Figure 4.

Mechanism of action of acyclovir and foscarnet. Acyclovir is converted by viral thymidine kinase (TK) to acyclovir monophosphate encoded by the human herpesvirus 1 UL23 gene. Further phosphorylation steps occur through cellular TK lead to the formation of activated acyclovir triphosphate. The active form inhibits herpes simplex virus (HSV) replication by selective inhibition of viral DNA polymerase and by the termination of growing viral DNA strands. Foscarnet, a direct inhibitor of the virus DNA polymerases can be used as an alternative drug for the treatment of HSV-resistant patients.