Comparison of the 'Denver regimen' against acute tuberculosis in the mouse and guinea pig

The heme oxygenase-1 metalloporphyrin inhibitor stannsoporfin enhances the bactericidal activity of a novel regimen for multidrug-resistant tuberculosis in a murine model

Multidrug-resistant (MDR) Mycobacterium tuberculosis (Mtb) poses significant challenges to global tuberculosis (TB) control efforts. Host-directed therapies (HDTs) offer a novel approach to TB treatment by enhancing immune-mediated clearance of Mtb. Prior preclinical studies found that the inhibition of heme oxygenase-1 (HO-1), an enzyme involved in heme metabolism, with tin-protoporphyrin IX (SnPP) significantly reduced mouse lung bacillary burden when co-administered with the first-line antitubercular regimen. Here, we evaluated the adjunctive HDT activity of a novel HO-1 inhibitor, stannsoporfin (SnMP), in combination with a novel MDR-TB regimen comprising a next-generation diarylquinoline, TBAJ-876 (S), pretomanid (Pa), and a new oxazolidinone, TBI-223 (O) (collectively, SPaO), in Mtb-infected BALB/c mice. After 4 weeks of treatment, SPaO + SnMP 5mg/kg reduced mean lung bacillary burden by an additional 0.69 log10 (P = 0.01) relative to SPaO alone. As early as 2 weeks post-treatment initiation, SnMP adjunctive therapy differentially altered the expression of pro-inflammatory cytokine genes and CD38, a marker of M1 macrophages. Next, we evaluated the sterilizing potential of SnMP adjunctive therapy in a mouse model of microbiological relapse. After 6 weeks of treatment, SPaO + SnMP 10mg/kg reduced lung bacterial burdens to 0.71 ± 0.23 log10 colony-forming units (CFUs), a 0.78 log-fold greater decrease in lung CFU compared to SpaO alone (P = 0.005). However, adjunctive SnMP did not reduce microbiological relapse rates after 5 or 6 weeks of treatment. SnMP was well tolerated and did not significantly alter gross or histological lung pathology. SnMP is a promising HDT candidate requiring further study in combination with regimens for drug-resistant TB.

Pharmacokinetic Considerations for Optimizing Inhaled Spray-Dried Pyrazinoic Acid Formulations

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a leading cause of death with 1.6 million deaths worldwide reported in 2021. Oral pyrazinamide (PZA) is an integral part of anti-TB regimens, but its prolonged use has the potential to drive the development of PZA-resistant Mtb. PZA is converted to the active moiety pyrazinoic acid (POA) by the Mtb pyrazinamidase encoded by pncA, and mutations in pncA are associated with the majority of PZA resistance. Conventional oral and parenteral therapies may result in subtherapeutic exposure in the lung; hence, direct pulmonary administration of POA may provide an approach to rescue PZA efficacy for treating pncA-mutant PZA-resistant Mtb. The objectives of the current study were to (i) develop novel dry powder POA formulations, (ii) assess their feasibility for pulmonary delivery using physicochemical characterization, (iii) evaluate their pharmacokinetics (PK) in the guinea pig model, and (iv) develop a mechanism-based pharmacokinetic model (MBM) using in vivo PK data to select a formulation providing adequate exposure in epithelial lining fluid (ELF) and lung tissue. We developed three POA formulations for pulmonary delivery and characterized their PK in plasma, ELF, and lung tissue following passive inhalation in guinea pigs. Additionally, the PK of POA following oral, intravenous, and intratracheal administration was characterized in guinea pigs. The MBM was used to simultaneously model PK data following administration of POA and its formulations via the different routes. The MBM described POA PK well in plasma, ELF, and lung tissue. Physicochemical analyses and MBM predictions suggested that POA maltodextrin was the best among the three formulations and an excellent candidate for further development as it has: (i) the highest ELF-to-plasma exposure ratio (203) and lung tissue-to-plasma exposure ratio (30.4) compared with POA maltodextrin and leucine (75.7/16.2) and POA leucine salt (64.2/19.3) and (ii) the highest concentration in ELF (CmaxELF: 171 nM) within 15.5 min, correlating with a fast transfer into ELF after pulmonary administration (KPM: 22.6 1/h). The data from the guinea pig allowed scaling, using the MBM to a human dose of POA maltodextrin powder demonstrating the potential feasibility of an inhaled product.

Pharmacokinetic considerations for optimizing inhaled spray-dried pyrazinoic acid formulations

Tuberculosis (TB), caused by Mycobacterium tuberculosis ( Mtb ), remains a leading cause of death with 1.6 million deaths worldwide reported in 2021. Oral pyrazinamide (PZA) is an integral part of anti-TB regimens, but its prolonged use has the potential to drive development of PZA resistant Mtb . PZA is converted to the active moiety pyrazinoic acid (POA) by the Mtb pyrazinamidase encoded by pncA , and mutations in pncA are associated with the majority of PZA resistance. Conventional oral and parenteral therapies may result in subtherapeutic exposure in the lung, hence direct pulmonary administration of POA may provide an approach to rescue PZA efficacy for treating pncA- mutant PZA-resistant Mtb . The objectives of the current study were to i) develop novel dry powder POA formulations ii) assess their feasibility for pulmonary delivery using physicochemical characterization, iii) evaluate their pharmacokinetics (PK) in the guinea pig model and iv) develop a mechanism based pharmacokinetic model (MBM) using in vivo PK data to select a formulation providing adequate exposure in epithelial lining fluid (ELF) and lung tissue. We developed three POA formulations for pulmonary delivery and characterized their PK in plasma, ELF, and lung tissue following passive inhalation in guinea pigs. Additionally, the PK of POA following oral, intravenous and intratracheal administration was characterized in guinea pigs. The MBM was used to simultaneously model PK data following administration of POA and its formulations via the different routes. The MBM described POA PK well in plasma, ELF and lung tissue. Physicochemical analyses and MBM predictions suggested that POA maltodextrin was the best among the three formulations and an excellent candidate for further development as it has: (i) the highest ELF-to-plasma exposure ratio (203) and lung tissue-to-plasma exposure ratio (30.4) compared with POA maltodextrin and leucine (75.7/16.2) and POA leucine salt (64.2/19.3); (ii) the highest concentration in ELF ( Cmac ELF : 171 nM) within 15.5 minutes, correlating with a fast transfer into ELF after pulmonary administration ( k PM : 22.6 1/h). The data from the guinea pig allowed scaling, using the MBM to a human dose of POA maltodextrin powder demonstrating the potential feasibility of an inhaled product.

Table of Contents TOC/Abstract Graphic

The potential of microdialysis to estimate rifampicin concentrations in the lung of guinea pigs

Optimised pre-clinical models are required for TB drug development to better predict the pharmacokinetics of anti-tuberculosis (anti-TB) drugs to shorten the time taken for novel drugs and combinations to be approved for clinical trial. Microdialysis can be used to measure unbound drug concentrations in awake freely moving animals in order to describe the pharmacokinetics of drugs in the organs as a continuous sampling technique. The aim of this work was to develop and optimise the microdialysis methodology in guinea pigs to better understand the pharmacokinetics of rifampicin in the lung. In vitro experiments were performed before progressing into in vivo studies because the recovery (concentration of the drug in the tissue fluid related to that in the collected dialysate) of rifampicin was dependent on a variety of experimental conditions. Mass spectrometry of the dialysate was used to determine the impact of flow rate, perfusion fluid and the molecular weight cut-off and membrane length of probes on the recovery of rifampicin at physiologically relevant concentrations. Following determination of probe efficiency and identification of a correlation between rifampicin concentrations in the lung and skeletal muscle, experiments were conducted to measure rifampicin in the sacrospinalis of guinea pigs using microdialysis. Lung concentrations of rifampicin were estimated from the rifampicin concentrations measured in the sacrospinalis. These studies suggest the potential usefulness of the microdialysis methodology to determine drug concentrations of selected anti-TB drugs to support new TB drug development.

