GS-9191 is the prodrug of the nucleotide analog PMEG. GS-9191 is a topical agent to permeate skin and be metabolized to the active nucleoside triphosphate analog in the epithelial layer. GS-9191 inhibits NA synthesis , arrests cell in S phase, and induces cell apoptosis. GS-9191 can be used for research of human papillomaviruses ( HPV ) infection.