Postnatal Hepatobiliary and Gastrointestinal Systems Development and Impact on ADME

Toxicity can result from variable target organ sensitivity and exposure based on postnatal development. Changes in the gastrointestinal tract (GIT) in neonates are driven by initial enteral feedings. These are important for nutrient uptake as well as drug disposition and include motility, expansion of enzyme and transporter function, permeability, intestinal microbiome, and species-specific maturation. Some aspects of GIT function do not mature until driven by increased dietary complexity. As with the GIT, postnatal hepatic maturation in the rat includes a variety of anatomic and functional changes that include refinements in the activities or expression of drug transporters and drug-metabolizing enzymes. These changes may impact rodent pharmacokinetics, nonclinical toxicity profiles, and estimation of safe pediatric doses. Pilot or dose range finding studies can help characterize and mitigate toxicity related to drug disposition, especially in juvenile rodents. Interpretation of developmental toxicity requires knowledge of developing systems in humans and nonclinical models.