Preserved Efficacy and Reduced Toxicity with Intermittent Linezolid Dosing in Combination with Bedaquiline and Pretomanid in a Murine Tuberculosis Model

The novel regimen of bedaquiline, pretomanid, and linezolid (BPaL) is highly effective against drug-resistant tuberculosis, but linezolid toxicities are frequent. We hypothesized that, for a similar total weekly cumulative dose, thrice-weekly administration of linezolid would preserve efficacy while reducing toxicity compared with daily dosing, in the context of the BPaL regimen. Using C3HeB/FeJ and BALB/c mouse models of tuberculosis disease, thrice-weekly linezolid dosing was compared with daily dosing, with intermittent dosing introduced (i) from treatment initiation or (ii) after an initial period of daily dosing. In all animals, BPa was dosed daily throughout treatment. Blood counts were used to assess hematologic toxicity. After unexpected findings of apparent antagonism, we conducted additional experiments to investigate strain-to-strain differences in the contribution of linezolid to regimen efficacy by comparing each 1- and 2-drug component to the BPaL regimen in BALB/c mice infected with Mycobacterium tuberculosis H37Rv or HN878. Giving linezolid daily for 1 to 2 months achieved the greatest efficacy but, after that, results were similar if the drug was stopped, dosed thrice-weekly, or continued daily. Erythrocyte counts were lower with daily than thrice-weekly dosing. Linezolid had additive effects with BPa against M. tuberculosis H37Rv but antagonistic effects with BPa against M. tuberculosis HN878. However, the overall efficacy of BPaL was high and similar against both strains. Dosing linezolid daily for the first 2 months and then less frequently thereafter may optimize its therapeutic margin. Linezolid's contribution to BPaL regimens may depend on the M. tuberculosis strain.

Metformin as Host-Directed Therapy for TB Treatment: Scoping Review

Tuberculosis (TB) disease is an international health concern caused by the bacteria Mycobacterium tuberculosis (Mtb). Evolution of multi-drug-resistant strains may cause bacterial persistence, rendering existing antibiotics ineffective. Hence, development of new or repurposing of currently approved drugs to fight Mtb in combination with existing antibiotics is urgently needed to cure TB which is refractory to current therapy. The shortening of TB therapy and reduction in lung injury can be achieved using adjunctive host-directed therapies. There is a wide range of probable candidates which include numerous agents permitted for the treatment of other diseases. One potential candidate is metformin, a Food and Drug Administration (FDA)-approved drug used to treat type 2 diabetes mellitus (DM). However, there is a scarcity of evidence supporting the biological basis for the effect of metformin as a host-directed therapy for TB. This scoping review summarizes the current body of evidence and outlines scientific gaps that need to be addressed in determining the potential role of metformin as a host-directed therapy.

Pharmacodynamic Correlates of Linezolid Activity and Toxicity in Murine Models of Tuberculosis

Background

Linezolid (LZD) is bactericidal against Mycobacterium tuberculosis, but it has treatment-limiting toxicities. A better understanding of exposure-response relationships governing LZD efficacy and toxicity will inform dosing strategies. Because in vitro monotherapy studies yielded conflicting results, we explored LZD pharmacokinetic/pharmacodynamic (PK/PD) relationships in vivo against actively and nonactively multiplying bacteria, including in combination with pretomanid.

Methods

Linezolid multidose pharmacokinetics were modeled in mice. Dose-fractionation studies were performed in acute (net bacterial growth) and chronic (no net growth) infection models. In acute models, LZD was administered alone or with bacteriostatic or bactericidal pretomanid doses. Correlations between PK/PD parameters and lung colony-forming units (CFUs) and complete blood counts were assessed.

Results

Overall, time above minimum inhibitory concentration (T_>MIC) correlated best with CFU decline. However, in growth-constrained models (ie, chronic infection, coadministration with pretomanid 50 mg/kg per day), area under the concentration-time curve over MIC (AUC/MIC) had similar explanatory power. Red blood cell counts correlated strongly with LZD minimum concentration (C_min).

Conclusions

Although T_>MIC was the most consistent correlate of efficacy, AUC/MIC was equally predictive when bacterial multiplication was constrained by host immunity or pretomanid. In effective combination regimens, administering the same total LZD dose less frequently may be equally effective and cause less C_min-dependent toxicity.

Improving treatment outcome assessment in a mouse tuberculosis model

Preclinical treatment outcome evaluation of tuberculosis (TB) occurs primarily in mice. Current designs compare relapse rates of different regimens at selected time points, but lack information about the correlation between treatment length and treatment outcome, which is required to efficiently estimate a regimens’ treatment-shortening potential. Therefore we developed a new approach. BALB/c mice were infected with a Mycobacterium tuberculosis Beijing genotype strain and were treated with rifapentine-pyrazinamide-isoniazid-ethambutol (R_pZHE), rifampicin-pyrazinamide-moxifloxacin-ethambutol (RZME) or rifampicin-pyrazinamide-moxifloxacin-isoniazid (RZMH). Treatment outcome was assessed in n = 3 mice after 9 different treatment lengths between 2–6 months. Next, we created a mathematical model that best fitted the observational data and used this for inter-regimen comparison. The observed data were best described by a sigmoidal E_max model in favor over linear or conventional E_max models. Estimating regimen-specific parameters showed significantly higher curative potentials for RZME and R_pZHE compared to RZMH. In conclusion, we provide a new design for treatment outcome evaluation in a mouse TB model, which (i) provides accurate tools for assessment of the relationship between treatment length and predicted cure, (ii) allows for efficient comparison between regimens and (iii) adheres to the reduction and refinement principles of laboratory animal use.

Preclinical Efficacy Testing of New Drug Candidates

This is a review of the preclinical efficacy testing of new antituberculosis drug candidates. It describes existing dynamic in vitro and in vivo models of antituberculosis chemotherapy and their utility in preclinical evaluations of promising new drugs and combination regimens, with an effort to highlight recent developments. Emphasis is given to the integration of quantitative pharmacokinetic/pharmacodynamic analyses and the impact of lesion pathology on drug efficacy. Discussion also includes in vivo models of chemotherapy of latent tuberculosis infection.

Assessment of Bactericidal Drug Activity and Treatment Outcome in a Mouse Tuberculosis Model Using a Clinical Beijing Strain

Mycobacterium tuberculosis Beijing strains are associated with lower treatment success rates in tuberculosis (TB) patients. In contrast, laboratory strains such as H37Rv are often used in preclinical tuberculosis models. Therefore, we explored the impact of using a clinical Beijing strain on treatment outcome in our mouse tuberculosis model. Additionally, the predictive value of bactericidal activity on treatment outcome was assessed. BALB/c mice were infected with a Beijing strain and treated with one of 10 different combinations of conventional anti-TB drugs. Bactericidal activity was assessed by determining reductions in mycobacterial load after 7, 14, and 28 days and after 2, 3, and 6 months of treatment. Treatment outcome was evaluated after a 6-month treatment course and was based on lung culture status 3 months posttreatment. None of the anti-TB drug regimens tested could achieve 100% treatment success. Treatment outcome depended critically on rifampin. Four non-rifampin-containing regimens showed 0% treatment success compared to success rates between 81 and 95% for six rifampin-containing regimens. Bactericidal activity was predictive only for treatment outcome after 3 months of treatment. Our data advocate the use of multiple mycobacterial strains, including a Beijing strain, to increase the translational value of mouse TB models evaluating treatment outcome. Additionally, our findings support the notion that bactericidal activity in the first 2 months of treatment, as measured in clinical phase IIa/b trials, has limited predictive value for tuberculosis treatment outcome, thus emphasizing the need for better parameters to guide future phase IIII trials.

