Biochem/physiol Actions Famciclovir is an antiretroviral guanosine analog used to treat herpesvirus infections and hepatitis B. Famciclovir is rapidly converted to penciclovir. Viral thymidine kinase phosphorylates penciclovir to a monophosphate form that celular kinases convert in turn to penciclovir triphosphate. Penciclovir triphosphate competitively inhibits viral DNA polymerase and thus viral replication. Prolonged administration can lead to resistance; it is often manifested as selection of pre-existing resistant strains with mutations in the reverse transcriptase domain of the DNA polymerase gene.1 Other Notes Tandem Mass Spectrometry data independently generated by Scripps Center for Metabolomics is available to view or download in PDF. F7932.pdf Tested metabolites are featured on Scripps Center for Metabolomics METLIN Metabolite Database. To learn more, visit sigma.com/metlin.