0Acyclovir is a synthetic purine deoxynucleoside analogue (specifically acyclic guanosine analogue) with potent activity against herpes simplex viruses type 1 and 2 (HSV-1, HSV-2) and much less activity against varicella-zoster virus (VZV). Clinically, the drug is indicated to treat the following infections: herpes labialis, herpes genitalis, herpes simplex encephalitis, herpes simplex keratitis, neonatal herpes infection, chickenpox (varicella), and shingles (zoster). In order to be active, the drug must be phosphorylated by viral thymidine kinase to acyclovir monophosphate, and then further converted to acyclovir triphosphate. After phosphorylation, the drug binds to viral DNA polymerase, is incorporated into viral DNA, and ultimately inhibits viral DNA synthesis. The greater antiviral activity of acyclovir against HSV as compared to VZV is due to its more efficient phosphorylation by HSV thymidine kinase
