Developmental pharmacology and age-related pharmacokinetics — formative SAQs

SAQ 1 — Neonatal toxicity on a 'standard' weight-based dose (10 marks)

A 10-day-old term neonate weighing 3.2 kg becomes apnoeic shortly after a morphine infusion is started on the ward for post-operative pain. The nurse stops the infusion. The order was written at a weight-based dose, but the team is surprised because the dose was taken from the adult drug chart. The baby is dusky with a pulse. [1] [5]

Questions

1. Outline your immediate management. (3 marks) [1]
2. Explain how each of the four ADME phases contributed to this toxicity in a term neonate. (5 marks) [2] [4]
3. State two prescribing principles that would have prevented this event. (2 marks) [5]

Model answer

Immediate management (3). Support the airway and breathing with bag-mask ventilation; give naloxone titrated to respiratory effort rather than full alertness to avoid precipitating withdrawal; call for senior paediatric and neonatal or PICU help; keep the infusion stopped and preserve the syringe and order; check whether any other child is on the same protocol; document and report; open disclosure to the family once the child is stable. [1]

ADME contribution (5). Absorption was intravenous, so gut immaturity did not apply, but the neonate's high body surface area to mass ratio means topical and transdermal exposure would have been relevant if other routes had been used. Distribution contributed strongly: total body water and extracellular fluid are far higher in the neonate, the plasma protein binding is low, and the blood-brain barrier is immature, so a larger fraction of the morphine reached the central nervous system. Metabolism contributed through slow UGT2B7 maturation — morphine is cleared by glucuronidation, which is markedly reduced in the first weeks of life. Excretion contributed through low neonatal glomerular filtration rate (about 2 to 4 mL per minute per 1.73 m² at birth), so the morphine metabolites were cleared slowly. The combined effect is a narrow therapeutic window and prolonged exposure. [2] [4]

Prevention (2). Always confirm weight in kilograms and look up the dose in a paediatric formulary (BNFc, Royal Children's Hospital Melbourne) rather than reproducing an adult dose from memory; apply maturation adjustment using postmenstrual and postnatal age for narrow-therapeutic-index drugs, and require an independent double-check of high-risk infusions such as opioids. [5]

SAQ 2 — The school-age child with breakthrough seizures (10 marks)

An 8-year-old boy with known focal epilepsy has had three breakthrough seizures in the past month despite documented adherence to his regular carbamazepine dose of 20 mg per kg per day. His carbamazepine level is 6 mg per litre (target range 4 to 12 mg per litre). He has grown 4 centimetres and gained 3 kilograms in the past six months. [7] [9]

Questions

1. Explain the developmental pharmacology that accounts for breakthrough seizures despite an adequate-looking level. (4 marks) [7]
2. Outline your stepwise approach to dose revision and therapeutic drug monitoring. (4 marks) [9]
3. State two pitfalls to avoid in managing this child's medication. (2 marks) [9]

Model answer

Developmental pharmacology (4). Per-kilogram clearance of many drugs in mid-childhood can exceed adult clearance because hepatic capacity and renal tubular secretion, normalised to body size, peak around early-to-mid childhood. A dose that was adequate two years ago may now be subtherapeutic because the child's clearance has risen and his weight has increased. The measured level of 6 mg per litre sits in the lower half of the reference range, consistent with relatively rapid clearance rather than non-adherence. Growth of 4 centimetres and 3 kilograms means the absolute daily dose has effectively fallen behind his clearance needs. [7]

Stepwise dose revision (4). Confirm adherence and that the formulation has not changed; review for interacting drugs (enzyme inducers such as rifampicin or macrolides that inhibit carbamazepine metabolism); recalculate the weight-based dose against the formulary; consider an increase in mg per kg per day or a move to three-times-daily dosing to maintain peak exposure; re-check a level at steady state (about five half-lives after the change); titrate to clinical seizure control, not just to the lower edge of the range. Involve the family in a written plan with return triggers. [9]

Pitfalls (2). Do not assume that a level inside the reference range excludes under-treatment — the target is the level at which this child is seizure-free, not a population mean. Do not switch brands of a narrow-therapeutic-index anti-epileptic without re-checking a level, because formulation differences can shift exposure. [9]