Ethambutol optimal clinical dose and susceptibility breakpoint identification by use of a novel pharmacokinetic-pharmacodynamic model of disseminated intracellular Mycobacterium avium

Ceftriaxone Efficacy for Mycobacterium avium Complex Lung Disease in the Hollow Fiber and Translation to Sustained Sputum Culture Conversion in Patients

BACKGROUND

Only 35.6%-50.8% of patients with Mycobacterium avium complex (MAC) pulmonary disease achieve sustained sputum culture conversion (SSCC) on treatment with the azithromycin-ethambutol-rifabutin standard of care (SOC). We tested the efficacy of ceftriaxone, a β-lactam with a lung-to-serum penetration ratio of 12.18-fold.

METHODS

We mimicked lung concentration-time profiles of 7 ceftriaxone once-daily doses for 28 days in the hollow fiber system model of intracellular MAC (HFS-MAC). Monte Carlo experiments were used for dose selection. We also compared once-daily ceftriaxone monotherapy to 3-drug SOC against 5 MAC clinical isolates in HFS-MAC using γ (kill) slopes, and translated to SSCC rates.

RESULTS

Ceftriaxone killed 1.02-3.82 log10 colony-forming units (CFU)/mL, at optimal dose of 2 g once-daily. Ceftriaxone killed all 5 strains below day 0 versus 2 of 5 for SOC. The median γ (95% confidence interval [CI]) was 0.49 (.47-.52) log10 CFU/mL/day for ceftriaxone and 0.38 (.34-.43) log10 CFU/mL/day for SOC. In patients, the SOC was predicted to achieve SSCC rates (CI) of 39.3% (36%-42%) at 6 months. The SOC SSCC was 50% at 8.18 (3.64-27.66) months versus 3.58 (2.20-7.23) months for ceftriaxone, shortening time to SSCC 2.35-fold.

CONCLUSIONS

Ceftriaxone is a promising agent for creation of short-course chemotherapy.

Sarecycline pharmacokinetics/pharmacodynamics in the hollow-fibre model of Mycobacterium avium complex: so near and yet so far

BACKGROUND

Poor sustained sputum culture conversion rates with the standard-of-care therapy highlight the need for better drugs to treat Mycobacterium avium complex pulmonary disease (MAC-PD).

OBJECTIVE

To determine the pharmacokinetics/pharmacodynamics (PK/PD)-optimized exposure of sarecycline and its potential role in treating MAC-PD.

METHODS

We performed MIC studies with MAC ATCC 700898 and 19 clinical isolates and test-tube static concentration-response studies. A dynamic hollow-fibre system model of intracellular MAC (HFS-MAC) study was performed mimicking six human-equivalent sarecycline dose concentration-time profiles to identify the PK/PD optimal exposure of sarecycline for MAC kill. The inhibitory sigmoid maximal effect (Emax) model was used for PK/PD analysis.

RESULTS

The sarecycline MIC of MAC ATCC 700898 was 1 mg/L, while the MIC for the 19 clinical strains ranged between 32 and >256 mg/L. The concentration mediating 50% of Emax (EC50) was similar between intracellular and extracellular MAC. In the HFS-MAC, all six sarecycline doses killed intracellular MAC, with an Emax of 1.0 log10 cfu/mL below Day 0 burden (stasis). The sarecycline EC80 (optimal) exposure was identified as AUC0-24/MIC = 139.46.

CONCLUSIONS

Sarecycline demonstrated anti-MAC Emax in the HFS-MAC model better than ethambutol but worse than omadacycline (>5 log10 cfu/mL below stasis) in HFS-MAC. However, since currently approved highest oral sarecycline dose achieves an AUC0-24 of 48.2 mg·h/L and MAC MICs are >32 mg/L, the target AUC0-24/MIC of 139.46 is unlikely to be achieved in patients.

Correlation of drug exposure and bacterial susceptibility with treatment response for Mycobacterium avium complex lung disease: protocol for a prospective observational cohort study

INTRODUCTION

The burden of Mycobacterium avium complex (MAC) lung disease is increasing globally and treatment outcome is in general poor. Therapeutic drug monitoring has the potential to improve treatment outcome by ensuring adequate drug exposure. However, very limited population-based studies exist for MAC lung disease. This study aims to describe the distribution of drug exposure for key antimycobacterial drugs at population level, and to analyse them in relationship to treatment outcome in patients with MAC lung disease.

METHODS AND ANALYSIS

A prospective cohort aiming to include 100 adult patients diagnosed with and treated for MAC lung disease will be conducted in Shanghai Pulmonary Hospital, China. Blood samples will be collected after 1 month MAC treatment for measurement of macrolides, rifamycin, ethambutol, amikacin and/or fluoroquinolones, using a validated liquid-chromatography tandem mass spectrometry method. Respiratory samples will be collected at inclusion and once every 3 months for mycobacterial culture until treatment completion. Minimum inhibitory concentration (MIC) determination will be performed using a commercial broth microdilution plate. In addition to mycobacterial culture, disease severity and clinical improvement will be assessed from the perspective of lung function, radiological presentation and self-reported quality of life. Whole genome sequencing will be performed for any longitudinal isolates with significant change of MIC to explore the emergence of drug resistance-conferring mutations. The relationship between drug exposure and treatment outcome will be analysed and potential confounders will be considered for adjustment in multivariable models. Meanwhile, the associations between drug exposure in relation to MIC and markers of treatment response will be explored using Cox proportional hazards or binary logistic regression models, as appropriate.

ETHICS AND DISSEMINATION

This study has been approved by the ethics committee of Shanghai Pulmonary Hospital (No. K22-149Z). Written and oral informed consent will be obtained from all participants. The study results will be submitted to a peer-reviewed journal.

TRIAL REGISTERATION NUMBER

NCT05824988.

Structural Determinants of Indole-2-carboxamides: Identification of Lead Acetamides with Pan Antimycobacterial Activity

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M.tb), is one of the leading causes of death in developing countries. Non-tuberculous mycobacteria (NTM) infections are rising and prey upon patients with structural lung diseases such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis. All mycobacterial infections require lengthy treatment regimens with undesirable side effects. Therefore, new antimycobacterial compounds with novel mechanisms of action are urgently needed. Published indole-2-carboxamides (IC) with suggested inhibition of the essential transporter MmpL3 showed good potency against whole-cell M.tb, yet had poor aqueous solubility. This project focused on retaining the required MmpL3 inhibitory pharmacophore and increasing the molecular heteroatom percentage by reducing lipophilic atoms. We evaluated pyrrole, mandelic acid, imidazole, and acetamide functional groups coupled to lipophilic head groups, where lead acetamide-based compounds maintained high potency against mycobacterial pathogens, had improved in vitro ADME profiles over their indole-2-carboxamide analogs, were non-cytotoxic, and were determined to be MmpL3 inhibitors.

Towards Improved Management of Tuberculous Bloodstream Infections: Pharmacokinetic Considerations with Suggestions for Better Treatment Outcomes

Mycobacterium tuberculosis is the leading cause of sepsis among HIV-infected adults, yet effective treatment remains a challenge. Efficacy of antituberculous drugs is optimized by high Area Under Curve to Minimum Inhibitory Concentration (AUC/MIC) ratios, suggesting that both the drug concentration at the disease site and time above MIC are critical to treatment outcomes. We elaborate on sepsis pathophysiology and show how it adversely affects antituberculous drug kinetics. Expanding distribution volumes secondary to an increased vascular permeability prevents the attainment of target Cmax concentrations for nearly all drugs. Furthermore, sepsis-induced metabolic acidosis promotes protonation, which increases renal clearance of basic drugs such as isoniazid and ethambutol, and hence AUCs are substantially reduced. Compared with the treatment of non-sepsis TB disease, these distorted kinetics underlie the poor treatment outcomes observed with bloodstream infections. In addition to aggressive hemodynamic management, an increase in both the dose and frequency of drug administration are warranted, at least in the early phase of treatment.

OUP accepted manuscript

Objectives

The standard of care (SOC) for the treatment of pulmonary Mycobacterium avium complex (MAC) disease (clarithromycin, rifabutin, and ethambutol) achieves sustained sputum conversion rates of only 54%. Thus, new treatments should be prioritized.