Cordyceps sinensis promotes immune regulation and enhances bacteriostatic activity of PA-824 via IL-10 in Mycobacterium tuberculosis disease

PA-824 is a novel bicyclic nitroimidazole anti-tuberculosis (TB) drug. Cordyceps sinensis (Berk.) Sacc. (CS) was proven to be a good immunomodulatory compound. This research aimed to investigate the effect of CS on PA-824 in Mycobacterium tuberculosis (M.tb) infected mice (female CBA/J mice, 6 to 8 weeks of age and 20±2 g of weight). Mice were randomly assigned to 4 groups: PA-824, CS, PA-824+CS, and control. To verify the effect of PA-824 and CS on M.tb, after drug administration, mice lungs were harvested and bacterial colony formations were measured. Cells were isolated from infected lungs and spleens to analyze the percentage of CD4⁺ T cells (CD11a positive). Lung cells were cultured to detect the secretion of interferon-γ (IFN-γ) and interleukin-10 (IL-10) by ELISA. IFN-γ and IL-10 double-positive CD4⁺ cells in peripheral blood were measured by flow cytometry. The expression levels of IL-2 and IL-10 in mice lungs were analyzed by real-time PCR and western blot. Results showed that PA-824 combined with CS led to the lowest lung colony-forming units (CFU) counts among treated groups. Furthermore, this beneficial outcome might be associated with the decreased CD11a on CD4⁺ cells in mice lungs and spleens. Moreover, the suppressed secretion of IFN-γ and IL-10, and IL-10 expressions, as well as the decreased IFN-γ and IL-10 double-positive CD4⁺ cells in blood, could also be associated with the positive effect. However, no significant effect on IL-2 production was found. The combination of PA-824 and CS had more effective bacteriostatic and immunomodulatory effects on M.tb infected mice than PA-824 alone. In conclusion, CS has the potential to be an effective adjuvant in TB treatment.

Metformin Adjunctive Therapy Does Not Improve the Sterilizing Activity of the First-Line Antitubercular Regimen in Mice

Preliminary preclinical and observational studies suggest the potential utility of metformin as an adjunctive, host-directed agent for treatment of tuberculosis (TB). In this study, we sought to investigate the bactericidal and sterilizing activities of human-like exposures of metformin when given in combination with the first-line regimen against chronic tuberculosis in BALB/c mice. Mice receiving metformin adjunctive therapy had similar lung bacillary burdens with control mice during treatment, and the proportion of mice with microbiological relapse was similar between the two groups.

Delamanid Kills Dormant Mycobacteria In Vitro and in a Guinea Pig Model of Tuberculosis

Tuberculosis (TB) treatment is long and requires multiple drugs, likely due to various phenotypes of TB bacilli with variable drug susceptibilities. Drugs with broad activity are urgently needed. This study aimed to evaluate delamanid's activity against growing or dormant bacilli in vitro as well as in vivo Cultures of Mycobacterium bovis BCG Tokyo under aerobic and anaerobic conditions were used to study the activity of delamanid against growing and dormant bacilli, respectively. Delamanid exhibited significant bactericidal activity against replicating and dormant bacilli at or above concentrations of 0.016 and 0.4 mg/liter, respectively. To evaluate delamanid's antituberculosis activity in vivo, we used a guinea pig model of chronic TB infection in which the lung lesions were similar to those in human TB disease. In the guinea pig TB model, a daily dose of 100 mg delamanid/kg of body weight for 4 or 8 weeks demonstrated strong bactericidal activity against Mycobacterium tuberculosis Importantly, histological examination revealed that delamanid killed TB bacilli within hypoxic lesions of the lung. The combination regimens containing delamanid with rifampin and pyrazinamide or delamanid with levofloxacin, ethionamide, pyrazinamide, and amikacin were more effective than the standard regimen (rifampin, isoniazid, and pyrazinamide). Our data show that delamanid is effective in killing both growing and dormant bacilli in vitro and in the guinea pig TB model. Adding delamanid to current TB regimens may improve treatment outcomes, as demonstrated in recent clinical trials with pulmonary multidrug-resistant (MDR) TB patients. Delamanid may be an important drug for consideration in the construction of new regimens to shorten TB treatment duration.

Assessment of treatment response by colony forming units, time to culture positivity and the molecular bacterial load assay compared in a mouse tuberculosis model

The aim of the study is to compare counting of colony forming units (CFU), the time to positivity (TTP) assay and the molecular bacterial load (MBL) assay, and explore whether the last assays can detect a subpopulation which is unable to grown on solid media. CFU counting, TTP and the MBL assay were used to determine the mycobacterial load in matched lung samples of a murine tuberculosis model. Mice were treated for 24 weeks with 4 treatment arms: isoniazid (H) - rifampicin (R) - pyrazinamide (Z), HRZ-Streptomycin (S), HRZ - ethambutol (E) or ZES. Inverse relationships were observed when comparing TPP with CFU or MBL. Positive associations were observed when comparing CFU with MBL. Description of the net elimination of bacteria was performed for CFU vs. time, MBL vs. time and 1/TTP vs. time and fitted by nonlinear regression. CFU vs. time and 1/TTP vs. time showed bi-phasic declines with the exception of HRZE. A similar rank order, based on the alpha slope, was found comparing CFU vs. time and TTP vs. time, respectively HRZE, HRZ, HRZS and ZES. In contrast, MBL vs. time showed a mono-phasic decline with a flat gradient of elimination and a different rank order respectively, ZES, HRZ, HRZE and HRZS. The correlations found between methods reflects the ability of each to discern the general mycobacterial load. Based on the description of net elimination, we conclude that the MBL assay can detect a subpopulation of Mycobacterium tuberculosis which is not detected by the CFU or TTP assays.

In vitro and in vivo fitness costs associated with Mycobacterium tuberculosis RpoB mutation H526D

AIM

There is controversy regarding the potential fitness costs of rifampicin (RIF) resistance-conferring mutations in the Mycobacterium tuberculosis (Mtb) rpoB gene. We characterized the pathogenicity of an Mtb RpoB H526D mutant.

MATERIALS & METHODS

A mutant containing the RpoB H526D mutation was isolated from wild-type Mtb grown on RIF-containing plates and complemented for determination of in vitro and in vivo fitness costs.

RESULTS

The RpoB H526D mutant showed reduced survival relative to control strains during progressive hypoxia and delayed growth following resuscitation from nutrient starvation (p < 0.05), which was associated with reduced expression of the resuscitation-promoting factor genes rpfB, rpfC and rpfE. Relative to the isogenic wild-type strain, the mutant showed significantly attenuated growth and long-term survival as well as reduced inflammation in mouse lungs. Conclusion & future perspective: Our data suggest that RpoB H526D mutation confers a fitness cost during growth-limiting conditions in vitro and in mouse lungs.

