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

Genetic and clinical predictors of rifapentine and isoniazid pharmacokinetics in paediatrics with tuberculosis infection

OBJECTIVES

Twelve weekly doses of rifapentine and isoniazid (3HP regimen) are recommended for TB preventive therapy in children with TB infection. However, they present with variability in the pharmacokinetic profiles. The current study aimed to develop a pharmacokinetic model of rifapentine and isoniazid in 12 children with TB infection using NONMEM.

METHODS

Ninety plasma and 41 urine samples were collected at Week 4 of treatment. Drug concentrations were measured using a validated HPLC-UV method. MassARRAY® SNP genotyping was used to investigate genetic factors, including P-glycoprotein (ABCB1), solute carrier organic anion transporter B1 (SLCO1B1), arylacetamide deacetylase (AADAC) and N-acetyl transferase (NAT2). Clinically relevant covariates were also analysed.

RESULTS

A two-compartment model for isoniazid and a one-compartment model for rifapentine with transit compartment absorption and first-order elimination were the best models for describing plasma and urine data. The estimated (relative standard error, RSE) of isoniazid non-renal clearance was 3.52 L·h-1 (23.1%), 2.91 L·h-1 (19.6%), and 2.58 L·h-1 (20.0%) in NAT2 rapid, intermediate and slow acetylators. A significant proportion of the unchanged isoniazid was cleared renally (2.7 L·h-1; 8.0%), while the unchanged rifapentine was cleared primarily through non-renal routes (0.681 L·h-1; 3.6%). Participants with the ABCB1 mutant allele had lower bioavailability of rifapentine, while food prolonged the mean transit time of isoniazid.

CONCLUSIONS

ABCB1 mutant allele carriers may require higher rifapentine doses; however, this must be confirmed in larger trials. Food did not affect overall exposure to isoniazid and only delayed absorption time.

Efficacy, Safety, and Population Pharmacokinetics of Eltrombopag in Children with Different Severities of Aplastic Anemia

Eltrombopag was approved as a first-line treatment for patients older than 2 years old with severe aplastic anemia (SAA). However, data on eltrombopag in children with different types of aplastic anemia (AA), especially non-severe AA (NSAA), are limited. We performed a prospective, single-arm, and observational study to investigate eltrombopag's efficacy, safety, and pharmacokinetics in children with NSAA, SAA, and very severe AA (VSAA). The efficacy and safety were assessed every 3 months. The population pharmacokinetic (PPK) model was used to depict the pharmacokinetic profile of eltrombopag. Twenty-three AA children with an average age of 7.9 (range of 3.0-14.0) years were enrolled. The response (complete and partial response) rate was 12.5%, 50.0%, and 100.0% after 3, 6, and 12 months in patients with NSAA. For patients with SAA and VSAA, these response rates were 46.7%, 61.5%, and 87.5%. Hepatotoxicity occurred in one patient. Fifty-three blood samples were used to build the PPK model. Body weight was the only covariate for apparent clearance (CL/F) and volume of distribution. The allele-T carrier of adenosine triphosphate-binding cassette transporter G2 was found to increase eltrombopag's clearance. However, when normalized by weight, the clearance between the wild-type and variant showed no statistical difference. In patients with response, children with NSAA exhibited lower area under the curve from time zero to infinity, higher CL/F, and higher weight-adjusted CL/F than those with SAA or VSAA. However, the differences were not statistically significant. The results may support further individualized treatment of eltrombopag in children with AA.

Development of a population pharmacokinetic model of pyrazinamide to guide personalized therapy: impacts of geriatric and diabetes mellitus on clearance

Objectives: This study was performed to develop a population pharmacokinetic model of pyrazinamide for Korean tuberculosis (TB) patients and to explore and identify the influence of demographic and clinical factors, especially geriatric diabetes mellitus (DM), on the pharmacokinetics (PK) of pyrazinamide (PZA). Methods: PZA concentrations at random post-dose points, demographic characteristics, and clinical information were collected in a multicenter prospective TB cohort study from 18 hospitals in Korea. Data obtained from 610 TB patients were divided into training and test datasets at a 4:1 ratio. A population PK model was developed using a nonlinear mixed-effects method. Results: A one-compartment model with allometric scaling for body size effect adequately described the PK of PZA. Geriatric patients with DM (age >70 years) were identified as a significant covariate, increasing the apparent clearance of PZA by 30% (geriatric patients with DM: 5.73 L/h; others: 4.50 L/h), thereby decreasing the area under the concentration-time curve from 0 to 24 h by a similar degree compared with other patients (geriatric patients with DM: 99.87 μg h/mL; others: 132.3 μg h/mL). Our model was externally evaluated using the test set and provided better predictive performance compared with the previously published model. Conclusion: The established population PK model sufficiently described the PK of PZA in Korean TB patients. Our model will be useful in therapeutic drug monitoring to provide dose optimization of PZA, particularly for geriatric patients with DM and TB.

Parametric population pharmacokinetics of isoniazid: a systematic review

INTRODUCTION

Isoniazid (INH) plays an important role in prevention and treatment of tuberculosis (TB). However, large pharmacokinetic (PK) variations are observed in patients receiving standard INH dosages. Considering the influence of PK variations on INH efficacy or adverse reactions, we reviewed the population PK studies of INH and explored significant covariates that influence INH PK.

METHODS

The PubMed and Embase databases were systematically searched from their inception to 30 January 2023. PPK studies on INH using a parametric nonlinear mixed-effect approach were included in this review. The characteristics and identified significant covariates of the included studies were summarized.

RESULTS

Twenty-one studies conducted in adults, and seven in pediatrics were included. A two-compartment model with first-order absorption and elimination was the frequently used structural model for INH. NAT2 genotype, body size, and age were identified as significant covariates affecting INH PK variation. The median clearance (CL) value in the fast metabolizers was 2.55-fold higher than that in the slow metabolizers. Infants and children had higher CL per weight values than adults with the same metabolic phenotype. In pediatric patients, CL value increased with postnatal age.

CONCLUSIONS

Compared with slow metabolizers, the daily dose of INH should be increased by 200-600 mg in fast metabolizers. To achieve effective treatment, pediatric patients need a higher dose per kilogram than adults. Further PPK studies of anti-tuberculosis drugs are needed to comprehensively understand the covariates that affect their PK characteristics and to achieve accurate dose adjustments.

Population Pharmacokinetic Analysis of Rifampicin in Plasma, Cerebrospinal Fluid, and Brain Extracellular Fluid in South African Children with Tuberculous Meningitis

Limited knowledge is available on the pharmacokinetics of rifampicin in children with tuberculous meningitis (TBM) and its penetration into brain tissue, which is the site of infection. In this analysis, we characterize the distribution of rifampicin in cerebrospinal fluid (CSF), lumbar (LCSF) and ventricular (VCSF), and brain extracellular fluid (ECF). Children with TBM were included in this pharmacokinetic analysis. Sparse plasma, LCSF, and VCSF samples were collected opportunistically, as clinically indicated. Brain ECF was sampled using microdialysis (MD). Rifampicin was quantified with liquid chromatography with tandem mass spectrometry in all samples, and 25-desacetyl rifampicin in the plasma samples. The data were interpreted with nonlinear mixed-effects modeling, with the CSF and brain ECF modeled as "effect compartments." Data were available from 61 children, with median (min-max) age of 2 (0.3 to 10) years and weight of 11.0 (4.8 to 49.0) kg. A one-compartment model for parent and metabolite with first-order absorption and elimination via saturable hepatic clearance described the data well. Allometric scaling, maturation, and auto-induction of clearance were included. The pseudopartition coefficient between plasma and LCSF/VCSF was ~5%, while the value for ECF was only ~0.5%, possibly reflecting low recovery of rifampicin using MD. The equilibration half-life between plasma and LCSF/VCSF was ~4 h and between plasma and ECF ~2 h. Our study confirms previous reports showing that rifampicin concentrations in the LCSF are lower than in plasma and provides novel knowledge about rifampicin in the VCSF and the brain tissue. Despite MD being semiquantitative because the relative recovery cannot be quantified, our study presents a proof-of-concept that rifampicin reaches the brain tissue and that MD is an attractive technique to study site-of-disease pharmacokinetics in TBM.

Pharmacokinetics and pharmacodynamics of anti-tuberculosis drugs: An evaluation of in vitro, in vivo methodologies and human studies

There has been an increased interest in pharmacokinetics and pharmacodynamics (PKPD) of anti-tuberculosis drugs. A better understanding of the relationship between drug exposure, antimicrobial kill and acquired drug resistance is essential not only to optimize current treatment regimens but also to design appropriately dosed regimens with new anti-tuberculosis drugs. Although the interest in PKPD has resulted in an increased number of studies, the actual bench-to-bedside translation is somewhat limited. One of the reasons could be differences in methodologies and outcome assessments that makes it difficult to compare the studies. In this paper we summarize most relevant in vitro, in vivo, in silico and human PKPD studies performed to optimize the drug dose and regimens for treatment of tuberculosis. The in vitro assessment focuses on MIC determination, static time-kill kinetics, and dynamic hollow fibre infection models to investigate acquisition of resistance and killing of Mycobacterium tuberculosis populations in various metabolic states. The in vivo assessment focuses on the various animal models, routes of infection, PK at the site of infection, PD read-outs, biomarkers and differences in treatment outcome evaluation (relapse and death). For human PKPD we focus on early bactericidal activity studies and inclusion of PK and therapeutic drug monitoring in clinical trials. Modelling and simulation approaches that are used to evaluate and link the different data types will be discussed. We also describe the concept of different studies, study design, importance of uniform reporting including microbiological and clinical outcome assessments, and modelling approaches. We aim to encourage researchers to consider methods of assessing and reporting PKPD of anti-tuberculosis drugs when designing studies. This will improve appropriate comparison between studies and accelerate the progress in the field.