Methods

We identified the omadacycline MIC against one laboratory MAC strain and calculated drug half life in solution, which we compared with measured MAC doubling times. Next, we performed an omadacycline hollow fibre system model of intracellular MAC (HFS-MAC) exposure–effect study, as well as the three-drug SOC, using pharmacokinetics achieved in patient lung lesions. Data was analysed using bacterial kill slopes (γ-slopes) and inhibitory sigmoid E_max bacterial burden versus exposure analyses. Monte Carlo experiments (MCE) were used to identify the optimal omadacycline clinical dose.

Results

Omadacycline concentration declined in solution with a half-life of 27.7 h versus a MAC doubling time of 16.3 h, leading to artefactually high MICs. Exposures mediating 80% of maximal effect changed up to 8-fold depending on sampling day with bacterial burden versus exposure analyses, while γ-slope-based analyses gave a single robust estimate. The highest omadacycline monotherapy γ-slope was −0.114 (95% CI: −0.141 to −0.087) (r^2 =0.98) versus −0.114 (95% CI: −0.133 to −0.094) (r²= 0.99) with the SOC. MCEs demonstrated that 450 mg of omadacycline given orally on the first 2 days followed by 300 mg daily would achieve the AUC_0-24 target of 39.67 mg·h/L.

Conclusions

Omadacycline may be a potential treatment option for pulmonary MAC, possibly as a back-bone treatment for a new MAC regimen and warrants future study in treatment of this disease.

Clinical Pharmacokinetic and Pharmacodynamic Considerations in the Drug Treatment of Non-Tuberculous Mycobacteria in Cystic Fibrosis

Non-tuberculous mycobacteria (NTM) are an emerging group of pulmonary infectious pathogens of increasing importance to the management of patients with cystic fibrosis (CF). NTM include slow-growing mycobacteria such as Mycobacterium avium complex (MAC) and rapidly growing mycobacteria such as Mycobacterium abscessus. The incidence of NTM in the CF population is increasing and infection contributes to significant morbidity to the patient and costs to the health system. Treating M. abscessus requires the combination of multiple costly antibiotics for months, with potentially significant toxicity associated with treatment. Although international guidelines for the treatment of NTM infection in CF are available, there are a lack of robust pharmacokinetic studies in CF patients to inform dosing and drug choice. This paper aims to outline the pharmacokinetic and pharmacodynamic factors informing the optimal treatment of NTM infections in CF.

Comparison of Rifamycins for Efficacy Against Mycobacterium avium Complex and Resistance Emergence in the Hollow Fiber Model System

Rifamycins are integral part of the combination regimen for treatment of pulmonary Mycobacterium avium-complex [MAC] infection, but different practitioners prefer different rifamycins. The objective of the study was to compare microbial kill and resistance emergence of rifamycins using principles of pharmacokinetics/pharmacodynamics. First, we identified rifamycin MICs in 20 MAC isolates from patients followed by concentration-response studies in test-tubes. Next, we examined efficacy and resistance suppression of three doses of each rifamycin in the hollow fiber system model of pulmonary MAC [HFS-MAC], mimicking human like concentration-time profile of the drugs. HFS-MAC units were repetitively sampled for total and drug-resistant MAC burden and for drug concentration measurements. Inhibitory sigmoid E max model, linear regression, and analysis of variance was used for data analysis. For rifabutin 90% of isolates had MIC ≤ 0.125 mg/L while for both rifampin and rifapentine this was ≤2.0 mg/L. There was no statistically significant difference (p > 0.05) in maximal kill and effective concentration mediating 50% of the bacterial kill among three rifamycins in the static concentration experiment. In the HFS-MAC, the bactericidal kill (day 0-4) for rifampin was 0.89 (95% Confidence Interval (CI): 0.43-1.35), for rifapentine was 1.05 (95% CI: 0.08-1.23), and for rifabutin was 0.92 (95% CI: 0.61-1.24) log10 CFU/ml, respectively. Rifamycins monotherapy failed after just 4-days of treatment and entire MAC population was drug resistant on day 26 of the study. There was no dose dependent difference in MAC kill or resistance suppression among the three rifamycins tested in the HFS-MAC. Therefore, replacing one rifamycin, due to emergence of drug-resistance, with other may not be beneficial in clinical setting.

Therapeutic Drug Monitoring in Non-Tuberculosis Mycobacteria Infections

Nontuberculous mycobacteria can cause minimally symptomatic self-limiting infections to progressive and life-threatening disease of multiple organs. Several factors such as increased testing and prevalence have made this an emerging infectious disease. Multiple guidelines have been published to guide therapy, which remains difficult owing to the complexity of therapy, the potential for acquired resistance, the toxicity of treatment, and a high treatment failure rate. Given the long duration of therapy, complex multi-drug treatment regimens, and the risk of drug toxicity, therapeutic drug monitoring is an excellent method to optimize treatment. However, currently, there is little available guidance on therapeutic drug monitoring for this condition. The aim of this review is to provide information on the pharmacokinetic/pharmacodynamic targets for individual drugs used in the treatment of nontuberculous mycobacteria disease. Lacking data from randomized controlled trials, in vitro, in vivo, and clinical data were aggregated to facilitate recommendations for therapeutic drug monitoring to improve efficacy and reduce toxicity.

Drug Sensitivity Testing of Mycobacterium tuberculosis Growing in a Hollow Fiber Bioreactor

Hollow fiber systems (HFSs) have been widely applied to study pharmacokinetic-pharmacodynamic (PK-PD) relationships in antibiotic research and development. The system comprises a bundle of high-density hollow capillary fibers that conduct a flow of medium with or without drug and an extra-capillary space (ECS) inoculated with the pathogen of interest. The semipermeable membrane of the hollow fibers allows for rapid exchange of small molecule drugs and solutes, while the pathogen is restricted to the ECS. The unique properties of the HFS are (1) the ability to simulate any PK profile within the fibers and ECS, including plasma or site-of-disease PK profiles, (2) the ability to simultaneously input several drugs with different half-lives, (3) the ability to manipulate growth conditions such as medium composition, carbon source, and pH, and (4) the ability to sample in both compartments in order to monitor drug concentrations and bacterial growth kinetics over time. The system is particularly suited for Mycobacterium tuberculosis research in a biosafety level 3 (BSL3) environment since pathogenic bacteria are sequestered in an isolated compartment. The HFS was qualified by the European Medicines Agency for antituberculosis drug development in 2015. Here, we describe the standard procedures used to study the growth kinetics of M. tuberculosis in the HFS and the killing effect of first-line antituberculous drugs applied under simulated human PK conditions. This animal-sparing and economical tool can be applied to optimize dosing schedules that minimize emergence of resistance and to prioritize drug regimens that accelerate sterilization.

Optimizing ethambutol dosing among HIV/tuberculosis co-infected patients: a population pharmacokinetic modelling and simulation study

BACKGROUND

Reduced ethambutol serum concentrations are commonly observed among TB patients co-infected with HIV and may lead to treatment failure.

OBJECTIVES

To perform a population pharmacokinetic study of ethambutol in HIV/TB patients, and to evaluate an intensified ethambutol weight-based dosing strategy to support pharmacokinetic target attainment.

METHODS

We conducted a prospective study of ethambutol pharmacokinetics among HIV/TB patients administered first-line TB treatment in Botswana, with study visits before and after initiation of ART. Clinical and disease status markers, including HIV-associated systemic immune activation and gut dysfunction biomarkers, were evaluated as covariates of ethambutol pharmacokinetic parameters in non-linear mixed effects analysis. Monte Carlo simulations were performed to compare pharmacokinetic target attainment under standard and intensified weight-based ethambutol dosing strategies.

RESULTS

We studied 40 HIV/TB patients prior to initiation of ART, of whom 24 returned for a second visit a median of 33 days following ART initiation. Ethambutol serum concentrations were best explained by a two-compartment model with first-order elimination, with a significant improvement in oral bioavailability following ART initiation. In Monte Carlo simulations, a supplementary ethambutol dose of 400 mg daily led to >2-fold improvements in pharmacokinetic target attainment probabilities in lung tissue, both before and after ART initiation.

