H. I. Seifart

Isoniazid pharmacokinetics in children treated for respiratory tuberculosis.

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.

High-performance liquid chromatographic determination of isoniazid, acetylisoniazid and hydrazine in biological fluids

The basic principle of derivatization of a hydrazide moiety with an aldehyde as applied in the method developed by Lacroix et al. [J. Chromatogr., 307 (1984) 137-144] for the quantitation of isoniazid and acetylisoniazid was improved by modification, standardization and extension to allow quantitation of hydrazine in patient samples. It could be shown that 40 microliters of 1% methanolic cinnamaldehyde per 200 microliters of deproteinized analysate gave maximal chromophoric isoniazid-cinnamaldehyde conjugate, read at 340 nm. The hydrolytic loss of isoniazid, crucial to the quantitation of acetylisoniazid, could be compensated for by introduction of an appropriate set of calibration curves. Although the method described here allows quantitation of monoacetylhydrazine and diacetylhydrazine, in addition to hydrazine, in mono-spiked samples, the method cannot be used for the quantitation of the acetylated metabolites of hydrazine in patient samples because of a lack of specificity. Linear calibration curves in the range 1-25 micrograms/ml for isoniazid and acetylisoniazid, 10-400 ng/ml for hydrazine and 50-1000 ng/ml for monoacetylhydrazine and diacetylhydrazine, could be constructed; analyte recoveries approaching 100% could be achieved in all instances.

Pharmacokinetics of Isoniazid, Rifampin, and Pyrazinamide in Children Younger than Two Years of Age with Tuberculosis: Evidence for Implementation of Revised World Health Organization Recommendations

ABSTRACT The World Health Organization (WHO) recently issued revised first-line antituberculosis (anti-TB) drug dosage recommendations for children. No pharmacokinetic studies for these revised dosages are available for children <2 years. The aim of the study was to document the pharmacokinetics of the first-line anti-TB agents in children <2 years of age comparing previous and revised WHO dosages of isoniazid (INH; 5 versus 10 mg/kg/day), rifampin (RMP; 10 versus 15 mg/kg/day), and pyrazinamide (PZA; 25 versus 35 mg/kg/day) and to investigate the effects of clinical covariates, including HIV coinfection, nutritional status, age, gender, and type of tuberculosis (TB), and the effect of NAT2 acetylator status. Serum INH, PZA, and RMP levels were prospectively assessed in 20 children <2 years of age treated for TB following the previous and the revised WHO dosage recommendations. Samples were taken prior to dosing and at 0.5, 1.5, 3, and 5 h following dosing. The maximum drug concentration in serum (Cmax), the time to Cmax (tmax), and the area under the concentration-time curve (AUC) were calculated. Eleven children had pulmonary and 9 had extrapulmonary TB. Five were HIV infected. The mean Cmax (μg/ml) following the administration of previous/revised dosages were as follows: INH, 3.19/8.11; RMP, 6.36/11.69; PZA, 29.94/47.11. The mean AUC (μg·h/ml) were as follows: INH, 8.09/20.36; RMP, 17.78/36.95; PZA, 118.0/175.2. The mean Cmax and AUC differed significantly between doses. There was no difference in the tmax 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.

The Influence of Human N-Acetyltransferase Genotype on the Early Bactericidal Activity of Isoniazid

The elimination of isoniazid is subject to the influence of the N-acetyltransferase 2 (NAT2) genotype, and individuals may be homozygotic slow, heterozygotic fast, or homozygotic fast acetylators of isoniazid. The early bactericidal activity (EBA) of an antituberculosis agent can be determined by quantitative culture of Mycobacterium tuberculosis in sputum samples obtained from patients with pulmonary tuberculosis during the first days of treatment. In these studies, the EBA of isoniazid during the first 2 days of treatment was determined for 97 patients with sputum smear-positive pulmonary tuberculosis following isoniazid doses of < or =37.5 mg, 75 mg, 150 mg, 300 mg, and 600 mg. The NAT2 genotype was determined in 70 patients, and the association between EBA and genotype was examined in this subgroup. Similarly, the relationship between EBA and isoniazid serum concentration was evaluated in 87 patients. The mean EBA of isoniazid increased with dose, but it levelled off between doses of 150 mg (mean EBA, 0.572) and 300 mg (mean EBA, 0.553). Significant differences were found in the mean EBA of isoniazid between the homozygous slow acetylator group and the heterozygous fast acetylator group and between the homozygous slow acetylator group and the homozygous fast acetylator group, but not between the heterozygous fast acetylator group and the homozygous fast acetylator group. The EBA appeared to reach a maximum at a 2-h isoniazid concentration of 2-3 microg/mL. These data confirm a significant effect of NAT2 genotype on the EBA of isoniazid over a range of doses.

