Penciclovir is an inhibitor of HSV-1 DNA synthesis [1].

Penciclovir is a guanine derivative and is used as a drug for the treatment against herpesviruses. Penciclovir has a similar spectrum of activity with acyclovir. In the plaque reduction assay, penciclovir shows potent inhibitory activities against HSV-1, HSV-2 and VZV with IC50 values of 0.4μg/ml, 1.5μg/ml and 3.1μg/ml, respectively. It also shows minimal activity against CMV. In MRC-5 cells, penciclovir inhibits the replication of HSV-1 with IC99 value of 0.4μg/ml. Moreover, penciclovir exerts persistent antiviral activity against HSV-1and HSV-2 in Vero cells. Besides that, penciclovir is found to be phosphorylated to the triphosphate in HSV-1-infected MRC-5 cells. The penciclovir triphosphate is the metabolite that actually inhibits herpesvirus replication. Penciclovir inhibits HSV-1 DNA synthesis with IC50 value of 0.16μM without effect on cellular DNA synthesis [1, 2].

References:
[1] Boyd M R, Bacon T H, Sutton D, et al. Antiherpesvirus activity of 9-(4-hydroxy-3-hydroxy-methylbut-1-yl) guanine (BRL 39123) in cell culture. Antimicrobial agents and chemotherapy, 1987, 31(8): 1238-1242.
[2] Hodge R A, Perkins R M. Mode of action of 9-(4-hydroxy-3-hydroxymethylbut-1-yl) guanine (BRL 39123) against herpes simplex virus in MRC-5 cells. Antimicrobial agents and chemotherapy, 1989, 33(2): 223-229.