Susceptibility of Mycobacterium tuberculosis strains to gatifloxacin and moxifloxacin by different methods

Structure-based virtual screening and biological evaluation of novel inhibitors of mycobacterium Z-ring formation

The major part of commercial prodrugs against Mycobacterium tuberculosis (Mtb) demonstrated a significant inhibitory effect on cell division and inhibition of bacterial growth in vitro. However, further implementation often failed to overcome the compensatory system of interchangeable cascades. This is the most common situation for the compounds, which hit the key enzymes activities involved in all basic stages of the cell cycle. We decided to find more compounds, which could affect a cytoskeleton complex playing important role in sensing the external signals, intracellular transport, and cell division. In general, the bacterial cytoskeleton is crucial for response to the environment and participates in cell-to-cell communication. In turn, filamentous temperature-sensitive Z (FtsZ) protein, a mycobacterial tubulin homolog, is essential for Z-ring formation and further bacteria cell division. We predicted the most preferable binding-sites and conducted a high-throughput virtual screening. Modeling results suggest that some compounds bind in a specific region on the surface Mtb FtsZ, which is absent in human, and other could hit GTPase activity of the FtsZ. Further in vitro studies confirmed that these novel molecules can efficiently bind to these pockets, demonstrating an effect on the polymerization state and kinetics mechanisms. The rescaling of the experiment on the cell line revealed that reported compounds are able to alter the polymerization level of the filamentous and, therefore, prevent mycobacteria reproduction.

Wild-Type MIC Distribution for Re-evaluating the Critical Concentration of Anti-TB Drugs and Pharmacodynamics Among Tuberculosis Patients From South India

The World Health Organization (WHO) has developed specific guidelines for critical concentrations (CCs) of antibiotics used for tuberculosis (TB) treatment, which is universally followed for drug susceptibility testing (DST) of clinical specimens. However, the CC of drugs can differ significantly among the mycobacterial species based on the population, geographic location, and the prevalence of the infecting strain in a particular area. The association between CC and the minimal inhibitory concentration (MIC) of anti-TB drugs is poorly understood. In this study, we assessed the MICs of anti-TB drugs, including isoniazid (INH), rifampicin (RMP), moxifloxacin (MXF), ethambutol (ETH), and p-aminosalicylic acid (PAS) on drug-sensitive Mtb isolates from pulmonary TB patients in South India. The MIC assays performed using solid- and liquid-growth media showed changes in the CC of a few of the tested antibiotics compared with the WHO-recommended levels. Our observation suggests that the WHO guidelines could potentially lead to overdiagnosis of drug-resistant cases, which can result in inappropriate therapeutic decisions. To evaluate the correlation between drug-resistance and CC, we performed the whole-genome sequencing for 16 mycobacterial isolates, including two wild-type and 14 resistant isolates. Our results showed that two of the isolates belonged to the W-Beijing lineage, while the rest were of the East-African-Indian type. We identified a total of 74 mutations, including five novel mutations, which are known to be associated with resistance to anti-TB drugs in these isolates. In our previous study, we determined the serum levels of INH and RMP among the same patients recruited in the current study and estimated the MICs of the corresponding infected isolates in these cases. Using these data and the CCs for INH and RMP from the present study, we performed pharmacodynamics (PD) evaluation. The results show that the PD of RMP was subtherapeutic. Together, these observations emphasize the need for optimizing the drug dosage based on the PD of large-scale studies conducted in different geographical settings.

Double mutation in DNA gyrase confers moxifloxacin resistance and decreased fitness of Mycobacterium smegmatis

Objectives

Ofloxacin and moxifloxacin are the most commonly used fluoroquinolones (FQs) for the treatment of tuberculosis. As a new generation FQ, moxifloxacin has been recommended for the treatment of ofloxacin-resistant TB. However, the mechanism by which ofloxacin-resistant Mycobacterium tuberculosis further gains resistance to moxifloxacin remains unclear.

