Polymorphic Expression of UGT1A9 is Associated with Variable Acetaminophen Glucuronidation in Neonates: A Population Pharmacokinetic and Pharmacogenetic Study

Introduction Acetaminophen (paracetamol, APAP) is widely used as an analgesic and antipyretic drug in children and neonates. A number of enzymes contribute to the metabolism of acetaminophen, and genetic factors might be important to explain variability in acetaminophen metabolism among individuals. Methods The current investigation utilized a previously published parent–metabolite population pharmacokinetic model describing acetaminophen glucuronidation, sulfation, and oxidation to examine the potential role of genetic variability on the relevant metabolic pathways. Neonates were administered 30-min intravenous infusions of acetaminophen 15 mg/kg every 12 h (< 28 weeks’ gestational age [GA]) or every 8 h (≥ 28 weeks GA) for 48 h. A total of 18 sequence variations (SVs) in UDP-glucuronosyltransferase ( UGT ), sulfotransferase ( SULT ), and cytochrome P450 ( CYP ) genes from 33 neonates (aged 1–26 days) were examined in a stepwise manner for an effect on the metabolic formation clearance of acetaminophen by glucuronidation (UGT), sulfation (SULT), and oxidation (CYP). The stepwise covariate modeling procedure was performed using NONMEM ® version 7.3. Results Incorporation of genotype as a covariate for one SV located in the UGT1A9 gene promoter region (rs3832043, − 118 > insT, T 9 > T 10 ) significantly improved model fit (likelihood ratio test, p < 0.001) and reduced between-subject variability in glucuronide formation clearance. Individuals with the UGT1A9 T 10 polymorphism, indicating insertion of an additional thymidine nucleotide, had a 42% reduction in clearance to APAP-glucuronide as compared to their wild-type counterparts. Conclusion This study shows a pharmacogenetic effect of an SV in the UGT1A9 promoter region on the metabolism of acetaminophen in neonates.