Influence of NAT2 Genotype and Maturation on Isoniazid Exposure in Low-Birth-Weight and Preterm Infants With or Without Human Immunodeficiency Virus (HIV) Exposure

BACKGROUND

Isoniazid (INH) metabolism depends on the N-acetyl transferase 2 (NAT2) enzyme, whose maturation process remains unknown in low birth weight (LBW) and preterm infants. We aimed to assess INH exposure and safety in infants receiving oral tuberculosis prevention.

METHODS

This population pharmacokinetics (PK) analysis used INH and N-acetyl-isoniazid (ACL) concentrations in infants (BW ≤ 4 kg), including preterm, with follow-up for 6 months. PK parameters were described using nonlinear mixed effects modeling. Simulations were performed to assess INH exposure and optimal dosing regimens, using 2 targets: Cmax at 3-6 mg/L and area under the curve (AUC) ≥ 10.52 mg h/L.

RESULTS

We included 57 infants (79% preterm, 84% LBW) in the PK analysis, with a median (range) gestational age of 34 (28.7-39.4) weeks. At the time of sampling, postnatal age was 2.3 (0.2-7.3) months and weight (WT) was 3.7 (0.9-9.3) kg. NAT2 genotype was available in 43 (75.4%) patients (10 slow, 26 intermediate, and 7 fast metabolizers). Ninety percent of NAT2 maturation was attained by 4.4 post-natal months. WT, postmenstrual age, and NAT2 genotype significantly influenced INH exposure, with a 5-fold difference in AUC between slow and fast metabolizers for the same dose. INH appeared safe across the broad range of exposure for 61 infants included in the safety analysis.

CONCLUSIONS

In LBW/preterm infants, INH dosing needs frequent adjustment to account for growth and maturation. Pharmacogenetics-based dosing regimens is the most powerful approach to deliver safe and equalized exposures for all infants, because NAT2 genotype highly impacts INH pharmacokinetic variability.

Population Pharmacokinetics and Pharmacodynamics of Isoniazid and its Metabolite Acetylisoniazid in Chinese Population

Objective: We aimed to establish a population pharmacokinetic (PPK) model for isoniazid (INH) and its major metabolite Acetylisoniazid (AcINH) in healthy Chinese participants and tuberculosis patients and assess the role of the NAT2 genotype on the transformation of INH to AcINH. We also sought to estimate the INH exposure that would achieve a 90% effective concentration (EC₉₀) efficiency for patients with various NAT2 genotypes.

Method: A total of 45 healthy participants and 157 tuberculosis patients were recruited. For healthy subjects, blood samples were collected 0–14 h after administration of 300 mg or 320 mg of the oral dose of INH; for tuberculosis patients who received at least seven days therapy with INH, blood samples were collected two and/or six hours after administration. The plasma concentration of INH and AcINH was determined by the reverse-phase HPLC method. NAT2 genotypes were determined by allele-specific amplification. The integrated PPK model of INH and AcINH was established through nonlinear mixed-effect modeling (NONMEM). The effect of NAT2 genotype and other covariates on INH and AcINH disposition was evaluated. Monte Carlo simulation was performed for estimating EC90 of INH in patients with various NAT2 genotypes.

Results: The estimated absorption rate constant (K_a), oral clearance (CL/F), and apparent volume of distribution (V₂/F) for INH were 3.94 ± 0.44 h⁻¹, 18.2 ± 2.45 L⋅h⁻¹, and 56.8 ± 5.53 L, respectively. The constant of clearance (K₃₀) and the volume of distribution (V₃/F) of AcINH were 0.33 ± 0.11 h⁻¹ and 25.7 ± 1.30 L, respectively. The fraction of AcINH formation (F_M) was 0.81 ± 0.076. NAT2 genotypes had different effects on the CL/F and F_M. In subjects with only one copy of NAT2 *5, *6, and *7 alleles, the CL/F values were approximately 46.3%, 54.9%, and 74.8% of *4/*4 subjects, respectively. The F_M values were approximately 48.7%, 63.8%, and 86.9% of *4/*4 subjects, respectively. The probability of target attainment of INH EC₉₀ in patients with various NAT2 genotypes was different.

Conclusion: The integrated parent-metabolite PPK model accurately characterized the disposition of INH and AcINH in the Chinese population sampled, which may be useful in the individualized therapy of INH.

Pharmacometrics in tuberculosis: progress and opportunities

Tuberculosis remains one of the leading causes of death by a communicable agent, infecting up to one-quarter of the world's population, predominantly in disadvantaged communities. Pharmacometrics employs quantitative mathematical models to describe the relationships between pharmacokinetics and pharmacodynamics, and to predict drug doses, exposures, and responses. Pharmacometric approaches have provided a scientific basis for improved dosing of antituberculosis drugs and concomitantly administered antiretrovirals at the population level. The development of modelling frameworks including physiologically-based pharmacokinetics, quantitative systems pharmacology and machine learning provides an opportunity to extend the role of pharmacometrics to in silico quantification of drug-drug interactions, prediction of doses for special populations, dose optimization and individualization, and understanding the complex exposure-response relationships of multidrug regimens in terms of both efficacy and safety, informing regimen design for future study. In this short clinically-focused review, we explore what has been done, and what opportunities exist for pharmacometrics to impact tuberculosis pharmacotherapy.

Leveraging physiologically based pharmacokinetic modeling to optimize dosing for lopinavir/ritonavir with rifampin in pediatric patients

STUDY OBJECTIVE

Treatment of HIV and tuberculosis co-infection leads to significant mortality in pediatric patients, and treatment can be challenging due to the clinically significant drug-drug interaction (DDI) between lopinavir/ritonavir (LPV/RTV) and rifampin. Doubling LPV/RTV results in insufficient lopinavir trough concentrations in pediatric patients. The objective of this study was to leverage physiologically based pharmacokinetic (PBPK) modeling to optimize the adjusted doses of LPV/RTV in children receiving the WHO-revised doses of rifampin (15 mg/kg daily).

DESIGN

Adult and pediatric PBPK models for LPV/RTV with rifampin were developed, including CYP3A and P-glycoprotein inhibition and induction.

SETTING (OR DATA SOURCE)

Data for LPV/RTV model development and evaluation were available from the pediatric AIDS Clinical Trials Group.

PATIENTS

Dosing simulations were next performed to optimize dosing in children (2 months to 8 years of age).

INTERVENTION

Exposure following super-boosted LPV/RTV with 10 and 15 mg/kg PO daily rifampin was simulated.

MEASUREMENTS AND MAIN RESULTS

Simulated parameters were within twofold observations for LPV, RTV, and rifampin in adults and children ≥2 weeks old. The model predicted that, in healthy adults receiving 400/100 mg oral LPV/RTV twice daily (BID), co-treatment with 600 mg oral rifampin daily decreased the steady-state area under the concentration vs. time curve of LPV by 79%, in line with the observed change of 75%. Simulated and observed concentration profiles were comparable for LPV/RTV (230/57.5 mg/m² ) PO BID without rifampin and 230/230 mg/m² LPV/RTV PO BID with 10 mg/kg PO daily rifampin in pediatric patients. Sixteen mg/kg of super-boosted LPV (LPV/RTV 1:1) PO BID with 15 mg/kg PO daily rifampin achieved simulated LPV troughs >1 mg/L in ≥93% of virtual children weighing 3.0-24.9 kg, which was comparable with 10 mg/kg PO daily rifampin.

CONCLUSIONS

Super-boosted LPV/RTV with 15 mg/kg rifampin achieves therapeutic LPV troughs in HIV/TB-infected simulated children.

Drug concentration at the site of disease in children with pulmonary tuberculosis

BACKGROUND

Current TB treatment for children is not optimized to provide adequate drug levels in TB lesions. Dose optimization of first-line antituberculosis drugs to increase exposure at the site of disease could facilitate more optimal treatment and future treatment-shortening strategies across the disease spectrum in children with pulmonary TB.

OBJECTIVES

To determine the concentrations of first-line antituberculosis drugs at the site of disease in children with intrathoracic TB.

METHODS

We quantified drug concentrations in tissue samples from 13 children, median age 8.6 months, with complicated forms of pulmonary TB requiring bronchoscopy or transthoracic surgical lymph node decompression in a tertiary hospital in Cape Town, South Africa. Pharmacokinetic models were used to describe drug penetration characteristics and to simulate concentration profiles for bronchoalveolar lavage, homogenized lymph nodes, and cellular and necrotic lymph node lesions.

