Penciclovir (BRL 39123) is a Herpesvirus Nucleoside Analog DNA Polymerase Inhibitor. In HSV infected cells, penciclovir is phosphorylated by viral thymidine kinase and subsequently converted by cellular kinases into the active metabolite, penciclovir triphosphate, which competitively inhibits viral HSV polymerase by blocking deoxyguanosine triphosphate substrate binding. As a result, herpes viral DNA synthesis and replication are selectively inhibited.

Penciclovir exhibits antiviral activity against a variety of herpesviruses, demonstrating efficacy against HHV-6A (IC50: 37.9 μM) and HHV-6B (IC50: 77.8 μM). It shows in vitro effectiveness against both type I and type II herpes simplex viruses and possesses a beneficial effect on varicella-zoster virus, without affecting DNA synthesis in uninfected cells. As a guanine analog, Penciclovir features low toxicity and high selectivity. It is converted by cellular kinases into an active triphosphate form that competitively inhibits viral DNA polymerase, thereby suppressing DNA synthesis in virus-infected cells. Studies related to herpes simplex virus type I have indicated that Penciclovir can induce apoptosis in cells with minimal genotoxic effects.

Penciclovir is dissolved with DMSO and diluted with appropriate media[2]. Human breast cancer cell lines MCF-7 and MDA-MB-435, glioblastoma U87 mg, and embryonic kidney cells 293T are cultured at 37°C in a humidified atmosphere containing 5% CO2 in Iscove's modified Dulbecco medium or Leibovitz's L-medium and 5% fetal bovine serum (FBS). The assays are performed with slight modifications. In brief, cells are seeded into 24-well plates (5×104 cells/well) and infected 48 h later with 103 particles per cell of unmodified virus (Adtk), PEGylated virus (PEG-Adtk), or RGD-PEG-modified virus (RGD-PEG-Adtk) in triplicates in culture medium with 2% FBS and incubated for 4 h at 37°C. The incubation medium is then replaced by normal medium and cells are further incubated for 48 h. Cells are harvested and lysed with 500 μL of TK lysis buffer that contained 0.5% Nonidet P-40 (NP-40), 20 mM N-(2-hydroxyethyl) piperazine-N′-(2-ethanesulfonic acid) (HEPES) (pH 7.6), 2 mM Mg(OAc)2, 1 mM dithiothreitol, and 50 μM thymidine. The supernatant is collected after centrifugation. The samples are kept at -80°C until use. The modified and unmodified adenovirus protein concentrations are determined by the Micro BCA assay. One microgram of cell extract is incubated with HSV1-tk substrate 8-3H-Penciclovir (8-3 H-PCV). The phosphorylated tracer is separated from unphosphorylated 8-3 H-PCV with DE-81 filters. TK activity is expressed as the percentage of conversion of substrate per minute per microgram protein[2].