polymerase inhibitor
A polymerase inhibitor is a drug that blocks a viral polymerase — the enzyme a virus uses to copy its genome — so the virus cannot make new copies of itself. If the polymerase is the photocopier the virus needs to reproduce, this drug jams the machine.
There are two main chemical strategies. Nucleoside and nucleotide analogues disguise themselves as the natural DNA or RNA building blocks; the polymerase grabs one and stitches it into the growing genome, but the fake block lacks the hook needed to add the next one, so the chain stops dead (chain termination). Non-nucleoside inhibitors instead bind a nearby pocket on the enzyme and lock its moving parts so it cannot work.
These drugs are central to treating HIV, hepatitis B and C, herpes, and were vital against COVID-19. Their selectivity hinges on viral polymerases differing from our own — but it is imperfect: nucleoside analogues can sometimes be mistaken for substrates by human polymerases, including the one in our mitochondria, which is a recognized source of toxicity. Single-agent use also invites resistance, so they are usually combined.
Acyclovir is activated only inside herpes-infected cells, where it becomes a chain-terminating nucleotide that the viral polymerase incorporates and stalls on.
Selective activation plus chain termination yields a clean antiviral.
Reverse-transcriptase inhibitors used against HIV are a special case: the viral polymerase copies RNA into DNA, a direction human cells normally do not use, which sharpens the drug's selectivity.