Pharmacology · Class
Anti-herpesvirus agents !
Antivirals that stop herpesviruses (herpes simplex, varicella-zoster, cytomegalovirus) from copying their DNA. They control outbreaks but do not cure.
Updated 2026-11-01
What they are
Medicines for infections by herpesviruses: herpes simplex (cold sores, genital herpes, encephalitis), varicella-zoster (chickenpox, shingles) and cytomegalovirus (CMV) in transplant recipients and people with advanced HIV. The virus stays latent in the body, so these drugs treat episodes or suppress them; they do not remove the virus.
Mechanism of action
Almost all of them end up blocking the viral DNA polymerase. Aciclovir, valaciclovir and famciclovir (penciclovir) are first phosphorylated by the viral thymidine kinase, so they act mainly in infected cells. Ganciclovir and valganciclovir are activated in CMV by the viral kinase UL97. Cidofovir is a nucleotide that only needs cellular kinases, and foscarnet binds the polymerase directly at its pyrophosphate site, so both still work when the viral kinase is missing or mutated.
Target: Viral DNA polymerase (activated by viral thymidine kinase or UL97 kinase)
Drugs in this class
- AciclovirThe classic antiviral for herpes simplex and varicella-zoster. Given early, it shortens outbreaks; IV it treats herpes…Viral DNA polymerase (herpes simplex and varicella-zoster)
- ValaciclovirAn oral prodrug that the body turns into aciclovir. It is much better absorbed, so it is taken less often for herpes…Viral DNA polymerase (through its active form, aciclovir)
- FamciclovirAn oral prodrug of penciclovir for shingles and herpes simplex. Its active form stays inside infected cells for many…Viral thymidine kinase (activation) and viral DNA polymerase
- GanciclovirAn intravenous antiviral for cytomegalovirus (CMV) in transplant recipients and people with AIDS. Its main danger is…CMV DNA polymerase (activated by the viral UL97 kinase)
- ValganciclovirThe oral prodrug of ganciclovir. It prevents and treats cytomegalovirus (CMV) disease and can lower all blood cell…CMV DNA polymerase (through ganciclovir, activated by UL97)
- CidofovirAn intravenous antiviral for CMV retinitis in AIDS. It works without the viral kinase but is very toxic to the kidneys.Viral DNA polymerase (CMV)
- FoscarnetAn intravenous antiviral for resistant herpes simplex and CMV. It blocks the viral polymerase directly but harms the…Viral DNA polymerase (pyrophosphate-binding site)
Others in the class
- Penciclovir (topical)
- Letermovir
- Maribavir
- Brincidofovir
- Trifluridine (ophthalmic)
Test yourself
Think of the answer, then tap the question to see it.
Which viral enzyme must phosphorylate aciclovir before it can act?
The viral thymidine kinase. That is why it acts mainly in infected cells and why thymidine kinase-deficient strains resist it.
Which two drugs still work against aciclovir-resistant, thymidine kinase-deficient herpes simplex?
Foscarnet, which binds the polymerase directly, and cidofovir, which is phosphorylated only by cellular enzymes.
What is the main dose-limiting toxicity of ganciclovir?
Bone marrow suppression, above all neutropenia, plus anaemia and thrombocytopenia.
Which electrolyte problems are typical of foscarnet?
Low ionized calcium, low magnesium, low potassium and changes in phosphate, which can cause tingling around the mouth and seizures.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Herpes simplex virus (World Health Organization, 2025); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 62 (McGraw Hill, 2023); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.