High prevalence of the CYP2B6 516G→T(*6) variant and effect on the population pharmacokinetics of efavirenz in HIV/AIDS outpatients in Zimbabwe
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- Nyakutira, C., Röshammar, D., Chigutsa, E. et al. Eur J Clin Pharmacol (2008) 64: 357. doi:10.1007/s00228-007-0412-3
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The study sought to investigate the relationship between efavirenz exposure and the CYP2B6 516G→T(*6) genotype in HIV/AIDS outpatients, using pharmacokinetic modelling and simulation.
Blood samples where obtained from 74 outpatients treated with a combination regimen including 600 mg efavirenz daily for a duration of at least 3 weeks at clinics in Harare, Zimbabwe. The subjects were genotyped for the major CYP2B6 variant, CYP2B6*6, associated with reduced enzyme activity, using a PCR-RFLP method. Efavirenz plasma concentrations were determined by HPLC-UV. Population pharmacokinetic modelling and simulation of the data were performed in NONMEM VI.
A high allele frequency of the CYP2B6*6 allele of 49% was observed. Efavirenz plasma concentrations were above 4 mg/L in 50% of the patients. Genotype and sex were identified as predictive covariates of efavirenz disposition. Pharmacokinetic parameter estimates indicate that a dose reduction to 400 mg efavirenz per day is possible in patients homozygous for the CYP2B6*6 genotype without compromising therapeutic efficacy.
The CYP2B6*6 allele occurs at a high frequency in people of African origin and is associated with high efavirenz concentrations. Simulations indicate that an a priori 35% dose reduction in homozygous CYP2B6*6 patients would maintain drug exposure within the therapeutic range in this group of patients. Our preliminary results suggest the conduct of a prospective clinical dose optimization study to evaluate the utility of genotype-driven dose adjustment in this population.