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

The active form of Ganciclovir is Ganciclovir triphosphate. Ganciclovir is converted into its active form by the addition of three phosphate groups, allowing it to inhibit viral DNA synthesis by acting as a chain terminator.

Answer 2

The question asks for the active, pharmacologically functional form of the antiviral drug ganciclovir. Let's assess each option.

1. Phosphate: This is only the first, mono-phosphorylated intermediate. A viral or cellular kinase adds a single phosphate group to ganciclovir to give ganciclovir monophosphate, but this form is not yet able to interfere with viral DNA synthesis on its own.
2. Tetraphosphate: Ganciclovir does not carry four phosphate groups at any stage of its activation. This option does not correspond to any real metabolite in the pathway.
3. Biphosphate: A diphosphate (two phosphate groups) does form as a mid-pathway intermediate after the monophosphate is further phosphorylated by cellular kinases, but this species is still not the final active drug; a third phosphorylation step is still needed.
4. Triphosphate: After sequential phosphorylation, first to the mono- then the di-phosphate, a final phosphorylation yields ganciclovir triphosphate. This triphosphate form competes with the natural substrate deoxyguanosine triphosphate for incorporation into growing viral DNA by viral DNA polymerase, and once incorporated it slows chain elongation, giving ganciclovir its antiviral effect.

Because the drug needs all three phosphate groups attached before it can interfere with viral DNA polymerase, only the fully phosphorylated species is pharmacologically active.

Therefore, the correct answer is Triphosphate.