RESULTS

Isoniazid, rifampicin and pyrazinamide showed lower penetration in most lymph node areas compared with plasma, while ethambutol accumulated in tissue. None of the drugs studied was able to reach target concentration in necrotic lesions.

CONCLUSIONS

Despite similar penetration characteristics compared with adults, low plasma exposures in children led to low site of disease exposures for all drugs except for isoniazid.

Emerging data on rifampicin pharmacokinetics and approaches to optimal dosing in children with tuberculosis

INTRODUCTION

Despite its longstanding role in tuberculosis (TB) treatment, there continues to be emerging rifampicin research that has important implications for pediatric TB treatment and outstanding questions about its pharmacokinetics and optimal dose in children.

AREAS COVERED

This review aims to summarize and discuss emerging data on the use of rifampicin for: 1) routine treatment of drug-susceptible TB; 2) special subpopulations such as children with malnutrition, HIV, or TB meningitis; 3) treatment shortening. We also highlight the implications of these new data for child-friendly rifampicin formulations and identify future research priorities.

EXPERT OPINION

New data consistently show low rifampicin exposures across all pediatric populations with 10-20 mg/kg dosing. Although clinical outcomes in children are generally good, rifampicin dose optimization is needed, especially given a continued push to shorten treatment durations and for specific high-risk populations of children who have worse outcomes. A pooled analysis of existing data using applied pharmacometrics would answer many of the important questions remaining about rifampicin pharmacokinetics needed to optimize doses, especially in special populations. Targeted clinical studies in children with TB meningitis and treatment shortening with high-dose rifampicin are also priorities.

Population Pharmacokinetics Analyses of Rifampicin in Adult and Children Populations: A Systematic Review

AIMS

Rifampicin has become an essential component as the first-line therapy for pulmonary tuberculosis (PTB). Several population pharmacokinetic (PK) studies on rifampicin in the adult and children population have been studied previously. Therefore, the aims of the systematic review were (i) to summarize the relevant published studies and significant covariates that influence the PK of rifampicin across different populations, (ii) to identify any knowledge gap that requires additional research in the future.

METHODS

A total of 121 relevant population PK articles were systematically identified using PubMed and Scopus from inception to October 2021. Review articles, in-vitro, and physiological methods, animal studies, and noncompartmental analysis were excluded.

RESULTS

19 studies which 16 involved adults, two involved children, and one involved both adults and children were included in the review. The structural model of rifampicin can be described as one compartment with a transient compartment absorption model and first-order elimination in most of the studies. Pharmaceutical formulation, body weight, gender, pregnancy status, diabetes, and nutritional supplementation were found to be the significant covariates that affect the PK parameters. External validation of the developed PK model was only conducted in two studies.

CONCLUSIONS

The source of variability for PK parameters of rifampicin remains inconsistent and poorly understood even though there were many potential covariates investigated in the selected studies. Exploring other possible factors and implementation a strict sampling strategy by considering the induction effects might unravel precise and reliable information. Furthermore, external validation should be frequently conducted to produce better predictability of model performance.

Development of population pharmacokinetics model of isoniazid in Indonesian patients with tuberculosis

OBJECTIVES

No population pharmacokinetics (PK) model of isoniazid (INH) has been reported for the Indonesian population with tuberculosis (TB). Therefore, we aimed to develop a population PK model to optimize pharmacotherapy of INH on the basis of therapeutic drug monitoring (TDM) implementation in Indonesian patients with TB.

MATERIALS AND METHODS

INH concentrations, N-acetyltransferase 2 (NAT2) genotypes, and clinical data were collected from Dr. Soetomo General Academic Hospital, Indonesia. A nonlinear mixed-effect model was used to develop and validate the population PK model.

RESULTS

A total of 107 patients with TB (with 153 samples) were involved in this study. A one-compartment model with allometric scaling for bodyweight effect described well the PK of INH. The NAT2 acetylator phenotype significantly affected INH clearance. The mean clearance rates for the rapid, intermediate, and slow NAT2 acetylator phenotypes were 55.9, 37.8, and 17.7 L/h, respectively. Our model was well-validated through visual predictive checks and bootstrapping.

CONCLUSIONS

We established the population PK model for INH in Indonesian patients with TB using the NAT2 acetylator phenotype as a significant covariate. Our Bayesian forecasting model should enable optimization of TB treatment for INH in Indonesian patients with TB.

Evaluation of the estimation and classification performance of NONMEM when applying mixture model for drug clearance

Maximum likelihood estimation of parameters involving mixture model is known to have significant and specific patterns of errors. Population pharmacokinetic (PopPK) modeling using NONMEM is no exception. A few relevant studies on estimation and classification performance were done, but a comprehensive study was not yet available. The current study aims to evaluate performance and likelihood ratio test (LRT)‐based true covariate detection rate when fitting a bimodal mixture of drug clearance (CL) in NONMEM. A large number of PopPK datasets with various settings were simulated and then estimated. The estimates were compared to the simulated values and summarized. The separation between the CL distributions of the two subpopulations is systematically overestimated. The major factor associated with the performance is the change in the minimum objective function value after removing the mixture component (dOFV). Other significant factors include estimated disparity index (DI), estimated mixing proportion, and number of subjects in the dataset. Small dOFV and large estimated DI are associated with the worst performance. Omitting a true mixture resulted in reduced true covariate detection rates. It is recommended that on top of routinely generated standard errors and model diagnostics, dOFV, and other factors when necessary, should be taken into account for the evaluation of performance when fitting mixture model using NONMEM. In addition, when fitting mixture model for CL is intended, the mixture component should be introduced prior to LRT‐based covariate model development for CL.

Therapeutic drug monitoring of intravenous anti-tuberculous therapy: management of an 8-month-old child with tuberculous meningitis

Tuberculous meningitis (TBM) is now uncommon in high-income countries. It is the most severe form of extrapulmonary tuberculosis with high rates of mortality and morbidity if diagnosis and treatment are delayed. An 8-month-old girl with TBM who was treated with high-dose intravenous anti-tuberculous drugs (ATD) is reported. Therapeutic drug monitoring (TDM) of isoniazid and rifampicin was undertaken by measuring serial drug concentrations in serum and cerebrospinal fluid (CSF). There was rapid eradication of Mycobacterium tuberculosis from the CSF with a good clinical outcome and no adverse effects. Using high-dose regimens of intravenous ATD to treat TBM is an important option in order to obtain sufficient CSF diffusion. When available, TDM and a multidisciplinary approach are essential for efficient therapeutic management.

Optimizing dosing and fixed-dose combinations of rifampicin, isoniazid, and pyrazinamide in pediatric patients with tuberculosis: a prospective population pharmacokinetic study

BACKGROUND

In 2010, the WHO revised dosing guidelines for treatment of childhood tuberculosis. Our aim was to investigate first-line antituberculosis drug exposures under these guidelines, explore dose optimization using the current dispersible fixed-dose combination (FDC) table of rifampicin/isoniazid/pyrazinamide; 75/50/150 mg , and suggest a new FDC with revised weight-bands.

METHODS

Children with drug-susceptible tuberculosis in Malawi and South Africa underwent pharmacokinetic sampling while receiving first-line tuberculosis drugs as single formulations according the 2010 WHO recommended doses. Nonlinear mixed-effects modelling and simulation was used to design the optimal FDC and weight-band dosing strategy for achieving the pharmacokinetic targets based on literature-derived adult AUC0-24h for rifampicin (38.7-72.9) isoniazid (11.6-26.3) and pyrazinamide (233-429 mg∙h/L).

RESULTS

180 children (42% female; 13.9% HIV-infected; median [range] age 1.9 [0.22-12] years; weight 10.7 [3.20-28.8] kg) were administered 1, 2, 3, or 4 FDC tablets (rifampicin/isoniazid/pyrazinamide 75/50/150 mg) daily for 4-8, 8-12, 12-16, and 16-25 kg weight-bands, respectively. Rifampicin exposure (for weight and age) was up to 50% lower than in adults. Increasing the tablet number resulted in adequate rifampicin but relatively high isoniazid and pyrazinamide exposures. Administering 1, 2, 3, or 4 optimized FDC tablets (rifampicin/isoniazid/pyrazinamide 120/35/130 mg) to children <6, 6-13, 13-20 and 20-25 kg, and 0.5 tablet in <3-month-olds with immature metabolism, improved exposures to all three drugs.

CONCLUSION

Current pediatric FDC doses resulted in low rifampicin exposures. Optimal dosing of all drugs cannot be achieved with the current FDCs. We propose a new FDC formulation and revised weight-bands.

Pharmacokinetics and safety of high-dose rifampicin in children with TB: the Opti-Rif trial

BACKGROUND

Rifampicin doses of 40 mg/kg in adults are safe and well tolerated, may shorten anti-TB treatment and improve outcomes, but have not been evaluated in children.

OBJECTIVES

To characterize the pharmacokinetics and safety of high rifampicin doses in children with drug-susceptible TB.

