Pharmacokinetic Modeling, Simulation, and Development of a Limited Sampling Strategy of Cycloserine in Patients with Multidrug-/Extensively Drug-Resistant Tuberculosis

Optimizing dosing of the cycloserine pro-drug terizidone in children with rifampicin-resistant tuberculosis

There are no pharmacokinetic data in children on terizidone, a pro-drug of cycloserine and a World Health Organization (WHO)-recommended group B drug for rifampicin-resistant tuberculosis (RR-TB) treatment. We collected pharmacokinetic data in children <15 years routinely receiving 15-20 mg/kg of daily terizidone for RR-TB treatment. We developed a population pharmacokinetic model of cycloserine assuming a 2-to-1 molecular ratio between terizidone and cycloserine. We included 107 children with median (interquartile range) age and weight of 3.33 (1.55, 5.07) years and 13.0 (10.1, 17.0) kg, respectively. The pharmacokinetics of cycloserine was described with a one-compartment model with first-order elimination and parallel transit compartment absorption. Allometric scaling using fat-free mass best accounted for the effect of body size, and clearance displayed maturation with age. The clearance in a typical 13 kg child was estimated at 0.474 L/h. The mean absorption transit time when capsules were opened and administered as powder was significantly faster compared to when capsules were swallowed whole (10.1 vs 72.6 min) but with no effect on bioavailability. Lower bioavailability (-16%) was observed in children with weight-for-age z-score below -2. Compared to adults given 500 mg daily terizidone, 2022 WHO-recommended pediatric doses result in lower exposures in weight bands 3-10 kg and 36-46 kg. We developed a population pharmacokinetic model in children for cycloserine dosed as terizidone and characterized the effects of body size, age, formulation manipulation, and underweight-for-age. With current terizidone dosing, pediatric cycloserine exposures are lower than adult values for several weight groups. New optimized dosing is suggested for prospective evaluation.

Limited sampling strategies for therapeutic drug monitoring of anti-tuberculosis medications: A systematic review of their feasibility and clinical utility

Therapeutic drug monitoring (TDM) is recommended for medications with high inter-individual variability, narrow therapeutic index drugs, possible drug-drug interactions, drug toxicity, and subtherapeutic concentrations, as well as to assess noncompliance. The area under the plasma concentration-time curve (AUC) is a significant pharmacokinetic parameter since it calculates the drug's total systematic exposure in the body. However, multiple blood samples from the patient are required to calculate the area under the curve, which is inconvenient for both the patient and the healthcare professional. To alleviate the issue, the limited sampling strategy (LSS) was devised, in which sampling is minimized while obtaining complete and precise findings to anticipate the area under the curve. One can reduce costs, labor, and discomfort for patients and healthcare workers by applying this limited sampling strategy. This article examines a systematic evaluation of all the limited sampling done in anti-tuberculosis (anti-TB) medications resulting from the literature search of several research papers. This article also briefly describes the two methodologies: Multiple regression analysis (MRA) and the Bayesian approach used to develop a limited sampling strategy model. Anti-TB medications have been found to have considerable inter-individual variability, and isoniazid has a narrow therapeutic index, both of which are criteria for therapeutic drug monitoring. To avoid multi-drug resistance and therapy failure, it is proposed that limited sampling strategy-based therapeutic drug monitoring of anti-TB medications be undertaken to generate an individualized dose regimen, particularly for individuals at high risk of treatment failure or delayed response.

Population Pharmacokinetics and Dose Evaluation of Cycloserine among Patients with Multidrug-Resistant Tuberculosis under Standardized Treatment Regimens

Although cycloserine is a recommended drug for the treatment of multidrug-resistant tuberculosis (MDR-TB) according to World Health Organization (WHO), few studies have reported on pharmacokinetics (PK) and/or pharmacodynamics (PD) data of cycloserine in patients with standardized MDR-TB treatment. This study aimed to estimate the population PK parameters for cycloserine and to identify clinically relevant PK/PD thresholds, as well as to evaluate the current recommended dosage. Data from a large cohort with full PK curves was used to develop a population PK model. This model was used to estimate drug exposure in patients with MDR-TB from a multicentre prospective study in China. The classification and regression tree was used to identify the clinically relevant PK/PD thresholds. Probability of target attainment was analyzed to evaluate the currently recommended dosing strategy. Cycloserine was best described by a two-compartment disposition model. A percentage of time concentration above MICs (T>MIC) of 30% and a ratio of area under drug concentration-time curve (AUC0-24h) over MIC of 36 were the valid predictors for 6-month sputum culture conversion and final treatment outcome. Simulations showed that with WHO-recommended doses (500 mg and 750 mg for patients weighing <45 kg and ≥45 kg), the probability of target attainment exceeded 90% at MIC ≤16 mg/L in MGIT for both T>MIC of 30% and AUC0-24h/MIC of 36. New clinically relevant PK/PD thresholds for cycloserine were identified in patients with standardized MDR-TB treatment. WHO-recommended doses were considered adequate for the MGIT MIC distribution in our cohort of Chinese patients with MDR-TB.

