Step by Step
TK
The activation trick — viral thymidine kinase
Acyclovir actually enters every cell in the body, infected or not. But it only becomes active in cells that contain viral thymidine kinase (TK) — an enzyme that only herpesviruses produce. Uninfected human cells simply don't have the enzyme needed to activate the drug, which is the entire basis of acyclovir's selective toxicity.
Mech
Mechanism once activated
Once phosphorylated by viral thymidine kinase, the activated form of acyclovir inhibits viral DNA polymerase, halting viral replication specifically within infected cells.
Use
Clinical uses
Acyclovir treats HSV-1 and HSV-2 (cold sores, genital herpes), VZV (chickenpox, shingles), and, given intravenously, HSV encephalitis — a serious, potentially fatal infection.
Val
Valacyclovir and resistance
Valacyclovir is simply an oral prodrug of acyclovir, offering better bioavailability and less frequent dosing. Resistance can develop through mutations in viral thymidine kinase, most often seen in immunocompromised patients — since without functional TK, the drug can no longer be activated inside the infected cell at all.
An immunocompromised patient with recurrent herpes outbreaks stops responding to acyclovir over time — a thymidine kinase mutation has likely developed, meaning the virus itself has become resistant to the drug's core activation mechanism.
Applied Walkthrough
1
A patient with HSV encephalitis is started on intravenous acyclovir.
2
Ask: why doesn't this drug cause the same widespread toxicity that many antivirals do, given that it reaches every cell in the body? Because acyclovir only becomes active in cells containing viral thymidine kinase — an enzyme that only herpes-infected cells possess. Healthy, uninfected cells throughout the body simply can't activate the drug, so it does no harm there.
3
Contrast: if a patient's herpes infection develops resistance over time, particularly if immunocompromised, the mechanism is usually a mutation in the viral thymidine kinase itself — since the drug depends entirely on that enzyme for activation, a mutated or absent TK renders acyclovir ineffective regardless of dose.
4
This selective-activation mechanism — rather than a selective-target mechanism — is what makes acyclovir a particularly elegant example of selective toxicity in pharmacology, and it's exactly the kind of underlying reasoning exams want you to understand, not just memorize.
Exam Application
Exams test the mechanism of selective toxicity (acyclovir requires viral thymidine kinase for activation, an enzyme unique to herpes-infected cells), what the activated drug does once phosphorylated (inhibits viral DNA polymerase), the difference between acyclovir and valacyclovir (the latter is simply an oral prodrug), and the resistance mechanism (TK mutation, especially in immunocompromised patients).
⚠ Common Trap
The most common trap is assuming acyclovir works broadly against many types of viruses the way a general antiviral might. It specifically and only works against viruses that produce thymidine kinase — herpesviruses (HSV, VZV) — and has no meaningful activity against viruses lacking this enzyme.
✓ Quick Self-Check
1. Why does acyclovir only affect herpes-infected cells and not healthy cells, even though it enters all cells?
Because it only becomes active once phosphorylated by viral thymidine kinase, an enzyme that only herpes-infected cells produce; uninfected cells can't activate the drug.
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2. What does activated acyclovir do once inside an infected cell?
It inhibits viral DNA polymerase, halting viral replication.
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3. What is valacyclovir, and how does it differ from acyclovir?
It's an oral prodrug of acyclovir, offering better bioavailability and less frequent dosing — otherwise the same active drug.
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4. What is the most common mechanism of acyclovir resistance, and in which patients is it most often seen?
Mutation in viral thymidine kinase, most often seen in immunocompromised patients.
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5. What conditions does acyclovir treat?
HSV-1 and HSV-2 (cold sores, genital herpes), VZV (chickenpox, shingles), and HSV encephalitis (IV).
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