Cerebrospinal fluid concentrations of ethionamide in children with tuberculous meningitis

Cerebrospinal fluid ethionamide concentrations were determined in 18 children (median age 26.5 months) with tuberculous meningitis complicated by raised intracranial pressure. Lumbar spinal fluid specimens were obtained before and after weekly hour-long monitoring of intracranial pressure. Thirty-five paired and four single specimens were evaluated. A dosage schedule of 15 mg/kg was used on 26 occasions, and a spinal fluid ethionamide concentration of 2.5 micrograms/ml, the in vitro minimal inhibitory concentration for Mycobacterium tuberculosis, was exceeded on only seven occasions (27%). A dosage of 20 mg/kg was administered on 13 occasions, and in only two instances (15%) was a concentration of 2.5 micrograms/ml not achieved. Ethionamide in a single daily dosage of 20 mg/kg should be considered for the initial treatment of tuberculous meningitis when the presence of isoniazid-resistant M. tuberculosis cannot be excluded.

Early bactericidal activity of antituberculosis agents.

The early bactericidal activity (EBA) of an antituberculosis agent is arbitrarily defined as the fall in log10 colony forming units (cfu) of Mycobacterium tuberculosis per ml sputum per day during the first 2 days of treatment. Determining the EBA is an important preliminary step in the clinical evaluation of an antituberculosis agent. We review the results of eight published studies of the EBA of different antituberculosis agents, the impact of these results on our understanding of the actions of the respective agents, the clinical characteristics and sputum findings of patients included in these studies, and explore sources of variation in the EBA results. Patients in these studies had a mean age of 31–36 years, a mean weight of 50–57 kg, 67% were male and 56% had lung involvement covering an area of more than one lung, and 90% had multicavitary disease. None of these findings were related to EBA in any study. The mean log10 cfu per ml sputum in the first specimen was 6.474. This was related to radiological extent of disease and cavity size in one study (p < 0.001) and, in the case of isoniazid to EBA with a rise in EBA of 0.094 (95% CL 0.029–0.158) for each tenfold rise in cfu counts/ml sputum. The overall variation in EBA in these studies was 0.0303, that due to laboratory processing of specimens was 0.0011, and due to patient characteristics and sputum sampling 0.0212. The EBA is a reproducible investigation that has contributed significantly to our knowledge of the actions and characteristics of both established and new antituberculosis agents. The greatest source of variation in EBA results appears to be that due to interpatient variation in disease characteristics and sputum sampling.

Fluctuation of the volume of distribution of amikacin and its effect on once-daily dosage and clearance in a seriously ill patient

ObjectiveThe main aim of the trial was to determine the extent to which the volume of distribution of amikacin fluctuates in a seriously ill patient receiving copious quantities of i.v. fluid over an extended term of treatment. The impact of the volume fluctuation on amikacin therapeutic peak concentrations was also assessed.Design and settingThe case report describes a young, previously healthy male adult admitted to the surgical ICU of a teaching hospital following trauma to the head and central nervous system.InterventionThe patient received 1 g of amikacin once-daily i. v. for 35 consecutive days as part of an antimicrobial regimen. Blood samples were drawn for routine amikacin concentration determinations on 14 occasions, extending over the entire term of treatment, from which the required pharmacokinetic parameters were determined.ResultsThe volume of distribution of amikacin varied extensively from 0.27 to 0.61 l/kg (normal range 0.27±0.06 l/kg) notwithstanding the fact that amikacin clearance remained satisfactorily high throughout the term of treatment.ConclusionsOnce-daily therapeutic amikacin concentrations fluctuate extensively and rapidly in the seriously ill patient receiving copious quantities of i.v. fluids, despite competent renal function. The volume expansion seen in our patient is difficult to account for in terms of the extracellular fluid compartment only.Recommendations(a) Once-daily regimen amikacin peak concentrations should be frequently monitored in the seriously ill patients; (b) once-daily amikacin regimens are best monitored using blood specimens drawn at 1 and 6–8 h post administration.

