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.

Isoniazid pharmacokinetics, pharmacodynamics, and dosing in South African infants.

AIMS: There are limited data on isoniazid (INH) pharmacokinetics in infants and young children and, therefore, uncertainty on appropriate dosing. METHODS: Pharmacokinetic data were obtained from perinatally HIV-exposed South African infants aged 3-24 months receiving INH 10-20 mg·kg·d orally for Mycobacterium tuberculosis prophylaxis. INH pharmacokinetic parameters were characterized using a population pharmacokinetic approach. Dosing simulations were performed to evaluate weight-based INH doses in children based on N-acetyltransferase 2 enzyme (NAT2) genotype, age, maximum concentrations (Cmax) ≥3 mg/L, and area under the curve (AUC0-24) ≥10.52 mg·h/L. RESULTS: In 151 infants (53% female, 48% HIV positive) receiving a mean INH dose of 14.5 mg·kg·d, mean (±SD) Cmax at 3, 6, and 23 months of age were 10.0 (3.5), 8.6 (2.6), and 9.3 (3.8) mg/L, respectively, mean (±SD) AUC0-24 were 53.6 (26.8), 42 (19.9), and 44 (30.7) mg·h/L, respectively, and mean (±SD) half-lives were 2.1 (0.7), 1.9 (0.6), and 1.8 (0.9) hours, respectively. A trimodal apparent oral clearance of INH as a function of the NAT2 genotype was apparent as early as 3 months. INH was well tolerated. At an average INH dose of 14.5 mg·kg·d, 99% of infants aged 3-24 months have an INH Cmax ≥3 mg/L, and 98% have an INH AUC0-24 ≥10.52 mg·h/L. CONCLUSIONS: INH at an average dose of 14.5 mg/kg once daily was well tolerated in infants and achieved INH Cmax values ≥3 mg/L and AUC0-24 values ≥10.52 mg·h/L.

Comparative diffusion of drugs through bronchial tissue.

The purpose of the study was to investigate the molecular diffusion of drugs across porcine bronchial tissue. Using an in vitro flow-through diffusion system, a series of model compounds were tested. These included theophylline, caffeine, theobromine, enprofylline, salbutamol, ipratropium bromide, and trimethoprim. All drugs were assayed by HPLC in conjunction with UV/vis or MS/MS detection. The results indicated that the mean flux value of theophylline was higher than that of all the other drugs listed above. Within the log10P range from -2.21 (ipratropium bromide) to 1.364 (trimethoprim), a sigmoidal relationship was found to exist between the apparent permeability coefficients (Papp) and the octanol/water partition coefficients across the bronchial tissue. The diffusion of ipratropium bromide (Papp 1.6 x 10(-8)cm/s) across bronchial tissue was similar to that of salbutamol (Papp 1.5 x 10(-8)cm/s). The data obtained in this study indicate that although lipophilicity is a main determinant in the diffusion of drug compounds across bronchial tissue, the number and position of alkyl groups also reflect the ability of the latter to cross membrane barriers.

In vitro transcorneal penetration of metronidazole and its potential use as adjunct therapy in Acanthamoeba keratitis.

PURPOSE: To investigate the in vitro permeation of metronidazole through rabbit and human corneas in the presence and absence of 0.01% benzalkonium chloride and to suggest its use as adjunct therapy in the treatment of Acanthamoeba keratitis. METHODS: Metronidazole permeation through rabbit and human corneas, the latter being unsuitable for transplantation, were used for all permeability experiments. Flux rates for metronidazole from 0.5% and 1.0% aqueous solutions in the presence and absence of 0.01% benzalkonium chloride (BZCL) were determined. ANOVA and the Duncan multiple range test were used to test for steady state and an unpaired t test with the Welch correction was used to test for differences between the mean flux values at each time point. A significance level of 5% was used for all the statistical tests. In the clinical cases described, 0.5% aqueous solution was used. RESULTS: Steady-state flux rates for metronidazole from 0.5% and 1.0% solutions across both rabbit and human corneas were achieved after 6 and 4 hours, respectively. No statistically significant differences were obtained in the presence and absence of 0.01% BZCl (P < 0.05) between the steady state flux values at both concentrations of metronidazole of human and rabbit corneas, except for 0.5% metronidazole across rabbit corneas. Flux rates of metronidazole across human corneas were 12%-33% higher than those across rabbit corneas. For both rabbit and human corneas, flux rates of metronidazole from 0.5% and 1.0% solutions were reduced by between 4% and 11%, respectively, in the presence of 0.01% benzalkonium chloride. CONCLUSIONS: Although statistically significant differences in flux values were obtained between human and rabbit corneas, the study supports the suitability of the in vitro rabbit cornea as a model for investigating permeation of drugs through human corneas. However, direct extrapolation of animal data to humans must be approached cautiously. The metronidazole from a 1% solution had a steady-state flux rate approximately double that from the 0.5% solution. Higher concentrations of up to 1% may be considered for clinical use for treating Acanthamoeba keratitis infections. It would appear to be prudent to omit benzalkonium chloride as a preservative from preparations of metronidazole formulated for topical ophthalmologic use. Early clinical experience with the topical solution as adjunct therapy in the treatment of Acanthamoeba keratitis is encouraging.

