Developmental expression of azathioprine-metabolizing enzymes in mice liver and its implication for fetal drug toxicity.

Azathioprine is occasionally prescribed to pregnant women following organ transplantation or for the management of autoimmune diseases. Azathioprine and its metabolites have been reported to be transferred to the fetus via the placenta. Therefore, the expression of enzymes responsible for azathioprine metabolism in the liver from fetal to adult life is crucial for assessing its potential toxicity. However, the developmental patterns of enzyme expression in the fetal and neonatal liver remain poorly understood. Liver samples were collected from male and female ICR mice at different developmental stages. The mRNA and protein expression of azathioprine-metabolizing enzymes was quantitatively analyzed. Additionally, pregnant mice were orally administered azathioprine, and the morphology, body weight, and survival of their offspring were evaluated. The expression of glutathione S-transferase Mu1, which metabolizes azathioprine to its active metabolite 6-MP, was found to be low in the fetal. We also found that the expression of xanthine oxidase, which metabolizes 6-MP to inactive metabolites, as well as thiopurine S-methyltransferase in males, were lower in the fetal and neonatal liver than in adulthood. Furthermore, azathioprine administration to pregnant mice resulted in offspring growth retardation, accompanied by reduced body weight and decreased survival rates. Azathioprine-metabolizing enzymes were expressed at lower levels in fetal and neonatal livers. These findings raise safety concerns and support the need for therapeutic strategies that take immature hepatic metabolism into account during early development. Although this murine ICR model provides mechanistic insight into developmental changes in thiopurine metabolism, species differences and the lack of pharmacokinetic data limit direct extrapolation to human fetal drug metabolism.