Children are not simply small adults, and their unique physiological characteristics require special consideration in drug dosing. The pediatric population spans a wide range of ages from preterm neonates to adolescents, each with distinct pharmacokinetic profiles. Developmental changes in drug absorption, distribution, metabolism, and excretion occur throughout childhood, and dosing must account for these age-related differences to achieve safe and effective therapy.
Developmental Changes in Absorption
Gastric pH is neutral at birth and gradually decreases to adult values over the first two years of life. This higher gastric pH in neonates and infants increases the absorption of acid-labile drugs and decreases the absorption of weak acids. Gastric emptying is delayed in neonates, slowing the rate of drug absorption. Intestinal motility is also reduced, prolonging transit time. These differences are most pronounced in preterm infants and gradually normalize with age.
Percutaneous absorption is enhanced in neonates because their stratum corneum is thinner and more hydrated. This increased permeability can lead to systemic toxicity from topically applied drugs, such as corticosteroids or antiseptics containing hexachlorophene. Intramuscular absorption is more variable in infants because of reduced muscle mass and unpredictable blood flow.
Developmental Changes in Distribution
Body composition changes significantly during development. Neonates have a higher proportion of total body water, approximately 75% of body weight compared to 60% in adults, which increases the volume of distribution for water-soluble drugs. Body fat content increases from approximately 15% in preterm infants to 25% in full-term infants, then decreases in early childhood before increasing again at puberty.
Plasma protein binding is reduced in neonates due to lower concentrations of albumin and alpha-1-acid glycoprotein and the presence of fetal albumin with lower binding affinity. This reduced binding increases the free fraction of highly bound drugs such as phenytoin and ceftriaxone, potentially increasing both pharmacological effect and toxicity. The blood-brain barrier is more permeable in neonates, allowing greater penetration of drugs into the central nervous system.
Developmental Changes in Metabolism
Drug-metabolizing enzymes mature at different rates, creating windows of vulnerability and altered drug handling. CYP3A7 is the predominant CYP enzyme in fetal liver and is gradually replaced by CYP3A4 during the first weeks to months of life. CYP2D6 activity reaches adult levels by approximately one year of age, while CYP1A2 activity, which metabolizes caffeine and theophylline, matures more slowly and may not reach adult levels until adolescence.
Phase II conjugation reactions also show developmental patterns. Glucuronidation is immature at birth, leading to reduced clearance of drugs such as morphine and chloramphenicol. The gray baby syndrome associated with chloramphenicol in neonates is a classic example of toxicity resulting from immature glucuronidation capacity. Sulfation, in contrast, is relatively well developed at birth and may serve as a compensatory pathway when glucuronidation is deficient.
Developmental Changes in Excretion
Renal function is substantially reduced at birth and matures over the first year of life. The glomerular filtration rate in neonates is approximately 2 to 4 mL per minute per 1.73 square meters, compared to the adult value of 120 mL per minute per 1.73 square meters. Glomerular filtration reaches adult levels by approximately one to two years of age. Tubular secretion matures more slowly, reaching adult capacity by approximately one year of age.
The reduced renal clearance in neonates requires dose adjustment for drugs eliminated primarily by the kidney, such as aminoglycoside antibiotics. Extended dosing intervals are typically used in neonates compared to older children and adults. The dosing interval is gradually shortened as renal function matures.
Dosing Methods in Pediatrics
Weight-based dosing is the most common approach in pediatrics, typically expressed as milligrams per kilogram. However, the relationship between weight and drug clearance is not always linear, and a fixed milligram-per-kilogram dose may not produce equivalent exposure across all ages. Body surface area-based dosing using the Mosteller formula is preferred for drugs with a narrow therapeutic index and for chemotherapy agents, as BSA correlates better with physiological processes relevant to drug disposition.
Allometric scaling, which relates dose to body weight raised to a power such as 0.75, has been proposed as a more physiologically rational approach for some drugs. This method accounts for the nonlinear relationship between body size and metabolic rate and may better predict clearance across the pediatric age range.
Ethical and Practical Considerations
Conducting clinical trials in children presents ethical and practical challenges that have historically resulted in limited pediatric pharmacokinetic data. Many drugs used in children are prescribed off-label, relying on extrapolation from adult data with dose adjustments based on weight or age. The US Food and Drug Administration and European Medicines Agency have implemented regulatory incentives to encourage pediatric studies, improving the evidence base for pediatric drug therapy.
The development of age-appropriate formulations is another important consideration. Many drugs are available only as tablets or capsules that cannot be swallowed by young children. Liquid formulations, chewable tablets, and sprinkle capsules address this need, but their pharmacokinetics may differ from the adult formulation. Extemporaneous compounding of suspensions from crushed tablets is common but raises concerns about stability, dosing accuracy, and bioavailability.
