Mechanistic Prediction of Maternal, Fetal, and Amniotic Fluid Betamethasone Concentrations to Support Dosing Optimization in Pregnancy.
Mahdi Alaa A, Abduljalil Khaled K, Moreira Fernanda de Lima FL, Mehta Hiral Raju HR et al.
Respiratory distress syndrome (RDS) remains a major cause of neonatal mortality following premature birth. Antenatal betamethasone is the standard of care to accelerate fetal lung maturation and improve neonatal outcomes; however, current dosing recommendations assume uniform maternal and fetal exposure across pregnancies. Gestational age (GA)-dependent physiological changes and maternal obesity may alter betamethasone disposition and fetal exposure. This study developed a maternal-fetal physiologically based pharmacokinetic (m-f-PBPK) model to quantify the effects of GA and maternal obesity on betamethasone exposure and evaluate current dosing recommendations between 24 and 34 gestational weeks. The m-f-PBPK model incorporated maternal CYP3A-mediated metabolism, placental passive diffusion and active efflux via P-glycoprotein (P-gp), placental metabolic clearance, GA-dependent physiology, and formulation-specific release rates of betamethasone phosphate and acetate. Model performance was evaluated using maternal and fetal exposure metrics across gestation. At GA 30 weeks in non-obese pregnancies, placental metabolic clearance contributed approximately 17% of maternal clearance, whereas fetal hepatic metabolism accounted for only 2.4%. Maternal obesity decreased maternal and fetal exposure by approximately 9%, while morbid obesity reduced exposure by 23%. Simulated fetal concentrations remained above the proposed pre-clinical-derived surrogate marker of 2 ng/mL for a median duration of 38-40 h in non-obese, 37.5-39 h in obese, and 30-32 h in morbidly obese populations. This m-f-PBPK model successfully characterized maternal-fetal pharmacokinetics and provides a mechanistic basis for understanding betamethasone fetal exposure across GAs with and without maternal obesity, supporting optimization of antenatal corticosteroid therapy.