleavage of the modification is found in phosphate buffer. In addition, antiviral activities against HIV are presented.