Antituberculosis activity of clarithromycin

Beyond antibiotic resistance: The whiB7 transcription factor coordinates an adaptive response to alanine starvation in mycobacteria

Pathogenic mycobacteria are a significant cause of morbidity and mortality worldwide. The conserved whiB7 stress response reduces the effectiveness of antibiotic therapy by activating several intrinsic antibiotic resistance mechanisms. Despite our comprehensive biochemical understanding of WhiB7, the complex set of signals that induce whiB7 expression remain less clear. We employed a reporter-based, genome-wide CRISPRi epistasis screen to identify a diverse set of 150 mycobacterial genes whose inhibition results in constitutive whiB7 expression. We show that whiB7 expression is determined by the amino acid composition of the 5' regulatory uORF, thereby allowing whiB7 to sense amino acid starvation. Although deprivation of many amino acids can induce whiB7, whiB7 specifically coordinates an adaptive response to alanine starvation by engaging in a feedback loop with the alanine biosynthetic enzyme, aspC. These findings describe a metabolic function for whiB7 and help explain its evolutionary conservation across mycobacterial species occupying diverse ecological niches.

Beyond antibiotic resistance: the whiB7 transcription factor coordinates an adaptive response to alanine starvation in mycobacteria

Pathogenic mycobacteria are a significant cause of morbidity and mortality worldwide. These bacteria are highly intrinsically drug resistant, making infections challenging to treat. The conserved whiB7 stress response is a key contributor to mycobacterial intrinsic drug resistance. Although we have a comprehensive structural and biochemical understanding of WhiB7, the complex set of signals that activate whiB7 expression remain less clear. It is believed that whiB7 expression is triggered by translational stalling in an upstream open reading frame (uORF) within the whiB7 5' leader, leading to antitermination and transcription into the downstream whiB7 ORF. To define the signals that activate whiB7, we employed a genome-wide CRISPRi epistasis screen and identified a diverse set of 150 mycobacterial genes whose inhibition results in constitutive whiB7 activation. Many of these genes encode amino acid biosynthetic enzymes, tRNAs, and tRNA synthetases, consistent with the proposed mechanism for whiB7 activation by translational stalling in the uORF. We show that the ability of the whiB7 5' regulatory region to sense amino acid starvation is determined by the coding sequence of the uORF. The uORF shows considerable sequence variation among different mycobacterial species, but it is universally and specifically enriched for alanine. Providing a potential rationalization for this enrichment, we find that while deprivation of many amino acids can activate whiB7 expression, whiB7 specifically coordinates an adaptive response to alanine starvation by engaging in a feedback loop with the alanine biosynthetic enzyme, aspC. Our results provide a holistic understanding of the biological pathways that influence whiB7 activation and reveal an extended role for the whiB7 pathway in mycobacterial physiology, beyond its canonical function in antibiotic resistance. These results have important implications for the design of combination drug treatments to avoid whiB7 activation, as well as help explain the conservation of this stress response across a wide range of pathogenic and environmental mycobacteria.

CRISPRi chemical genetics and comparative genomics identify genes mediating drug potency in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) infection is notoriously difficult to treat. Treatment efficacy is limited by Mtb's intrinsic drug resistance, as well as its ability to evolve acquired resistance to all antituberculars in clinical use. A deeper understanding of the bacterial pathways that influence drug efficacy could facilitate the development of more effective therapies, identify new mechanisms of acquired resistance, and reveal overlooked therapeutic opportunities. Here we developed a CRISPR interference chemical-genetics platform to titrate the expression of Mtb genes and quantify bacterial fitness in the presence of different drugs. We discovered diverse mechanisms of intrinsic drug resistance, unveiling hundreds of potential targets for synergistic drug combinations. Combining chemical genetics with comparative genomics of Mtb clinical isolates, we further identified several previously unknown mechanisms of acquired drug resistance, one of which is associated with a multidrug-resistant tuberculosis outbreak in South America. Lastly, we found that the intrinsic resistance factor whiB7 was inactivated in an entire Mtb sublineage endemic to Southeast Asia, presenting an opportunity to potentially repurpose the macrolide antibiotic clarithromycin to treat tuberculosis. This chemical-genetic map provides a rich resource to understand drug efficacy in Mtb and guide future tuberculosis drug development and treatment.

Improved survival in patients with severe tuberculosis after exposure to fluoroquinolones

BACKGROUND

The clinical impact of prior exposure to antibiotics on patients with tuberculosis (TB) is largely unknown. This study investigated the survival of patients with severe TB after exposure to a variety of antibiotics.

METHODS

A retrospective cohort study was conducted in TB patients with prior exposure to fluoroquinolones (FQs) (FQ group), to third-generation cephalosporins (CEPH group), and to third-generation penicillins (PCN group). To understand the impact of monotherapy with antibiotics on survival, patients with prior exposure to only moxifloxacin, ceftriaxone, or piperacillin were investigated.

