Penciclovir ester prodrug

Famciclovir is the diacetyl ester prodrug of 6-deoxypencidovir, an acyclic guanosine analog (Figure 49-2). After oral administration, famciclovir is rapidly deacetylated and oxidized by first-pass metabolism to penciclovir. It is active in vitro against HSV-1, HSV-2, VZV, EBV, and HBV. As with acyclovir, activation by phosphorylation is catalyzed by the virus-specified thymidine kinase in infected cells, followed by competitive inhibition of the viral DNA polymerase to block DNA synthesis. Unlike acyclovir, however, penciclovir does not cause chain termination. Penciclovir triphosphate has lower affinity for the viral DNA polymerase than acyclovir triphosphate, but it achieves higher intracellular concentrations. The most commonly encountered clinical mutants of HSV are thymidine kinase-deficient these are cross-resistant to acyclovir and famciclovir. 

Famciclovir is the diacetyl ester prodrug of 6-deoxy penciclovir and lacks intrinsic antiviral activity. Penciclovir (9-[4-hydroxy-3-hydroxymethylbut-l-yl] guanine) is an acyclic guanine nucleoside analog. The side chain differs structurally in that the oxygen has been replaced by a carbon and an additional hydroxymethyl group is present. 

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