Methods

We used Mycobacterium smegmatis as a model for studying FQ resistance in M. tuberculosis . Moxifloxacin-resistant M. smegmatis was selected in vitro based on strains with primary ofloxacin resistance. The gyrA and gyrB genes of the resistant strains were sequenced to identify resistance-associated mutations. An in vitro competition assay was applied to explore the influence of gyrA / gyrB mutations on bacterial fitness. Finally, we evaluated the clinical relevance of our findings by analysing the WGS data of 1984 globally collected M. tuberculosis strains.

Results

A total of 57 moxifloxacin-resistant M. smegmatis strains based on five ofloxacin-resistant strains were obtained. Sequencing results revealed that all moxifloxacin-resistant strains harboured second-step mutations in gyrA or gyrB . The relative fitnesses of the double-mutation strains varied from 0.65 to 0.93 and were mostly lower than those of their mono-mutation parents. From the genomic data, we identified 37 clinical M. tuberculosis strains harbouring double mutations in gyrA and/or gyrB and 36 of them carried at least one low-level FQ-resistance mutation.

Conclusions

Double mutation in DNA gyrase leads to moxifloxacin resistance and decreased fitness in M. smegmatis . Under current dosing of moxifloxacin, double mutations mainly happened in M. tuberculosis strains with primary low-level resistance mutations.

The role of moxifloxacin in tuberculosis therapy

Tuberculosis (TB) remains a global threat with more than 9 million new infections. Treatment remains difficult and there has been no change in the duration of the standard regimen since the early 1980s. Moreover, many patients are unable to tolerate this treatment and discontinue therapy, increasing the risk of resistance. There is a growing tide of multidrug resistance and few effective antibiotics to tackle the problem. Since the turn of the millennium there has been a surge in interest in developing new therapies for TB and a number of new drugs have been developed. In this review the repurposing of moxifloxacin, an 8-methoxy-fluoroquinolone, for TB treatment is discussed. The evidence that underpins the development of this agent is reviewed. The results of the recently completed phase III trials are summarised and the reasons for the unexpected outcome are explored. Finally, the design of new trials that incorporate moxifloxacin, and that address both susceptible disease and multidrug resistance, is described.

Correlation of different phenotypic drug susceptibility testing methods for four fluoroquinolones in Mycobacterium tuberculosis

Background

Molecular resistance testing fails to explain all fluoroquinolone resistance, with a continued need for a suitable rapid phenotypic drug susceptibility testing method.

Objective

To evaluate the optimal method for phenotypic fluoroquinolone susceptibility testing.

Methods

Using Löwenstein–Jensen medium, Middlebrook 7H11 agar, BACTEC-MGIT 960 and the resazurin microtitre plate assay, we determined susceptibility to fluoroquinolones in Mycobacterium tuberculosis and investigated cross-resistance between ofloxacin, levofloxacin, moxifloxacin and gatifloxacin. We compared MICs of all four fluoroquinolones for 91 strains on Löwenstein–Jensen (as the gold standard) with their MICs in resazurin plates, and with ofloxacin susceptibility at a single concentration in MGIT and on 7H11 agar, in addition to sequencing of the gyrAB genes.

Results and conclusions

Applying a cut-off of 2 mg/L ofloxacin, 1 mg/L levofloxacin and 0.5 mg/L moxifloxacin and gatifloxacin in all methods, some discordance between solid medium and MGIT methods was observed, yet this tended to be explained by MICs around the cut-off. The high discordance between Löwenstein–Jensen (LJ) and resazurin plates suggests that the currently applied cut-offs for all fluoroquinolones in the resazurin method should decrease and minor changes in colour (from blue to purple) be considered as meaningful. High-level resistance in all assays to all drugs correlated well with the presence of gyrA mutations, in support of recent findings that fluoroquinolone resistance should be tested at different concentrations, as patients with lower levels of resistance may continue to benefit from high-dose fluoroquinolone-based therapy.