Infant Exposure to Antituberculosis Drugs via Breast Milk and Assessment of Potential Adverse Effects in Breastfed Infants: Critical Review of Data

Infants of mothers treated for tuberculosis might be exposed to drugs via breast milk. The existing information on the exposure of breastfed infants lacks a critical review of the published data. We aimed to evaluate the quality of the existing data on antituberculosis (anti-TB) drug concentrations in the plasma and milk as a methodologically sound basis for the potential risk of breastfeeding under therapy. We performed a systematic search in PubMed for bedaquiline, clofazimine, cycloserine/terizidone, levofloxacin, linezolid, pretomanid/pa824, pyrazinamide, streptomycin, ethambutol, rifampicin and isoniazid, supplemented with update references found in LactMed^®. We calculated the external infant exposure (EID) for each drug and compared it with the recommended WHO dose for infants (relative external infant dose) and assessed their potential to elicit adverse effects in the breastfed infant. Breast milk concentration data were mainly not satisfactory to properly estimate the EID. Most of the studies suffer from limitations in the sample collection, quantity, timing and study design. Infant plasma concentrations are extremely scarce and very little data exist documenting the clinical outcome in exposed infants. Concerns for potential adverse effects in breastfed infants could be ruled out for bedaquiline, cycloserine/terizidone, linezolid and pyrazinamide. Adequate studies should be performed covering the scenario in treated mothers, breast milk and infants.

The Correlations of Minimal Inhibitory Concentration Values of Anti-TB Drugs with Treatment Outcomes and Clinical Profiles in Patients with Multidrug-Resistant Tuberculosis (MDR-TB) in China

Objective

It is a challenge to obtain satisfactory treatment outcomes for patients with multidrug-resistant/rifampicin-resistant tuberculosis (MDR/RR-TB); the study aims to correlate the Minimum Inhibitory Concentration (MIC) value of drugs with the outcome of patients with MDR/RR-TB to obtain an understanding for better regimens and optimal outcomes.

Methods

The patients diagnosed with MDR/RR-TB were retrospectively enrolled from January 1, 2018 to December 31, 2019, recorded clinical characteristics, MIC DST (Drug Susceptibility Test) results, and followed the treatment outcome. The data were analyzed on the correlations of MIC DST values with outcomes and clinical characteristics.

Results

A total of 276 patients with MDR/RR-TB were included, containing 98 cases (35.5%) with newly treated patients and 178 cases (64.5%) with re-treated patients. A total of 220 cases recorded treatment success (79.7%) and 49 cases recorded treatment failure or died. MIC values of isoniazid (H), moxifloxacin (Mfx), and ethionamide (Eto) in newly treated patients were lower than those in retreated patients, and resistance levels of Mfx and H were closely associated with the treatment outcome (P < 0.05) while those of other drugs had no close association with treatment outcome.

Conclusions

MIC values of some anti-TB drugs, such as fluoroquinolones (FQs) and H, can reflect the treatment outcome for patients with MDR/RR-TB, which can contribute to making regimens for better treatment outcomes.

Bone Penetration of Cycloserine in Osteoarticular Tuberculosis Patients of China

The cycloserine concentrations in plasma and bone that were collected during operations on 28 osteoarticular tuberculosis (TB) patients treated daily with a 500-mg cycloserine-containing regimen were determined. The median concentrations in plasma and bone were 16.29 μg/mL (interquartile range [IQR], 6.47 μg/mL) and 24.33 μg/g (IQR, 14.68 μg/g), respectively. The median bone/plasma penetration ratio was 0.76 (range, 0.33 to 1.98). Cycloserine could effectively penetrate bone and acquire concentrations comparable to those in plasma, which favors its usage in osteoarticular TB treatment.

Cycloserine did not increase depression incidence or severity at standard dosing for multidrug-resistant tuberculosis

In 2018 cycloserine was elevated to World Health Organization (WHO) group B status for multidrug-resistant tuberculosis (MDR-TB), and is recommended in longer MDR-TB treatment regimens [1]. Inclusion of cycloserine is associated with improved MDR-TB treatment success and reduced mortality, but is limited by treatment-associated depression, psychosis and neuropathy, forcing 9% of patients to stop therapy [1–3]. Cycloserine also demonstrates wide interindividual pharmacokinetic variation, with significant food and drug interactions, leaving nearly half of patients with inappropriate drug levels [4, 5]. Optimal dosing is unknown [6], but modelling studies suggest doses from 250 mg to 750 mg twice daily, with 500 mg twice daily for paucibacillary disease and 750 mg twice daily for cavitary pulmonary disease [7]. Therefore, clinicians must balance the known benefits of cycloserine with the dearth of susceptibility- and drug-monitoring capacity and the spectre of treatment-limiting side-effects. To evaluate the impact of cycloserine prescription and dose on incident depression during MDR-TB treatment, we analysed longitudinal cohort data from India.

