Background:  Efavirenz (EFV) is metabolized mainly by CYP2B6 into 8-hydroxy-EFV (8-OH-EFV), and to a lesser extent by accessory pathways involving the CYP3A4 and 2A6 as well as the UDP glucuronosyl transferases (UGT), leading, respectively, to 8-OH-, 7-OH- and N-Glucuronide-EFV (N-gln-EFV). We hypothesized that diminished loss of function of CYP2B6 would redirect metabolism to accessory pathways.

Methods:  We assessed the absolute or relative quantity of 8-OH, 7-OH, and N-gln-EFV metabolites in vivo, in individuals (n = 48) representative of the various genetic profiles of CYP2B6, CYP2A6, and CYP3A4, by liquid chromatography coupled with triple quadripole tandem mass spectrometry.

Results:  CYP2B6 loss of function homozygocity, when compared to common CYP2B6 allele homozygocity, was associated with a 45% decrease in 8-OH-EFV. In addition, we observed a 32% decrease in 7-OH-EFV—consistent with a role of CYP2B6 in this minor pathway—and a 150% increase in N-gln-EFV. Among CYP2B6 slow metabolizers, carriers of CYP2A6 or CYP3A4 loss of function alleles (1, in the figure) presented a higher median log₁₀ N-gln-EFV concentration (a.u.) than individuals with common alleles (0, in the figure): respectively 1.65 and 1.51 vs 1.40, and carriers of both CYP2A6 and CYP3A4 LOF alleles exhibited the greatest median log₁₀ N-gln-EFV concentration (a.u.) of 1.80.

Conclusions:  These results are consistent with the role of CYP2A6 and CYP3A4 in EFV metabolism as accessory isoenzymes. Their function becomes increasingly relevant in the presence of impaired CYP2B6 function. Furthermore, the simultaneous occurrence of limited function in CYP2B6 and the accessory hydroxylating pathways (CYP2A6 and CYP3A4), results in the redirection of EFV metabolism from hydroxylation to N-glucuronidation. However the UGT do not compensate the loss of the others isoenzymes, leading to marked EFV accumulation.