Drug regimens identified and optimized by output-driven platform markedly reduce tuberculosis treatment time

The current drug regimens for treating tuberculosis are lengthy and onerous, and hence complicated by poor adherence leading to drug resistance and disease relapse. Previously, using an output-driven optimization platform and an in vitro macrophage model of Mycobacterium tuberculosis infection, we identified several experimental drug regimens among billions of possible drug-dose combinations that outperform the current standard regimen. Here we use this platform to optimize the in vivo drug doses of two of these regimens in a mouse model of pulmonary tuberculosis. The experimental regimens kill M. tuberculosis much more rapidly than the standard regimen and reduce treatment time to relapse-free cure by 75%. Thus, these regimens have the potential to provide a markedly shorter course of treatment for tuberculosis in humans. As these regimens omit isoniazid, rifampicin, fluoroquinolones and injectable aminoglycosides, they would be suitable for treating many cases of multidrug and extensively drug-resistant tuberculosis.

Current antibiotic therapies for tuberculosis are lengthy and onerous. Here, the authors use an output-driven approach to optimize drug doses for two experimental drug regimens in a mouse model of tuberculosis, leading to improved regimens that reduce treatment time by 75%.

Bedaquiline and Pyrazinamide Treatment Responses Are Affected by Pulmonary Lesion Heterogeneity in Mycobacterium tuberculosis Infected C3HeB/FeJ Mice

BALB/c and Swiss mice are routinely used to validate the effectiveness of tuberculosis drug regimens, although these mouse strains fail to develop human-like pulmonary granulomas exhibiting caseous necrosis. Microenvironmental conditions within human granulomas may negatively impact drug efficacy, and this may not be reflected in non-necrotizing lesions found within conventional mouse models. The C3HeB/FeJ mouse model has been increasingly utilized as it develops hypoxic, caseous necrotic granulomas which may more closely mimic the pathophysiological conditions found within human pulmonary granulomas. Here, we examined the treatment response of BALB/c and C3HeB/FeJ mice to bedaquiline (BDQ) and pyrazinamide (PZA) administered singly and in combination. BALB/c mice consistently displayed a highly uniform treatment response to both drugs, while C3HeB/FeJ mice displayed a bimodal response composed of responsive and less-responsive mice. Plasma pharmacokinetic analysis of dissected lesions from BALB/c and C3HeB/FeJ mice revealed that PZA penetrated lesion types from both mouse strains with similar efficiency. However, the pH of the necrotic caseum of C3HeB/FeJ granulomas was determined to be 7.5, which is in the range where PZA is essentially ineffective under standard laboratory in vitro growth conditions. BDQ preferentially accumulated within the highly cellular regions in the lungs of both mouse strains, although it was present at reduced but still biologically relevant concentrations within the central caseum when dosed at 25 mg/kg. The differential treatment response which resulted from the heterogeneous pulmonary pathology in the C3HeB/FeJ mouse model revealed several factors which may impact treatment efficacy, and could be further evaluated in clinical trials.

Mycobacterial Protein Tyrosine Phosphatases A and B Inhibitors Augment the Bactericidal Activity of the Standard Anti-tuberculosis Regimen

Novel drugs are required to shorten the duration of treatment for tuberculosis (TB) and to combat the emergence of drug resistance. One approach has been to identify and target Mycobacterium tuberculosis (Mtb) virulence factors, which promote the establishment of TB infection and pathogenesis. Mtb produces a number of virulence factors, including two protein tyrosine phosphatases (PTPs), mPTPA and mPTPB, to evade the antimicrobial functions of host macrophages. To assess the therapeutic potential of targeting the virulent Mtb PTPs, we developed highly potent and selective inhibitors of mPTPA (L335-M34) and mPTPB (L01-Z08) with drug-like properties. We tested the bactericidal activity of L335-M34 and L01-Z08 alone or together in combination with the standard antitubercular regimen of isoniazid-rifampicin-pyrazinamide (HRZ) in the guinea pig model of chronic TB infection, which faithfully recapitulates some of the key histological features of human TB lesions. Following a single dose of L335-M34 50mg/kg and L01-Z08 20 mg/kg, plasma levels were maintained at levels 10-fold greater than the biochemical IC₅₀ for 12-24 hours. Although neither PTP inhibitor alone significantly enhanced the antibacterial activity of HRZ, dual inhibition of mPTPA and mPTPB in combination with HRZ showed modest synergy, even after 2 weeks of treatment. After 6 weeks of treatment, the degree of lung inflammation correlated with the bactericidal activity of each drug regimen. This study highlights the potential utility of targeting Mtb virulence factors, and specifically the Mtb PTPs, as a strategy for enhancing the activity of standard anti-TB treatment.

Toward an Evidence-Based Nonclinical Road Map for Evaluating the Efficacy of New Tuberculosis (TB) Drug Regimens: Proceedings of a Critical Path to TB Drug Regimens-National Institute of Allergy and Infectious Diseases In Vivo Pharmacology Workshop for TB Drug Development

Novel tuberculosis (TB) drug regimens are urgently needed, and their development will be enabled by improved preclinical approaches that more effectively inform and ensure safe selection of clinical candidates and drug combination/regimens. An evidence-based approach for the assessment of nonclinical models supporting TB drug development has been proposed by a joint partnership between the National Institute of Allergy and Infectious Diseases (NIAID) and the Critical Path to TB Drug Regimens (CPTR) Consortium.

Pharmacokinetics of Inhaled Rifampicin Porous Particles for Tuberculosis Treatment: Insight into Rifampicin Absorption from the Lungs of Guinea Pigs

Tuberculosis (TB) is a life-threatening infection that requires a lengthy treatment process that is often associated with adverse effects. Pulmonary delivery of anti-TB drugs has the potential to increase efficacy of treatment by increasing drug concentrations at the lungs, the primary site of infection. The aim of the present study is to evaluate the disposition of rifampicin (RIF) after its pulmonary administration as porous particles (PPs) to guinea pigs and contrast it to that after oral administration. RIF microparticles were prepared by spray drying a solution of RIF and L-leucine (9:1), and the resulting particles were characterized for their physicochemical properties. Animals received RIF either as intravenous solution (iv), as oral suspension of micronized RIF (ORS) and RIF-PPs (ORPP), or by insufflation of the PPs (IRPP). Plasma samples were collected at preselected time points, and bronchoalveolar lavage (BAL) was performed at the end of the study. RIF concentrations in biological samples were analyzed by HPLC. Plasma concentration versus time data was analyzed by compartmental and noncompartmental methods. RIF PPs were thin walled porous particles with mass median aerodynamic diameter (MMAD) of 4.8±0.1 μm, GSD=1.29±0.03, and fine particle fraction below 5.8 μm of 52.9±2.0%. RIF content in the resulting particles was 91.8±0.1%. Plasma concentration vs time profiles revealed that the terminal slope of the iv group was different from that of the oral or pulmonary groups, indicating the possibility of flip-flop kinetics. RIF from IRPP appeared to be absorbed faster than that of ORPP or ORS as evidenced by higher RIF plasma concentrations up until 2 h. Notably, similar AUC (when corrected by dose), similar CL, λ, and half-life were obtained after oral administration of RIF at 40 mg/kg and pulmonary administration of RIF at 20 mg/kg. However, RIF in the IRPP group had a shorter Tmax and higher bioavailability than orally dosed groups. In addition, RIF concentrations in the BAL of animals in the IRPP group were 3-4-fold larger than those in the orally dosed groups. The disposition in ORS and ORPP were best described by a model with two sequential compartments, whereas the disposition of IRPP was best described by a two parallel compartment model. The advantages of delivering RIF by the pulmonary route are demonstrated in the present study. These include achieving higher RIF concentrations in the lungs and similar systemic levels after pulmonary delivery of one-half of the oral nominal dose. This is expected to result in a more effective treatment of pulmonary TB, as shown previously in published efficacy studies.

