Famciclovir and Penciclovir

The antiviral agent penciclovir is a synthetic acyclic guanine analog that inhibits the replication of several herpesviruses. Structurally, it is very similar to aciclovir; however, these two drugs differ in pharmacokinetics and antiviral effects (Tyring, 1998). Penciclovir (9-(4-hydroxy-3-hydroxymethylbut-1-yl) guanine) has poor oral absorption, which led to the development of famciclovir (2-[2-(2-amino-9H-purin-9-yl)ethyl]-1,3-propanediol diacetate), a penciclovir prodrug with enhanced bioavailability. When administered orally, famciclovir is readily absorbed and thereafter rapidly metabolized to penciclovir, resulting in therapeutic blood levels of the latter. Penciclovir inhibits herpesvirus replication by selectively inhibiting herpes-viral DNA polymerase without affecting mammalian DNA polymerases. Famciclovir is approved in most countries for treatment of infections due to herpes simplex and varicella-zoster viruses.