acyclovir
Meaning of acyclovir: Its safety rests on a trick of chemistry: the drug stays inert until a viral enzyme switches it on, so uninfected cells ignore it.
Definition of acyclovir
Acyclovir is an antiviral medicine for infections caused by herpes simplex virus and varicella-zoster virus, the virus behind chickenpox and shingles. It acts as a decoy building block: activated inside an infected cell, it jams the enzyme that copies viral DNA and halts the growing chain. It shortens and softens an outbreak without removing the virus.
How does it work?
Acyclovir looks like guanosine, one of the four units of DNA, but is missing part of the sugar ring. In an ordinary cell it does almost nothing, because human enzymes barely recognize it. In a cell infected by herpes simplex or varicella-zoster the virus supplies its own enzyme, thymidine kinase, which attaches the first phosphate group; the cell's own enzymes add two more. The finished molecule is then picked up by the viral DNA-copying enzyme in preference to the real unit, and because the missing part of the ring is exactly where the next unit would attach, DNA replication stops dead. The enzyme is left stuck to the broken chain. Two layers of selectivity, activation only in infected cells and a viral enzyme that prefers the fake, are why an antiviral this disruptive can be taken with so little effect on the person.
Which viruses does it actually cover?
| Virus | Activity |
|---|---|
| Herpes simplex types 1 and 2 | Strong; cold sores, genital herpes, eye infection, encephalitis |
| Varicella-zoster | Active, but higher doses are needed; chickenpox and shingles |
| Epstein-Barr virus | Weak, with little clinical benefit |
| Cytomegalovirus | Very little, because this virus lacks the enzyme that activates the drug |
That last row corrects a common misunderstanding: cytomegalovirus is treated with different agents such as ganciclovir, not with acyclovir.
Why does the infection come back?
Herpesviruses do not leave. After a first infection they retreat into nerve cell bodies and sit dormant, their genetic material present but not being copied and no viral proteins being made. Acyclovir acts only on virus that is actively replicating, so it has nothing to attack in that reservoir. Taken continuously it can suppress recurrences and lower the chance of passing the virus on, but stopping restores the earlier pattern. Practical points follow from its handling: absorption by mouth is poor, which is why valacyclovir, converted to acyclovir after absorption, is often given instead; clearance is by the kidneys, so doses are reduced when kidney function is impaired; and given rapidly into a vein without enough fluid it can crystallize in the kidney.
Also known as
Used in a sentence
Acyclovir was prescribed for treatment of the patient's herpes simplex outbreak.
Good to know
Acyclovir reduces the severity and duration of an active outbreak but does not clear the latent virus from nerve cells, which is why recurrent episodes remain possible after treatment.