- Design
- Case report with therapeutic drug monitoring
- Population
- One adult switching to lenacapavir while on primidone 50 mg daily
- Primary outcome
- Plasma lenacapavir concentrations over 6 months
- Effect
- 24.9 to 58.3 ng/mL, all above 4× the protein-adjusted EC95; viral load undetectable
Lenacapavir, a twice-yearly HIV capsid inhibitor, is a CYP3A4 substrate, and co-administration with primidone, a CYP3A inducer whose metabolite phenobarbital also induces it, is not recommended. A case report followed a man in his sixties who switched to lenacapavir while taking long-standing primidone 50 mg daily, with levels measured over the first six-month interval.
Lenacapavir concentrations stayed above four times the protein-adjusted effective concentration throughout, ranging from 24.9 ng/mL at month one to 58.3 ng/mL at month three, with a projected 40.6 ng/mL at six months. These were broadly in line with levels seen without interacting drugs, and the viral load remained undetectable.
The dose of primidone was low, and one case cannot be generalised to higher doses or stronger inducers. But it offers some reassurance where stopping an established anticonvulsant would be harmful.
- Low-dose primidone did not lower lenacapavir below target in this one patient
- Do not extrapolate to higher primidone doses, phenobarbital, carbamazepine or rifampicin
- Consider drug level monitoring when an inducer cannot be avoided
- Seek specialist HIV pharmacology advice before combining lenacapavir with any enzyme inducer
Why it matters
It gives the first measured data for a combination that labelling simply advises against.
Don't overread it
One case on a low dose does not make the combination safe in general.
The statistics, in plain English
A single patient's levels show what is possible, not what is typical. Inducer effects depend on dose, so a 50 mg daily dose may behave very differently from usual anticonvulsant doses.
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