Pharmacokinetics in Children

Absorption from the gastrointestinal tract is affected by

• Gastric acid secretion

• Bile salt formation

• Gastric emptying time

• Intestinal motility

• Bowel length and effective absorptive surface

• Microbial flora

• Illness

All these factors vary with age (1).

Reduced gastric emptying and intestinal motility increase the time it takes to reach therapeutic concentrations when enteral drugs are given to infants < 3 months old. Drug-metabolizing enzymes present in the intestines of young infants are another cause of reduced drug absorption. Infants with congenital atretic bowel or surgically removed bowel or who have jejunal feeding tubes may have specific absorptive defects depending on the length of bowel lost or bypassed and the location of the lost segment. How the type of food consumed may alter gastric emptying should also be considered (eg, solid versus liquid).

Alterations in intestinal flora that aid metabolism may also affect absorption in the gut.

Injected drugs are often erratically absorbed because of

• Variability in their chemical characteristics

• Differences in absorption by site of injection (intramuscular or subcutaneous)

• Variability in muscle mass among children

• Illness (eg, compromised circulatory status)

• Variability in depth of injection (too deep or too shallow)

Intramuscular injections are generally avoided in children because of pain and the possibility of tissue damage, but, when needed, water-soluble drugs are best because they do not precipitate at the injection site.

Transdermal absorption may be enhanced in neonates and young infants because the stratum corneum is thin and because the ratio of surface area to weight is much greater than for older children and adults. Skin disruptions (eg, abrasions, eczema, burns) increase absorption in children of any age.

Transrectal drug therapy

Absorption of inhaled drugs from the lungs (eg, beta-agonists for asthma, pulmonary surfactant for respiratory distress syndrome) may vary less by physiologic parameters and more by reliability of the delivery device and patient or caregiver technique.

1. 1. van den Anker J, Reed MD, Allegaert K, Kearns GL: Developmental changes in pharmacokinetics and pharmacodynamics. J Clin Pharmacol 58 (supplement 10):S10–S25, 2018. doi: 10.1002/jcph.1284

The volume of distribution of drugs changes in children with aging. These age-related changes are due to changes in body composition (especially the extracellular and total body water spaces) and plasma protein binding.

Higher doses (per kg of body weight) of water-soluble drugs are required in younger children because a higher percentage of their body weight is water (see figure Changes in body composition with growth and aging). Conversely, lower doses are required to avoid toxicity as children grow older because of the decline in water as a percentage of body weight. Additionally, children with obesity have been shown to have significantly higher percentages of total body water, body volume, lean body mass, and fat mass compared to children without obesity (1).

Changes in body composition with growth and aging

Adapted from Puig M: Body composition and growth. In Nutrition in Pediatrics, ed. 2, edited by WA Walker and JB Watkins. Hamilton, Ontario, BC Decker, 1996.

1. 1. Vaughns JD, Conklin LS, Long Y, et al: Obesity and pediatric drug development. J Clin Pharmacol 58(5):650–661, 2018. doi: 10.1002/jcph.1054

The cytochrome P-450 (CYP450) enzyme system in the small bowel and liver is the most important known system for drug metabolism. CYP450 enzymes inactivate drugs via

• Oxidation, reduction, and hydrolysis (phase I metabolism)

• Hydroxylation and conjugation (phase II metabolism)

Phase I metabolism1). Kidneys, lungs, and skin also play a role in the metabolism of some drugs, as do intestinal drug-metabolizing enzymes in neonates.

Phase II metabolism varies considerably by substrate. Maturation of enzymes responsible for bilirubin and acetaminophen

Drug metabolites are eliminated primarily through bile or the kidneys. Renal elimination depends on

• Plasma protein binding

• Renal blood flow

• Glomerular filtration rate

• Tubular secretion

All of these factors are altered in the first 2 years of life. Renal plasma flow is low at birth (12 mL/minute) and reaches adult levels of 140 mL/minute by age 1 year. Similarly, glomerular filtration rate is 2 to 4 mL/minute at birth, increases to 8 to 20 mL/minute by 2 to 3 days, and reaches adult levels of 120 mL/minute by 3 to 5 months.

1. 1. Blake JB, Abdel-Rahman SM, Pearce RE, et al: Effect of diet on the development of drug metabolism by cytochrome P-450 enzymes in healthy infants. Pediatr Res 60(6):717–723, 2006. doi: 10.1203/01.pdr.0000245909.74166.00

Because of the above factors, drug dosing in children < 12 years old is frequently a function of age, body weight, or both. This approach is practical but not ideal. Even within a population of similar age and weight, drug requirements may differ because of maturational differences in absorption, metabolism, and elimination. Thus, when practical, dose adjustments should be based on plasma drug concentration (however, plasma drug concentration may not reflect the drug concentration in the target organ). Unfortunately, these adjustments are not feasible for most drugs. However, in the US, because of the Best Pharmaceuticals for Children Act of 2001 and the Pediatric Research Equity Act of 2003 (both made permanent in 2012 [1]), more complete pediatric dosing, pharmacokinetic, and safety information is available for over 900 drugs for use in children (see also the U.S. Food and Drug Administration [FDA] 2020 status report).

Physiologically based pharmacokinetic modeling is a mathematical technique that uses known principles of biochemistry and physiology to predict how a drug will be absorbed, distributed, metabolized, and excreted. Results of this modeling can help support decisions on whether, when, and how to conduct a clinical trial and can help improve the safety and efficiency of pediatric clinical trials.

1. 1. Bourgeois FT, Kesselheim AS: Promoting pediatric drug research and labeling—Outcomes of legislation. N Engl J Med 381(9):875–881, 2019. doi: 10.1056/NEJMhle1901265