CONCLUSIONS

Low serum ethambutol concentrations were commonly observed among HIV/TB patients in Botswana, and the oral bioavailability of ethambutol increased following ART initiation. Supplementary ethambutol dosing among HIV/TB patients may provide a strategy to optimize anti-TB treatment regimens in this high-risk population.

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.

Minocycline treatment for pulmonary Mycobacterium avium complex disease based on pharmacokinetics/pharmacodynamics and Bayesian framework mathematical models

OBJECTIVES

Our aim was to identify the pharmacokinetic/pharmacodynamic parameters of minocycline in the hollow-fibre system (HFS) model of pulmonary Mycobacterium avium complex (MAC) and to identify the optimal clinical dose.

METHODS

Minocycline MICs for 55 MAC clinical isolates from the Netherlands were determined. We also co-incubated primary isolated macrophages infected with MAC with minocycline. Next, we performed a 28 day HFS-MAC model dose-response study in which we mimicked pulmonary concentration-time profiles achieved in patients. The HFS-MAC model was sampled at intervals to determine the minocycline pharmacokinetics and MAC burden. We identified the AUC0-24/MIC ratios associated with 1.0 log10 cfu/mL kill below day 0 (stasis), defined as a bactericidal effect. We then performed 10000 Monte Carlo experiments to identify the optimal dose for a bactericidal effect in patients.

RESULTS

The MIC for 50% and 90% of cumulative clinical isolates was 8 and 64 mg/L, respectively. Minocycline decreased MAC bacterial burden below stasis in primary isolated macrophages. In the HFS-MAC model, minocycline achieved a microbial kill of 3.6 log10 cfu/mL below stasis. The AUC0-24/MIC exposure associated with a bactericidal effect was 59. Monte Carlo experiments identified a minocycline susceptibility MIC breakpoint of 16 mg/L. At this proposed breakpoint, the clinical dose of 200 mg/day achieved the bactericidal effect exposure target in ∼50% of patients, while 400 mg/day achieved this in 73.6% of patients, in Monte Carlo experiments.

CONCLUSIONS

Minocycline at a dose of 400 mg/day is expected to be bactericidal. We propose a clinical trial for validation.

Preclinical Models of Nontuberculous Mycobacteria Infection for Early Drug Discovery and Vaccine Research

Nontuberculous mycobacteria (NTM) represent an increasingly prevalent etiology of soft tissue infections in animals and humans. NTM are widely distributed in the environment and while, for the most part, they behave as saprophytic organisms, in certain situations, they can be pathogenic, so much so that the incidence of NTM infections has surpassed that of Mycobacterium tuberculosis in developed countries. As a result, a growing body of the literature has focused attention on the critical role that drug susceptibility tests and infection models play in the design of appropriate therapeutic strategies against NTM diseases. This paper is an overview of the in vitro and in vivo models of NTM infection employed in the preclinical phase for early drug discovery and vaccine development. It summarizes alternative methods, not fully explored, for the characterization of anti-mycobacterial compounds.

Linezolid as treatment for pulmonary Mycobacterium avium disease

Objectives

To identify the pharmacokinetic/pharmacodynamic parameters and exposures of linezolid in the treatment of pulmonary Mycobacterium avium complex (MAC) disease.

Methods

Human-derived monocytes infected with MAC were inoculated into hollow-fibre systems for dose-effect and dose-scheduling studies. We mimicked linezolid concentration-time profiles achieved in adult human lungs treated for 28 days. Sampling to confirm that the intended linezolid pharmacokinetics had been achieved, and for enumeration of MAC colony-forming units, was performed based on repetitive sampling from each system over the 28 days. We then performed 10 000 patient Monte Carlo simulations to identify doses associated with optimal effect in the clinic.

Results

Linezolid achieved a hitherto unprecedented feat of at least 1.0 log10 cfu/mL reduction. Efficacy was most closely linked to the AUC0-24/MIC ratio. The AUC0-24/MIC ratio associated with no change in bacterial burden or bacteriostasis was 7.82, while that associated with 1.0 log10 cfu/mL kill was 42.06. The clinical dose of 600 mg/day achieved or exceeded the bacteriostasis exposure in 98.73% of patients. The proportion of 10 000 patients treated with the standard 1200 mg/day who achieved the exposure for 1.0 log10 cfu/mL kill was 70.64%, but was 90% for 1800 mg/day. The proposed MIC breakpoint for linezolid is 16 mg/L, with which 49%-80% of clinical isolates would be considered resistant.

Conclusions

Linezolid is associated with a bactericidal effect in pulmonary MAC that is greater than that seen with other recommended drugs. However, because of the MIC distribution, doses that would optimize the bactericidal effect would be associated with a high adverse event rate.

A programme to create short-course chemotherapy for pulmonary Mycobacterium avium disease based on pharmacokinetics/pharmacodynamics and mathematical forecasting

Objectives

Pulmonary Mycobacterium avium complex (MAC) prevalence is on the rise worldwide. The average therapy duration is 1.5 years, which is associated with poor cure rates. Our objective was to develop a programme to design a combination therapy regimen for pulmonary MAC to be administered for 6 months or less with efficacy in > 90% of patients.

Methods

We performed a literature search for the following MeSH headings 'Mycobacterium avium' AND 'pharmacokinetics/pharmacodynamics' in PubMed up to 2016. The findings were then used to identify steps in the programme to design new regimens with faster microbial kill rates than the current standard regimen.

Results

First, we designed a strategy for rapid in vitro screening of all antibiotic classes for repurposing against pulmonary MAC. Secondly, we identified and compared maximal microbial kill rates (Emax), and optimal exposures of eight different antibiotics. These studies had all been performed in the hollow-fibre system model of pulmonary MAC (HFS-MAC). Thirdly, all drugs with a high Emax at clinically achievable optimal exposures will be chosen, and exposures associated with synergy or additivity for two/three drugs identified based on Bliss independence. Fourthly, the time-kill slopes and resistance suppression of the chosen combinations will be compared with those of standard combination therapy in the HFS-MAC. Finally, we will identify the clinical doses best able to achieve synergistic or additive combination exposures by taking into account pharmacokinetic variability.

Conclusions

Our stepwise pharmacokinetics/pharmacodynamics approach provides a scientific rationale and a strategy for achieving short-course chemotherapy for pulmonary MAC disease within a few years.

A 'shock and awe' thioridazine and moxifloxacin combination-based regimen for pulmonary Mycobacterium avium-intracellulare complex disease

Objectives

To develop a thioridazine/moxifloxacin-based combination regimen for treatment of pulmonary infection due to Mycobacterium avium-intracellulare complex (MAC) that kills bacteria faster than the standard treatment regimen.

Methods

Monocytes were infected with MAC and inoculated into the hollow-fibre system model for pulmonary MAC disease (HFS-MAC). We co-administered ethambutol plus azithromycin daily for 28 days, to achieve the same human concentration-time profiles that result from standard doses, in three HFS-MAC systems. Two experimental regimens consisted of thioridazine at an exposure associated with optimal kill, given intermittently on days 0, 3, 7 and 10. Regimen A consisted of thioridazine in combination with standard dose azithromycin for the entire study duration. Regimen B was thioridazine plus moxifloxacin at concentration-time profiles achieved by the standard daily dose administered for 14 days, followed by daily azithromycin. Each HFS-MAC was sampled for bacterial burden every 7 days.

Results

The bacteria in the non-treated HFS-MAC grew at a rate of 0.11 ± 0.01 log10 cfu/mL/day. The azithromycin/ethambutol regimen decreased bacterial burden by 1.21 ± 0.74 log10 cfu/mL below baseline during the first 7 days, after which it failed. Regimen A killed 3.28 ± 0.32 log10 cfu/mL below baseline up to day 14, after which regrowth occurred once thioridazine treatment stopped. Regimen B killed bacteria to below the limits of detection in 7 days (≥5.0 log10 cfu/mL kill), with rebound in the azithromycin continuation phase.

Conclusions

The thioridazine/moxifloxacin regimen demonstrated that rapid microbial kill could be achieved within 7 days. This is a proof of principle that short-course chemotherapy for pulmonary MAC is possible.