RESULTS

Patients in the FQ group (N= 401) had a significantly higher survival rate (82.5%) than patients in the CEPH (N = 210) and PCN (N = 172) groups (67.6% and 62.8%, respectively; both p < 0.0001) at 180 d after TB diagnosis. Adjusted odds ratio (AOR) logistic regression analysis demonstrated that patients in the FQ group had significantly more favourable outcomes than those in the CEPH and PCN groups in terms of intensive care unit (ICU) admission rate (versus CEPH cohort: AOR, 1.70; 95% CI: 1.16-2.50; p = 0.0067, versus PCN cohort: AOR, 3.58; 95% CI: 2.42-5.29; both p < 0.0001), mechanical ventilation rate (versus CEPH cohort: AOR, 1.70; 95% CI: 1.09-2.66; p = 0.0205, versus PCN cohort: AOR, 3.92; 95% CI: 2.54-6.05; both p < 0.0001), and acute respiratory failure rate (versus CEPH cohort: AOR, 1.62; 95% CI: 1.07-2.45; p = 0.0223, versus PCN cohort: AOR, 4.29; 95% CI: 2.86-6.43; both p < 0.0001). TB patients with prior exposure to moxifloxacin (N = 198) had a significantly higher survival rate (85.9%) than that of patients with exposure to ceftriaxone (N = 119) and piperacillin (N = 172) monotherapy (survival rates: 69.8% and 62.8%, respectively; both p < 0.001).

CONCLUSIONS

TB patients with prior exposure to FQs had more favourable outcomes compared with patients who had prior exposure to third-generation cephalosporins or third-generation penicillins. This study provides new insights into the impact of previous exposure to FQs on the survival of TB patients.

Network Analysis of Human Genes Influencing Susceptibility to Mycobacterial Infections

Tuberculosis and nontuberculous mycobacterial infections constitute a high burden of pulmonary disease in humans, resulting in over 1.5 million deaths per year. Building on the premise that genetic factors influence the instance, progression, and defense of infectious disease, we undertook a systems biology approach to investigate relationships among genetic factors that may play a role in increased susceptibility or control of mycobacterial infections. We combined literature and database mining with network analysis and pathway enrichment analysis to examine genes, pathways, and networks, involved in the human response to Mycobacterium tuberculosis and nontuberculous mycobacterial infections. This approach allowed us to examine functional relationships among reported genes, and to identify novel genes and enriched pathways that may play a role in mycobacterial susceptibility or control. Our findings suggest that the primary pathways and genes influencing mycobacterial infection control involve an interplay between innate and adaptive immune proteins and pathways. Signaling pathways involved in autoimmune disease were significantly enriched as revealed in our networks. Mycobacterial disease susceptibility networks were also examined within the context of gene-chemical relationships, in order to identify putative drugs and nutrients with potential beneficial immunomodulatory or anti-mycobacterial effects.

In vitro and in vivo Evaluation of Synergism between Anti-Tubercular Spectinamides and Non-Classical Tuberculosis Antibiotics

Spectinamides are new semi-synthetic spectinomycin derivatives with potent anti-tubercular activity. The reported synergism of the precursor spectinomycin with other antibiotics prompted us to examine whether spectinamides sensitize M. tuberculosis to other antibiotics not traditionally used in the treatment of tuberculosis to potentially expand therapeutic options for MDR/XDR Tuberculosis. Whole cell synergy checkerboard screens were performed using the laboratory strain M. tuberculosis H37Rv, lead spectinamide 1599, and a broad panel of 27 antibiotics. In vitro, 1599 synergized with 11 drugs from 6 antibiotic classes. The observed synergy was tested against clinical isolates confirming synergy with Clarithromycin, Doxycycline and Clindamycin, combinations of which were taken forward for in vivo efficacy determination. Co-administration of 1599 and clarithromycin provided additional bacterial killing in a mouse model of acute tuberculosis infection, but not in a chronic infection model. Further studies indicated that mismatched drug exposure profiles likely permitted induction of phenotypic clarithromycin resistance and subsequent loss of synergism. These studies highlight the importance of validating in vitro synergism and the challenge of matching drug exposures to obtain a synergistic outcome in vivo. Results from this study indicate that a 1599 clarithromycin combination is potentially viable, providing the drug exposures can be carefully monitored.

Evaluation of macrolides for possible use against multidrug-resistant Mycobacterium tuberculosis

Multidrug-resistant tuberculosis (MDR-TB) is a major global health problem. The loss of susceptibility to an increasing number of drugs behoves us to consider the evaluation of non-traditional anti-tuberculosis drugs.

Clarithromycin, a macrolide antibiotic, is defined as a group 5 anti-tuberculosis drug by the World Health Organization; however, its role or efficacy in the treatment of MDR-TB is unclear. A systematic review of the literature was conducted to summarise the evidence for the activity of macrolides against MDR-TB, by evaluating in vitro, in vivo and clinical studies. PubMed and Embase were searched for English language articles up to May 2014.

Even though high minimum inhibitory concentration values are usually found, suggesting low activity against Mycobacterium tuberculosis, the potential benefits of macrolides are their accumulation in the relevant compartments and cells in the lungs, their immunomodulatory effects and their synergistic activity with other anti-TB drugs.

A future perspective may be use of more potent macrolide analogues to enhance the activity of the treatment regimen.

