The antiretroviral
drug efavirenz remains widely used in children and mothers during breastfeeding in
tuberculosis-endemic areas. Evaluating the safety of
efavirenz during breastfeeding requires an understanding of its pharmacokinetics (PKs) in breast milk, its exposure in the breastfed infant, and the potential influence of polymorphisms in
drug disposition genes. The interplay of these factors between the mother and the nursing infant is a complex scenario that can be readily investigated using physiologically-based PK (PBPK) modeling. A verified PBPK model for
efavirenz describing the CYP3A4- and CYP2B6-mediated auto-induction during multiple dosing was reported previously and was applied in this study to predict the exposure of
efavirenz in vulnerable populations, including children (down to the age of 3 months), mothers, and breastfeeding infants, accounting for the various
CYP2B6 genotypes. Predicted pharmacokinetic parameters for mothers, breastfeeding infants, and children aged ≥ 3 months were reasonably consistent with observed data, irrespective of
CYP2B6 genotype. The clinically significant trend toward higher infant
efavirenz exposure from GG/GG to TT/TT composite maternal/infant
CYP2B6 genotypes was captured reasonably well by the PBPK model. Thereafter, simulations were performed to determine the adequacy of the current World Health Organization (WHO; ≥ 3 years) and the US Food and Drug Administration (FDA; ≥ 3 months) weight-based dosing regimens for
efavirenz in children according to
CYP2B6 genotype. The findings of this study indicate that PBPK models can be used in designing studies in vulnerable populations and providing guidance on optimal doses based on developmental physiology and pharmacogenetics.