5′Prime;-Ester Prodrugs of the Varicella-Zoster Antiviral Agent, 6-Methoxypurine Arabinoside

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Abstract
The potent and selective activity of 6-methoxypurine arabinoside (9-[β-D-arabinofuranosyl]-6-methoxy-9H-purine; 1) and its pharmacokinetic limitations have been described previously. In an attempt to circumvent first-pass catabolism following oral administration, a series of 5′-esters (2a-t) was prepared by dicyclohexyl-carbodiimide-promoted condensation with formic acid in the case of 2a, or by direct acylation of the parent nucleoside with the corresponding acyl chloride. These compounds were evaluated in rats for efficacy as oral prodrugs of 1 by measuring the amount of 1 excreted in the urine. Both aliphatic and aromatic esters exhibited a range of systemic availabilities. The substituted benzoate esters showed a correlation between the amount of 1 excreted and the electron-donating properties of the substituent. A similar relationship was observed for the relative stabilities of this series of esters to non-enzymatic hydrolysis at neutral pH. No clear relationship between systemic availability and solubility, partition coefficient, or stability was observed for the remaining esters of 1. Although significant enhancements in systemic availability were observed with some of the 5′-esters of this series, their limited water solubility precluded their use in intravenous formulations.