Activity of 10 antimicrobial agents against intracellular Rhodococcus equi

Studies with facultative intracellular bacterial pathogens have shown that evaluation of the bactericidal activity of antimicrobial agents against intracellular bacteria is more closely associated with in vivo efficacy than traditional in vitro susceptibility testing. The objective of this study was to determine the relative activity of 10 antimicrobial agents against intracellular Rhodococcus equi. Equine monocyte-derived macrophages were infected with virulent R. equi and exposed to erythromycin, clarithromycin, azithromycin, rifampin, ceftiofur, gentamicin, enrofloxacin, vancomycin, imipenem, or doxycycline at concentrations achievable in plasma at clinically recommended dosages in foals. The number of intracellular R. equi was determined 48h after infection by counting colony forming units (CFUs). The number of R. equi CFUs in untreated control wells were significantly higher than those of monolayers treated with antimicrobial agents. Numbers of R. equi were significantly lower in monolayers treated with enrofloxacin followed by those treated with gentamicin, and vancomycin, when compared to monolayers treated with other antimicrobial agents. Numbers of R. equi in monolayers treated with doxycycline were significantly higher than those of monolayers treated with other antimicrobial agents. Differences in R. equi CFUs between monolayers treated with other antimicrobial agents were not statistically significant. Enrofloxacin, gentamicin, and vancomycin are the most active drugs in equine monocyte-derived macrophages infected with R. equi. Additional studies will be needed to determine if these findings correlate with in vivo efficacy.

Rifampin induces hydroxyl radical formation in Mycobacterium tuberculosis

The antituberculosis (anti-TB) drug rifampin (RIF) binds to the beta subunit of the RNA polymerase (RpoB) of Mycobacterium tuberculosis, but the bactericidal responses triggered after target interaction are not known. To evaluate whether RIF induced an oxidative burst, lysates of RIF-treated M. tuberculosis were tested for determination of reactive oxygen species (ROS) by the electron paramagnetic resonance (EPR) technique using 1-hydroxy-3-carboxy-pyrrolidine (CPH) and 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as spin traps. M. tuberculosis killing by RIF stimulated an increase in the rate of formation of the CPH radical (CP·). Lysate pretreatment with the O2·(-) and ·OH scavengers superoxide dismutase (SOD) and thiourea (THIO), respectively, or with the metal chelator diethylene triamine pentaacetic acid (DTPA) inhibited CP· formation, arguing in favor of a metal-catalyzed ROS response. Formation of CP· did not increase following treatment of RIF-resistant strains with RIF, indicating that the ROS were induced after RpoB binding. To identify the ROS formed, lysates of RIF-treated bacilli were incubated with DMPO, a spin trap specific for ·OH and O2·(-), with or without pretreatment with SOD, catalase, THIO, or DTPA. Superoxide dismutase, catalase, and THIO decreased formation of the DMPO-OH adduct, and SOD plus DTPA completely suppressed it, suggesting that RIF activated metal-dependent O2·(-)-mediated mechanisms producing ·OH inside tubercle bacilli. The finding that the metal chelator DTPA reduced the bactericidal activity of RIF supported the possibility that ·OH was generated through these mechanisms and that it participated at least in part in M. tuberculosis killing by the drug.

Synthetic lethality reveals mechanisms of Mycobacterium tuberculosis resistance to β-lactams

Most β-lactam antibiotics are ineffective against Mycobacterium tuberculosis due to the microbe’s innate resistance. The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains has prompted interest to repurpose this class of drugs. To identify the genetic determinants of innate β-lactam resistance, we carried out a synthetic lethality screen on a transposon mutant library for susceptibility to imipenem, a carbapenem β-lactam antibiotic. Mutations in 74 unique genes demonstrated synthetic lethality. The majority of mutations were in genes associated with cell wall biosynthesis. A second quantitative real-time PCR (qPCR)-based synthetic lethality screen of randomly selected mutants confirmed the role of cell wall biosynthesis in β-lactam resistance. The global transcriptional response of the bacterium to β-lactams was investigated, and changes in levels of expression of cell wall biosynthetic genes were identified. Finally, we validated these screens in vivo using the MT1616 transposon mutant, which lacks a functional acyl-transferase gene. Mice infected with the mutant responded to β-lactam treatment with a 100-fold decrease in bacillary lung burden over 4 weeks, while the numbers of organisms in the lungs of mice infected with wild-type bacilli proliferated. These findings reveal a road map of genes required for β-lactam resistance and validate synthetic lethality screening as a promising tool for repurposing existing classes of licensed, safe, well-characterized antimicrobials against tuberculosis.

The global emergence of multidrug-resistant and extensively drug-resistant M. tuberculosis strains has threatened public health worldwide, yet the pipeline of new tuberculosis drugs under development remains limited. One strategy to cope with the urgent need for new antituberculosis agents is to repurpose existing, approved antibiotics. The carbapenem class of β-lactam antibiotics has been proposed as one such class of drugs. Our study identifies molecular determinants of innate resistance to β-lactam drugs in M. tuberculosis, and we demonstrate that functional loss of one of these genes enables successful treatment of M. tuberculosis with β-lactams in the mouse model.

