The effect of penciclovir (BRL 39123) on the replication of varicella-zoster virus (VZV) in human embryonic lung fibroblasts (MRC-5 cells) was similar to aciclovir when the compounds were present continuously. However, when the compounds were withdrawn the antiviral activity of penciclovir was maintained more effectively than that of aciclovir. In the plaque reduction assay, median 50% effective concentrations (EC50s) were 3.8 μg ml−1 for penciclovir and 4.2 μg ml−1 for aciclovir ( n = 29 clinical isolates). Similarly, penciclovir and aciclovir were equally effective in reducing the numbers of VZV-infected MRC-5 cells and in reducing VZV DNA synthesis within infected cells following continuous treatment. Within VZV-infected cells (S)-penciclovir-triphosphate was formed from penciclovir with >95% enantiomeric purity, and the concentration of penciclovir-triphosphate was 360-fold greater than aciclovir-triphosphate immediately after treatment. This phosphorylation ratio compensates for the lower affinity of VZV DNA polymerase for penciclovir-triphosphate compared with aciclovir-triphosphate (Kis = 7.5 μM and 0.2 μM, respectively). When VZV-infected cultures were treated for 3 days, followed by withdrawal of the compound, inhibition of viral DNA synthesis by penciclovir was maintained for 24 h, whereas viral DNA synthesis resumed more readily after removal of aciclovir. Furthermore, following 8 h daily pulse treatment for 5 days, penciclovir was significantly more active than aciclovir in reducing VZV DNA synthesis ( p = 0.006, n = 10 clinical isolates). The long intracellular half-life of penciclovir-triphosphate (9.1 h) compared with that of aciclovir-triphosphate (0.8 h) accounts for the sustained inhibition of virus replication by penciclovir. This property may contribute to the clinical efficacy of famciclovir, the oral form of penciclovir.
Inhibition of Varicella-Zoster Virus by Penciclovir in Cell Culture and Mechanism of Action