Amino Acid Ester Prodrugs of Acyclovir

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Abstract
Eighteen amino acid esters of the antiherpetic drug, acyclovir, were synthesized as potential prodrugs for oral administration. The esters were examined for in vitro antiviral activity against herpes simplex virus Type 1 (HSV-1). They were found to have less potency than the parent compound. Their efficiencies as prodrugs were evaluated in rats by measuring the urinary recovery of acyclovir. Ten prodrugs produced greater amounts of the parent drug in the urine. The L-amino acid esters were better prodrugs than the corresponding D- or D, L-isomers, suggesting the involvement of a stereoselective transporter. The L-valyl ester, 256U87, was the best prodrug. Sixty three per cent of its administered dose was excreted as acyclovir in the urine, a considerable improvement over acyclovir itself, for which this value was 19%. Since 256U87 was stable in aqueous solutions, its conversion to acyclovir in vivo was probably enzyme catalyzed. This L-valyl ester prodrug of acyclovir is now undergoing clinical evaluation.