WHO group 5 drugs and difficult multidrug-resistant tuberculosis: a systematic review with cohort analysis and meta-analysis

It is often necessary to include WHO group 5 drugs in the treatment of extensively drug-resistant tuberculosis (XDR-TB) and fluoroquinolone-resistant multidrug-resistant tuberculosis (MDR-TB). As clinical evidence about the use of group 5 drugs is scarce, we conducted a systematic review using published individual patient data. We searched PubMed and OvidSP through 7 April 2013 for publications in English to assemble a cohort with fluoroquinolone-resistant MDR-TB treated with group 5 drugs. Favorable outcome was defined as sputum culture conversion, cure, or treatment completion in the absence of death, default, treatment failure, or relapse. A cohort of 194 patients was assembled from 20 articles involving 12 geographical regions. In descending order of frequency, linezolid was used in treatment of 162 (84%) patients, macrolides in 84 (43%), clofazimine in 65 (34%), amoxicillin with clavulanate in 56 (29%), thioridazine in 18 (9%), carbapenem in 16 (8%), and high-dose isoniazid in 16 (8%). Cohort analysis with robust Poisson regression models and random-effects meta-analysis similarly suggested that linezolid use significantly increased the probability (95% confidence interval) of favorable outcome by 57% (10% to 124%) and 55% (10% to 121%), respectively. Defining significant associations by risk ratios ≥ 1.2 or ≤ 0.9, neither cohort analysis nor meta-analysis demonstrated any significant add-on benefit from the use of other group 5 drugs with respect to outcome for patients treated with linezolid, although selection bias might have led to underestimation of their effects. Our findings substantiated the use of linezolid in the treatment of XDR-TB or fluoroquinolone-resistant MDR-TB and call for further studies to evaluate the roles of other group 5 drugs.

Paediatric use of second-line anti-tuberculosis agents: a review

Childhood multidrug-resistant tuberculosis (MDR-TB) is an emerging global epidemic. With the imminent roll-out of rapid molecular diagnostic tests, more children are likely to be identified and require treatment. As MDR-TB is resistant to the most effective first-line drugs, clinicians will have to rely on second-line medications which are less effective and often associated with more pronounced adverse effects than first-line therapy. Despite the fact that most of these agents were discovered many years ago, robust information is lacking regarding their pharmacokinetic and pharmacodynamic properties, adverse effects and drug interactions, especially in children. Children differ from adults in the way that drugs are administered, the manner in which they are metabolised and in the adverse effects experienced. The interaction of these drugs with human immunodeficiency virus infection and antiretroviral therapy is also poorly documented. This article reviews the available second-line drugs currently used in the treatment of MDR-TB in children and discusses medication properties and adverse effects while potential interactions with antiretroviral therapy are explored.

Long-term linezolid treatment in a young child with extensively drug-resistant tuberculosis

Treatment of extensively drug-resistant tuberculosis (XDR-TB) is a challenge. We describe a young child with pulmonary XDR-TB who did not respond to an aggressive multidrug-resistant TB treatment regimen, but was cured with linezolid in combination with other reserve antituberculous drugs. No serious adverse events were observed during 19 months of treatment for XDR-TB.

Structure-activity relationships of macrolides against Mycobacterium tuberculosis

Existing 14, 15 and 16-membered macrolide antibiotics, while effective for other bacterial infections, including some mycobacteria, have not demonstrated significant efficacy in tuberculosis. Therefore an attempt was made to optimize this class for activity against Mycobacterium tuberculosis through semisyntheses and bioassay. Approximately 300 macrolides were synthesized and screened for anti-TB activity. Structural modifications on erythromycin were carried out at positions 3, 6, 9, 11, and 12 of the 14-membered lactone ring; as well as at position 4'' of cladinose and position 2' of desosamine. In general, the synthesized macrolides belong to four subclasses: 9-oxime, 11,12-carbamate, 11,12-carbazate, and 6-O-substituted derivatives. Selected compounds were assessed for mammalian cell toxicity and in some cases were further assessed for CYP3A4 inhibition, microsome stability, in vivo tolerance and efficacy. The activity of 11,12-carbamates and carbazates as well as 9-oximes is highly influenced by the nature of the substitution at these positions. For hydrophilic macrolides, lipophilic substitution may result in enhanced potency, presumably by enhanced passive permeation through the cell envelope. This strategy, however, has limitations. Removal of the C-3 cladinose generally reduces the activity. Acetylation at C-2' or 4'' maintains potency of C-9 oximes but dramatically decreases that of 11,12-substituted compounds. Further significant increases in the potency of macrolides for M. tuberculosis may require a strategy for the concurrent reduction of ribosome methylation.

Medications for Extensively Drug-Resistant Tuberculosis: Back to the Future?

Objective:

To reexamine the existing medications for the potential treatment of extensively drug-resistant tuberculosis (XDR-TB), based on susceptibility data, and to identify potential future medications from the literature.

Data Sources:

Relevant information was identified through a search of MEDLINE (1966–November 2007), PubMed (1955–November 2007), American Search Premier (1975–November 2007), International Pharmaceutical Abstracts (1960–November 2007), Science Citation Index Expanded (1996–November 2007), Cochrane Databases (publications archived until November 2007), and various tertiary sources as listed in the references, using the terms extensively drug-resistant tuberculosis (XDR-TB), ethambutol, pyrazinamide, para-aminosalicylic acid, cycloserine, linezolid, diarylquinoline, nitroimidazopyran, fluoroquinolones, β-lactams, new treatments, and ethionamide alone or in combination regimens.