senX3-independent contribution of regX3 to Mycobacterium tuberculosis virulence

Background

Mycobacterium tuberculosis (Mtb) must adapt to various stress conditions during host infection. The two-component regulatory system (2CRS) SenX3-RegX3 is required for Mtb virulence. We showed recently that the senX3-regX3 intergenic region contains promoter activity, driving senX3-independent regX3 expression. In the current study, we tested the hypothesis that RegX3 has a SenX3-independent role in Mtb virulence. The gene expression patterns, growth, and survival of mutants containing transposon insertions in senX3 (senX3::Tn) and regX3 (regX3::Tn) were compared to those of their respective complemented strains and the isogenic wild-type parent strain during axenic growth in nutrient-rich broth, phosphate depletion, nutrient starvation, and in the lungs of BALB/c mice.

Results

regX3 expression was reduced in senX3::Tn during phosphate depletion and nutrient starvation, and expression of the phosphate-specific transport gene pstC2 was reduced similarly in senX3::Tn and regX3::Tn during phosphate depletion. Although senX3 and regX3 were each dispensable for Mtb growth in nutrient-rich broth, disruption of senX3 or regX3 caused a similar growth defect during phosphate depletion. Interestingly, senX3::Tn, in which monocistronic regX3 expression is preserved, showed significantly higher survival relative to regX3::Tn after 7 days of nutrient starvation (p <0.01), and in mouse lungs at Day 31 (p < 0.01), Day 62 (p < 0.01), and Day 124 (p = 0.05) after aerosol infection.

Conclusion

Our data demonstrate the specificity of the senX3-regX3 2CRS for sensing and responding to low ambient phosphate, but also raise the possibility that RegX3 may function independently of its cognate sensor histidine kinase.

Reduced emergence of isoniazid resistance with concurrent use of thioridazine against acute murine tuberculosis

The repurposing of existing drugs is being pursued as a means by which to accelerate the development of novel regimens for the treatment of drug-susceptible and drug-resistant tuberculosis (TB). In the current study, we assessed the activity of the antipsychotic drug thioridazine (TRZ) in combination with the standard regimen in a well-validated murine TB model. Single-dose and steady-state pharmacokinetic studies were performed in BALB/c mice to establish human-equivalent doses of TRZ. To determine the bactericidal activity of TRZ against TB in BALB/c mice, three separate studies were performed, including a dose-ranging study of TRZ monotherapy and efficacy studies of human-equivalent doses of TRZ with and without isoniazid (INH) or rifampin (RIF). Therapeutic efficacy was assessed by the change in mycobacterial load in the lung. The human-equivalent dose of thioridazine was determined to be 25 mg/kg of body weight, which was well tolerated in mice. TRZ was found to accumulate at high concentrations in lung tissue relative to serum levels. We observed modest synergy during coadministration of TRZ with INH, and the addition of TRZ reduced the emergence of INH-resistant mutants in mouse lungs. In conclusion, this study further illustrates the opportunity to reevaluate the contribution of TRZ to the sterilizing activity of combination regimens to prevent the emergence of drug-resistant M. tuberculosis.

Differential regulation of the two-component regulatory system senX3-regX3 in Mycobacterium tuberculosis

The highly successful pathogen Mycobacterium tuberculosis (Mtb) has evolved strategies to adapt to various stress conditions, thus promoting survival within the infected host. The two-component regulatory system (2CRS) senX3-regX3, which has been implicated in the Mtb response to inorganic phosphate depletion, is believed to behave as an auto-regulatory bicistronic operon. Unlike other 2CRS, Mtb senX3-regX3 features an intergenic region (IR) containing several mycobacterium interspersed repetitive units (MIRU) of unknown function. In this study, we used a lacZ reporter system to study the promoter activity of the 5' untranslated region of senX3, and that of various numbers of MIRUs in the senX3-regX3 IR, during axenic Mtb growth in nutrient-rich broth, and upon exposure to growth-restricting conditions. Activity of the senX3 promoter was induced during phosphate depletion and nutrient starvation, and IR promoter activity under these conditions was directly proportional to the number of MIRUs present. Quantitative reverse transcriptase (qRT)-PCR analysis of exponentially growing Mtb revealed monocistronic transcription of senX3 and regX3, and, to a lesser degree, bicistronic transcription of the operon. In addition, we observed primarily monocistronic upregulation of regX3 during phosphate depletion of Mtb, which was confirmed by Northern analysis in wild-type Mtb and by RT-PCR in a senX3-disrupted mutant, while upregulation of regX3 in nutrient-starved Mtb was chiefly bicistronic. Our findings of differential regulation of senX3-regX3 highlight the potential regulatory role of MIRUs in the Mtb genome and provide insight into the regulatory mechanisms underlying Mtb adaptation to physiologically relevant conditions.

An update on the use of rifapentine for tuberculosis therapy

INTRODUCTION

Tuberculosis (TB) remains rampant throughout the world, in large part due to the lengthy treatment times of current therapeutic options. Rifapentine, a rifamycin antibiotic, is currently approved for intermittent dosing in the treatment of TB. Recent animal studies have shown that more frequent administration of rifapentine could shorten treatment times, for both latent and active TB infection. However, these results were not replicated in a subsequent human clinical trial.

AREAS COVERED

This review analyses the evidence for more frequent administration of rifapentine and the reasons for the apparent lack of efficacy in shortening treatment times in human patients. Inhaled delivery is discussed as a potential option to achieve the therapeutic effect of rifapentine by overcoming the barriers associated with oral administration of this drug. Avenues for developing an inhalable form of rifapentine are also presented.

EXPERT OPINION

Rifapentine is a promising active pharmaceutical ingredient with potential to accelerate treatment of TB if delivered by inhaled administration. Progression of current fundamental work on inhaled anti-tubercular therapies to human clinical trials is essential for determining their role in future treatment regimens. While the ultimate goal for global TB control is a vaccine, a short and effective treatment option is equally crucial.

Contribution of moxifloxacin or levofloxacin in second-line regimens with or without continuation of pyrazinamide in murine tuberculosis

RATIONALE

High-dose levofloxacin (L) (1,000 mg) was as active as moxifloxacin (M) (400 mg) in an early bactericidal activity trial, suggesting these fluoroquinolones could be used interchangeably. Whether pyrazinamide (Z) contributes sterilizing activity beyond the first 2 months in fluoroquinolone-containing second-line regimens remains unknown.

OBJECTIVES

We compared the efficacy of M and high-dose L alone or in combination with ethionamide (Et), amikacin (A), and Z given for 2 or 7 months.

METHODS

A pharmacokinetic study was performed to determine the L dose equivalent to 1,000 mg in humans. Treatment started 2 weeks after aerosol infection with Mycobacterium tuberculosis H37Rv. Mice received M or L alone or in combination with 2 months of EtZA followed by 5 months of Et or EtZ.