Factors in hydrazine formation from isoniazid by paediatric and adult tuberculosis patients

SummaryAn HPLC method is described for measurement of plasma hydrazine (Hz) concentrations (CHz) at the same time as isoniazid (INH) levels (CINH). Study has been made of CHz during 2–5 h after dose in healthy adults (A,n = 34), in adult pulmonary TB patients (B,n = 18) and in paediatric tuberculous meningitis patients (C,n = 25). Although the population has about equal proportions of ‘slow’ (52%) and ‘fast’ acetylators, in none of the groups could a correlation be shown between CHz levels or rates of Hz accumulation and any measure of acetylator type. Consequently Hz must be derived both from INH and from its metabolites during the first hours post-dose.For group A and ca. 70% of groups B and C a constant and maximal fraction of dose (ca. 0.6% for adults and 0.4% for paediatric patients) appeared as Hz at 4–5 h. For group B patients small pre-dose concentrations increased with duration of treatment. Four patients in group B showed the highest levels of CHz and rates of Hz accumulation some three times greater than the rest; all four had been identified as alcoholics and one showed evidence of hepatotoxicity at CHz (5 h) = 1.3% of dose. Amongst group C (9/25) episodes of high CHz > 0.5% of dose occurred during the first weeks of treatment and one developed CHz ca. 100 ng/ml = 1.3% of dose coincidentally with indications of hepatic damage.

Cerebrospinal fluid pyrazinamide concentrations in children with tuberculous meningitis

Cerebrospinal fluid pyrazinamide concentrations were determined by high pressure liquid chromatography in 53 samples from 13 children who had tuberculous meningitis complicated by increased intracranial pressure. Peak concentrations of up to 50 micrograms/ml were achieved between 1 1/2 and 2 1/2 hours after pyrazinamide administration and in most cases a concentration of 20 micrograms/ml or more was achieved. We conclude that pyrazinamide easily gains entry into the cerebrospinal fluid of children with tuberculous meningitis and should be included in treatment regimens for that disease.

Population pharmacokinetics of rifampicin, pyrazinamide and isoniazid in children with tuberculosis: in silico evaluation of currently recommended doses

OBJECTIVES To describe the population pharmacokinetics of rifampicin, pyrazinamide and isoniazid in children and evaluate the adequacy of steady-state exposures. PATIENTS AND METHODS We used previously published data for 76 South African children with tuberculosis to describe the population pharmacokinetics of rifampicin, pyrazinamide and isoniazid. Monte Carlo simulations were used to predict steady-state exposures in children following doses in fixed-dose combination tablets in accordance with the revised guidelines. Reference exposures were derived from an ethnically similar adult population with tuberculosis taking currently recommended doses. RESULTS The final models included allometric scaling of clearance and volume of distribution using body weight. Maturation was included for clearance of isoniazid and clearance and absorption transit time of rifampicin. For a 2-year-old child weighing 12.5 kg, the estimated typical oral clearances of rifampicin and pyrazinamide were 8.15 and 1.08 L/h, respectively. Isoniazid typical oral clearance (adjusted for bioavailability) was predicted to be 4.44, 11.6 and 14.6 L/h for slow, intermediate and fast acetylators, respectively. Higher oral clearance values in intermediate and fast acetylators also resulted from 23% lower bioavailability compared with slow acetylators. CONCLUSIONS Simulations based on our models suggest that with the new WHO dosing guidelines and utilizing available paediatric fixed-dose combinations, children will receive adequate rifampicin exposures when compared with adults, but with a larger degree of variability. However, pyrazinamide and isoniazid exposures in many children will be lower than in adults. Further studies are needed to confirm these findings in children administered the revised dosages and to optimize pragmatic approaches to dosing.