PATIENTS AND METHODS

The Opti-Rif trial enrolled dosing cohorts of 20 children aged 0-12 years, with incremental dose escalation with each subsequent cohort, until achievement of target exposures or safety concerns. Cohort 1 opened with a rifampicin dose of 15 mg/kg for 14 days, with a single higher dose (35 mg/kg) on day 15. Pharmacokinetic data from days 14 and 15 were analysed using population modelling and safety data reviewed. Incrementally increased rifampicin doses for the next cohort (days 1-14 and day 15) were simulated from the updated model, up to the dose expected to achieve the target exposure [235 mg/L·h, the geometric mean area under the concentration-time curve from 0 to 24 h (AUC0-24) among adults receiving a 35 mg/kg dose].

RESULTS

Sixty-two children were enrolled in three cohorts. The median age overall was 2.1 years (range = 0.4-11.7). Evaluated doses were ∼35 mg/kg (days 1-14) and ∼50 mg/kg (day 15) for cohort 2 and ∼60 mg/kg (days 1-14) and ∼75 mg/kg (day 15) for cohort 3. Approximately half of participants had an adverse event related to study rifampicin; none was grade 3 or higher. A 65-70 mg/kg rifampicin dose was needed in children to reach the target exposure.

CONCLUSIONS

High rifampicin doses in children achieved target exposures and the doses evaluated were safe over 2 weeks.

Management of childhood and adolescent latent tuberculous infection (LTBI) in Germany, Austria and Switzerland

Background

Majority of active tuberculosis (TB) cases in children in low-incidence countries are due to rapid progression of infection (latent TB infection (LTBI)) to disease. We aimed to assess common practice for managing paediatric LTBI in Austria, Germany and Switzerland prior to the publication of the first joint national guideline for paediatric TB in 2017.

Methods

Online-based survey amongst pediatricians, practitioners and staff working in the public health sector between July and November 2017. Data analysis was conducted using IBM SPSS.

Results

A total of 191 individuals participated in the survey with 173 questionnaires included for final analysis. Twelve percent of respondents were from Austria, 60% from Germany and 28% from Switzerland. Proportion of children with LTBI and migrant background was estimated by the respondents to be >50% by 58%. Tuberculin skin test (TST) and interferon-γ-release-assay (IGRA), particularly Quantiferon-gold-test, were reported to be used in 86% and 88%, respectively. In children > 5 years with a positive TST or IGRA a chest x-ray was commonly reported to be performed (28%). Fifty-three percent reported to take a different diagnostic approach in children ≤ 5 years, mainly combining TST, IGRA and chest x-ray for initial testing (31%). Sixty-eight percent reported to prescribe isoniazid-monotherapy: for 9 (62%), or 6 months (6%), 31% reported to prescribe combination therapy of isoniazid and rifampicin. Dosing of isoniazid and rifampicin below current recommendations was reported by up to 22% of respondents. Blood-sampling before/during LTBI treatment was reported in >90% of respondents, performing a chest-X-ray at the end of treatment by 51%.

Conclusion

This survey showed reported heterogeneity in the management of paediatric LTBI. Thus, regular and easily accessible educational activities and national up-to-date guidelines are key to ensure awareness and quality of care for children and adolescents with LTBI in low-incidence countries.

Determination of Rifampin Concentrations by Urine Colorimetry and Mobile Phone Readout for Personalized Dosing in Tuberculosis Treatment

BACKGROUND

Individual pharmacokinetic variability is a driver of poor tuberculosis (TB) treatment outcomes. We developed a method for measurement of rifampin concentrations by urine colorimetry and a mobile phone photographic application to predict clinically important serum rifampin pharmacokinetic measurements in children treated for TB.

METHODS

Among spiked urine samples, colorimetric assay performance was tested with conventional spectrophotometric and the mobile phone/light box methods under various environmental and biologic conditions. Urine rifampin absorbance (Abs) was then determined from timed specimens from children treated for TB in Tanzania, and compared to serum pharmacokinetic measurements collected throughout the dosing interval.

RESULTS

Both the mobile phone/light box and spectrophotometry demonstrated excellent correlation across a wide range of urine rifampin concentrations (7.8-1000 mg/L) in intra- and interday trials, 24-hour exposure to ambient light or darkness, and varying urinalysis profiles (all r ≥ 0.98). In 12 Tanzanian children, the urine mobile phone/light box measurement and serum peak concentration (Cmax) were significantly correlated (P = .004). Using a Cmax target of 8 mg/L, the area under the receiver operating characteristic curve was 80.1% (range, 47.2%-100%). A urine mobile phone/light box threshold of 50 Abs correctly classified all patients (n = 6) with serum measurements below target.

CONCLUSIONS

The urine colorimetry with mobile phone/light box assay accurately measured rifampin absorbance in varying environmental and biological conditions that may be observed clinically. Among children treated for TB, the assay was sensitive for detection of low rifampin serum concentrations. Future work will identify the optimal timing for urine collection, and operationalize use in TB-endemic settings.

Population Pharmacokinetics and Bayesian Dose Adjustment to Advance TDM of Anti-TB Drugs

Tuberculosis (TB) is still the number one cause of death due to an infectious disease. Pharmacokinetics and pharmacodynamics of anti-TB drugs are key in the optimization of TB treatment and help to prevent slow response to treatment, acquired drug resistance, and adverse drug effects. The aim of this review was to provide an update on the pharmacokinetics and pharmacodynamics of anti-TB drugs and to show how population pharmacokinetics and Bayesian dose adjustment can be used to optimize treatment. We cover aspects on preclinical, clinical, and population pharmacokinetics of different drugs used for drug-susceptible TB and multidrug-resistant TB. Moreover, we include available data to support therapeutic drug monitoring of these drugs and known pharmacokinetic and pharmacodynamic targets that can be used for optimization of therapy. We have identified a wide range of population pharmacokinetic models for first- and second-line drugs used for TB, which included models built on NONMEM, Pmetrics, ADAPT, MWPharm, Monolix, Phoenix, and NPEM2 software. The first population models were built for isoniazid and rifampicin; however, in recent years, more data have emerged for both new anti-TB drugs, but also for defining targets of older anti-TB drugs. Since the introduction of therapeutic drug monitoring for TB over 3 decades ago, further development of therapeutic drug monitoring in TB next steps will again depend on academic and clinical initiatives. We recommend close collaboration between researchers and the World Health Organization to provide important guideline updates regarding therapeutic drug monitoring and pharmacokinetics/pharmacodynamics.

A Model-Informed Method for the Purpose of Precision Dosing of Isoniazid in Pulmonary Tuberculosis

Background and Objective

This study aimed to develop and evaluate a population pharmacokinetic model and limited sampling strategy for isoniazid to be used in model-based therapeutic drug monitoring.

Methods

A population pharmacokinetic model was developed based on isoniazid and acetyl-isoniazid pharmacokinetic data from seven studies with in total 466 patients from three continents. Three limited sampling strategies were tested based on the available sampling times in the dataset and practical considerations. The tested limited sampling strategies sampled at 2, 4, and 6 h, 2 and 4 h, and 2 h after dosing. The model-predicted area under the concentration–time curve from 0 to 24 h (AUC₂₄) and the peak concentration from the limited sampling strategies were compared to predictions using the full pharmacokinetic curve. Bias and precision were assessed using the mean error (ME) and the root mean square error (RMSE), both expressed as a percentage of the mean model-predicted AUC₂₄ or peak concentration on the full pharmacokinetic curve.

Results

Performance of the developed model was acceptable and the uncertainty in parameter estimations was generally low (the highest relative standard error was 39% coefficient of variation). The limited sampling strategy with sampling at 2 and 4 h was determined as most suitable with an ME of 1.1% and RMSE of 23.4% for AUC₂₄ prediction, and ME of 2.7% and RMSE of 23.8% for peak concentration prediction. For the performance of this strategy, it is important that data on both isoniazid and acetyl-isoniazid are used. If only data on isoniazid are available, a limited sampling strategy using 2, 4, and 6 h can be employed with an ME of 1.7% and RMSE of 20.9% for AUC₂₄ prediction, and ME of 1.2% and RMSE of 23.8% for peak concentration prediction.

Conclusions

A model-based therapeutic drug monitoring strategy for personalized dosing of isoniazid using sampling at 2 and 4 h after dosing was successfully developed. Prospective evaluation of this strategy will show how it performs in a clinical therapeutic drug monitoring setting.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40262-020-00971-2.

Population Pharmacokinetic Models of Antituberculosis Drugs in Patients: A Systematic Critical Review

BACKGROUND

Tuberculosis (TB) remains one of the most important infectious diseases. Population pharmacokinetic (pop-PK) models are widely used to individualize dosing regimens of several antibiotics, but their application in anti-TB drug studies is scant. The aim of this study was to provide an insight regarding the status of pop-PK for these drugs and to compare results obtained through both parametric and nonparametric approaches to design precise dosage regimens.

METHODS

First, a systematic approach was implemented, searching in PubMed and Google Scholar. Articles that did not include human patients, that lacked an explicit structural model, that analyzed drugs inactive against M. tuberculosis, or were without full-text access, were excluded. Second, the PK parameters were summarized and categorized as parametric versus nonparametric results. Third, a Monte Carlo simulation was performed in Pmetrics using the results of both groups, and an error term was built to describe the imprecision of each PK modeling approach.