Management of patients with multidrug-resistant/extensively drug-resistant tuberculosis in Europe: a TBNET consensus statement

The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB) substantially challenges TB control, especially in the European Region of the World Health Organization, where the highest prevalence of MDR/XDR cases is reported. The current management of patients with MDR/XDR-TB is extremely complex for medical, social and public health systems. The treatment with currently available anti-TB therapies to achieve relapse-free cure is long and undermined by a high frequency of adverse drug events, suboptimal treatment adherence, high costs and low treatment success rates. Availability of optimal management for patients with MDR/XDR-TB is limited even in the European Region. In the absence of a preventive vaccine, more effective diagnostic tools and novel therapeutic interventions the control of MDR/XDR-TB will be extremely difficult. Despite recent scientific advances in MDR/XDR-TB care, decisions for the management of patients with MDR/XDR-TB and their contacts often rely on expert opinions, rather than on clinical evidence.

This document summarises the current knowledge on the prevention, diagnosis and treatment of adults and children with MDR/XDR-TB and their contacts, and provides expert consensus recommendations on questions where scientific evidence is still lacking.

TBNET consensus statement on the management of patients with MDR/XDR-TB has been released in theEur Respir Jhttp://ow.ly/uizRD

Rapid detection of isoniazid, rifampin, and ofloxacin resistance in Mycobacterium tuberculosis clinical isolates using high-resolution melting analysis

A high-resolution melting analysis (HRMA) assay was developed to detect isoniazid, rifampin, and ofloxacin resistance in Mycobacterium tuberculosis by targeting resistance-associated mutations in the katG, mabA-inhA promoter, rpoB, and gyrA genes. A set of 28 (17 drug-resistant and 11 fully susceptible) clinical M. tuberculosis isolates was selected for development and evaluation of HRMA. PCR amplicons from the katG, mabA-inhA promoter, rpoB, and gyrA genes of all 28 isolates were sequenced. HRMA results matched well with 18 mutations, identified by sequencing, in 17 drug-resistant isolates and the absence of mutations in 11 susceptible isolates. Among 87 additional isolates with known resistance phenotypes, HRMA identified katG and/or mabA-inhA promoter mutations in 66 of 69 (95.7%) isoniazid-resistant isolates, rpoB mutations in 51 of 54 (94.4%) rifampin-resistant isolates, and gyrA mutations in all of 41 (100%) ofloxacin-resistant isolates. All mutations within the HRMA primer target regions were detected as variant HRMA profiles. The corresponding specificities were 97.8%, 100%, and 98.6%, respectively. Most false-positive results were due to synonymous mutations, which did not affect susceptibility. HRMA is a rapid, sensitive method for detection of drug resistance in M. tuberculosis which could be used routinely for screening isolates in countries with a high prevalence of tuberculosis and drug resistance or in individual isolates when drug resistance is suspected.

Molecular basis and mechanisms of drug resistance in Mycobacterium tuberculosis: classical and new drugs

Tuberculosis (TB) remains one of the leading public health problems worldwide. Declared as a global emergency in 1993 by the WHO, its control is hampered by the emergence of multidrug resistance (MDR), defined as resistance to at least rifampicin and isoniazid, two key drugs in the treatment of the disease. More recently, severe forms of drug resistance such as extensively drug-resistant (XDR) TB have been described. After the discovery of several drugs with anti-TB activity, multidrug therapy became fundamental for control of the disease. Major advances in molecular biology and the availability of new information generated after sequencing the genome of Mycobacterium tuberculosis increased our knowledge of the mechanisms of resistance to the main anti-TB drugs. Better knowledge of the mechanisms of drug resistance in TB and the molecular mechanisms involved will help us to improve current techniques for rapid detection and will also stimulate the exploration of new targets for drug activity and drug development. This article presents an updated review of the mechanisms and molecular basis of drug resistance in M. tuberculosis. It also comments on the several gaps in our current knowledge of the molecular mechanisms of drug resistance to the main classical and new anti-TB drugs and briefly discusses some implications of the development of drug resistance and fitness, transmission and pathogenicity of M. tuberculosis.