Tedizolid is highly bactericidal in the treatment of pulmonary Mycobacterium avium complex disease

Objectives

To determine if tedizolid is effective for pulmonary Mycobacterium avium complex (MAC) disease, and to use pharmacokinetics/pharmacodynamics to design optimal doses.

Methods

We performed an exposure-response experiment in the hollow-fibre system model of intracellular MAC (HFS-MAC). We mimicked the tedizolid concentration-time profiles achieved in the lungs of patients treated once daily for 28 days. The HFS-MAC was sampled at intervals to determine the tedizolid pharmacokinetics and MAC intracellular burden. We identified the 0-24 h area under the concentration-time curves to MIC (AUC0-24/MIC) ratios associated with the following targets: 80% of maximal kill (EC80), bacteriostasis, and 1.0 and 2.0 log10 cfu/mL kill. We then performed 10 000 patient Monte Carlo simulations to identify the optimal dose for each of the exposure targets.

Results

Tedizolid achieved the feat of 2.0 log10 cfu/mL kill below initial bacterial burden, an effect not seen before in this model with other antibiotics. The tedizolid exposure associated with 1.0 log10 cfu/mL kill was a non-protein bound AUC0-24/MIC ratio of 23.46, while that associated with 2.0 log10 cfu/mL kill was 37.50, and the EC80 was 21.71. The clinical dose of 200 mg achieved each of these targets in ∼100% of the 10 000 patients, except the 2.0 log10 cfu/mL kill which required 300 mg/day. A tedizolid susceptibility MIC breakpoint of 1 mg/L is proposed.

Conclusions

Tedizolid, at standard clinical doses, is expected to be bactericidal, and even achieved an unprecedented 2.0 log10 cfu/mL kill of MAC as monotherapy. We propose it as the backbone of short-course anti-MAC chemotherapy.

Meta-analyses and the evidence base for microbial outcomes in the treatment of pulmonary Mycobacterium avium-intracellulare complex disease

Objectives

To perform a systematic review and meta-analysis of the level of funding support and the sputum culture conversion rates in pulmonary Mycobacterium avium-intracellulare complex (P-MAC) disease in adult patients without cystic fibrosis or HIV infection, treated with recommended antibiotic regimens.

Methods

We performed a literature search to identify clinical trials, prospective studies and registries that reported outcomes in P-MAC patients. Studies that reported P-MAC diagnosis and treatments based on established guidelines met the inclusion criteria and were examined for bias and quality. We modified existing quality scales and came up with a 10 star quality score. Outcomes meta-analysed were sputum conversion incidence ratios (IR) and their 95% CI, weighted for study quality.

Results

Twenty-one studies that examined 28 regimens, including 2534 patients in intent-to-treat analyses and 1968 in per-protocol analyses, were identified. The study quality mean ± SD scores were 5.4 ± 2.2 out of 10 stars. Only two (9.5%) studies received public funding. There was significant heterogeneity of microbial effect among treatment regimens (I2 > 40%; P > 0.001). The pooled IR for sustained sputum conversion was 0.54 (95% CI 0.45-0.63) for macrolide-containing regimens versus 0.38 (0.25-0.52) with macrolide-free regimens. Prolonging therapy duration beyond 12 months was associated with an average decline in sputum conversion to 22% (95% CI 1%-44%).

Conclusions

Researchers working on P-MAC therapy have received very little public funding support. As a result, the evidence base for treatment guidelines is based on studies of relatively small numbers of patients in low-quality studies. Nevertheless, these studies showed poor sputum conversion rates in patients receiving recommended treatment regimens.

A novel ceftazidime/avibactam, rifabutin, tedizolid and moxifloxacin (CARTM) regimen for pulmonary Mycobacterium avium disease

Objectives

To compare the efficacy of ceftazidime/avibactam plus tedizolid-based combination regimens with the standard therapy of azithromycin, ethambutol and rifabutin for the treatment of pulmonary Mycobacterium avium complex (MAC) disease.

Methods

We mimicked the human pulmonary concentration-time profiles of ceftazidime/avibactam and tedizolid in combination, ceftazidime/avibactam, rifabutin, tedizolid and moxifloxacin (CARTM), and the standard regimen and examined microbial kill in triplicate hollow-fibre system model of intracellular pulmonary MAC (HFS-MAC) units. The tedizolid and moxifloxacin doses used were non-optimized; the tedizolid dose was that associated with bacteriostasis. Drugs were administered daily for 28 days. Each HFS-MAC was sampled in the central and peripheral compartment to ascertain that the intended drug exposures had been achieved. The peripheral compartments were sampled at regular intervals over the 28 days to quantify the burden of MAC.

Results

MAC-infected macrophages in the HFS-MAC achieved multi-fold higher intracellular versus extracellular concentrations of rifabutin, moxifloxacin, ceftazidime/avibactam. The non-optimized ceftazidime/avibactam plus tedizolid dual therapy held the bacterial burden at the same level as day 0 (stasis) throughout the 28 days. The standard therapy reduced the bacterial load 2 log10 cfu/mL below stasis on day 14 but started failing after that. The CARTM regimen achieved 3.2 log10 cfu/mL kill below stasis on day 21, but had started to fail by day 28.

Conclusions

The CARTM regimen promises to have kill rates better than standard therapy. Experiments to identify exposures of each of the four drugs associated with optimal effect in the CARTM combination are needed in order to design a short-course chemotherapy regimen.

Failure of the azithromycin and ethambutol combination regimen in the hollow-fibre system model of pulmonary Mycobacterium avium infection is due to acquired resistance

Objectives

To investigate the performance of the two backbone drugs in the standard combination therapy regimen in the hollow-fibre system (HFS) model of pulmonary Mycobacterium avium complex (MAC) infection.

Methods

Six HFS were inoculated with human-derived monocytes infected with MAC, and treated with 15 mg/kg of ethambutol and 500 mg of azithromycin daily for 28 days to recapitulate the concentration-time profiles seen in the lungs of humans treated with these drugs and doses. The concentration-time profiles achieved were validated by sampling the central compartment at seven timepoints over 24 h. The total MAC burden, as well as the subpopulation resistant to 3 × MIC of each drug, was identified based on sampling the peripheral compartment of each system on days 0, 3, 7, 14, 21 and 28 of therapy. The experiment was performed twice.

Results

In non-treated control HFS, MAC grew from 5.0 to 8.53 log10 cfu/mL in 28 days. The dual therapy killed a maximum of 1.52 ± 0.43 log10 cfu/mL during the first 7 days, after which it failed. By day 28 there was no difference in MAC burden between the combination-therapy-treated and non-treated systems. Failure arose in parallel with the emergence of acquired ethambutol resistance. By day 28, 100% of the bacterial population was ethambutol resistant in the combination-therapy-treated HFS replicates.

Conclusions

The backbone combination of macrolide and ethambutol has poor MAC kill rates and is ineffective. Microbial kill is rapidly abrogated by acquired drug resistance. This backbone should be replaced.

Linezolid for Infants and Toddlers With Disseminated Tuberculosis: First Steps

BACKGROUND

 Infants and toddlers often present with disseminated and lymph node tuberculosis, in which Mycobacterium tuberculosis (Mtb) is predominantly intracellular. Linezolid, used to treat tuberculosis in adults, has not been formally studied in infants. Infants clear linezolid 5 times faster than adults and achieve lower 0- to 24-hour area under the concentration-time curves (AUC_0-24).

METHODS

 To mimic intracellular disease, we infected human-derived THP-1 macrophages with Mtb and inoculated hollow fiber systems. We performed dose-effect and dose-scheduling studies in which we recapitulated the linezolid half-life of 3 hours encountered in infants. Repetitive sampling for linezolid pharmacokinetics, Mtb intracellular burden, viable monocyte count, and RNA sequencing reads were performed up to 28 days.

RESULTS

 The linezolid extracellular half-life was 2.64 ± 0.38 hours, whereas intracellular half-life was 8.93 ± 1.30 hours (r² = 0.89). Linezolid efficacy was linked to the AUC_0-24 to minimum inhibitory concentration (MIC) ratio (r² = 0.98). The exposure associated with maximal Mtb kill was an AUC_0-24/MIC of 23.37 ± 1.16. We identified a 414-gene transcript on exposure to toxic linezolid doses. The largest number of genes mapped to ribosomal proteins, a signature hitherto not associated with linezolid toxicity. The second-largest number of differentially expressed genes mapped to mitochondrial enzyme inhibition. Linezolid AUC_0-24 best explained the mitochondrial gene inhibition, with 50% inhibition at 94 mg × hour/L (highest r² = 0.98).