RESULTS

Thirty-three articles reporting at least 1 pop-PK model of 19 anti-TB drug were found; 46 different models including PK parameter estimates and their relevant covariates were also reported. Only 9 models were based on nonparametric approaches. Rifampin was the drug most studied, but only using parametric approaches. The simulations showed that nonparametric approaches improve the error term compared with parametric approaches.

CONCLUSIONS

More and better models, ideally using nonparametric approaches linked with clear pharmacodynamic goals, are required to optimize anti-TB drug dosing, as recommended in the WHO End TB strategy.

Population Pharmacokinetic Properties of Antituberculosis Drugs in Vietnamese Children with Tuberculous Meningitis

Optimal dosing of children with tuberculous meningitis (TBM) remains uncertain and is currently based on the treatment of pulmonary tuberculosis in adults. This study aimed to investigate the population pharmacokinetics of isoniazid, rifampin, pyrazinamide, and ethambutol in Vietnamese children with TBM, to propose optimal dosing in these patients, and to determine the relationship between drug exposure and treatment outcome. A total of 100 Vietnamese children with TBM were treated with an 8-month antituberculosis regimen.

Optimal dosing of children with tuberculous meningitis (TBM) remains uncertain and is currently based on the treatment of pulmonary tuberculosis in adults. This study aimed to investigate the population pharmacokinetics of isoniazid, rifampin, pyrazinamide, and ethambutol in Vietnamese children with TBM, to propose optimal dosing in these patients, and to determine the relationship between drug exposure and treatment outcome. A total of 100 Vietnamese children with TBM were treated with an 8-month antituberculosis regimen. Nonlinear mixed-effects modeling was used to evaluate the pharmacokinetic properties of the four drugs and to simulate different dosing strategies. The pharmacokinetic properties of rifampin and pyrazinamide in plasma were described successfully by one-compartment disposition models, while those of isoniazid and ethambutol in plasma were described by two-compartment disposition models. All drug models included allometric scaling of body weight and enzyme maturation during the first years of life. Cerebrospinal fluid (CSF) penetration of rifampin was relatively poor and increased with increasing protein levels in CSF, a marker of CSF inflammation. Isoniazid and pyrazinamide showed good CSF penetration. Currently recommended doses of isoniazid and pyrazinamide, but not ethambutol and rifampin, were sufficient to achieve target exposures. The ethambutol dose cannot be increased because of ocular toxicity. Simulation results suggested that rifampin dosing at 50 mg/kg of body weight/day would be required to achieve the target exposure. Moreover, low rifampin plasma exposure was associated with an increased risk of neurological disability. Therefore, higher doses of rifampin could be considered, but further studies are needed to establish the safety and efficacy of increased dosing.

A population approach of rifampicin pharmacogenetics and pharmacokinetics in Mexican patients with tuberculosis

The aim of this study was to develop a population pharmacokinetic model of rifampicin (RMP) in Mexican patients with tuberculosis (TB) to evaluate the influence of anthropometric and clinical covariates, as well as genotypic variants associated with MDR1 and OATP1B1 transporters. A prospective study approved by Research Ethics Committee was performed at Hospital Central in San Luis Potosí, Mexico. TB patients under DOTS scheme and who signed informed consent were consecutively included. Anthropometric and clinical information was retrieved from medical records. Single nucleotide polymorphisms in MDR1 (C3435T) and SLCO1B1 (A388G and T521C) genes were evaluated. RMP plasma concentrations and time data were assessed with NONMEM software. A total of 71 Mexican TB patients from 18 to 72 years old were included for RMP quantification from 0.3 to 12 h after dose; 329 and 97 plasma concentrations were available for model development and validation, respectively. Sequential process includes a typical lag time of 0.25 h prior to absorption start with a Ka of 1.24 h^-1 and a zero-order absorption of 0.62 h to characterize the gradual increase in RMP plasma concentrations. Final model includes total body weight in volume of distribution (0.7 L/kg, CV = 26.8%) and a total clearance of 5.96 L/h (CV = 38.5%). Bioavailability was modified according to time under treatment and generic formulation administration. In conclusion, a population pharmacokinetic model was developed to describe the variability in RMP plasma concentrations in Mexican TB patients. Genetic variants evaluated did not showed significant influence on pharmacokinetic parameters. Final model will allow therapeutic drug monitoring at early stages.

Mathematical model and tool to explore shorter multi-drug therapy options for active pulmonary tuberculosis

Standard treatment for active tuberculosis (TB) requires drug treatment with at least four drugs over six months. Shorter-duration therapy would mean less need for strict adherence, and reduced risk of bacterial resistance. A system pharmacology model of TB infection, and drug therapy was developed and used to simulate the outcome of different drug therapy scenarios. The model incorporated human immune response, granuloma lesions, multi-drug antimicrobial chemotherapy, and bacterial resistance. A dynamic population pharmacokinetic/pharmacodynamic (PK/PD) simulation model including rifampin, isoniazid, pyrazinamide, and ethambutol was developed and parameters aligned with previous experimental data. Population therapy outcomes for simulations were found to be generally consistent with summary results from previous clinical trials, for a range of drug dose and duration scenarios. An online tool developed from this model is released as open source software. The TB simulation tool could support analysis of new therapy options, novel drug types, and combinations, incorporating factors such as patient adherence behavior.

A comprehensive in-silico model of pulmonary tuberculosis successfully predicted previous clinical trials and could simulate future therapeutics.

A systematic review and meta-analysis of first-line tuberculosis drug concentrations and treatment outcomes

Low serum concentrations of first-line tuberculosis (TB) drugs have been widely reported. However, the impact of low serum concentrations on treatment outcome is less well studied. A systematic search of MEDLINE/Pubmed and the Cochrane Central Register of Controlled Trials up to 31 March 2018 was conducted for articles describing drug concentrations of first-line TB drugs and treatment outcome in adult patients with drug-susceptible TB. The search identified 3073 unique publication abstracts, which were reviewed for suitability: 21 articles were acceptable for inclusion in the qualitative analysis comprising 13 prospective observational cohorts, 4 retrospective observational cohorts, 1 case-control study and 3 randomised controlled trials. Data for meta-analysis were available for 15 studies, 13 studies of rifampicin (RMP), 10 of isoniazid (INH), 8 of pyrazinamide (PZA) and 4 of ethambutol (EMB). This meta-analysis revealed that low PZA concentration appears to increase the risk of poor outcomes (8 studies, n = 2727; RR 1.73, 95%CI 1.10-2.72), low RMP concentrations may slightly increase the risk of poor outcomes (13 studies, n = 2753; RR 1.40, 95%CI 0.91-2.16), whereas low concentrations of INH (10 studies, n = 2640; RR 1.32, 95%CI 0.66-2.63) and EMB (4 studies, n = 551; RR 1.12, 95%CI 0.41-3.05) appear to make no difference to treatment outcome. There was no significant publication bias or between-study heterogeneity in any of the analyses. The potential clinical impact of low concentrations of PZA and RMP warrants further evaluation. Also, comprehensive assessments of the complex pharmacokinetic-pharmacodynamic relationships in the treatment of TB are urgently needed.

Optimization of a paediatric fixed dose combination mini-tablet and dosing regimen for the first line treatment of tuberculosis

We aim to optimize the paediatric dosing regimen of isoniazid, rifampicin and pyrazinamide for the first-line treatment of tuberculosis, based on a fixed dose combination (FDC) mini-tablet using simulations. An optimization problem was set up to determine the 3 strengths of the drugs of the mini-tablet and 4 cutoff points that define the weight bands of a dosing chart, simultaneously. Using Monte Carlo simulations, first, exposure targets were determined for the 3 drugs, from published population pharmacokinetic models for adults, assuming that the approved doses for adults are de facto efficacious. Then optimal strengths and cutoff points were determined by matching children exposures generated from population pharmacokinetic models to the adults targets. The optimal dosing strengths of the FDC tablet were found to be 95 mg of rifampicin, 200 mg of pyrazinamide and 75 mg of isoniazid, and the 4 body weight bands for 1 to 4 mini-tablets, respectively were: 4 to 8 kg, 8 to 12 kg, 12 to 18 kg and 18 to 28 kg. Children with body weight ≥ 28 kg will be treated with adult dosages. The higher doses proposed were evaluated to be much closer to the adult targets compared to the existing recommended by WHO paediatric doses.

Nonlinear Mixed-Effects Model Development and Simulation Using nlmixr and Related R Open-Source Packages

nlmixr is a free and open-source R package for fitting nonlinear pharmacokinetic (PK), pharmacodynamic (PD), joint PK-PD, and quantitative systems pharmacology mixed-effects models. Currently, nlmixr is capable of fitting both traditional compartmental PK models as well as more complex models implemented using ordinary differential equations. We believe that, over time, it will become a capable, credible alternative to commercial software tools, such as NONMEM, Monolix, and Phoenix NLME.

Alternative dosing guidelines to improve outcomes in childhood tuberculosis: a mathematical modelling study

BACKGROUND

Malnourished and young children are particularly susceptible to severe forms of tuberculosis and poor treatment response. WHO dosing guidelines for drugs for tuberculosis treatment are based only on weight, which might lead to systematic underdosing and poor outcomes in these children. We aimed to assess and quantify the population effect of WHO guidelines for drug-susceptible tuberculosis in children in the 20 countries with the highest disease burden.

