13.13: Pharmacokinetics in Pediatric Patients: Drug Metabolism

In pediatric care, understanding the nuances of hepatic drug metabolism is crucial, as it significantly differs from that of adults. This divergence is primarily due to the developmental stage of drug-metabolizing enzymes, which affects how medications are processed in the body. In neonates, for instance, the activity of Phase I enzymes—critical for the initial breakdown of drugs—is markedly reduced, functioning at just 20–40% of the levels seen in adults. This reduction poses a challenge in the dosing and effectiveness of medications for newborns.

There's a noticeable shift as infants grow from 6 months to a year old. The activity of these Phase I enzymes gradually increases, eventually aligning with adult levels. This progression underscores the age-dependent nature of drug metabolism, which has significant implications for pharmacokinetics—the study of how drugs move through the body. Understanding these changes is vital for healthcare professionals in optimizing drug therapy, ensuring that pediatric patients' medications are safe and effective.

Children between 3 and 10 years demonstrate an increased hepatic metabolism compared to adults. This increase means that certain drugs, such as carbamazepine, a common anticonvulsant, are metabolized more rapidly in children, requiring adjustments in dosing to achieve therapeutic effects without causing harm.

Phase II enzymes, responsible for further processing drugs into forms more easily excreted from the body, also exhibit varied activity across ages. Neonates and young children show lower activity levels of enzymes like glucuronosyltransferase, only reaching adult capabilities by adolescence. This developmental timeline has clinical implications, as seen in conditions like kernicterus, where high levels of unconjugated bilirubin—typically processed by Phase II enzymes—accumulate in the blood, potentially crossing the underdeveloped blood-brain barrier and causing brain damage.

In summary, the pediatric body's capacity to metabolize drugs evolves significantly from birth through adolescence. This evolution impacts drug selection, dosing, and management strategies, emphasizing the importance of age-specific considerations in pediatric pharmacotherapy.

Pediatric hepatic drug metabolism and metabolizing enzymes differ significantly from those in adults.

In neonates, Phase I enzyme activities operate at a fraction of adult levels, ranging from 20– 40%.

By 6 months to 1 year of age, hepatic enzyme activity in infants begins to approach adult levels but becomes fully functional only by age 2.

This age-dependent development of Phase I enzymes significantly affects drug pharmacokinetics, impacting drug metabolism.

Notably, children 3–10 years old exhibit higher hepatic metabolism than adults. For instance, the metabolism of carbamazepine is notably increased in children, often necessitating higher doses in this subgroup.

Similar to Phase I enzymes, Phase II enzymes also show significant age-dependent variation, with glucuronosyltransferase activity being low in neonates and young children but approaching adult levels by adolescence.

In neonates, variations in enzyme activity can cause high levels of unconjugated bilirubin in the blood. It can also cross immature brain barriers, which may lead to kernicterus.