MEASUREMENTS AND MAIN RESULTS

After 2 months of treatment, lung colony-forming unit (CFU) counts were similar in mice receiving either fluoroquinolone alone, but, after 4 and 5 months, CFU counts were 2 log10 lower in mice receiving M. Mice receiving 2MEtZA/3MEt and 2LEtZA/3LEt had 1.0 and 2.7 log10 lung CFUs, respectively. When Z was given throughout, both regimens rendered mice culture negative by 5 months, and most mice did not relapse after 7 months of treatment, with fewer relapses observed in the M group after 6 and 7 months of treatment.

CONCLUSIONS

In murine tuberculosis, M had superior efficacy compared with L despite lower serum drug exposures and may remain the fluoroquinolone of choice for second-line regimens. Z contributed substantial sterilizing activity beyond 2 months in fluoroquinolone-containing second-line regimens, largely compensating for L's weaker activity.

Assessment of clofazimine activity in a second-line regimen for tuberculosis in mice

RATIONALE

Although observational studies suggest that clofazimine-containing regimens are highly active against drug-resistant tuberculosis, the contribution of clofazimine for the treatment of this disease has never been systematically evaluated.

OBJECTIVES

Our goal was to directly compare the activity of a standard second-line drug regimen with or without the addition of clofazimine in a mouse model of multidrug-resistant tuberculosis. Our comparative outcomes included time to culture conversion in the mouse lungs and the percentage of relapses after treatment cessation.

METHODS

Mice were aerosol-infected with an isoniazid-resistant (as a surrogate of multidrug-resistant) strain of Mycobacterium tuberculosis. Treatment, which was administered for 5 to 9 months, was initiated 2 weeks after infection and comprised the following second-line regimen: daily (5 d/wk) moxifloxacin, ethambutol, and pyrazinamide, supplemented with amikacin during the first 2 months. One-half of the mice also received daily clofazimine. The decline in lung bacterial load was assessed monthly using charcoal-containing agar to reduce clofazimine carryover. Relapse was assessed 6 months after treatment cessation.

MEASUREMENTS AND MAIN RESULTS

After 2 months, the bacillary load in lungs was reduced from 9.74 log10 at baseline to 3.61 and 4.68 in mice treated with or without clofazimine, respectively (P < 0.001). Mice treated with clofazimine were culture-negative after 5 months, whereas all mice treated without clofazimine remained heavily culture-positive for the entire 9 months of the study. The relapse rate was 7% among mice treated with clofazimine for 8 to 9 months.

CONCLUSIONS

The clofazimine contribution was substantial in these experimental conditions.

A medicinal chemists' guide to the unique difficulties of lead optimization for tuberculosis

Tuberculosis is a bacterial disease that predominantly affects the lungs and results in extensive tissue pathology. This pathology contributes to the complexity of drug development as it presents discrete microenvironments within which the bacterium resides, often under conditions where replication is limited and intrinsic drug susceptibility is low. This consolidated pathology also results in impaired vascularization that limits access of potential lead molecules to the site of infection. Translating these considerations into a target-product profile to guide lead optimization programs involves implementing unique in vitro and in vivo assays to maximize the likelihood of developing clinically meaningful candidates.

Potent rifamycin-sparing regimen cures guinea pig tuberculosis as rapidly as the standard regimen

Strategies involving new drug combinations, as well as new uses of existing drugs, are urgently needed to reduce the time required to cure patients with drug-sensitive or multidrug-resistant (MDR) tuberculosis (TB). We compared the sterilizing activity of the standard first-line antitubercular regimen, rifampin-isoniazid-pyrazinamide (RHZ), with that of the novel regimen PA-824-moxifloxacin-pyrazinamide (PaMZ), which is currently being studied in clinical trials (NCT01498419), in the guinea pig model of chronic TB infection, in which animals develop necrotic granulomas histologically resembling their human counterparts. Guinea pigs were aerosol infected with ~2 log10 bacilli of wild-type Mycobacterium tuberculosis H37Rv, and antibiotic treatment was initiated 6 weeks after infection. Separate groups of animals received RHZ, PaMZ, or single or two-drug components of the latter regimen administered at human-equivalent doses 5 days/week for a total of 8 weeks. Relapse rates were assessed 3 months after discontinuation of treatment to determine the sterilizing activity of each combination regimen. PaMZ given at human-equivalent doses was safe and well tolerated for the entire treatment period and rendered guinea pig lungs culture negative more rapidly than RHZ did. After 1 month of treatment, 80% and 50% of animals in the RHZ and PaMZ groups, respectively, had lung culture-positive relapse. Both combination regimens prevented microbiological relapse when administered for a total of 2 months. Our data support the use of PaMZ as a novel isoniazid- and rifamycin-sparing regimen suitable for treatment of both drug-sensitive TB and MDR-TB.

Thioridazine lacks bactericidal activity in an animal model of extracellular tuberculosis

OBJECTIVES

The antipsychotic drug thioridazine is active in the murine model of tuberculosis infection, which is predominantly intracellular in nature. Recent clinical reports suggest that thioridazine may play a role in the treatment of drug-resistant tuberculosis. We studied the tuberculocidal activity of thioridazine in guinea pigs, which develop necrotic lung granulomas histologically resembling their human counterparts.

METHODS

Pharmacokinetic studies were performed in guinea pigs to establish human-equivalent doses of thioridazine. Guinea pigs were aerosol-infected with ∼100 bacilli of Mycobacterium tuberculosis and single-drug treatment was started 4 weeks later with a range of thioridazine doses daily (5 days/week) for up to 4 weeks. Control animals received no treatment or 60 mg/kg isoniazid.

RESULTS

The human-equivalent dose of thioridazine was determined to be 5 mg/kg with saturable absorption noted above 50 mg/kg. At the start of treatment, the lung bacterial burden was ∼6.2 log10 cfu. Although isoniazid reduced bacillary counts more than 10-fold, thioridazine monotherapy showed limited killing over the range of doses tested, reducing lung bacillary counts by 0.3-0.5 log10 following 1 month of treatment. Thioridazine was tolerated up to 40 mg/kg.

CONCLUSIONS

Thioridazine has limited bactericidal activity against extracellular bacilli within necrotic granulomas. Its contribution to the sterilizing activity of combination regimens against drug-susceptible and drug-resistant tuberculosis remains to be determined.

Preliminary pharmacokinetic study of repeated doses of rifampin and rifapentine in guinea pigs

Substitution of rifapentine (RFP) for rifampin (RIF) in the standard antituberculous regimen reduces the time required to cure chronic tuberculosis (TB) infection in mice, but not in guinea pigs. In order to gain insight into these discrepant findings, we conducted a steady-state pharmacokinetic (PK) study in healthy guinea pigs to study the metabolism and autoinduction of RIF and RFP. Both RFP and RIF 25-desacetyl metabolites (desRFP and desRIF, respectively), were detected at low concentrations in the serum of guinea pigs. The metabolite concentrations in guinea pigs are much lower than those seen in humans at steady state.

Activity of 5-chloro-pyrazinamide in mice infected with Mycobacterium tuberculosis or Mycobacterium bovis

BACKGROUND & OBJECTIVES

Pyrazinamide is an essential component of first line anti-tuberculosis regimen as well as most of the second line regimens. This drug has a unique sterilizing activity against Mycobacterium tuberculosis. Its unique role in tuberculosis treatment has lead to the search and development of its structural analogues. One such analogue is 5-chloro-pyrazinamide (5-Cl-PZA) that has been tested under in vitro conditions against M. tuberculosis. The present study was designed with an aim to assess the activity of 5-Cl-PZA, alone and in combination with first-line drugs, against murine tuberculosis.