CONCLUSIONS

 We identified the linezolid AUC_0-24/MIC target for optimal efficacy against pediatric intracellular tuberculosis, and an AUC_0-24 threshold associated with mitochondrial inhibition. These constitute a therapeutic window to be targeted for optimal linezolid doses in children with tuberculosis.

The discovery of ceftazidime/avibactam as an anti-Mycobacterium avium agent

OBJECTIVES

To determine if ceftaroline and ceftazidime combined with avibactam are efficacious against pulmonary Mycobacterium avium complex (MAC) disease.

METHODS

First, we performed a concentration-effect study of ceftaroline and ceftaroline/avibactam against extracellular MAC in test tubes. Given the difficulty of obtaining avibactam at the time of experimentation, we used a single concentration of commercial ceftazidime/avibactam, and two sets of non-treated controls, one with ceftazidime/avibactam and the other without. After finding antimicrobial activity with the ceftazidime/avibactam 'control', we performed ceftazidime/avibactam dose-effect studies in test tubes against extracellular MAC and in 24-well plates against intracellular MAC. We then performed a ceftazidime/avibactam exposure-effect and dose-fractionation studies in the hollow-fibre system model of intracellular pulmonary MAC (HFS-MAC). In each experiment, we repetitively sampled each HFS-MAC at specified times to validate ceftazidime/avibactam pharmacokinetics and to quantify bacterial burden.

RESULTS

Ceftaroline killed extracellular MAC with maximal microbial kill (Emax) of 4.87 ± 0.26 log10 cfu/mL. However, the ceftazidime/avibactam 'control' also killed MAC compared with the non-treated control. Ceftazidime/avibactam Emax was 3.8 log10 cfu/mL against extracellular bacilli and 3.6 log10 cfu/mL against intracellular MAC. In the HFS-MAC, ceftazidime/avibactam achieved a half-life of 2.5-3.3 h and killed MAC 0.61-2.40 log10 cfu/mL below the starting bacterial burden. The ceftazidime/avibactam efficacy was linked to the proportion of the dosing interval for which the concentration persists above the MIC (fT>MIC), with optimal efficacy at free-drug fT>MIC of 52% (r2 = 0.95).

CONCLUSIONS

Ceftazidime/avibactam effectively kills MAC at exposures easily achieved in the lung by clinical doses. Efficacy was higher than with clinically achievable doses of azithromycin and ethambutol.

Lactoferricin Peptides Increase Macrophages' Capacity To Kill Mycobacterium avium

The genus Mycobacterium comprises several pathogenic species, including M. tuberculosis, M. leprae, M. avium, etc. Infections caused by these bacteria are particularly difficult to treat due to their intrinsic impermeability, low growth rate, and intracellular localization. Antimicrobial peptides are increasingly acknowledged as potential treatment tools, as they have a high spectrum of activity, low tendency to induce bacterial resistance, and immunomodulatory properties. In this study, we show that peptides derived from bovine lactoferricin (LFcin) improve the antimicrobial activity of ethambutol against Mycobacterium avium growing inside macrophages. Moreover, the d-enantiomer of a short version of lactoferricin containing amino acids 17 to 30 (d-LFcin17–30) causes intramacrophagic death of M. avium by increasing the formation of lysosomes and autophagosomes. This work opens the way to the use of lactoferricin-derived peptides to treat infections caused by mycobacteria and highlights important modulatory effects of d-FLcin17–30 on macrophages, which may be useful under other conditions in which macrophage activation is needed.

Mycobacterial infections cause a significant burden of disease and death worldwide. Their treatment is long, toxic, costly, and increasingly prone to failure due to bacterial resistance to currently available antibiotics. New therapeutic options are thus clearly needed. Antimicrobial peptides represent an important source of new antimicrobial molecules, both for their direct activity and for their immunomodulatory potential. We have previously reported that a short version of the bovine antimicrobial peptide lactoferricin with amino acids 17 to 30 (LFcin17–30), along with its variants obtained by specific amino acid substitutions, killed Mycobacterium avium in broth culture. In the present work, those peptides were tested against M. avium living inside its natural host cell, the macrophage. We found that the peptides increased the antimicrobial action of the conventional antibiotic ethambutol inside macrophages. Moreover, the d-enantiomer of the lactoferricin peptide (d-LFcin17–30) was more stable and induced significant killing of intracellular mycobacteria by itself. Interestingly, d-LFcin17–30 did not localize to M. avium-harboring phagosomes but induced the production of proinflammatory cytokines and increased the formation of lysosomes and autophagosome-like vesicles. These results lead us to conclude that d-LFcin17–30 primes macrophages for intracellular microbial digestion through phagosomal maturation and/or autophagy, culminating in mycobacterial killing.

IMPORTANCE The genus Mycobacterium comprises several pathogenic species, including M. tuberculosis, M. leprae, M. avium, etc. Infections caused by these bacteria are particularly difficult to treat due to their intrinsic impermeability, low growth rate, and intracellular localization. Antimicrobial peptides are increasingly acknowledged as potential treatment tools, as they have a high spectrum of activity, low tendency to induce bacterial resistance, and immunomodulatory properties. In this study, we show that peptides derived from bovine lactoferricin (LFcin) improve the antimicrobial activity of ethambutol against Mycobacterium avium growing inside macrophages. Moreover, the d-enantiomer of a short version of lactoferricin containing amino acids 17 to 30 (d-LFcin17–30) causes intramacrophagic death of M. avium by increasing the formation of lysosomes and autophagosomes. This work opens the way to the use of lactoferricin-derived peptides to treat infections caused by mycobacteria and highlights important modulatory effects of d-FLcin17–30 on macrophages, which may be useful under other conditions in which macrophage activation is needed.

Ethambutol Is Cleared by a Contemporary High-Flux Hemodialyzer, and Drug Monitoring Ensures Safety and Therapeutic Effect

It is uncertain, given the lack of recent data and the inconclusive nature of previous data, whether ethambutol is cleared by hemodialysis using contemporary dialyzers. We measured serum ethambutol concentrations before, during, and 1 h after hemodialysis in a 75-year-old Caucasian man receiving ethambutol for disseminated Bacille Calmette-Guérin infection. There was a mean 41% decrease in serum ethambutol concentration during dialysis, confirming the hemodialyzability of ethambutol and the utility of drug monitoring in ensuring safety.

The Role of Therapeutic Drug Monitoring in Mycobacterial Infections

Tuberculosis (TB) is a leading cause of infectious death. Nontuberculous mycobacteria (NTM) cause a wide variety of difficult-to-treat infections in various human hosts. Therapeutic drug monitoring (TDM) remains a standard clinical technique that uses plasma drug concentrations to determine dose. The reason to do this is simple: drug exposure (that is, the free drug area under the plasma concentration-time curve) relative to the MIC and not the dose per se largely determines the outcome of the infections. TDM provides objective information that clinician can use to make informed dosing decisions. The normal plasma concentration ranges provide reasonable guidance for initial target concentrations. Clinicians then combine concentration data with knowledge about the patients, in order to decide how aggressive to be with dosing. With sicker patients, who are closer to a poor outcome, one may be willing to accept an increased risk of potential toxicity in order to secure patient survival. In the clinic, time and resources are limited, so typically only two samples are collected postdose. The 2-h postdose concentrations approach the peak for most TB and NTM drugs. A 6-h sample allows the clinician to distinguish between delayed absorption and malabsorption, because patients with the latter need higher doses in order to gain the benefit associated with standard doses. Plasma concentrations do not account for all of the variability in patient responses to TB or NTM treatment, and concentrations cannot guarantee patient outcomes. However, combined with clinical and bacteriological data, TDM can be a decisive tool, allowing clinicians to look inside of their patients and adjust doses based on objective data. Knowing the dose, rather than guessing at the dose, is the path to shorter and more successful treatment regimens.

