Aims: The aims of this study were to describe paracetamol pharmacokinetics in neonates and infants.
Methods: Infants in their first 3 months of life (n = 30) were randomised to sequentially receive one of three paracetamol formulations (dose 30-40 mg kg-1) over a 2 day period. The formulations were (a) elixir, (b) glycogelatin capsule suppository and (c) triglyceride base suppository. Approximately six blood samples were taken after each dose over the subsequent 10-16 h. Data were analysed using a nonlinear mixed effect model. These neonatal and infant data were then included with data from four published studies of paracetamol pharmacokinetics (n = 221) and age-related pharmacokinetic changes investigated.
Results: Population pharmacokinetic parameter estimates and their coefficients of variation (CV%) for a one compartment model with first order input, lag time and first order elimination were volume of distribution 69.9 (18%) l and clearance 13.0 (41%) l h-1 (standardized to a 70 kg person). The volume of distribution decreased exponentially with a half-life of 1.9 days from 120 l 70 kg-1 at birth to 69.9 l 70 kg-1 by 14 days. Clearance increased from birth (4.9 l h-1 70 kg-1) with a half-life of 3.25 months to reach 12.4 l h-1 70 kg-1 by 12 months. The absorption half-life (tabs) for the oral preparation was 0.13 (154%) h with a lag time (tlag) of 0.39 h (31%). Absorption parameters for the triglyceride base and capsule suppositories were tabs 1.34 (90%) h, tlag 0.14 h (31%) and tabs 0.65 (63%) h, tlag 0.54 h (31%), respectively. The tabs for elixir and capsule suppository in children under 3 months were 3.68 and 1.51 times greater than children over 3 months. The relative bioavailability of rectal formulations compared with elixir were 0.67 (30%) and 0.61 (23%) for the triglyceride base and capsule suppositories, respectively.
Conclusions: Total body clearance of paracetamol at birth is 62% and volume of distribution 174% that of older children. A target concentration above 10 mg l-1 in approximately 50% subjects can be achieved by a dose from 45 mg kg-1 day-1 at birth and up to 90 mg kg-1 day-1 in 5-year-old children. A reduced dose of 75 mg kg-1 day-1 in an 8-year-old child is sufficient because clearance is a nonlinear function of weight.