This is a single case report with drug monitoring, from the British Journal of Clinical Pharmacology. A man in his sixties switched from a boosted darunavir regimen to lenacapavir while taking long-standing primidone 50 mg daily. Primidone and its metabolite phenobarbital induce CYP3A, and lenacapavir labelling does not recommend the combination because of the risk of subtherapeutic concentrations.
Plasma lenacapavir was measured throughout the first six-month dosing interval. Concentrations stayed above four times the protein-adjusted 95% effective concentration. The lowest was 24.9 ng/mL at month 1 and the highest 58.3 ng/mL at month 3, with a projected 6-month value of 40.6 ng/mL, in line with levels reported without interacting drugs. The viral load remained undetectable.
This is one patient on a low primidone dose. It shows the interaction was not large in this case; it cannot show that the combination is safe in general, and the label advice stands.
- Lenacapavir concentrations stayed above four times the protein-adjusted 95% effective concentration for six months.
- The lowest measured concentration was 24.9 ng/mL (month 1) and the highest was 58.3 ng/mL (month 3).
- Viral load stayed undetectable in this single patient.
- Primidone was taken at 50 mg daily, which is a low dose.
- Labelling still advises against the combination, so seek specialist advice and consider drug monitoring if it cannot be avoided.
Why it matters
It gives a measured data point where the only guidance has been a precaution based on mechanism.
Don't overread it
A single case report cannot support co-administration of lenacapavir with CYP3A inducers.
The statistics, in plain English
One patient gives no estimate of how often or how much concentrations fall, because individual induction varies with dose, timing and genetics. The finding is reassuring for this patient and should be treated as a prompt for monitoring, not a rule.
Read the rest in the app
You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.
Scan to keep reading on your phone. No account needed to start.
Tomorrow morning, before your first patient
One edition a day for clinical pharmacology, written by the desk, every claim tied to its paper. Six minutes.
Get the app — free