Pharmacology
Why do people on the same HIV drug dose reach different blood levels?
Open access · cc by · source: Europe PMC
Two variants of the liver enzyme gene CYP2B6, along with body weight and sex, together explained about 55% of the person-to-person variation in how quickly patients cleared efavirenz.
Study at a glance
- Design
- Cross-sectional — Single-timepoint plasma efavirenz measurement (12-15 h post-dose) with TaqMan genotyping, analysed by nonlinear mixed-effects population PK modelling in NONMEM plus Monte Carlo simulations
- N
- N=185 · 185 patients analysed: 95 recruited on efavirenz-based ART alone and 90 with HIV/TB co-infection also receiving rifampicin-containing TB therapy (60 men, 125 women)
- Population
- Adults with HIV, some co-infected with tuberculosis, attending two hospitals in Zimbabwe while taking efavirenz-based antiretroviral therapy
- Outcome
- Apparent oral clearance of efavirenz and plasma concentration; secondary outcomes were reported central nervous system adverse effects
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
Patients homozygous for the CYP2B6*6, CYP2B6*18 and CYP2A6*9 variants had significantly higher average efavirenz levels, and those carrying at least two mutant alleles across CYP2B6*6 and *18 had roughly fourfold higher concentrations than wild-type patients. In the final model, CYP2B6*6, CYP2B6*18, body weight and sex together explained up to 55% of between-subject variability in clearance; clearance fell from 7.01 L/h in extensive metabolisers to 0.539 L/h in poor metabolisers, rose 21% per 10 kg of weight, and was 22% higher in women. CYP2A6 and ABCB1 variants were not retained, co-treatment with rifampicin made no significant difference, and average levels did not differ between patients with and without CNS side effects.
Methodology
The researchers recruited 185 patients taking efavirenz-based therapy at two Zimbabwean hospitals, 90 of whom were also on rifampicin-containing tuberculosis treatment. They measured plasma efavirenz 12-15 hours after a dose by HPLC and genotyped each patient for variants in CYP2B6, CYP2A6 and ABCB1. A one-compartment population pharmacokinetic model was fitted in NONMEM, testing genetic and demographic variables as covariates on drug clearance, and the fitted model was then used for simulation.
Limitations
Each patient contributed a single sparse concentration measurement, and absorption rate and volume of distribution were fixed from earlier literature rather than estimated, so the clearance estimates depend on those assumptions. The design is observational and cross-sectional with no virological endpoint measured, so it cannot show that any particular exposure level improves treatment outcomes; the simulated exposures are model output, not trial results, and the authors call for controlled clinical studies to test them. CNS side effects were assessed once by score chart although symptoms in this cohort appeared anywhere from four weeks to many months, so the genotype-symptom analysis was poorly timed. About 45% of variability remained unexplained, and results come from one country and may not transfer to other populations.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Genotype mattered more than the TB drug for efavirenz exposure.
TB co-treatment complicates HIV drug therapy less than expected for efavirenz: rifampicin lowered efavirenz levels only in the first week in a Ugandan cohort (symptoms 74% vs 72%), and made no significant difference to clearance in a Zimbabwean population-pharmacokinetic study.
Evidence for the claim as stated.
Genes can change exposure more than tenfold at the same dose.
CYP2B6 genotype is a major determinant of efavirenz exposure: in 185 Zimbabwean adults, CYP2B6*6, *18, body weight and sex explained up to 55% of between-person variability in clearance, which fell from 7.01 L/h in extensive to 0.539 L/h in poor metabolisers; carriers of two or more variant alleles had about fourfold higher concentrations.
Evidence for the claim as stated.
Exposure links genotype to side effects, imperfectly.
Higher efavirenz exposure tracked neuropsychiatric symptoms in a Ugandan cohort of 197 adults: 74% developed a symptom within 12 weeks, symptomatic patients had higher plasma levels, and CYP2B6 genotype predicted levels at every time point, though the direct genotype-symptom link was only a trend.
Scope note — Cross-sectional Zimbabwean data found no difference in average levels between patients with and without CNS effects.
Limits the claim's scope: a different population, assay, or outcome.
Whether efavirenz levels explain CNS side effects differs between studies: the Ugandan prospective cohort linked higher levels to symptoms, while the Zimbabwean cross-sectional study of patients already on therapy found no difference. Timing matters, as Ugandan symptoms arose mostly in the first two weeks.
Same question, contrary or null result.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Whether efavirenz levels explain CNS side effects differs between studies: the Ugandan prospective cohort linked higher levels to symptoms, while the Zimbabwean cross-sectional study of patients already on therapy found no difference. Timing matters, as Ugandan symptoms arose mostly in the first two weeks.
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Same topic cluster — not a recommendation engine.