Precursor to metabolite: Famciclovir to Penciclovir

Famciclovir is converted to Penciclovir by esterase + aldehyde oxidase.

The pair

Famciclovir — Oral prodrug of penciclovir for herpes and shingles.
Penciclovir — Topical antiviral for cold sores and the active drug released from famciclovir; modest benefit with frequent application.