Valaciclovir
An oral prodrug of aciclovir, better absorbed, used for cold sores, genital herpes and shingles.
Also called valacyclovir (US name).
Drug card
- How it works
- Blocking herpesvirus DNA polymerase after activation by viral thymidine kinase
- Used for
- Cold sores and genital herpes, including suppression
- Shingles (herpes zoster)
- Chickenpox in children
- Preventing CMV disease after organ transplant (UK)
- Known for
- Acute kidney injury
- Agitation, hallucinations, confusion, encephalopathy
- Its name
- -vir means an antiviral; -ciclovir means an antiherpes nucleoside analogue
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does valaciclovir work?
The L-valyl ester of aciclovir. Gut and liver enzymes convert it almost completely to aciclovir and valine, giving 3 to 5.5 times the aciclovir bioavailability of oral aciclovir. The aciclovir is then activated by viral thymidine kinase and blocks viral DNA polymerase.
Like other antiherpes nucleoside analogues: These guanosine analogs are switched on mainly in infected cells: the thymidine kinase of herpes simplex or varicella-zoster virus adds the first phosphate, and cell enzymes make the triphosphate. That triphosphate competes for viral DNA polymerase, is built into the growing viral DNA and ends the chain. Most resistant viruses lack a working thymidine kinase.
Uses
What is valaciclovir used for?
- Cold sores and genital herpes, including suppression
- Shingles (herpes zoster)
- Chickenpox in children
- Preventing CMV disease after organ transplant (UK)
Side effects
Side effects of valaciclovir
- Acute kidney injuryKnown forSerious
- Agitation, hallucinations, confusion, encephalopathyKnown forSerious
- Headache
- Nausea
- Abdominal pain
- Dizziness
- TTP/HUS at very high dosesSerious
- Severe skin reactions: SJS, TEN, DRESSSerious
- Raised liver enzymes
- Rash
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Reduce the dose in kidney impairment and take care in older people, who are prone to kidney failure and confusion.
- Keep patients hydrated, as aciclovir can crystallize in the kidney tubules.
- Very high doses in advanced HIV or transplant patients have caused TTP/HUS.
- For shingles, start within 72 hours of the rash; later benefit is unproven.
Good to know: Valaciclovir is the INN; valacyclovir is the US name. Its better absorption lets it be taken fewer times a day than aciclovir.
Remember it
A way to remember it
The virus loads its own trap.
Viral thymidine kinase adds the first phosphate, so aciclovir is activated mainly in infected cells, where it then jams the viral DNA polymerase. Viruses that lose thymidine kinase escape it.
Written for biofideX
Its name
-vir means antiviral
Sub-stems name the target: -ciclovir, -amivir, -fovir, -tegravir, -navir, -buvir, -virine (NNRTIs). Lamivudine and zidovudine are antivirals without it.
-ciclovir means antiherpes nucleoside analogue
Aciclovir, valaciclovir, ganciclovir and penciclovir share it. The US names of the first two are acyclovir and valacyclovir.
- Every drug name stem
Its class
Antiherpes nucleoside analogues
Other antiherpes nucleoside analogues with a card:
Easily mixed up with nucleoside reverse transcriptase inhibitors and NS5B polymerase inhibitors.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Valacyclovir tablets: label (DailyMed)
- Valacyclovir (NCI Thesaurus)
- Valaciclovir 500mg tablets: SmPC (emc)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Antivirals
Valaciclovir is one of 10 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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