METHODS

We used an integrated model that linked country-specific demographic data at the individual level from the 20 countries with the highest disease burden to pharmacokinetic, outcome, and epidemiological models. We estimated tuberculosis treatment outcomes in children younger than 5 years following WHO guidelines (children are dosed by weight bands corresponding to the number of fixed-dose combination tablets [75 mg rifampicin, 50 mg isoniazid, 150 mg pyrazinamide]) and two alternative dosing strategies: one based on a proposed algorithm that uses age, weight, and available formulations, in which underweight children would receive the same drug doses as would normal weight children of the same age; and another based on an individualised algorithm without dose limitations, in which derived doses results in target exposure attainment for the typical child.

FINDINGS

We estimated that 57 234 (43%) of 133 302 children younger than 5 years who were treated for tuberculosis in 2017 were underdosed with WHO dosing and only 47% of children would reach the rifampicin exposure target. Underdosing and subtherapeutic exposures were more common among malnourished children than among age-matched healthy children. The proposed dosing approach improved estimated rifampicin target exposure attainment to 62% and equalised outcomes by nutritional status. An estimated third of unfavourable treatment outcomes might be resolved with this dosing strategy, saving the lives of a minimum of 2423 children in these countries annually. With individualised dosing approaches, almost all children could achieve adequate exposure for cure.

INTERPRETATION

This work shows that a simple change in dosing procedure to include age and nutritional status, requiring no additional measurements or new drug formulations, is one approach to improve tuberculosis treatment outcomes in children, especially malnourished children who are at high risk of mortality.

FUNDING

Eunice Kennedy Shriver National Institute of Child Health and Human Development and UK Medical Research Council.

Optimization of dosing regimens of isoniazid and rifampicin in children with tuberculosis in India

AIMS

Pharmacokinetic studies in the past have shown inadequate antituberculosis drug levels in children with the currently available dosing regimens. This study attempted to investigate the pharmacokinetics of isoniazid and rifampicin, when used in children, and to optimize their dosing regimens.

METHODS

Data were collected from 41 children, aged 2-16 years, who were being treated with antituberculosis drugs for at least 2 months. Concentration measurements were done for 6 h and analysed using a nonlinear, mixed-effects model.

RESULTS

Isoniazid pharmacokinetics were described by a one-compartment disposition model with a transit absorption model (fixed, n = 5). A mixture model was used to identify the slow and fast acetylator subgroups. Rifampicin was described by a one-compartment disposition model with a transit absorption model (fixed, n = 9). Body weight was added to the clearance and volume of distribution of both the drugs using an allometric function. Simulations with the isoniazid model showed that 84.9% of the population achieved therapeutic peak serum concentration with the planned fixed-dose combination regimen. Simulations with the rifampicin model showed that only about 28.8% of the simulated population achieve the therapeutic peak serum concentration with the fixed-dose combination regimen. A novel regimen for rifampicin, with an average dose of 35 mg kg^-1 , was found to provide adequate drug exposure in most children.

CONCLUSIONS

The exposure to isoniazid is adequate with present regimens. For rifampicin, a novel dosing regimen was developed to ensure adequate drug concentrations in children. However, further studies are required to assess the dose-effect relationship of higher doses of rifampicin.

Current research toward optimizing dosing of first-line antituberculosis treatment

Introduction: Drug concentrations in tuberculosis patients on standard regimens vary widely with clinically important consequences.

Areas covered: We review the available literature identifying factors correlated with pharmacokinetic variability of antituberculosis drugs. Based on population pharmacokinetic models and the weight, height, and sex distributions in a large data base of African tuberculosis patients, we propose simplified weight-based doses of the available fixed dose combination(FDC) for adults with drug susceptible tuberculosis. Emerging studies will support optimized weight-based dosing for children. Other sources of important pharmacokinetic variability include genetic variants, drug-drug interactions, formulation quality, and methods of preparation and administration.

Expert commentary: Optimized weight band-based dosing will result in more equitable distribution of drug exposures by weight. The use of high doses of isoniazid in patients with drug-resistant tuberculosis would be safer and more effective if a feasible test was developed to allow stratified dosing according to acetylator type. There is an urgent need for more suitable formulations of many second-line drugs for children. The adoption of new technologies and efficient FDC design may allow further advances for patients and treatment programs. Lastly, current efforts to ensure adequate quality of antituberculosis drug products are not preventing the use of substandard products to treat patients with tuberculosis.

Population Pharmacokinetics of the Antimalarial Amodiaquine: a Pooled Analysis To Optimize Dosing

Amodiaquine plus artesunate is the recommended antimalarial treatment in many countries where malaria is endemic. However, pediatric doses are largely based on a linear extrapolation from adult doses.

Amodiaquine plus artesunate is the recommended antimalarial treatment in many countries where malaria is endemic. However, pediatric doses are largely based on a linear extrapolation from adult doses. We pooled data from previously published studies on the pharmacokinetics of amodiaquine, to optimize the dose across all age groups. Adults and children with uncomplicated malaria received daily weight-based doses of amodiaquine or artesunate-amodiaquine over 3 days. Plasma concentration-time profiles for both the parent drug and the metabolite were characterized using nonlinear mixed-effects modeling. Amodiaquine pharmacokinetics were adequately described by a two-compartment disposition model, with first-order elimination leading to the formation of desethylamodiaquine, which was best described by a three-compartment disposition model. Body size and age were the main covariates affecting amodiaquine clearance. After adjusting for the effect of weight, clearance rates for amodiaquine and desethylamodiaquine reached 50% of adult maturation at 2.8 months (95% confidence interval [CI], 1.5 to 3.7 months) and 3.9 months (95% CI, 2.6 to 5.3 months) after birth, assuming that the baby was born at term. Bioavailability was 22.4% (95% CI, 15.6 to 31.9%) lower at the start of treatment than during convalescence, which suggests a malaria disease effect. Neither the drug formulation nor the hemoglobin concentration had an effect on any pharmacokinetic parameters. Results from simulations showed that current manufacturer dosing recommendations resulted in low desethylamodiaquine exposure in patients weighing 8 kg, 15 to 17 kg, 33 to 35 kg, and >62 kg compared to that in a typical 50-kg patient. We propose possible optimized dosing regimens to achieve similar drug exposures among all age groups, which require further validation.

Extrapolation of praziquantel pharmacokinetics to a pediatric population: a cautionary tale

L-praziquantel (PZQ) pharmacokinetic data were analyzed from two relative bioavailability Phase 1 studies in adult, healthy subjects with two new oral dispersion tablet (ODT) formulations of L-PZQ administered under various combinations of co-administration with food, water, and/or crushing. Linear mixed effects models adequately characterized the noncompartmental estimates of the pharmacokinetic profiles in both studies. Dose, food, and formulation were found to significantly affect L-PZQ exposure in both studies. The model for AUC was then extrapolated to children 2–5 years old accounting for enzyme maturation and weight. The predicted exposures were compared to an external Phase 1 study conducted by the Swiss Tropical and Public Health Institute using a currently marketed formulation (Cesol 600 mg immediate-release tablets) and found to be substantially lower than observed. A root cause analysis was completed to identify the reason for failure of the models. Various scenarios were proposed and tested. Two possible reasons for the failure were identified. One reason was that the model did not account for the reduced hepatic clearance seen in patients compared to the healthy volunteer population used to build the model. The second possible reason was that PZQ absorption appears sensitive to meal composition and the model did not account for differences in meals between a standardized Phase 1 unit and clinical sites in Africa. Further studies are needed to confirm our hypotheses.

Evaluation of the Adequacy of WHO Revised Dosages of the First-Line Antituberculosis Drugs in Children with Tuberculosis Using Population Pharmacokinetic Modeling and Simulations

Optimal doses for antituberculosis (anti-TB) drugs in children have yet to be established. In 2010, the World Health Organization (WHO) recommended revised dosages of the first-line anti-TB drugs for children. Pharmacokinetic (PK) studies that investigated the adequacy of the WHO revised dosages to date have yielded conflicting results. We performed population PK modeling using data from one of these studies to identify optimal dosage ranges. Ghanaian children with tuberculosis on recommended therapy with rifampin (RIF), isoniazid (INH), pyrazinamide (PZA), and ethambutol (EMB) for at least 4 weeks had blood samples collected predose and at 1, 2, 4, and 8 hours postdose. Drug concentrations were determined by validated liquid chromatography-mass spectrometry methods. Nonlinear mixed-effects models were applied to describe the population PK of those drugs using MonolixSuite2016R1 (Lixoft, France). Bayesian estimation was performed, the correlation coefficient, bias, and precision between the observed and predicted areas under the concentration-time curve (AUCs) were calculated, and Bland-Altman plots were analyzed. The population PK of RIF and PZA was described by a one-compartment model and that for INH and EMB by a two-compartment model. Plasma maximum concentration (C_max) and AUC targets were based on published results for children from India. The lowest target values for pediatric TB patients were attainable at the WHO-recommended dosage schedule for RIF and INH, except for N-acetyltransferase 2 non-slow acetylators (rapid and intermediate acetylators) in the lower-weight bands. However, higher published adult targets were not attainable for RIF and INH. The targets were not achieved for PZA and EMB. (This study has been registered at ClinicalTrials.gov under identifier NCT01687504.).