METHODS

The minimum inhibitory concentration (MIC) of 5-Cl-PZA in Middlebrook 7H9 broth (neutral pH) and the inhibitory titre of serum from mice that received a 300 mg/kg oral dose of 5-Cl-PZA 30 min before cardiac puncture were determined. To test the tolerability of orally administered 5-Cl-PZA, uninfected mice received doses up to 300 mg/kg for 2 wk. Four weeks after low-dose aerosol infection either with M. tuberculosis or M. bovis, mice were treated 5 days/wk with 5-Cl-PZA, at doses ranging from 37.5 to 150 mg/kg, either alone or in combination with isoniazid and rifampicin. Antimicrobial activity was assessed by colony-forming unit counts in lungs after 4 and 8 wk of treatment.

RESULTS

The MIC of 5-Cl-PZA against M. tuberculosis was between 12.5 and 25 μg/ml and the serum inhibitory titre was 1:4. Under the same experimental conditions, the MIC of pyrazinamide was >100 μg/ml and mouse serum had no inhibitory activity after a 300 mg/kg dose; 5-Cl-PZA was well tolerated in uninfected and infected mice up to 300 and 150 mg/kg, respectively. While PZA alone and in combination exhibited its usual antimicrobial activity in mice infected with M. tuberculosis and no activity in mice infected with M. bovis, 5-Cl-PZA exhibited antimicrobial activity neither in mice infected with M. tuberculosis nor in mice infected with M. bovis.

INTERPRETATION & CONCLUSION

Our findings showed that 5-Cl-PZA at doses up to 150 mg/kg was not active in chronic murine TB model. Further studies need to be done to understand the mechanism and mode of inactivation in murine model of tuberculosis.

Rifapentine is not more active than rifampin against chronic tuberculosis in guinea pigs

Rifamycins are key sterilizing drugs in the current treatment of active tuberculosis (TB). Daily dosing of rifapentine (P), a potent rifamycin with high intracellular accumulation, in place of rifampin (R) in the standard antitubercular regimen significantly shortens the duration of treatment needed to prevent relapse in a murine model of active TB. We undertook the current study to compare directly the activities of human-equivalent doses of P and R in a guinea pig model of chronic TB, in which bacilli are predominantly extracellular within human-like necrotic granulomas. Hartley strain guinea pigs were aerosol infected with ~200 bacilli of Mycobacterium tuberculosis H37Rv, and treatment given 5 days/week was initiated 6 weeks later. R at 100 mg/kg of body weight and P at 100 mg/kg were given orally alone or in combination with isoniazid (H) at 60 mg/kg and pyrazinamide (Z) at 300 mg/kg. Culture-positive relapse was assessed in subgroups of guinea pigs after completion of 1 and 2 months of treatment. Human-equivalent doses of R and P showed equivalent bactericidal activity when used alone and in combination therapy. In guinea pigs treated with rifampin, isoniazid, and pyrazinamide (RHZ) or PHZ, microbiological relapse occurred in the lungs of 8/10 animals treated for 1 month and in 0/10 animals treated for 2 months. Substitution of P for R in the standard antitubercular regimen did not shorten the time to cure in this guinea pig model of chronic TB. Data from ongoing clinical trials comparing the activity of these two drugs are awaited to determine the relevance of the guinea pig TB model in preclinical drug screening.

Evaluation of a mouse model of necrotic granuloma formation using C3HeB/FeJ mice for testing of drugs against Mycobacterium tuberculosis

Persistence of Mycobacterium tuberculosis remains a significant challenge for the effective treatment of tuberculosis in humans. In animals that develop necrotic lung lesions following infection with M. tuberculosis, drug-tolerant bacilli are present and persist in an extracellular microenvironment within the necrotic cores. In this study, we examined the efficacy of drug treatment in C3HeB/FeJ (Kramnik) mice that develop lesions with liquefactive necrosis, in comparison to BALB/c mice that develop nonnecrotic lesions following aerosol challenge. To accomplish this, Kramnik and BALB/c mice were infected by aerosol with M. tuberculosis and treated for 7 to 8 weeks with monotherapy using drugs with different modes of action. The efficacy of drug therapy was quantified by enumeration of bacterial load. The progression of disease and location and distribution of bacilli within lesions were visualized using various staining techniques. In the late stages of infection, Kramnik mice developed fibrous encapsulated lung lesions with central liquefactive necrosis containing abundant extracellular bacilli, whereas BALB/c mice formed nonnecrotic lesions with primarily intracellular bacilli. Necrotic lesions in Kramnik mice showed evidence of hypoxia by pimonidazole staining. Kramnik mice were significantly more refractory to drug therapy, especially for pyrazinamide. Metronidazole showed no bactericidal activity in either model. There were significantly higher numbers of drug-resistant colonies isolated from the Kramnik mice compared to BALB/c mice. These results suggest that the Kramnik mouse model will be a valuable model to test antituberculosis drugs, especially against bacilli that persist within necrotic lesions.

Models and approaches for anti-TB drug testing

Unique challenges remain in the development of new drugs for the treatment of TB. While existing multidrug treatment regimens are prolonged and difficult for patients to adhere to, they are highly efficacious, setting a high bar for the performance of new agents. Complicating matters more, regulatory standards have changed since the first drugs for TB were introduced, with a rigorous characterization of the effect of a new drug within a combination regimen expected. If these demands are to be satisfied, innovative models will be needed to demonstrate drug efficacy. In the past, mycobacterial cultures performed on solid media at the end of treatment have been used as critical biomarkers of drug efficacy, but their inability to predict long-term outcomes with precision has limited their utility. This article reviews a range of nonclinical and clinical models to characterize the bactericidal and/or sterilizing activity of new compounds. Novel approaches, using in vitro and animal models, sensitive biomarkers, as well as creative new clinical trial designs, are discussed. These promise a timely expansion of our TB treatment armamentarium to include potent new drugs and shorter, simpler treatment regimens.

Antitubercular effect of 8-[(4-Chloro phenyl) sulfonyl]-7-Hydroxy-4-Methyl-2H-chromen-2-One in guinea pigs

OBJECTIVE

TO EVALUATE THE ANTITUBERCULAR EFFICACY AND SAFETY OF NEW CHEMICAL ENTITY (NCE): 8-[(4-Chloro phenyl) sulfonyl]-7-Hydroxy-4-Methyl-2H-chromen-2-One (CSHMC) in guinea pigs.

MATERIALS AND METHODS

This pilot study was carried out in guinea pigs. They were infected with M. tuberculosis H(37)Rv (1.5 × 10(4) cfu/guinea pig) via intramuscular route. After 30 days, infections were confirmed in 6 guinea pigs by histopathology of spleen, lung, and liver. After that CSHMC (5 and 20 mg/kg) was administered for 1 month and its effect was compared with vehicle, rifampicin (60 mg/kg) and isoniazid (30 mg/kg). Efficacy of CSHMC was evaluated on the basis of histopathologic scoring of lesion in lung, spleen, liver, and safety on the basis of measuring hemogram, liver and renal function parameters.

RESULTS

Isoniazid, rifampicin, and CSHMC (20 mg/kg) significantly reduce the median lesion score in lung, spleen, and liver as compared to disease control group. Reduction in median lesion score for lung and spleen were not statistically significant for CSHMC 5 mg/kg. CSHMC (20 and 5 mg/kg) did not produce any changes in hemogram, liver and renal function parameters with respect to normal values.

CONCLUSIONS

CSHMC had shown significant antitubercular efficacy comparable to isoniazid and rifampicin and did not show hematological, hepato- and nephrotoxicity.

