Background:  The antiretroviral drug efavirenz (EFV) presents wide inter-individual variability in exposure. This is explained to a great extent by polymorphisms of CYP2B6, coding for the isoenzyme responsible for EFV metabolism. When CYP2B6 function is impaired, CYP3A isoenzymes may control the rate of 8-hydroxylation, and CYP2A6 may modulate an alternative pathway to 7-hydroxy-EFV. We hypothesize that genetic variability in these accessory pathways may contribute to the remaining, unexplained variability in EFV exposure in subjects with limited CYP2B6 function.
Methods:  Participants (n = 167), fully characterized for CYP2B6 variation, were genotyped for the decreased function alleles CYP3A4*1B, CYP3A5*3, *6, and *7; and for the increased function alleles CYP3A7*1C and *2 by Taqman allelic discrimination. CYP2A6 was assessed by full resequencing (promoter region, all 9 exons and exon-intron boundaries) of the gene in a well-characterized subgroup of 21 subjects homozygous for a loss or diminished function CYP2B6 allele (*6, *11, *18, *27, or *28), and by genotyping loss of function CYP2A6 alleles and by determining the gene copy number in the complete study population.
Results:  Analysis of the 146 subjects without impaired CYP2B6 function did not reveal a correlation between the CYP3A family, CYP2A6 genotype, and EFV exposure. However, a significant correlation was observed between CYP3A4*1B allele and high EFV exposure among CYP2B6 slow metabolizers. Median log₁₀ EFV AUC values for carriers of 3A4*1B (n = 6) was 2.48 mg·h/mL, vs 2.12 for non-carriers (n = 15), p=0.016. Resequencing of CYP2A6 revealed 16 polymorphisms in the exons, 16 in the exon-intron boundaries, and 1 in the promoter region. Median log₁₀ EFV AUC values for subjects (n = 5) with alleles associated with decreased 2A6 function (2A6*7, *9, *17) was 2.50 mg·h/mL, vs 2.15 for non-carriers (n = 16), p = 0.018. Combining CYP3A4 and 2A6 information provided the best discrimination of subjects with extreme EFV AUC: median log₁₀ EFV AUC was 2.46 mg·h/mL for CYP2B6 slow metabolizers with reduced CYP3A4/2A6 function (n = 8), 2.11 for CYP2B6 slow metabolizers (n = 13), and 1.65 for the reference population (all common alleles, n = 53).
Conclusions:  In the setting of limited CYP2B6 function, functional polymorphisms in accessory metabolic pathways may contribute to extremely high EFV exposure. However, functional studies are needed to confirm these associations because of a strong confounding factor of ethnicity.