The pharmacokinetics of intravenous paracetamol in neonates: size matters most

Background The aim was to describe intravenous paracetamol pharmacokinetics, determine major covariates and suggest a dosing regimen for (pre) term neonates. Methods A population pharmacokinetic analysis of 943 paracetamol observations from 158 neonates (27-45 weeks' postmenstrual age (PMA)) was undertaken using non-linear mixed effects models. Data from three published studies were pooled with newly collected time-concentration points during repeated intravenous paracetamol administration. Results A two-compartment (central, peripheral) linear disposition model was used. Population parameter estimates (between-subject variability, %) were central volume 51.9 l/70 kg (21.6%), peripheral volume of distribution 22.7 l/70 kg, clearance 5 l/h/70 kg (40%) and intercompartment clearance 16.2 l/h/70 kg. Covariate information predicts 60.9% of clearance variance. Weight was used to predict patient size and was the major covariate contributing 57.5% of variance. Clearance expressed as mg/kg/h increases only slightly with PMA (0.138 l/kg/h at 28 weeks' PMA to 0.167 l/kg/h at 44 weeks' PMA) and contributes only 2.2% of variance. High unconjugated bilirubin levels contributed an additional 1.2%. Conclusions Patient size (predicted by weight) is the major covariate of clearance variance in neonates. Using these estimates, a mean paracetamol serum concentration of 11 mg/l is predicted in neonates of 32-44 weeks' PMA given a standard dose of intravenous paracetamol of 10 mg/kg every 6 h. Safety data for this drug are limited in neonates. Continued surveillance therefore remains essential.