Effectiveness of tuberculosis chemotherapy correlates with resistance to Mycobacterium tuberculosis infection in animal models

OBJECTIVES

It is widely believed that persistent Mycobacterium tuberculosis inhabits necrotic lung granulomas in humans and that the microenvironmental conditions encountered therein render the bacilli phenotypically tolerant to antibiotics, accounting for the long duration required for successful treatment of tuberculosis (TB). To validate this belief, we directly compared the activity of rifampicin/isoniazid/pyrazinamide (RHZ) against chronic TB infection in guinea pigs, which exhibit caseous granulomas histologically resembling human caseous foci, and in mice, which lack necrotic granulomas.

METHODS

Guinea pigs and mice were aerosol-infected with M. tuberculosis CDC1551 and twice weekly treatment with RHZ was started 4 weeks later. Culture-positive relapse was assessed in subgroups of guinea pigs after 3 months and 4 months of treatment.

RESULTS

All guinea pig lungs exhibited histological evidence of granulomas with central caseation, while mouse lungs exhibited cellular lesions at the initiation of antibiotic treatment. Guinea pig lungs became culture-negative after 2 months of RHZ given twice weekly at human-equivalent doses. Relapse rates in guinea pigs were 0% (0/10) both after 3 months and 4 months of treatment. In contrast, all mouse lungs remained culture-positive after 4 months of equivalent RHZ exposures.

CONCLUSIONS

Caseous necrosis does not reduce the sterilizing activity of the standard antituberculosis regimen of RHZ. Our findings have important implications for the use of alternative animal models in testing novel TB drug regimens and for modelling M. tuberculosis persistence.

Dose-dependent activity of pyrazinamide in animal models of intracellular and extracellular tuberculosis infections

Recent in vitro pharmacokinetic data suggest that the currently recommended dose of pyrazinamide may be suboptimal for killing intracellular bacilli in humans. We evaluated a range of pyrazinamide doses against intracellular and extracellular Mycobacterium tuberculosis in chronically infected mice and guinea pigs, respectively. Antibiotics were given five times weekly for 4 weeks beginning 28 days after infection. Human-equivalent doses of isoniazid reduced lung bacterial counts 10-fold in each species. Pyrazinamide given at 1/4 and 1/2 the human-equivalent dose was minimally active, while human-equivalent doses reduced lung bacterial counts by ∼1.0 log(10) in each species. Doubling the human-equivalent dose of pyrazinamide reduced the lung bacillary burden by 1.7 and 3.0 log(10) in mice and guinea pigs, respectively. As in humans and mice, pyrazinamide showed significant synergy with rifampin in guinea pigs. Clinical studies are warranted to investigate the sterilizing activity and tolerability of higher doses of pyrazinamide in combination tuberculosis regimens.

PA-824 exhibits time-dependent activity in a murine model of tuberculosis

PA-824 is one of two nitroimidazoles in phase II clinical trials to treat tuberculosis. In mice, it has dose-dependent early bactericidal and sterilizing activity. In humans with tuberculosis, PA-824 demonstrated early bactericidal activity (EBA) at doses ranging from 200 to 1,200 mg per day, but no dose-response effect was observed. To better understand the relationship between drug exposure and effect, we performed a dose fractionation study in mice. Dose-ranging pharmacokinetic data were used to simulate drug exposure profiles. Beginning 2 weeks after aerosol infection with Mycobacterium tuberculosis, total PA-824 doses from 144 to 4,608 mg/kg were administered as 3, 4, 8, 12, 24, or 48 divided doses over 24 days. Lung CFU counts after treatment were strongly correlated with the free drug T(>MIC) (R(2) = 0.87) and correlated with the free drug AUC/MIC (R(2) = 0.60), but not with the free drug C(max)/MIC (R(2) = 0.17), where T(>MIC) is the cumulative percentage of the dosing interval that the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions and AUC is the area under the concentration-time curve. When the data set was limited to regimens with dosing intervals of ≤72 h, both the T(>MIC) and the AUC/MIC values fit the data well. Free drug T(>MIC) of 22, 48, and 77% were associated with bacteriostasis, a 1-log kill, and a 1.59-log kill (or 80% of the maximum observed effect), respectively. Human pharmacodynamic simulations based on phase I data predict 200 mg/day produces free drug T(>MIC) values near the target for maximal observed bactericidal effect. The results support the recently demonstrated an EBA of 200 mg/day and the lack of a dose-response between 200 and 1,200 mg/day. T(>MIC), in conjunction with AUC/MIC, is the parameter on which dose optimization of PA-824 should be based.

Activities of TMC207, rifampin, and pyrazinamide against Mycobacterium tuberculosis infection in guinea pigs

The experimental compound TMC207 is showing promise against infections caused by Mycobacterium tuberculosis both in a variety of animal studies and in the field. In this study, we used the guinea pig model, a species that shows several similarities to human tuberculosis, including the hallmark of primary granuloma necrosis, to determine the efficacy of a combination regimen combining TMC207 with rifampin and pyrazinamide. This drug regimen rapidly reduced the bacterial load in the lungs to undetectable levels by 8 weeks of treatment. This reduction was associated with a substantial improvement in lung pathology, but despite this effect areas of residual necrosis still remained. In the draining lymph nodes, however, tissue damage was rapid and not significantly reversed by the drug treatment. Approximately 10 to 11 months after the treatment had ended, the animals began to trigger a Karnovsky scale indicating bacterial regrowth and potential relapse, an event confirmed by the new development of both pulmonary and extrapulmonary granulomatous lesions. Interestingly, a similar rate of relapse was also seen in animals receiving 24 weeks of rifampin, pyrazinamide, and isoniazid standard chemotherapy. These data indicate that TMC207 could be a useful addition to current treatment regimens for tuberculosis.

The potent bactericidal activity of streptomycin in the guinea pig model of tuberculosis ceases due to the presence of persisters

OBJECTIVES

The biphasic kill curve of isoniazid against Mycobacterium tuberculosis in guinea pigs is due to the presence of persisters rather than selection of isoniazid-resistant mutants. To determine whether this phenomenon is common to other bactericidal drugs, we studied the activity of streptomycin and its ability to select for streptomycin-resistant mutants in the guinea pig model of tuberculosis.

METHODS

Pharmacokinetic studies were performed to establish the human-equivalent dose of streptomycin. Guinea pigs were aerosol-infected with M. tuberculosis and 2 weeks later streptomycin was given for 5 days/week via intramuscular injection. Bactericidal activity was assessed by homogenizing and plating lungs for cfu until 10 weeks of treatment. At each timepoint, cfu were isolated, suspended in normal saline and re-plated on plates containing 0.5, 1.0, 2.0 or 10.0 mg/L streptomycin.

RESULTS

The human-equivalent dose of streptomycin was determined to be 70 mg/kg. Streptomycin showed potent activity during the first 14 days of treatment, rescuing all animals from acute tuberculosis-related death and reducing lung cfu by ∼4 log(10). However, streptomycin activity was dramatically reduced thereafter, as lung cfu declined by only ∼1 log(10) over the next 56 days of treatment. Although streptomycin-resistant mutants were detectable, their frequency of isolation was identical at treatment initiation and after 70 days of treatment.

CONCLUSIONS

The reduced activity of streptomycin during the second phase of monotherapy is not associated with the selection of streptomycin-resistant mutants but, rather, with the presence of phenotypically tolerant 'persisters'.