A Multilaboratory, Multicountry Study To Determine MIC Quality Control Ranges for Phenotypic Drug Susceptibility Testing of Selected First-Line Antituberculosis Drugs, Second-Line Injectables, Fluoroquinolones, Clofazimine, and Linezolid

Our objective was to establish reference MIC quality control (QC) ranges for drug susceptibility testing of antimycobacterials, including first-line agents, second-line injectables, fluoroquinolones, and World Health Organization category 5 drugs for multidrug-resistant tuberculosis using a 7H9 broth microdilution MIC method. A tier-2 reproducibility study was conducted in eight participating laboratories using Clinical Laboratory and Standards Institute (CLSI) guidelines. Three lots of custom-made frozen 96-well polystyrene microtiter plates were used and prepared with 2× prediluted drugs in 7H9 broth-oleic acid albumin dextrose catalase. The QC reference strain was Mycobacterium tuberculosis H37Rv. MIC frequency, mode, and geometric mean were calculated for each drug. QC ranges were derived based on predefined, strict CLSI criteria. Any data lying outside CLSI criteria resulted in exclusion of the entire laboratory data set. Data from one laboratory were excluded due to higher MIC values than other laboratories. QC ranges were established for 11 drugs: isoniazid (0.03 to 0.12 μg/ml), rifampin (0.03 to 0.25 μg/ml), ethambutol (0.25 to 2 μg/ml), levofloxacin (0.12 to 1 μg/ml), moxifloxacin (0.06 to 0.5 μg/ml), ofloxacin (0.25 to 2 μg/ml), amikacin (0.25 to 2 μg/ml), kanamycin (0.25 to 2 μg/ml), capreomycin (0.5 to 4 μg/ml), linezolid (0.25 to 2 μg/ml), and clofazimine (0.03 to 0.25 μg/ml). QC ranges could not be established for nicotinamide (pyrazinamide surrogate), prothionamide, or ethionamide, which were assay nonperformers. Using strict CLSI criteria, QC ranges against the M. tuberculosis H37Rv reference strain were established for the majority of commonly used antituberculosis drugs, with a convenient 7H9 broth microdilution MIC method suitable for use in resource-limited settings.

Thioridazine as Chemotherapy for Mycobacterium avium Complex Diseases

Mycobacterium avium-intracellulare complex (MAC) causes an intractable intracellular infection that presents as chronic pulmonary disease. Currently, therapy consists of ethambutol and macrolides and takes several years to complete. The neuroleptic phenothiazine thioridazine kills mycobacteria by inhibiting the electron transport chain. In several experiments with bacterial populations of up to 10¹² CFU/ml, we failed to isolate any bacteria resistant to 3 times the MIC of thioridazine, suggesting the absence of resistant mutants at bacterial burdens severalfold higher than those encountered in patients. In the hollow-fiber model of intracellular MAC (HFS-MAC), thioridazine achieved an extracellular half-life of 16.8 h and an intracellular half-life of 19.7 h. Thioridazine concentrations were >28,000-fold higher inside infected macrophages than in the HFS-MAC central compartment (equivalent to plasma). Thioridazine maximal kill was 5.20 ± 0.75 log₁₀ CFU/ml on day 7 (r² = 0.96) and 7.19 ± 0.31 log₁₀ CFU/ml on day 14 (r² = 0.99), the highest seen with any drug in the system. Dose fractionation studies revealed that thioridazine efficacy and acquired drug resistance were driven by the peak concentation-to-MIC ratio, with a 50% effective concentration (EC₅₀) of 2.78 ± 0.44 for microbial killing. Acquired drug resistance was encountered by day 21 with suboptimal doses, demonstrating that fluctuating drug concentrations drive evolution faster than static concentrations in mutation frequency studies. However, the thioridazine EC₅₀ changed 16.14-fold when the concentration of fetal bovine serum was changed from 0% to 50%, suggesting that intracellular potency could be heavily curtailed by protein binding. Efficacy in patients will depend on the balance between trapping of the drug in the pulmonary system and the massive intracellular concentrations versus very high protein binding of thioridazine.

Moxifloxacin's Limited Efficacy in the Hollow-Fiber Model of Mycobacterium abscessus Disease

Current regimens used to treat pulmonary Mycobacterium abscessus disease have limited efficacy. There is an urgent need for new drugs and optimized combinations and doses. We performed hollow-fiber-system studies in which M. abscessus was exposed to moxifloxacin lung concentration-time profiles similar to human doses of between 0 and 800 mg/day. The minimum bactericidal concentration and MIC were 8 and 2 mg/liter, respectively, in our M. abscessus strain, suggesting bactericidal activity. Measurement of the moxifloxacin concentrations in each hollow-fiber system revealed an elimination rate constant (kel) of 0.11 ± 0.05 h(-1) (mean ± standard deviation) (half-life of 9.8 h). Inhibitory sigmoid maximal effect (Emax) modeling revealed that the highest Emax was 3.15 ± 1.84 log10 CFU/ml on day 3, and the exposure mediating 50% of Emax (EC50) was a 0- to 24-h area under the concentration time curve (AUC0-24)-to-MIC ratio of 41.99 ± 31.78 (r(2) = 0.99). The EC80 was an AUC0-24/MIC ratio of 102.11. However, no moxifloxacin concentration killed the bacteria to burdens below the starting inoculum. There was regrowth beyond day 3 in all doses, with replacement by a resistant subpopulation that had an MIC of >32 mg/liter by the end of the experiment. A quadratic function best described the relationship between the AUC0-24/MIC ratio and the moxifloxacin-resistant subpopulation. Monte Carlo simulations of 10,000 patients revealed that the 400- to 800-mg/day doses would achieve or exceed the EC80 in ≤12.5% of patients. The moxifloxacin susceptibility breakpoint was 0.25 mg/liter, which means that almost all M. abscessus clinical strains are moxifloxacin resistant by these criteria. While moxifloxacin's efficacy against M. abscessus was poor, formal combination therapy studies with moxifloxacin are still recommended.

Azithromycin Dose To Maximize Efficacy and Suppress Acquired Drug Resistance in Pulmonary Mycobacterium avium Disease

Mycobacterium aviumcomplex is now the leading mycobacterial cause of chronic pneumonia in the United States. Macrolides and ethambutol form the backbone of the regimen used in the treatment of pulmonary disease. However, therapy outcomes remain poor, with microbial cure rates of 4% in cavitary disease. The treatment dose of azithromycin has mostly been borrowed from that used to treat other bacterial pneumonias; there are no formal dose-response studies in pulmonaryM. aviumdisease and the optimal dose is unclear. We utilized population pharmacokinetics and pharmacokinetics/pharmacodynamics-derived azithromycin exposures associated with optimal microbial kill or resistance suppression to perform 10,000 patient Monte Carlo simulations of dose effect studies for daily azithromycin doses of 0.5 to 10 g. The currently recommended dose of 500 mg per day achieved the target exposures in 0% of patients. Exposures associated with optimal kill and resistance suppression were achieved in 87 and 54% of patients, respectively, only by the very high dose of 8 g per day. The azithromycin susceptibility breakpoint above which patients failed therapy on the very high doses of 8 g per day was an MIC of 16 mg/liter, suggesting a critical concentration of 32 mg/liter, which is 8-fold lower than the currently used susceptibility breakpoint of 256 mg/liter. If the standard dose of 500 mg a day were used, then the critical concentration would fall to 2 mg/liter, 128-fold lower than 256 mg/liter. The misclassification of resistant isolates as susceptible could explain the high failure rates of current doses.

