Acyclovir is a guanosine analog with antiviral activity in vitro against the herpes simplex viruses, varicella zoster virus, Epstein-Barr virus, cytomegalovirus, and human herpes virus 6 (ID50s = 0.1 - 63.1 μM). Acyclovir differs from most nucleoside analogs in that it contains only a partial nucleoside structure with an acyclic side chain. It diffuses freely into cells and is selectively converted into acyclo-guanosine monophosphate by a virus-specific thymidine kinase. During DNA replication, the phosphorylated form of acyclovir is preferentially incorporated into viral DNA, resulting in premature chain termination and inhibition of further activity of DNA polymerase.