Study Selection and Data Extraction:

After identification of the relevant information, the data presented in this article were selected based on clinical relevance and value of information.

Data Synthesis:

Based on susceptibility data, pyrazinamide, ethambutol, para-aminosalicylic acid, cycloserine, and ethionamide may be used for the treatment of tuberculosis. However, due to the emergence of XDR-TB, many of these agents are no longer successful treatment regimens. We have found limited data supporting potential future use of β-lactams, clarithromycin, and linezolid in resistant TB infections. TMC207, nitroimidazopyran, and SQ109 compounds may also prove to be viable options in the near future for treatment of tuberculosis, especially in cases with resistance to mainstay medications.

Conclusions:

Extensively resistant tuberculosis appears to be a potentially catastrophic disease if allowed to spread. Due to its resistance profile, very few potentially effective agents are available, calling attention to this growing problem.

Antituberculosis drugs: ten years of research

Tuberculosis is today amongst the worldwide health threats. As resistant strains of Mycobacterium tuberculosis have slowly emerged, treatment failure is too often a fact, especially in countries lacking the necessary health care organisation to provide the long and costly treatment adapted to patients. Because of lack of treatment or lack of adapted treatment, at least two million people will die of tuberculosis this year. Due to this concern, this infectious disease was the focus of renewed scientific interest in the last decade. Regimens were optimized and much was learnt on the mechanisms of action of the antituberculosis drugs used. Moreover, the quest for original drugs overcoming some of the problems of current regimens also became the focus of research programmes and many new series of M. tuberculosis growth inhibitors were reported. This review presents the drugs currently used in antituberculosis treatments and the most advanced compounds undergoing clinical trials. We then provide a description of their mechanism of action along with other series of inhibitors known to act on related biochemical targets. This is followed by other inhibitors of M. tuberculosis growth, including recently reported compounds devoid of a reported mechanism of action.

Synergistic antimycobacterial activities of sesquiterpene lactones from Laurus spp

OBJECTIVES

The aim of this study was to determine the antimycobacterial potential of laurel oil, its fractions and its two sesquiterpene lactones against several mycobacterial strains and clinical isolates, and to establish the possibility of occurrence of some synergistic effects between those lactones using a modification of the fluorometric Alamar Blue microassay (FMABA).

METHODS

The in vitro antimycobacterial activity of whole oil and its fractions and pure active compounds were determined by FMABA. A bioassay-guided fractionation of the traditional preparation of laurel oil from Madeira Islands was performed, yielding pure compounds chemically identified by standard procedures. Synergism of pure compounds was established by X/Y quotient analysis adapted to FMABA.

RESULTS

Sesquiterpene lactones, costunolide and dehydrocostuslactone, were the compounds responsible for the antimycobacterial activity against Mycobacterium tuberculosis H37Rv with MICs of 6.25 and 12.5 mg/L, respectively. Antimycobacterial activity against drug-resistant M. tuberculosis clinical isolates was better for the mixture than for pure compounds.

CONCLUSIONS

Both lactones presented synergistic activity, i.e. analysis of relative fluorescence units presented an X/Y value <0.5 at a concentration of 1/8 MIC of each compound in the combination. Establishment of synergism by FMABA represents another application of the microplate Alamar Blue assay.

Tuberculosis chemotherapy: current drug delivery approaches

Tuberculosis is a leading killer of young adults worldwide and the global scourge of multi-drug resistant tuberculosis is reaching epidemic proportions. It is endemic in most developing countries and resurgent in developed and developing countries with high rates of human immunodeficiency virus infection. This article reviews the current situation in terms of drug delivery approaches for tuberculosis chemotherapy. A number of novel implant-, microparticulate-, and various other carrier-based drug delivery systems incorporating the principal anti-tuberculosis agents have been fabricated that either target the site of tuberculosis infection or reduce the dosing frequency with the aim of improving patient outcomes. These developments in drug delivery represent attractive options with significant merit, however, there is a requisite to manufacture an oral system, which directly addresses issues of unacceptable rifampicin bioavailability in fixed-dose combinations. This is fostered by the need to deliver medications to patients more efficiently and with fewer side effects, especially in developing countries. The fabrication of a polymeric once-daily oral multiparticulate fixed-dose combination of the principal anti-tuberculosis drugs, which attains segregated delivery of rifampicin and isoniazid for improved rifampicin bioavailability, could be a step in the right direction in addressing issues of treatment failure due to patient non-compliance.

Intrinsic macrolide resistance of the Mycobacterium tuberculosis complex is inducible

Mycobacterium tuberculosis is intrinsically resistant to macrolides, a characteristic associated with expression of the erm(37) gene. This intrinsic resistance was found to be inducible with clarithromycin and the ketolide HMR3004. Furthermore, underlying the phenotypic induction was an increase in erm(37) mRNA levels.

Clinical guidelines to diagnose smear-negative pulmonary tuberculosis in Pakistan, a country with low-HIV prevalence

OBJECTIVES

To develop and validate clinical guidelines for diagnosis of smear-negative pulmonary tuberculosis (TB) in developing countries with low-HIV prevalence.

METHODS

We developed diagnostic guidelines for smear-negative TB. Clinical diagnoses based on these guidelines were compared with sputum culture, chest X-rays and reports of an expert panel.