A Long-term Co-perfused Disseminated Tuberculosis-3D Liver Hollow Fiber Model for Both Drug Efficacy and Hepatotoxicity in Babies

Treatment of disseminated tuberculosis in children ≤ 6 years has not been optimized. The pyrazinamide-containing combination regimen used to treat disseminated tuberculosis in babies and toddlers was extrapolated from adult pulmonary tuberculosis. Due to hepatotoxicity worries, there are no dose–response studies in children. We designed a hollow fiber system model of disseminated intracellular tuberculosis with co-perfused three-dimensional organotypic liver modules to simultaneously test for efficacy and toxicity. We utilized pediatric pharmacokinetics of pyrazinamide and acetaminophen to determine dose-dependent pyrazinamide efficacy and hepatotoxicity. Acetaminophen concentrations that cause hepatotoxicity in children led to elevated liver function tests, while 100 mg/kg pyrazinamide did not. Surprisingly, pyrazinamide did not kill intracellular Mycobacterium tuberculosis up to fourfold the standard dose as monotherapy or as combination therapy, despite achieving high intracellular concentrations. Host-pathogen RNA-sequencing revealed lack of a pyrazinamide exposure transcript signature in intracellular bacteria or of phagolysosome acidification on pH imaging. Artificial intelligence algorithms confirmed that pyrazinamide was not predictive of good clinical outcomes in children ≤ 6 years who had extrapulmonary tuberculosis. Thus, adding a drug that works inside macrophages could benefit children with disseminated tuberculosis. Our in vitro model can be used to identify such new regimens that could accelerate cure while minimizing toxicity.

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We designed a pre-clinical of disseminated for simultaneous identification of toxicity and efficacy in children.

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The system is a co-culture of infected monocytes and 3 dimensional organotypic liver recapitulating children pharmacokinetics.

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Pyrazinamide, central drug in treatment regimen, had no effect as monotherapy or contribute to the combination therapy.

Due to fear of toxicity children are often not involved in clinical trials, and as a result the optimal treatment regimens are often lacking. As an example, toddlers and babies develop disseminated tuberculosis but are treated with regimens designed for adults with lung cavity disease. We designed a “glass-mouse” model of disseminated tuberculosis that simultaneously tests for the efficacy and toxicity of the anti-tuberculosis drugs for children with disseminated disease. We found that while not causing dose-dependent liver toxicity, one of the central drugs used to treat this children is likely not efficacious.

Amikacin Pharmacokinetics/Pharmacodynamics in a Novel Hollow-Fiber Mycobacterium abscessus Disease Model

The treatment of pulmonary Mycobacterium abscessus disease is associated with very high failure rates and easily acquired drug resistance. Amikacin is the key drug in treatment regimens, but the optimal doses are unknown. No good preclinical model exists to perform formal pharmacokinetics/pharmacodynamics experiments to determine these optimal doses. We developed a hollow-fiber system model of M. abscessus disease and studied amikacin exposure effects and dose scheduling. We mimicked amikacin human pulmonary pharmacokinetics. Both amikacin microbial kill and acquired drug resistance were linked to the peak concentration-to-MIC ratios; the peak/MIC ratio associated with 80% of maximal kill (EC80) was 3.20. However, on the day of the most extensive microbial kill, the bacillary burden did not fall below the starting inoculum. We performed Monte Carlo simulations of 10,000 patients with pulmonary M. abscessus infection and examined the probability that patients treated with one of 6 doses from 750 mg to 4,000 mg would achieve or exceed the EC80. We also examined these doses for the ability to achieve a cumulative area under the concentration-time curve of 82,232 mg · h/liter × days, which is associated with ototoxicity. The standard amikacin doses of 750 to 1,500 mg a day achieved the EC80 in ≤ 21% of the patients, while a dose of 4 g/day achieved this in 70% of the patients but at the cost of high rates of ototoxicity within a month or two. The susceptibility breakpoint was an MIC of 8 to 16 mg/liter. Thus, amikacin, as currently dosed, has limited efficacy against M. abscessus. It is urgent that different antibiotics be tested using our preclinical model and new regimens developed.

Drug susceptibility testing of nontuberculous mycobacteria

Diseases caused by nontuberculous mycobacteria are emerging in many settings. With an increased number of patients needing treatment, the role of drug susceptibility testing is again in the spotlight. This articles covers the history and methodology of drug susceptibility tests for nontuberculous mycobacteria, but focuses on the correlations between in vitro drug susceptibility, pharmacokinetics and in vivo outcomes of treatment. Among slow-growing nontuberculous mycobacteria, clear correlations have been established for macrolides and amikacin (Mycobacterium avium complex) and for rifampicin (Mycobacterium kansasii). Among rapid-growing mycobacteria, correlations have been established in extrapulmonary disease for aminoglycosides, cefoxitin and co-trimoxazole. In pulmonary disease, correlations are less clear and outcomes of treatment are generally poor, especially for Mycobacterium abscessus. The clinical significance of inducible resistance to macrolides among rapid growers is an important topic. The true role of drug susceptibility testing for nontuberculous mycobacteria still needs to be addressed, preferably within clinical trials.

Rapid drug tolerance and dramatic sterilizing effect of moxifloxacin monotherapy in a novel hollow-fiber model of intracellular Mycobacterium kansasii disease

Mycobacterium kansasii is the second most common mycobacterial cause of lung disease. Standard treatment consists of rifampin, isoniazid, and ethambutol for at least 12 months after negative sputum. Thus, shorter-duration therapies are needed. Moxifloxacin has good MICs for M. kansasii. However, good preclinical models to identify optimal doses currently are lacking. We developed a novel hollow fiber system model of intracellular M. kansasii infection. We indexed the efficacy of the standard combination regimen, which was a kill rate of -0.08 ± 0.05 log10 CFU/ml/day (r(2) = 0.99). We next performed moxifloxacin dose-effect and dose-scheduling studies at a half-life of 11.1 ± 6.47 h. Some systems also were treated with the efflux pump inhibitor reserpine. The highest moxifloxacin exposure, as well as lower exposures plus reserpine, sterilized the cultures by day 7. This suggests that efflux pump-mediated tolerance at low ratios of the area under the concentration-time curve from 0 to 24 h (AUC0 - 24) to MICs is an early bacterial defense mechanism but is overcome by higher exposures. The highest rate of moxifloxacin monotherapy sterilization was -0.82 ± 0.15 log10 CFU/ml/day (r(2) = 0.97). The moxifloxacin exposure associated with 80% of maximal kill (EC80) was an AUC0-24/MIC of 317 (the non-protein-bound moxifloxacin AUC0-24/MIC was 158.5). We performed Monte Carlo simulations of 10,000 patients in order to identify the moxifloxacin dose that would achieve or exceed the EC80. The simulations revealed an optimal moxifloxacin dose of 800 mg a day. The MIC susceptibility breakpoint at this dose was 0.25 mg/liter. Thus, moxifloxacin, at high enough doses, is suitable to study in patients for the potential to add rapid sterilization to the standard regimen.

Mycobacterium avium complex (MAC) Immune Reconstitution Syndrome (IRIS) With Reduced Susceptibility to Ethambutol in an HIV-Infected Patient

Objective: To describe a case of Mycobacterium avium complex (MAC) lymphadenitis complicated by immune reconstitution syndrome (IRIS) and reduced susceptibility to ethambutol. Case Summary: A 24-year-old man was diagnosed in October 2012 with advanced HIV infection upon hospitalization for multiple opportunistic infections (OIs). Within 5 months of starting antiretroviral therapy, the patient developed significant cervical lymphadenopathy concerning for MAC/IRIS. Acid-fast bacilli were detected in the primary lymph node biopsy smear, and culture results confirmed the presence of MAC. Susceptibility testing revealed an organism susceptible to azithromycin, with an elevated minimum inhibitory concentration (MIC) to ethambutol (8 µg/mL). Currently, there is no interpretation for an ethambutol MIC of 8 µg/mL for MAC. A review of the primary literature revealed the possibility of decreased ethambutol susceptibility when the MIC is above 1 µg/mL, and therefore, therapy was replaced by rifabutin in combination with azithromycin. Discussion: Current guidelines recommend a 2-drug regimen for the treatment of MAC, specifically a macrolide plus ethambutol. Guidelines also emphasize MAC susceptibility testing for macrolides only. Susceptibility results from this patient’s biopsy prompted an evaluation of the effectiveness of his antimycobacterial regimen. Conclusions: Reduced ethambutol susceptibility in this patient triggered a search of the primary literature that resulted in the decision to replace ethambutol with rifabutin. Additional clinical trials are needed to define susceptibility breakpoints for ethambutol and other antimycobacterial agents used for MAC infection treatment and to direct clinical decisions when elevated MICs to primary agents are identified.