Evidence-Based Design of Fixed-Dose Combinations: Principles and Application to Pediatric Anti-Tuberculosis Therapy

BACKGROUND AND OBJECTIVES

Fixed-dose combination formulations where several drugs are included in one tablet are important for the implementation of many long-term multidrug therapies. The selection of optimal dose ratios and tablet content of a fixed-dose combination and the design of individualized dosing regimens is a complex task, requiring multiple simultaneous considerations.

METHODS

In this work, a methodology for the rational design of a fixed-dose combination was developed and applied to the case of a three-drug pediatric anti-tuberculosis formulation individualized on body weight. The optimization methodology synthesizes information about the intended use population, the pharmacokinetic properties of the drugs, therapeutic targets, and practical constraints. A utility function is included to penalize deviations from the targets; a sequential estimation procedure was developed for stable estimation of break-points for individualized dosing. The suggested optimized pediatric anti-tuberculosis fixed-dose combination was compared with the recently launched World Health Organization-endorsed formulation.

RESULTS

The optimized fixed-dose combination included 15, 36, and 16% higher amounts of rifampicin, isoniazid, and pyrazinamide, respectively. The optimized fixed-dose combination is expected to result in overall less deviation from the therapeutic targets based on adult exposure and substantially fewer children with underexposure (below half the target).

CONCLUSION

The development of this design tool can aid the implementation of evidence-based formulations, integrating available knowledge and practical considerations, to optimize drug exposures and thereby treatment outcomes.

Suboptimal Antituberculosis Drug Concentrations and Outcomes in Small and HIV-Coinfected Children in India: Recommendations for Dose Modifications

This work aimed to evaluate the once‐daily antituberculosis treatment as recommended by the new Indian pediatric guidelines. Isoniazid, rifampin, and pyrazinamide concentration–time profiles and treatment outcome were obtained from 161 Indian children with drug‐sensitive tuberculosis undergoing thrice‐weekly dosing as per previous Indian pediatric guidelines. The exposure–response relationships were established using a population pharmacokinetic‐pharmacodynamic approach. Rifampin exposure was identified as the unique predictor of treatment outcome. Consequently, children with low body weight (4–7 kg) and/or HIV infection, who displayed the lowest rifampin exposure, were associated with the highest probability of unfavorable treatment (therapy failure, death) outcome (P_unfavorable). Model‐based simulation of optimized (P_unfavorable ≤ 5%) rifampin once‐daily doses were suggested per treatment weight band and HIV coinfection status (33% and 190% dose increase, respectively, from the new Indian guidelines). The established dose‐exposure–response relationship could be pivotal in the development of future pediatric tuberculosis treatment guidelines.

Partnerships to Design Novel Regimens to Treat Childhood Tuberculosis, Sui Generis: The Road Ahead

There has been a recent expansion of preclinical models to predict the efficacy of regimens to treat adults with tuberculosis. Despite increasing global interest in childhood tuberculosis, these same tools have not been employed to develop pediatric regimens. Children differ from adults in bacillary burden, spectrum of disease, the metabolism and distribution of antituberculosis drugs, and the toxicity experienced. The studies documented in this series describe a proof-of-concept approach to pediatric regimen development. We propose a program of investigation that would take this forward into a systematic and comprehensive method to find optimal drug combinations to use in children, ideal exposures, and required dosing. Although the number of possible drug combinations is extensive, a series of principles could be employed to select likely effective regimens. Regimens should avoid drugs with overlapping toxicity or linked mechanisms of resistance and should aim to include drugs with different mechanisms of action and ones that are able to target different subpopulations of mycobacteria. Finally drugs should penetrate into body sites necessary for treating pediatric disease. At an early stage, this body of work would need to engage with regulatory agencies and bodies that formulate guidelines, so that once regimens and dosages are identified, translation into clinical studies and clinical practice can be rapid. The development of child-friendly drug formulations would need to be carried out in parallel so that pharmacokinetic studies can be undertaken as formulations are created. Significant research and development would be required and a wide range of stakeholders would need to be engaged. The time is right to consider a more thoughtful and systematic approach toward identifying, testing, and comparing combinations of drugs for children with tuberculosis.

Pretreatment With Rifampicin and Tyrosine Kinase Inhibitor Dasatinib Potentiates the Inhibitory Effects Toward OATP1B1- and OATP1B3-Mediated Transport

Present studies determined the effects of pretreatment with rifampicin, an organic anion-transporting polypeptide (OATP) inhibitor, and the tyrosine kinase inhibitor dasatinib on OATP1B1- and OATP1B3-mediated transport, and evaluated the OATP-mediated drug-drug interaction potential of dasatinib using the static R-value and dynamic physiologically based pharmacokinetic models. Rifampicin and dasatinib pretreatment significantly decreased OATP1B1- and OATP1B3-mediated transport. Rifampicin pretreatment also significantly decreased [³H]-pitavastatin and [³H]-CCK-8 accumulation in human sandwich-cultured hepatocytes. Present studies revealed that estrone-3-sulfate is a less-sensitive OATP1B1 substrate than estradiol-17β-glucuronide in assessing rifampicin pretreatment effects. Pretreatment with rifampicin and dasatinib reduced the inhibition constant (K_i) values against OATP1B1 by 3 and 2.1 fold and against OATP1B3 by 2.4 and 2.1 fold, respectively. The in vitro rifampicin K_i values after preincubation are comparable to the estimated in vivo K_i reported previously. Models predict that dasatinib has a low potential to cause OATP1B1- and OATP1B3-mediated drug-drug interactions. Time-lapse confocal microscopy demonstrated that rifampicin and dasatinib pretreatment did not affect plasma membrane localization of green-fluorescent protein-tagged OATP1B1 (GFP-OATP1B1) and GFP-OATP1B3 in human embryonic kidney 293 stable cell lines. In summary, we report novel findings that pretreatment with rifampicin and dasatinib potentiates the inhibitory effects toward OATP1B1 and OATP1B3 without affecting plasma membrane levels of the transporters.

The epidemiology, pathogenesis, transmission, diagnosis, and management of multidrug-resistant, extensively drug-resistant, and incurable tuberculosis

Global tuberculosis incidence has declined marginally over the past decade, and tuberculosis remains out of control in several parts of the world including Africa and Asia. Although tuberculosis control has been effective in some regions of the world, these gains are threatened by the increasing burden of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis. XDR tuberculosis has evolved in several tuberculosis-endemic countries to drug-incurable or programmatically incurable tuberculosis (totally drug-resistant tuberculosis). This poses several challenges similar to those encountered in the pre-chemotherapy era, including the inability to cure tuberculosis, high mortality, and the need for alternative methods to prevent disease transmission. This phenomenon mirrors the worldwide increase in antimicrobial resistance and the emergence of other MDR pathogens, such as malaria, HIV, and Gram-negative bacteria. MDR and XDR tuberculosis are associated with high morbidity and substantial mortality, are a threat to health-care workers, prohibitively expensive to treat, and are therefore a serious public health problem. In this Commission, we examine several aspects of drug-resistant tuberculosis. The traditional view that acquired resistance to antituberculous drugs is driven by poor compliance and programmatic failure is now being questioned, and several lines of evidence suggest that alternative mechanisms-including pharmacokinetic variability, induction of efflux pumps that transport the drug out of cells, and suboptimal drug penetration into tuberculosis lesions-are likely crucial to the pathogenesis of drug-resistant tuberculosis. These factors have implications for the design of new interventions, drug delivery and dosing mechanisms, and public health policy. We discuss epidemiology and transmission dynamics, including new insights into the fundamental biology of transmission, and we review the utility of newer diagnostic tools, including molecular tests and next-generation whole-genome sequencing, and their potential for clinical effectiveness. Relevant research priorities are highlighted, including optimal medical and surgical management, the role of newer and repurposed drugs (including bedaquiline, delamanid, and linezolid), pharmacokinetic and pharmacodynamic considerations, preventive strategies (such as prophylaxis in MDR and XDR contacts), palliative and patient-orientated care aspects, and medicolegal and ethical issues.

