- Design
- Case report with therapeutic drug monitoring
- Population
- One man in his sixties with HIV on primidone 50 mg daily
- Primary outcome
- Plasma lenacapavir concentrations over six months
- Effect
- Lowest 24.9 ng/mL (month 1); projected 40.6 ng/mL at 6 months; viral load undetectable
Lenacapavir, the twice-yearly HIV capsid inhibitor, is a CYP3A4 substrate, and labelling advises against combining it with strong inducers. Primidone, and its metabolite phenobarbital, induce CYP3A.
A case report describes a man in his sixties switched to lenacapavir while continuing primidone 50 mg daily. Plasma lenacapavir was measured through the first six-month interval: the lowest level was 24.9 ng/mL at month one and the projected six-month level 40.6 ng/mL, all above four times the protein-adjusted effective concentration, and his viral load stayed undetectable.
One patient on a low primidone dose cannot establish safety, and higher doses or phenobarbital itself may induce more strongly. But it shows the interaction may be manageable with drug-level monitoring where switching the anticonvulsant is not an option.
- Avoid combining lenacapavir with strong CYP3A inducers where alternatives exist
- If co-prescription is unavoidable, consider measuring lenacapavir levels
- This case used low-dose primidone; higher doses may induce more strongly
- Monitor viral load closely in any induced patient
Why it matters
Long-acting antiretrovirals leave no room to correct underexposure quickly, so interaction evidence matters.
Don't overread it
This is a single case report; it does not show the combination is safe in general.
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