RESULTS

The guidelines achieved a sensitivity of 0.59 [confidence interval (CI) 0.46-0.66] and a specificity of 0.86 (CI 0.84-0.88) in diagnosing smear-negative TB. A total of 6.8% of patients who initially improved after a course of antibiotics were later confirmed to have TB. Clinicians detected an abnormal chest X-ray in 92% (CI 88-96%) and radiological signs of pulmonary TB in 98% (CI 94-100%) of cases.

CONCLUSIONS

Our experience highlights a number of dilemmas faced in developing, testing and implementing diagnostic guidelines in poorly resourced conditions. Using radiological criteria for TB and appropriate training can help in improving the diagnostic skills of primary care clinicians working in low-HIV settings with access to X-ray facilities. But a significant number of apparently smear-negative TB cases may in fact be smear positive and TB programmes should focus on improving the quality of direct acid-fast bacilli microscopy. The value of an antibiotic trial is questionable due to the relatively large number of false negatives generated by this approach.

Determination of in vitro synergy when three antimicrobial agents are combined against Mycobacterium tuberculosis

We determined the in vitro antimycobacterial activity of rifampicin, isoniazid and a third agent in combination using a three-dimensional chequerboard in Middlebrook 7H9 broth microdilutions. Of 28 agents screened, ethambutol, streptomycin, clarithromycin, minocycline, ciprofloxacin, levofloxacin, sparfloxacin, gatifloxacin and sitafloxacin were potentially synergistic. A further three-dimensional chequerboard assay quantitatively looked for synergy against ten clinical isolates of Mycobacterium tuberculosis, including seven multidrug-resistant isolates. Sitafloxacin, gatifloxacin and clarithromycin showed significant synergy, with fractional inhibitory concentration indices ranging from 0.41 to 0.79, 0.39 to 0.90 and 0.48 to 0.95, respectively. It is concluded that three-dimensional chequerboard assay can quantitatively determine antimycobacterial synergy, and that fluoroquinolones and antibacterial agents such as clarithromycin are effective against multidrug-resistant isolates of M. tuberculosis when combined with rifampicin and isoniazid.

Activity of RBx 7644 and RBx 8700, new investigational oxazolidinones, against Mycobacterium tuberculosis infected murine macrophages

The decision to develop a new chemical entity should not only be based on its ability to inhibit multidrug-resistant tuberculosis (MDR-TB) strains but also on its ability to enter macrophages and be active against intracellular bacteria. RBx 7644 and RBx 8700, two novel extended spectrum oxazolidinones, were investigated for their activity against sensitive and MDR isolates of Mycobacterium tuberculosis and for activity against bacteria within a macrophage cell line. RBx 8700 showed excellent in vitro activity against sensitive as well as MDR M. tuberculosis strains with MIC(50) and MIC(90) values of 0.032 and 0.25mg/L (sensitive) and 0.25 and 1.0mg/L (MDR) strains. The corresponding MIC(50) and MIC(90) values of RBx 7644, linezolid, rifampicin and isoniazid were 8 and 16; 32 and 64; 64 and 64; 64 and 64 mg/L, respectively. RBx 8700 and rifampicin were bactericidal at 0.5 and 0.25mg/L when tested intracellularly whereas linezolid reduced the count by 100-fold at a concentration of 8 mg/L. In combination studies with standard antimycobacterial drugs, RBx 8700 did not show any antagonistic effect.

In vitro and in vivo activities of macrolide derivatives against Mycobacterium tuberculosis

Existing macrolides have never shown definitive clinical efficacy in tuberculosis. Recent reports suggest that ribosome methylation is involved in macrolide resistance in Mycobacterium tuberculosis, a mechanism that newer macrolides have been designed to overcome in gram-positive bacteria. Therefore, selected macrolides and ketolides (descladinose) with substitutions at positions 9, 11,12, and 6 were assessed for activity against M. tuberculosis, and those with MICs of < or = 4 microM were evaluated for cytotoxicity to Vero cells and J774A.1 macrophages. Several compounds with 9-oxime substitutions or aryl substitutions at position 6 or on 11,12 carbamates or carbazates demonstrated submicromolar MICs. For the three macrolide-ketolide pairs, macrolides demonstrated superior activity. Four compounds with low MICs and low cytotoxicity also effected significant reductions in CFU in infected macrophages. Active compounds were assessed for tolerance and the ability to reduce CFU in the lungs of BALB/c mice in an aerosol infection model. A substituted 11,12 carbazate macrolide demonstrated significant dose-dependent inhibition of M. tuberculosis growth in mice, with a 10- to 20-fold reduction of CFU in lung tissue. Structure-activity relationships, some of which are unique to M. tuberculosis, suggest several synthetic directions for further improvement of antituberculosis activity. This class appears promising for yielding a clinically useful agent for tuberculosis.

Molecular basis of intrinsic macrolide resistance in clinical isolates of Mycobacterium fortuitum

OBJECTIVES

Some clinical isolates of Mycobacterium fortuitum are naturally resistant to macrolides, e.g. clarithromycin. Thus, the aim of this study was to identify the gene(s) conferring this resistance.

