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Isoniazid pharmacokinetics in children treated for respiratory tuberculosis
  1. H S Schaaf1,
  2. D P Parkin2,
  3. H I Seifart2,
  4. C J Werely3,
  5. P B Hesseling1,
  6. P D van Helden3,
  7. J S Maritz4,
  8. P R Donald1
  1. 1Department of Paediatrics and Child Health, Faculty of Health Sciences, Stellenbosch University and Tygerberg Children’s Hospital, South Africa
  2. 2Department of Pharmacology, Faculty of Health Sciences, Stellenbosch University, South Africa
  3. 3Department of Medical Biochemistry, Faculty of Health Sciences, Stellenbosch University, South Africa
  4. 4Biostatistics Unit of the South African Medical Research Council, South Africa
  1. Correspondence to:
    Dr H S Schaaf
    Paediatrics and Child Health, Stellenbosch University, PO Box 19063, 7505 Tygerberg, South Africa;


Aims: To define the pharmacokinetics of isoniazid (INH) in children with tuberculosis in relation to the N-acetyltransferase 2 (NAT2) genotype.

Methods: The first order elimination rate constant (k) and area under the concentration curve (AUC) were calculated in 64 children <13 years of age (median 3.8) with respiratory tuberculosis from INH concentrations determined 2–5 hours after a 10 mg/kg INH dose. The NAT2 genotype was determined; 25 children were classified as homozygous slow (SS), 24 as heterozygous fast (FS), and 15 as homozygous fast (FF) acetylators.

Results: The mean (SD) k values of the genotypes differed significantly from one another: SS 0.254 (0.046), FS 0.513 (0.074), FF 0.653 (0.117). Within each genotype a median regression of k on age showed a significant decrease in k with age. The mean (SD) INH concentrations (mg/l) two hours after INH administration were SS 8.599 (1.974), FS 5.131 (1.864), and FF 3.938 (1.754). A within genotype regression of 2-hour INH concentrations on age showed a significant increase with age. A within genotype regression of 3-hour, 4-hour, and 5-hour concentrations on age also showed a significant increase with age in each instance. In ethnically similar adults, mean (SD) 2-hour INH concentrations (mg/l) for each genotype were significantly higher than the children’s: SS 10.942 (1.740), FS 8.702 (1.841), and FF 6.031 (1.431).

Conclusions: Younger children eliminate INH faster than older children and, as a group, faster than adults, and require a higher mg/kg body weight INH dose to achieve serum concentrations comparable to adults.

  • AUC, area under the concentration curve
  • FF, homozygous fast acetylator
  • FS, heterozygous fast acetylator
  • INH, isoniazid
  • NAT2, N-acetyltransferase 2
  • SS, homozygous slow acetylator
  • isoniazid
  • pharmacokinetics
  • acetylation
  • NAT2 genotype

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  • Financial assistance: Harry and Doris Crossley Foundation (HS Schaaf) and National Research Foundation (PD van Helden)

  • Competing interests: none declared

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