Drug treatment of pulmonary nontuberculous mycobacterial disease in HIV-negative patients: the evidence

Pulmonary disease (PD) caused by nontuberculous mycobacteria is an emerging infection mainly in countries where the incidence of tuberculosis is in decline. It affects an elderly population, often with underlying chronic lung diseases, but its epidemiology shows significant regional variation. Guidelines and recommendations for treatment of these infections exist, but build strongly on expert opinion, as very few good quality clinical trials have been performed in this field. Only for the most frequent causative agents, the Mycobacterium avium complex, Mycobacterium kansasii and Mycobacterium abscessus, a reasonable number of trials and case series is now available. For the less frequent causative agents of pulmonary nontuberculous mycobacterial (NTM) disease (Mycobacterium xenopi, Mycobacterium malmoense, Mycobacterium fortuitum, Mycobacterium chelonae) data is mostly limited to a few very small case series. Within this review, we have collected and combined evidence from all available trials and case series. From the data of these trials and case series, we reconstruct a more evidence-based overview of possible drug treatment regimens and their outcomes.

Synergistic activity of rifampicin and ethambutol against slow-growing nontuberculous mycobacteria is currently of questionable clinical significance

A key issue in the treatment of disease caused by slow-growing nontuberculous mycobacteria is the limited association between in vitro minimum inhibitory concentrations (MICs) of rifampicin and ethambutol alone and the in vivo outcome of treatment with these drugs. Combined susceptibility testing to rifampicin and ethambutol could provide a more realistic view of the efficacy of these drugs. In this study, Mycobacterium avium (n = 5), Mycobacterium chimaera (n = 6), Mycobacterium intracellulare (n = 4), Mycobacterium xenopi (n = 4), Mycobacterium malmoense (n = 3) and Mycobacterium simiae (n = 2) clinical isolates were selected and the MICs of rifampicin and ethambutol alone and in combination were measured using the Middlebrook 7H10 agar dilution method. Synergy was defined as a fractional inhibitory concentration index ≤ 0.5. Rifampicin and ethambutol showed synergistic activity against the majority of M. avium (4/5), M. chimaera (5/6) and M. intracellulare (3/4) isolates and 1 of 2 eligible M. malmoense isolates. No synergistic activity was measured against M. xenopi and M. simiae. Synergy was neither universal for all species nor for all isolates of one species; it thus needs to be tested for rather than assumed. Even if this synergy exists in vivo, it is questionable whether the MICs to the combined drugs can be overcome by the drug exposure attained by current regimens at the recommended dosages. New dosing strategies for rifampicin and ethambutol should be studied to increase the exposure to these drugs and thus maximise their impact.

The antibiotic resistance arrow of time: efflux pump induction is a general first step in the evolution of mycobacterial drug resistance

We hypothesize that low-level efflux pump expression is the first step in the development of high-level drug resistance in mycobacteria. We performed 28-day azithromycin dose-effect and dose-scheduling studies in our hollow-fiber model of disseminated Mycobacterium avium-M. intracellulare complex. Both microbial kill and resistance emergence were most closely linked to the within-macrophage area under the concentration-time curve (AUC)/MIC ratio. Quantitative PCR revealed that subtherapeutic azithromycin exposures over 3 days led to a 56-fold increase in expression of MAV_3306, which encodes a putative ABC transporter, and MAV_1406, which encodes a putative major facilitator superfamily pump, in M. avium. By day 7, a subpopulation of M. avium with low-level resistance was encountered and exhibited the classic inverted U curve versus AUC/MIC ratios. The resistance was abolished by an efflux pump inhibitor. While the maximal microbial kill started to decrease after day 7, a population with high-level azithromycin resistance appeared at day 28. This resistance could not be reversed by efflux pump inhibitors. Orthologs of pumps encoded by MAV_3306 and MAV_1406 were identified in Mycobacterium tuberculosis, Mycobacterium leprae, Mycobacterium marinum, Mycobacterium abscessus, and Mycobacterium ulcerans. All had highly conserved protein secondary structures. We propose that induction of several efflux pumps is the first step in a general pathway to drug resistance that eventually leads to high-level chromosomal-mutation-related resistance in mycobacteria as ordered events in an "antibiotic resistance arrow of time."

Potential use of nitrate reductase as a biomarker for the identification of active and dormant inhibitors of Mycobacterium tuberculosis in a THP1 infection model

The development of a macrophage-based, antitubercular high-throughput screening system could expedite discovery programs for identifying novel inhibitors. In this study, the kinetics of nitrate reduction (NR) by Mycobacterium tuberculosis during growth in Thp1 macrophages was found to be almost parallel to viable bacilli count. NR in the culture medium containing 50 mM of nitrate was found to be optimum on the fifth day after infection with M. tuberculosis. The signal-to-noise (S/N) ratio and Z-factor obtained from this macrophage-based assay were 5.4 and 0.965, respectively, which confirms the robustness of the assay protocol. The protocol was further validated by using standard antitubercular inhibitors such as rifampicin, isoniazid, streptomycin, ethambutol, and pyrazinamide, added at their IC(90) value, on the day of infection. These inhibitors were not able to kill the bacilli when added to the culture on the fifth day after infection. Interestingly, pentachlorophenol and rifampicin killed the bacilli immediately after addition on the fifth day of infection. Altogether, this assay protocol using M. tuberculosis-infected Thp-1 macrophages provides a novel, cost-efficient, robust, and easy-to-perform screening platform for the identification of both active and hypoxic stage-specific inhibitors against tuberculosis.

Ethambutol pharmacokinetic variability is linked to body mass in overweight, obese, and extremely obese people

We conducted a prospective study of 18 adult volunteers (male-to-female ratio of 1) whose body mass index fell into categories of <25, 25 to 40, or >40 kg/m(2), who received a single oral dose of 1,600 mg ethambutol. Only individuals with normal renal function were recruited. The minimum body mass (M) was 45.6 kg, the median was 90.8 kg, and the maximum weight was 160.4 kg. Ethambutol pharmacokinetics were best described by a two-compartment model. Inclusion of weight as a covariate dramatically improved the model, with a relative likelihood approaching infinity. The typical clearance was 42.6 liters/h. Ethambutol systemic clearance was proportional to (M/45.6)(3/4) and thus obeyed fractal geometry-based laws. This means that the area under the concentration-time curve (AUC) actually decreased for obese patients compared to that for leaner patients, reducing chances of concentration-dependent toxicity. On the other hand, such reduced AUCs could lead to therapy failure. Thus, new and individualized ethambutol dosing regimens need to be designed for obese and extremely obese patients.

Pharmacokinetic/pharmacodynamic-based treatment of disseminated Mycobacterium avium

Disseminated Mycobacterium avium complex (MAC) is treated with a macrolide and ethambutol. However, the kill rates are extremely slow so that therapy takes many months to years to achieve and even then more than 40% of patients are not completely cured. Recent studies have demonstrated that assays that detect extracellular MAC have a limited predictive value. Antibiotics kill at a much slower and more disappointing rate against bacilli within macrophages. Use of pharmacodynamic/pharmacokinetic models has resulted in design of new doses and dosing schedules for disseminated MAC, as well as new susceptibility breakpoints for ethambutol and moxifloxacin.

An oracle: antituberculosis pharmacokinetics-pharmacodynamics, clinical correlation, and clinical trial simulations to predict the future

Antimicrobial pharmacokinetic-pharmacodynamic (PK/PD) science and clinical trial simulations have not been adequately applied to the design of doses and dose schedules of antituberculosis regimens because many researchers are skeptical about their clinical applicability. We compared findings of preclinical PK/PD studies of current first-line antituberculosis drugs to findings from several clinical publications that included microbiologic outcome and pharmacokinetic data or had a dose-scheduling design. Without exception, the antimicrobial PK/PD parameters linked to optimal effect were similar in preclinical models and in tuberculosis patients. Thus, exposure-effect relationships derived in the preclinical models can be used in the design of optimal antituberculosis doses, by incorporating population pharmacokinetics of the drugs and MIC distributions in Monte Carlo simulations. When this has been performed, doses and dose schedules of rifampin, isoniazid, pyrazinamide, and moxifloxacin with the potential to shorten antituberculosis therapy have been identified. In addition, different susceptibility breakpoints than those in current use have been identified. These steps outline a more rational approach than that of current methods for designing regimens and predicting outcome so that both new and older antituberculosis agents can shorten therapy duration.