METHODS

M. fortuitum ATCC 6841T DNA libraries were screened for plasmids that complemented the macrolide-susceptible phenotype of Mycobacterium smegmatis variant ermKO4 [erm(38)-negative]. Macrolide-resistant M. smegmatis transformants were selected on agar containing 128 mg/L erythromycin.

RESULTS

Genetic complementation identified an M. fortuitum rRNA methylase gene, termed erm(39), 69% identical to erm(38) of M. smegmatis. In addition, erm(39) was found to be in the same chromosomal location as erm(38) in their respective hosts. Like erm(38), erm(39) conferred resistance (MIC >128 mg/L) to macrolide-lincosamide (ML) agents, but not to streptogramin B. Analysis of erm gene expression in M. fortuitum showed that ML agents increased erm(39) RNA levels, reaching a steady state level approximately 20-fold higher than baseline. Screening of 32 M. fortuitum clinical isolates by PCR showed that all were positive for erm(39), irrespective of clarithromycin susceptibility. A majority of clarithromycin-susceptible (MIC < or = 2 mg/L) isolates were postulated to carry a disabled erm(39) gene as they had a GTG-->CTG mutation in the putative initiation codon of the erm(39) gene.

CONCLUSIONS

The similarity of the erm genes of M. smegmatis and M. fortuitum suggests that they were inherited from a common ancestor. Although the clinical impact of erm(39) on the therapeutic utility of clarithromycin is unclear, induction of this gene is consistent with the trailing end-points commonly seen during susceptibility testing of M. fortuitum isolates against macrolides.

Which agents should we use for the treatment of multidrug-resistant Mycobacterium tuberculosis?

The inappropriate treatment of drug-susceptible tuberculosis can lead to the selection and transmission of multidrug-resistant tuberculosis (MDR-TB), indicating resistance to at least isoniazid and rifampicin. In the treatment of MDR-TB, residual first-line drugs, such as ethambutol, pyrazinamide and streptomycin must be appropriately combined with additional second-line drugs, guided by individual susceptibility patterns. The clinical pharmacology of these second-line antituberculous drugs is reviewed. Fluoroquinolones represent the only substantial therapeutic advance in the last 20 years. Many factors potentially affect the outcome of MDR-TB. Treatment adherence, prior exposure to antituberculous drugs, the number of drugs to which the infection is still susceptible and the time since the first diagnosis of tuberculosis are the most relevant. The management of MDR-TB requires considerable expertise. When initiating or revising therapy for MDR-TB, the process of selecting drugs should rely on prior treatment history, results of susceptibility testing and an evaluation of the patient's adherence. In making drug selection, we propose to follow a hierarchy based on the intrinsic activity against Mycobacterium tuberculosis and the clinical evidence of efficacy of the available active compounds.

Intrinsic macrolide resistance in Mycobacterium smegmatis is conferred by a novel erm gene, erm(38)

High-level, acquired macrolide resistance in mycobacteria is conferred by mutation within the 23S rRNA gene. However, several mycobacteria are naturally resistant to macrolides, including the Mycobacterium smegmatis group and Mycobacterium tuberculosis complex. Thus, the aim of this study was to characterize this resistance. Intrinsic macrolide resistance in M. smegmatis was inducible and showed cross-resistance to lincosamides but not to streptogramin B (i.e., ML resistance). A similar phenotype was found with Mycobacterium microti and macrolide-resistant Mycobacterium fortuitum. A search of the DNA sequence data for M. smegmatis strain mc(2)155 identified a novel erm gene, erm(38), and expression analysis showed that erm(38) RNA levels increased >10-fold after a 2-h incubation with macrolide. Inducible ML resistance was not expressed by an erm(38) knockout mutant, and complementation of this mutant with intact erm(38) in trans resulted in high-level ML resistance (e.g., clarithromycin MIC of >512 micro g/ml). Thus, the results indicate that erm(38) confers the intrinsic ML resistance of M. smegmatis. Southern blot analysis with an erm(38)-specific probe indicated that a similar gene may be present in macrolide-resistant M. fortuitum. This finding, with the presence of the erm(37) gene (Rv1988) in the M. tuberculosis complex, suggests that such genes are widespread in mycobacteria with intrinsic macrolide resistance.

Molecular basis of intrinsic macrolide resistance in the Mycobacterium tuberculosis complex

The intrinsic resistance of the Mycobacterium tuberculosis complex (MTC) to most antibiotics, including macrolides, is generally attributed to the low permeability of the mycobacterial cell wall. However, nontuberculous mycobacteria (NTM) are much more sensitive to macrolides than members of the MTC. A search for macrolide resistance determinants within the genome of M. tuberculosis revealed the presence of a sequence encoding a putative rRNA methyltransferase. The deduced protein is similar to Erm methyltransferases, which confer macrolide-lincosamide-streptogramin (MLS) resistance by methylation of 23S rRNA, and was named ErmMT. The corresponding gene, ermMT (erm37), is present in all members of the MTC but is absent in NTM species. Part of ermMT is deleted in some vaccine strains of Mycobacterium bovis BCG, such as the Pasteur strain, which lack the RD2 region. The Pasteur strain was susceptible to MLS antibiotics, whereas MTC species harboring the RD2 region were resistant to them. The expression of ermMT in the macrolide-sensitive Mycobacterium smegmatis and BCG Pasteur conferred MLS resistance. The resistance patterns and ribosomal affinity for erythromycin of Mycobacterium host strains expressing ermMT, srmA (monomethyltransferase from Streptomyces ambofaciens), and ermE (dimethyltransferase from Saccharopolyspora erythraea) were compared, and the ones conferred by ErmMT were similar to those conferred by SrmA, corresponding to the MLS type I phenotype. These results suggest that ermMT plays a major role in the intrinsic macrolide resistance of members of the MTC and could be the first example of a gene conferring resistance by target modification in mycobacteria.

