The pharmacokinetics of methadone and its metabolites in neonates, infants, and children.

Background: The lack of methadone pharmacokinetic data in children and neonates restrains dosing to achieve the target concentration in these populations. A minimum effective analgesic concentration of methadone in opioid naïve adults is 0.058 mg·l(-1) , while no withdrawal symptoms were observed in neonates suffering opioid withdrawal if plasma concentrations of methadone were above 0.06 mg·l(-1) . The racemate of methadone which is commonly used in pediatric and anesthetic care is metabolized to 2-ethylidine-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenylpyrroline (EMDP).

Methods: Data from four studies (age 33-week PMA-15 years) were pooled (n = 56) for compartment analysis using nonlinear mixed effects modeling. Parameter estimates were standardized to a 70-kg person using an allometric model approach. Investigation was made of the racemate and metabolite (EDDP and EMDP) dispositions. In addition, neonatal data (n = 7) allowed further study of R- and S-enantiomer pharmacokinetics.

Results: A three-compartment linear disposition model best described the observed time-concentration profiles with additional compartments for metabolites. Population parameter estimates (between-subject variability) were central volume (V1) 21.5 (29%) l.70 kg(-1) , peripheral volumes of distribution V2 75.1 (23%) l.70 kg(-1) and V3 484 (8%) l.70 kg(-1) , clearance (CL) 9.45 (11%) l·h(-1) .70 kg(-1) , and intercompartment clearances Q2 325 (21%) l·h(-1) .70 kg(-1) and Q3 136 (14%) l·h(-1) .70 kg(-1) . EDDP formation clearance was 9.1 (11%) l·h(-1) .70 kg(-1) , formation clearance of EMDP from EDDP 7.4 (63%) l·h(-1) .70 kg(-1) , elimination clearance of EDDP was 40.9 (26%) l·h(-1) .70 kg(-1) and the rate constant for intermediate compartments 2.17 (43%) h(-1) .

Conclusions: Current pharmacokinetic parameter estimates in children and neonates are similar to those reported in adults. There was no clearance maturation with age. Neonatal enantiomer clearances were similar to those described in adults. A regimen of 0.2 mg·kg(-1) per 8 h in neonates achieves a target concentration of 0.06 mg·l(-1) within 36 h. Infusion, rather than intermittent dosing, should be considered if this target is to be achieved in older children after cardiac surgery.