Molecular characterization of fluoroquinolone resistance in Mycobacterium tuberculosis: functional analysis of gyrA mutation at position 74

A PCR-sequencing assay was evaluated for direct detection of mutations in the quinolone resistance-determining region (QRDR) of gyrase A (gyrA) gene in fluoroquinolone-resistant Mycobacterium tuberculosis in respiratory specimens. As determined by gyrA QRDR analysis, complete concordance of genotypic and phenotypic fluoroquinolone resistance was demonstrated. Our results indicate that the assay is a rapid and reliable method for the diagnosis of fluoroquinolone-resistant tuberculosis, facilitating timely clinical management and public health control. Using the assay, we detected a novel gyrA Ala74Ser mutation in M. tuberculosis directly from sputum specimens. The functional effect of the Ala74Ser mutant was verified through the study of the DNA supercoiling inhibitory activity of fluoroquinolones against the recombinant gyrase. The drug-mediated gyrase-DNA cleavage complex model suggests perturbation of the gyrA-gyrA dimer interface caused by the Ala74Ser mutation probably disturbs the putative quinolone binding pocket and leads to the reduction of the drug binding affinity. A number of gyrA mutations (Glu21Gln, Ser95Thr, and Gly668Asp) were also characterized to be natural polymorphisms not associated with fluoroquinolone resistance.

Fluoroquinolone resistance in Mycobacterium tuberculosis and mutations in gyrA and gyrB

This study evaluated cross-resistance of Mycobacterium tuberculosis strains to ofloxacin, moxifloxacin, and gatifloxacin and investigated the presence of mutations in gyrA and gyrB. Fluoroquinolone susceptibilities were determined for 41 M. tuberculosis strains by the proportion method on 7H11, and MICs were determined by the resazurin microtiter assay. Forty strains shared the same resistance results for the three fluoroquinolones. However, one strain, with an Asn-533 --> Thr mutation in gyrB, was susceptible to ofloxacin but resistant to moxifloxacin and gatifloxacin.

Fluoroquinolone resistance in Mycobacterium tuberculosis: an assessment of MGIT 960, MODS and nitrate reductase assay and fluoroquinolone cross-resistance

OBJECTIVES

The aim of this study was to assess the sensitivity, specificity and time to results of mycobacterial growth indicator tube (MGIT) 960, microscopic observation drug susceptibility (MODS) assay and nitrate reductase assay (NRA) compared with the gold standard agar proportion method (PM), and to determine whether there is cross-resistance between older-generation fluoroquinolones and moxifloxacin.

METHODS

Mycobacterium tuberculosis isolates from culture-confirmed tuberculosis patients from 2002 to 2007 were tested for ofloxacin (2 mg/L) resistance by PM and MGIT 960. All isolates from 2005 and 2006 were also tested by MODS and NRA. Ofloxacin-resistant isolates by PM were further tested by all four methods using ciprofloxacin, levofloxacin and moxifloxacin. For each ofloxacin-resistant isolate, two ofloxacin-susceptible isolates were tested against all three fluoroquinolones using all four methods.

RESULTS

Of the 797 M. tuberculosis isolates, 19 (2.4%) were ofloxacin-resistant by PM. MGIT 960 had 100% sensitivity (95% CI, 83%-100%) and specificity (95% CI, 99.5%-100%). Of the 797 isolates, 239 were from 2005 to 2006 and 6 of these (2.5%) were resistant by PM. MODS had 100% sensitivity (95% CI, 61%-100%) and specificity (95% CI, 98%-100%). NRA had 100% sensitivity (95% CI, 61%-100%) and 98.7% specificity (95% CI, 96%-99.6%). The median time to results was shorter using MGIT 960 (8 days), MODS (6 days) or NRA (9 days) compared with PM (21 days) (P < 0.001). All 19 ofloxacin-resistant isolates were resistant to ciprofloxacin, levofloxacin and moxifloxacin by PM.

CONCLUSIONS

MGIT 960, MODS and NRA are sensitive and specific and more rapid than PM for identifying fluoroquinolone resistance in M. tuberculosis. Ofloxacin resistance was associated with cross-resistance to ciprofloxacin, levofloxacin and moxifloxacin.