The study was undertaken to show that polymorphic isoniazid elimination in humans is trimodal; that the acetylator genotype and eliminator phenotype of the individual patient are concordant; and that the differences in the pharmacokinetic parameters of fast, intermediate, and slow eliminator subgroups are statistically significant. Sixty adult patients of both sexes and of mixed race with tuberculosis participated in the trial. The apparent elimination rate constant (k, h(-1)) and the area under the isoniazid concentration-time curve (AUC, mg/L/h), over the interval 2 to 6 h after oral isoniazid were determined in all patients; NAT2 allele composition was determined in 47 patients. Serum INH concentrations were determined by HPLC and genotypes by PCR/restriction enzyme analysis. Three eliminator phenotypes could be distinguished, and concordance between the phenotype and the genotype of the individual could be demonstrated. The isoniazid concentration-time profiles of the three eliminator subgroups were significantly different (p < 0.05). The NAT2*12A allele, which codes for fast acetylation, has a high frequency in the population studied, the intermediate acetylator genotype is constituted of codominant fast and slow alleles, and the distribution of phenotypes/genotypes in the population is consistent with Hardy-Weinberg predictions. The therapeutic implications of polymorphic isoniazid metabolism are discussed.

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.

There was no difference in the t max values achieved. Children less than 2 years of age achieve target concentrations of first-line anti-TB agents using revised WHO dosage recommendations. Our data provided supportive evidence for the implementation of the revised WHO guidelines for first-line anti-TB therapy in young children.Isoniazid (INH), rifampin (RMP), and pyrazinamide (PZA) are routinely used to treat tuberculosis (TB) in children (23, 44). Recommendations for pediatric dosages are based on a small number of pharmacokinetic studies, few of which included children younger than 2 years of age. During early life, children experience significant changes in the relative sizes of their body compartments and their ability to absorb, metabolize, and excrete drugs (5, 17). These changes are greatest within the first 2 years of life (4). Most published studies on first-line anti-TB drugs in children have not analyzed differences between older and younger children or the effect of HIV coinfection. The pharmacokinetics of INH are further complicated by genetic polymorphisms of N-acetyltransferase type 2 (NAT2) in the metabolic pathway of INH, which influences INH concentrations (18,26,46).In the absence of pharmacodynamic data for children and therefore data that demonstrate an association between serum drug concentration and clinical outcome, optimal anti-TB therapy should aim to produce the targeted serum drug concentrations that have been determined in adult pharmacokinetic and pharmacodynamic studies. For INH, the proposed optimal maximum serum drug concentration (C max ) for therapy is 3 to 5 g/ml (15, 27). Target serum RMP concentrations in adults after a standard oral dose of 600 mg are in the range of 8 to 24 g/ml; serum RMP concentrations below 8 g/ml are considered low, and those below 4 g/ml are considered very low (28,29). There is more uncertainty regarding the optimal therapeutic serum PZA concentration. In adults, serum PZA concentrations are targeted at 20 to 60 g/ml (11, 28). However, in a recent study of adults, poor treatment outcome of pulmonary TB was associated with serum PZA concentrations of Ͻ35 g/ml (8).Optimal anti-TB therapy is important in all children but particularly in young children (Ͻ2 years of age) and those HIV infected, where there is a high risk of progression to severe

Collections of sputum from 105 patients with newly diagnosed pulmonary tuberculosis were made before and at 1 and 2 d after the start of chemotherapy with isoniazid (INH) alone given to groups of patients in doses of 600 mg, 300 mg, 150 mg, 75 mg, 37.5 mg, 18.75 mg, and 9 mg daily, as well as from an untreated group. Counts of colony forming units (cfu) of Mycobacterium tuberculosis in the collections were set up on plates of selective 7H10 medium. The early bactericidal activity (EBA) of INH was defined as the decrease in log10 cfu/ml sputum/day during the first 2 d of treatment. A smooth curve relating EBA to log dose was obtained, with 600 mg INH yielding the highest mean EBA of 0.539, and 18.75 mg INH yielding the lowest EBA (0.111) that could be distinguished from the bactericidal activity of the untreated group. The ratio of the usual dose of 300 mg INH to the lowest dose, of 18.75 mg, that produced a detectable EBA, termed the therapeutic margin, was therefore 16, in contrast to the lower therapeutic margin of 4 for rifampin. The EBA was related to the INH acetylator genotype of patients treated with 600 mg or 9 mg INH.