Official American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America Clinical Practice Guidelines: Treatment of Drug-Susceptible Tuberculosis

The American Thoracic Society, Centers for Disease Control and Prevention, and Infectious Diseases Society of America jointly sponsored the development of this guideline for the treatment of drug-susceptible tuberculosis, which is also endorsed by the European Respiratory Society and the US National Tuberculosis Controllers Association. Representatives from the American Academy of Pediatrics, the Canadian Thoracic Society, the International Union Against Tuberculosis and Lung Disease, and the World Health Organization also participated in the development of the guideline. This guideline provides recommendations on the clinical and public health management of tuberculosis in children and adults in settings in which mycobacterial cultures, molecular and phenotypic drug susceptibility tests, and radiographic studies, among other diagnostic tools, are available on a routine basis. For all recommendations, literature reviews were performed, followed by discussion by an expert committee according to the Grading of Recommendations, Assessment, Development and Evaluation methodology. Given the public health implications of prompt diagnosis and effective management of tuberculosis, empiric multidrug treatment is initiated in almost all situations in which active tuberculosis is suspected. Additional characteristics such as presence of comorbidities, severity of disease, and response to treatment influence management decisions. Specific recommendations on the use of case management strategies (including directly observed therapy), regimen and dosing selection in adults and children (daily vs intermittent), treatment of tuberculosis in the presence of HIV infection (duration of tuberculosis treatment and timing of initiation of antiretroviral therapy), as well as treatment of extrapulmonary disease (central nervous system, pericardial among other sites) are provided. The development of more potent and better-tolerated drug regimens, optimization of drug exposure for the component drugs, optimal management of tuberculosis in special populations, identification of accurate biomarkers of treatment effect, and the assessment of new strategies for implementing regimens in the field remain key priority areas for research. See the full-text online version of the document for detailed discussion of the management of tuberculosis and recommendations for practice.

Validation and Application of a Dried Blood Spot Assay for Biofilm-Active Antibiotics Commonly Used for Treatment of Prosthetic Implant Infections

Dried blood spot (DBS) antibiotic assays can facilitate pharmacokinetic (PK)/pharmacodynamic (PD) studies in situations where venous blood sampling is logistically difficult. We sought to develop, validate, and apply a DBS assay for rifampin (RIF), fusidic acid (FUS), and ciprofloxacin (CIP). These antibiotics are considered active against organisms in biofilms and are therefore commonly used for the treatment of infections associated with prosthetic implants. A liquid chromatography-mass spectroscopy DBS assay was developed and validated, including red cell partitioning and thermal stability for each drug and the rifampin metabolite desacetyl rifampin (Des-RIF). Plasma and DBS concentrations in 10 healthy adults were compared, and the concentration-time profiles were incorporated into population PK models. The limits of quantification for RIF, Des-RIF, CIP, and FUS in DBS were 15 μg/liter, 14 μg/liter, 25 μg/liter, and 153 μg/liter, respectively. Adjusting for hematocrit, red cell partitioning, and relative recovery, DBS-predicted plasma concentrations were comparable to measured plasma concentrations for each antibiotic (r > 0.95; P < 0.0001), and Bland-Altman plots showed no significant bias. The final population PK estimates of clearance, volume of distribution, and time above threshold MICs for measured and DBS-predicted plasma concentrations were comparable. These drugs were stable in DBSs for at least 10 days at room temperature and 1 month at 4°C. The present DBS antibiotic assays are robust and can be used as surrogates for plasma concentrations to provide valid PK and PK/PD data in a variety of clinical situations, including therapeutic drug monitoring or studies of implant infections.

A simultaneous population pharmacokinetic analysis of rifampicin in Malawian adults and children

AIMS

Low rifampicin plasma concentrations can lead to treatment failure and increased risk of developing drug resistant tuberculosis. The objectives of this study were to characterize the population pharmacokinetics (popPK) of rifampicin in Malawian children and adults with tuberculosis, simulate exposures under revised WHO dosing guidelines that aim to reduce the risk of low exposures of rifampicin and examine predicted exposures using weight- and age-based dosing bands under new dosing recommendations.

METHODS

Patients were recruited at least two weeks after initiation of the intensive phase of treatment and received RIF in FDC of anti-TB drugs. A total of 5-6 rich and 1-2 sparse samples were collected. nonmem (v7.2) was used to build a population-PK model.

RESULTS

A 165 TB patients, 115 adults and 50 children, aged 7 months to 65 years and weighing 4.8 to 87 kg, were included in the one compartment model with first order absorption best described the data. The mean population estimate for CL/F was 23.9 (l h(-1)  70 kg(-1) ) with inter-individual variability of 46.6%. Exposure was unaffected by HIV status. Relative bioavailability in children was estimated at 49% lower compared to adults (100% relative bioavailability). Simulations showed significantly lower rifampicin exposure in children vs. adults. In children average AUC was 13.5 mg l(-1) h, which was nearly half that was observed in adults (26.3 mg l(-1)  h). Using age as a surrogate for weight in dosing bands gave similar results compared with the weight bands. Increasing dose to approximately 15 mg kg(-1) , increased AUC in children to an average of 22 mgl(-1)  h. bringing expected exposures in children closer to those predicted for adults.

CONCLUSION

The popPK model developed can be used to optimize rifampicin exposures through dosing simulations. WHO dosing recommendations may not be achieved using currently licensed fixed dose combination formulations of TB therapy.

Pediatric tuberculous meningitis: Model-based approach to determining optimal doses of the anti-tuberculosis drugs rifampin and levofloxacin for children

Pediatric tuberculous meningitis (TBM) is a highly morbid, often fatal disease. Standard treatment includes isoniazid, rifampin, pyrazinamide, and ethambutol. Current rifampin dosing achieves low cerebrospinal fluid (CSF) concentrations, and CSF penetration of ethambutol is poor. In adult trials, higher-dose rifampin and/or a fluoroquinolone reduced mortality and disability. To estimate optimal dosing of rifampin and levofloxacin for children, we compiled plasma and CSF pharmacokinetic (PK) and outcomes data from adult TBM trials plus plasma PK data from children. A population PK/pharmacodynamic (PD) model using adult data defined rifampin target exposures (plasma area under the curve (AUC)0-24 = 92 mg*h/L). Levofloxacin targets and rifampin pediatric drug disposition information were literature-derived. To attain target rifampin exposures, children require daily doses of at least 30 mg/kg orally or 15 mg/kg intravenously (i.v.). From our pediatric population PK model, oral levofloxacin doses needed to attain exposure targets were 19-33 mg/kg. Our results provide data-driven guidance to maximize pediatric TBM treatment while we await definitive trial results.

A Critical Review of the Current Evidence for Measuring Drug Concentrations of First-Line Agents Used to Treat Tuberculosis in Children

Tuberculosis is a leading cause of infectious disease-related morbidity and mortality worldwide. Additionally, treatment is complex with most patients requiring combination therapy of first-line agents for multiple months. Children are especially at risk from the medications used to treat tuberculosis and therefore interventions to optimize both efficacy and safety are needed. Protocols exist for therapeutic drug monitoring in tuberculosis patients yet there is a gap in knowledge regarding the extent of any benefits achieved, especially in children. This review aims to summarize and evaluate literature reporting outcomes related to the measurement of drug concentrations of first-line agents used to treat tuberculosis (rifampin, isoniazid, pyrazinamide, ethambutol) in children. Findings showed a lack of strong evidence to support therapeutic drug monitoring in children with tuberculosis. Standard weight-based dosing of first-line agents does not commonly achieve target concentrations yet the effect on clinical outcomes remains unclear. As such, therapeutic drug monitoring should not be recommended currently as a widespread practice for all children with tuberculosis. However, future research should assess any benefit in special populations such as those with relapsing or recurrent disease, or those presenting with adverse drug reactions.

Pharmacokinetics of isoniazid, rifampicin, pyrazinamide and ethambutol in Indian children

BACKGROUND

The available pharmacokinetic data on anti-tubercular drugs in children raises the concern of suboptimal plasma concentrations attained when doses extrapolated from adult studies are used. Also, there is lack of consensus regarding the effect of malnutrition on pharmacokinetics of anti-tubercular drugs in children. We conducted this study with the aims of determining the plasma concentrations of isoniazid, rifampicin, pyrazinamide and ethambutol achieved with different dosage of the anti-tubercular drugs so as to provide supportive evidence to the revised dosages and to evaluate the effects of malnutrition on the pharmacokinetics of these drugs in children. We also attempted to correlate the plasma concentrations of these drugs with clinical outcome of therapy.

METHOD

Prospective drug estimation study was conducted in two groups of children, age 6 months to 15 years, with tuberculosis, with or without severe malnutrition, receiving different dosage of daily anti- tubercular therapy. The dosage (range) of isoniazid was 5 (4-6) and 10 (7-15) mg/kg in the two groups, respectively, that of rifampicin-10 (8-12) and 15 (10-12) mg/kg, respectively, both the groups received same dose of pyrazinamide (30-35 mg/kg) and ethambutol (20-25 mg/kg). All four drugs were simultaneously estimated by liquid chromatography-mass spectrometry (LC-MS/MS).

RESULTS AND CONCLUSION

The median (IQR) Cmax of isoniazid increased significantly from 0.6 (0.3,1.2) μg/mL to 3.4 (1.8, 5.0) μg/mL with increase in the dose. Plasma rifampicin concentrations increased only marginally on increasing the dose [median (IQR) Cmax: 10.4 (7.2, 13.9) μg/mL vs. 12.0 (6.1, 24.3) μg/mL, p=0.08]. For ethambutol, 55.9% of the children had inadequate 2-hour concentrations. Two-hour plasma concentrations of at least one drug were low in 59 (92.2%) and 54 (85.7%) children in the two dosing regimen, respectively. We did not observe any effect of malnutrition on pharmacokinetic parameters of the drugs studied. We did not observe an association between low plasma drug concentrations and poor outcome. We may have to be cautious while increasing the doses and strive to asses other factors influencing the drug concentrations and treatment outcomes in children.