Neonatal Maturation of Paracetamol (Acetaminophen) Glucuronidation, Sulfation, and Oxidation Based on a Parent–Metabolite Population Pharmacokinetic Model

ObjectivesThis study aimed to model the population pharmacokinetics of intravenous paracetamol and its major metabolites in neonates and to identify influential patient characteristics, especially those affecting the formation clearance (CLformation) of oxidative pathway metabolites.MethodsNeonates with a clinical indication for intravenous analgesia received five 15-mg/kg doses of paracetamol at 12-h intervals (<28 weeks’ gestation) or seven 15-mg/kg doses at 8-h intervals (≥28 weeks’ gestation). Plasma and urine were sampled throughout the 72-h study period. Concentration–time data for paracetamol, paracetamol-glucuronide, paracetamol-sulfate, and the combined oxidative pathway metabolites (paracetamol-cysteine and paracetamol-N-acetylcysteine) were simultaneously modeled in NONMEM 7.2.ResultsThe model incorporated 259 plasma and 350 urine samples from 35 neonates with a mean gestational age of 33.6 weeks (standard deviation 6.6). CLformation for all metabolites increased with weight; CLformation for glucuronidation and oxidation also increased with postnatal age. At the mean weight (2.3 kg) and postnatal age (7.5 days), CLformation estimates (bootstrap 95% confidence interval; between-subject variability) were 0.049 L/h (0.038–0.062; 62 %) for glucuronidation, 0.21 L/h (0.17–0.24; 33 %) for sulfation, and 0.058 L/h (0.044–0.078; 72 %) for oxidation. Expression of individual oxidation CLformation as a fraction of total individual paracetamol clearance showed that, on average, fractional oxidation CLformation increased <15 % when plotted against weight or postnatal age.ConclusionsThe parent–metabolite model successfully characterized the pharmacokinetics of intravenous paracetamol and its metabolites in neonates. Maturational changes in the fraction of paracetamol undergoing oxidation were small relative to between-subject variability.