Question

Which is the active form of Ganciclovir?

• A. Phosphate
• B. Tetraphosphate
• C. Biphosphate
• D. Triphosphate

Hint:

For nucleoside analogs like Ganciclovir, the active form is typically the triphosphate form, which is essential for its antiviral activity.

Answer 1

The Correct Option is D

Ganciclovir itself is inactive when it enters a cell; it needs to be converted step by step into a form that can interact with viral DNA polymerase. Cytomegalovirus infected cells carry a viral kinase (UL97) that adds the first phosphate group, forming ganciclovir monophosphate. From there, ordinary cellular kinases already present in the host cell add a second and then a third phosphate group, producing the diphosphate and finally the triphosphate.

This stepwise build-up matters because only the fully phosphorylated triphosphate closely resembles deoxyguanosine triphosphate, the natural building block viral DNA polymerase normally uses. Because it looks like the real nucleotide, ganciclovir triphosphate gets pulled into the growing viral DNA chain, where it slows down and eventually stops further chain extension.

None of the earlier steps, the monophosphate or the diphosphate, can do this job; they still need further phosphorylation. There is also no tetraphosphate stage in this pathway.

So the biologically active form that actually blocks viral replication is the triphosphate, which is the correct answer.