Programmes and principles in treatment of multidrug-resistant tuberculosis

Multidrug-resistant tuberculosis (MDR-TB) presents an increasing threat to global tuberculosis control. Many crucial management issues in MDR-TB treatment remain unanswered. We reviewed the existing scientific research on MDR-TB treatment, which consists entirely of retrospective cohort studies. Although direct comparisons of these studies are impossible, some insights can be gained: MDR-TB can and should be addressed therapeutically in resource-poor settings; starting of treatment early is crucial; aggressive treatment regimens and high-end dosing are recommended given the lower potency of second-line antituberculosis drugs; and strategies to improve treatment adherence, such as directly observed therapy, should be used. Opportunities to treat MDR-TB in developing countries are now possible through the Global Fund to Fight AIDS, TB, and Malaria, and the Green Light Committee for Access to Second-line Anti-tuberculosis Drugs. As treatment of MDR-TB becomes increasingly available in resource-poor areas, where it is needed most, further clinical and operational research is urgently needed to guide clinicians in the management of this disease.

Drug treatment for tuberculosis during pregnancy: safety considerations

Untreated tuberculosis in pregnancy poses a significant threat to the mother, fetus and family. Adherence to treatment is especially difficult in pregnancy because of the general fear of any medication and pregnancy-related nausea. Supervised treatment is especially helpful in encouraging adherence. All 4 first line drugs [isoniazid, rifampicin (rifampin), ethambutol and pyrazinamide] have an excellent safety record in pregnancy and are not associated with human fetal malformations. Drug-induced hepatitis, especially with isoniazid, is a significant problem in treating tuberculosis, not peculiar to pregnancy; close monitoring of liver function is recommended. Liver enzyme induction by rifampicin alters the metabolism of other drugs, e.g. methadone doses will need to be increased. Streptomycin should not be used in pregnancy, as perhaps 1 in 6 babies will have problems with hearing and/or balance. Ciprofloxacin has the best safety profile of second line drugs in the treatment of drug-resistant tuberculosis. Preventive treatment with isoniazid can be undertaken safely during pregnancy. Pyridoxine (vitamin B6) should be added to the drug treatment of tuberculosis in all pregnant women taking isoniazid. Neither tuberculin nor the bacille Calmette Guérin (BCG) vaccine are treatments for tuberculosis, but they play an important role in the management of the disease. Tuberculin testing is safe, but BCG vaccination should be avoided in pregnancy and instead given earlier in life.

Effect of proton pump inhibitor alone or in combination with clarithromycin on mycobacterial growth in human alveolar macrophages

The effect of omeprazole, a clinically used proton pump inhibitor, alone or in combination with clarithromycin was evaluated against Mycobacterium avium, Mycobacterium intracellulare and Mycobacterium tuberculosis, using a human alveolar macrophage model of infection. Omeprazole exhibited no significant effect on the growth of the two M. avium complex strains or on the mycobactericidal activity of clarithromycin against them. In contrast, omeprazole significantly promoted the growth of Mycobacterium tuberculosis and the anti-mycobacterial activity of clarithromycin against it in human alveolar macrophages. It was speculated that intracellular acidic milieu around M. tuberculosis might be one reason for the lower activity of clarithromycin in the treatment of human tuberculosis.

Treatment and prevention of multidrug-resistant tuberculosis

Multidrug-resistant tuberculosis (MDRTB), which is defined as combined resistance to isoniazid and rifampicin, is a 'man-made' disease that is caused by improper treatment, inadequate drug supplies or poor patient supervision. Patients with MDRTB face chronic disability and death, and represent an infectious hazard for the community. Cure rates of 96% have been achieved but require prompt recognition of the disease, rapid accurate susceptibility results, and early administration of an individualised re-treatment regimen. Such regimens are usually based on a quinolone and an injectable agent (i.e. an aminoglycoside or capreomycin) supplemented by other 'second-line' drugs. This therapy is prolonged (e.g. 24 months), expensive, and has multiple adverse effects. Prevention of MDRTB is therefore of paramount importance. The World Health Organization (WHO) has recommended a multifaceted programme, known by the acronym DOTS (directly observed therapy, short-course), that promotes effective treatment of drug-susceptible TB as the prime method of limiting drug resistance. DOTS was part of a successful MDRTB control programme in New York City, which also included treatment of prevalent MDRTB cases, streamlined laboratory testing, effective infection control procedures and wider application of screening and preventive therapy (although the optimal chemotherapy for MDRTB infection remains undefined). Industrialised countries have the resources to treat patients with MDRTB and to mount these extensive control programmes. Unfortunately, MDRTB is also prevalent in Asia, South America and the former Soviet Union. First world countries have a vested interest, as well as a moral responsibility, to assist in controlling MDRTB in these 'hot spots'.