In a longitudinal cohort of MDR-TB patients receiving individualised, DST-based treatment, neither the inclusion of cycloserine in a multidrug regimen nor the dose used (up to 750 mg daily) significantly increased incidence of depression during treatment https://bit.ly/3GtQmOH

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.

Optimization of Busulfan Dosing Regimen in Pediatric Patients Using a Population Pharmacokinetic Model Incorporating GST Mutations

Purpose

The aim of this study was to develop a novel busulfan dosing regimen, based on a population pharmacokinetic (PPK) model in Chinese children, and to achieve better area under the concentration-time curve (AUC) targeting.

Patients and Methods

We collected busulfan concentration-time samples from 69 children who received intravenous busulfan prior to allogeneic hematopoietic stem cell transplantation (allo-HSCT). A population pharmacokinetic model for busulfan was developed by nonlinear mixed effect modelling and was validated by an external dataset (n=14). A novel busulfan dosing regimen was developed through simulated patients, and has been verified on real patients. Limited sampling strategy (LSS) was established by Bayesian forecasting. Mean absolute prediction error (MAPE) and relative root mean Squared error (rRMSE) were calculated to evaluate predictive accuracy.

Results

A one-compartment model with first-order elimination best described the data. GSTA1 genotypes, body surface area (BSA) and aspartate aminotransferase (AST) were found to be significant covariates of Bu clearance, and BSA had significant impact of the volume of distribution. Moreover, two equations were obtained for recommended dose regimens: dose (mg)=34.14×BSA (m²)+3.75 (for GSTA1 *A/*A), Dose (mg)=30.99×BSA (m²)+3.21 (for GSTA1 *A/*B). We also presented a piecewise dosage based on BSA categories for each GSTA1 mutation. A two-point LSS, two hours and four hours after dosing, behaved well with acceptable prediction precision (rRMSE=1.026%, MAPE=6.55%).

Conclusion

We recommend a GSTA1-BSA and BSA-based dosing (Q6 h) based on a PPK model for personalizing busulfan therapy in pediatric population. Additionally, an optimal LSS (C_2h and C_4h) provides convenience for therapeutic drug monitoring (TDM) in the future.

Therapeutic drug monitoring in patients with tuberculosis and concurrent medical problems

INTRODUCTION

Therapeutic drug monitoring (TDM) has been recommended for treatment optimization in tuberculosis (TB) but is only is used in certain countries e.g. USA, Germany, the Netherlands, Sweden and Tanzania. Recently, new drugs have emerged and PK studies in TB are continuing, which contributes further evidence for TDM in TB. The aim of this review is to provide an update on drugs used in TB, treatment strategies for these drugs, and TDM to support broader implementation.

AREAS COVERED

This review describes the different drug classes used for TB, multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB), along with their pharmacokinetics, dosing strategies, TDM and sampling strategies. Moreover, the review discusses TDM for patient TB and renal or liver impairment, patients co-infected with HIV or hepatitis, and special patient populations - children and pregnant women.

EXPERT OPINION

TB treatment has a long history of using 'one size fits all.' This has contributed to treatment failures, treatment relapses, and the selection of drug-resistant isolates. While challenging in resource-limited circumstances, TDM offers the clinician the opportunity to individualize and optimize treatment early in treatment. This approach may help to refine treatment and thereby reduce adverse effects and poor treatment outcomes. Funding, training, and randomized controlled trials are needed to advance the use of TDM for patients with TB.

Population Pharmacokinetics and Limited Sampling Strategy for Therapeutic Drug Monitoring of Polymyxin B in Chinese Patients With Multidrug-Resistant Gram-Negative Bacterial Infections

Polymyxin B is used as a last therapeutic option for the treatment of multidrug-resistant Gram-negative bacterial infections. This study aimed to develop a population pharmacokinetic model and limited sampling strategy, a method to estimate the area under the concentration curve (AUC) by using a limited number of samples, to assist therapeutic drug monitoring of polymyxin B in Chinese patients. Population pharmacokinetic analysis was performed using Phoenix^® NLME with data obtained from 46 adult patients at steady state. Various demographic variables were investigated as potential covariates for population pharmacokinetic modeling. The limited sampling strategies based on the Bayesian approach and multiple linear regression were validated using the intraclass correlation coefficient and Bland-Altman analysis. As a result, the data was described by a two-compartment population pharmacokinetic model. Through the modeling, creatinine clearance was found to be a statistically significant covariate influencing polymyxin B clearance. The limited sampling strategies showed the two-point model (C_0h and C_2h) could predict polymyxin B exposure with good linear relativity (r² > 0.98), and the four-point model (C_1h, C1_.5h, C_4h, and C_8h) performed best in predicting polymyxin B AUC (r² > 0.99). In conclusion, this study successfully developed a population pharmacokinetic model and limited sampling strategies that could be applied in clinical practice to assist in therapeutic drug monitoring of polymyxin B in Chinese patients.