Diffusion of metronidazole released from aqueous solution and a gel through human and rabbit corneas.

Metronidazole has been suggested as an adjunct in the treatment of Acanthamoeba keratitis. This study involves the permeation of metronidazole from topically applied solutions and gels through human and rabbit corneas. The permeation of metronidazole, from the solutions and gels through the corneas, was determined using a flow-through diffusion apparatus. Human (32) and rabbit (54) corneas were obtained from an eye bank and animal nursery farm, respectively. Experiments were conducted at 20 degrees C and over a time period of 24 hours. High-performance liquid chromatography (HPLC) analysis was used as a detection method. Statistical tests used included an Analysis of Variance (ANOVA) and Duncan's multiple range test, for a determination of the steady state, and a nonlinear regression analysis, using an F test, was used to compare entire curves. Flux rates for 0.5% and 1% metronidazole solutions across human corneas were found to be 14% and 30% higher, respectively, than for rabbit corneas. However, the flux rates of the 0.5% and 0.8% metronidazole gels across rabbit corneas were found to be 7% and 25% higher than those for human corneas. The flux rates measured for at least the first 10 hours were not, statistically, significantly different between the metronidazole released from aqueous solutions and from the gel. Steady-state flux rates of the 0.5% metronidazole aqueous and gel formulations were approximately half those of the 0.8% and 1% preparations. Because of the longer corneal contact times, it may be therapeutically advantageous to use 0.8% metronidazole concentrations in gels for treating Acanthamoeba keratitis infestations. Further in vivo studies on the ocular compatibility of these formulations need to be performed. The use of rabbit corneas as an in vitro permeability model for human corneas is supported by these studies.

Acquired drug resistance during inadequate therapy in a young child with tuberculosis.

Drug resistance in children with tuberculosis is usually primary (transmitted); however, resistance acquisition during treatment is possible. We describe a child with tuberculosis who acquired drug resistance while receiving directly observed but inadequate first-line therapy and the programmatic and clinical factors that may have contributed to resistance acquisition.

The pharmacogenetics of NAT2 enzyme maturation in perinatally HIV exposed infants receiving isoniazid.

The roles of the NAT2 genotype and enzyme maturation on isoniazid pharmacokinetics were investigated in South African infants with perinatal HIV exposure enrolled in a randomized, double-blind, controlled trial of isoniazid for prevention of tuberculosis disease and latent infection. Plasma concentration-time measurements of isoniazid from 151 infants (starting at 3-4 months of age) receiving isoniazid 10 to 20 mg/kg/d orally during the course of the 24-month study were incorporated in a population analysis along with NAT2 genotype, body weight, age, and sex. The results showed a different NAT2 enzyme maturation profile for each of the 3 acetylation groups, with the 70-kg body weight-normalized typical apparent clearance for the fast and intermediate acetylators increasing from 14.25 L/h and 10.88 L/h at 3 months of age to 22.84 L/h and 15.58 L/h at 24 months of age, respectively, with no significant change in the apparent clearance of the slow group during this period. A hypothesis is proposed to explain the genotype-dependent enzyme maturation processes for the NAT2 enzyme.

In vitro transcorneal diffusion of the antimicrobial macrolides azithromycin and clarithromycin and the impact on microbial keratitis.

PURPOSE: To compare the in vitro penetration of azithromycin and clarithromycin across both human and rabbit corneas. METHODS: In vitro diffusion studies were performed on frozen-thawed (liquid nitrogen, -80 degrees C) corneas using a flow-through diffusion apparatus (24 hours, 20 degrees C, 1.5 mL/h). Either phosphate-buffered saline (PBS) or 2 mg/g polyacrylic acid (Teargel) formulations of clarithromycin and azithromycin (1 or 4 mg/mL) were used in the donor chambers. Effluent samples collected (2 hours) were analyzed using binary high-performance liquid chromatography in conjunction with either UV/VIS or tandem mass spectrometry detection. RESULTS: The flux values of azithromycin (PBS and polyacrylic acid) across both types of corneas showed concentration dependence. Polyacrylic acid formulations showed a decrease in flux values of azithromycin across both types of corneas. For clarithromycin, flux values across both types of corneas were approximately 2.3-2.4 times higher than azithromycin. The flux values of azithromycin at 4 mg/mL (PBS and polyacrylic acid) across human cornea were higher than those across rabbit cornea, whereas the inverse was true at 1 mg/mL PBS for both drugs. CONCLUSIONS: Both macrolides penetrated the corneas, and the flux values were found to be concentration dependent (azithromycin). Clarithromycin had a higher diffusion rate across corneas than azithromycin. Although the human cornea had a higher permeability to azithromycin at a higher concentration, the inverse was found at lower concentrations for both drugs. Rabbit cornea can be used in general as an in vitro permeability model for human cornea; however, care must be taken with the extrapolation of results.