title: Clonidine disposition in children; a population analysis.
abstract: BACKGROUND
  There are few data describing clonidine population pharmacokinetics in children (0-15 years) despite common use. Current pediatric data, described in terms of elimination half-life or C(max) and T(max), poorly explain variability in drug responses among individuals representative of those in whom the drug will be used clinically.
  
  
  METHODS
  Published data from four studies investigating clonidine PK after intravenous (i.v.), rectal and epidural administration (n = 42) were combined with an open-label study undertaken to examine the pharmacokinetics of i.v. clonidine 1-2 microg x kg(-1) bolus in children after cardiac surgery (n = 41). A population pharmacokinetic analysis of clonidine time-concentration profiles (380 observations) was undertaken using nonlinear mixed effects modeling. Estimates were standardized to a 70-kg adult using allometric size models.
  
  
  RESULTS
  Children had a mean age of 4 (sd 3.6 years, range 1 week-14 years) years and weight 17.8 (sd 12.6, range 2.8-60) kg. A two compartment disposition model with first-order elimination was superior to a one compartment model. Population parameter estimates (between subject variability) were clearance (CL) 14.6 (CV 35.1%) l x h(-1 )70 kg(-1), central volume of distribution (V1) 62.5 (71.1%) l 70 kg(-1), intercompartment clearance (Q) 157 (77.3%) l x h(-1) 70 kg(-1) and peripheral volume of distribution (V2) 119 (22.9%) l 70 kg(-1). Clearance at birth was 3.8 l x h(-1) 70 kg(-1) and matured with a half-time of 25.7 weeks to reach 82% adult rate by 1 year of age. The volumes of distribution, but not clearance, were increased after cardiac surgery (V1 123%, V2 126%). There was a lag time of 2.3 (CV 73.2%) min before absorption began in the rectum. The absorption half-life from the epidural space was slower than that from the rectum (0.98 CV 24.5% h vs 0.26 CV 32.3% h). The relative bioavailability of epidural and rectal clonidine was unity (F = 1).
  
  
  CONCLUSIONS
  Clearance in neonates is approximately one-third that described in adults, consistent with immature elimination pathways. Maintenance dosing, which is a function of clearance, should be reduced in neonates and infants when using a target concentration approach.
authors.affiliation: Department of Anaesthesiology, University of Auckland, New Zealand.
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Potts
authors.forename: Peter
authors.initials: P
authors.lastname: Larsson
authors.forename: Staffan
authors.initials: S
authors.lastname: Eksborg
authors.forename: Guy
authors.initials: G
authors.lastname: Warman
authors.forename: Per-Arne
authors.initials: PA
authors.lastname: Lönnqvist
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Anderson
mesh_terms: D000293:Adolescent; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000493:pharmacokinetics*; D000367:Age Factors; D000758:Anesthesia / Q000379:methods*; D006348:Cardiac Surgical Procedures; D002648:Child; D002675:Child, Preschool; D003000:Clonidine / Q000008:administration & dosage / Q000493:pharmacokinetics*; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008657:Metabolic Clearance Rate; D013997:Time Factors
chemical_list: D000316:Adrenergic alpha-Agonists; D003000:Clonidine