gyrA mutations and phenotypic susceptibility levels to ofloxacin and moxifloxacin in clinical isolates of Mycobacterium tuberculosis

Mutations in gyrA and gyrB among Fluoroquinolone- and Multidrug-Resistant Mycobacterium tuberculosis Isolates

In order to correlate the mutations inside the entiregyrAandgyrBgenes with the level of resistance to ofloxacin (OFX) and moxifloxacin (MFX) in isolates of multidrug-resistantMycobacterium tuberculosis(MDR-TB), a total of 111 isolates were categorized into OFX-susceptible (MIC, ≤2 μg/ml) and low-level (MIC, 4 to 8 μg/ml) and high-level (MIC, ≥16 μg/ml) OFX-resistant isolates and MFX-susceptible (MIC, ≤0.5 μg/ml) and low-level (MIC, 1 to 2 μg/ml) and high-level (MIC, ≥4 μg/ml) MFX-resistant isolates. Resistance-associated mutations inside thegyrAgene were found in 30.2% of OFX-susceptible and 72.5% and 72.2% of low-level and high-level OFX-resistant isolates and in 28.6% of MFX-susceptible and 58.1% and 83.9% of low-level and high-level MFX-resistant isolates. Compared with OFX-susceptible isolates, low-level and high-level OFX-resistant isolates had a significantly higher prevalence of mutations atgyrAcodons 88 to 94 (17.0%, 65.0%, and 72.2%, respectively;P< 0.001) and a higher prevalence of thegyrBG512R mutation (0.0%, 2.5%, and 16.7%, respectively;P= 0.006). Similarly, compared with MFX-susceptible isolates, low-level and high-level MFX-resistant isolates had a significantly higher prevalence of mutations atgyrAcodons 88 to 94 (14.3%, 51.6%, and 80.6%, respectively;P< 0.001) as well as a higher prevalence of thegyrBG512R mutation (0.0%, 0.0%, and 12.9%, respectively;P= 0.011). D94G and D94N mutations ingyrAand the G512R mutation ingyrBwere correlated with high-level MFX resistance, while the D94A mutation was associated with low-level MFX resistance. The prevalence of mutations atgyrAcodons 88 to 94 and thegyrBG512R mutation were higher among fluoroquinolone (FQ)-susceptible East Asian (Beijing) and Indo-Oceanic strains than they were among Euro-American strains, implying that molecular techniques to detect FQ resistance may be less specific in areas with a high prevalence of East Asian (Beijing) and Indo-Oceanic strains.

Gyrase Mutations Are Associated with Variable Levels of Fluoroquinolone Resistance in Mycobacterium tuberculosis

Molecular diagnostics that rapidly and accurately predict resistance to fluoroquinolone drugs and especially later-generation agents promise to improve treatment outcomes for patients with multidrug-resistant tuberculosis and prevent the spread of disease. Mutations in the gyr genes are known to confer most fluoroquinolone resistance, but knowledge about the effects of gyr mutations on susceptibility to early- versus later-generation fluoroquinolones and about the role of mutation-mutation interactions is limited. Here, we sequenced the full gyrA and gyrB open reading frames in 240 multidrug-resistant and extensively drug-resistant tuberculosis strains and quantified their ofloxacin and moxifloxacin MIC by testing growth at six concentrations for each drug. We constructed a multivariate regression model to assess both the individual mutation effects and interactions on the drug MICs. We found that gyrB mutations contribute to fluoroquinolone resistance both individually and through interactions with gyrA mutations. These effects were statistically significant. In these clinical isolates, several gyrA and gyrB mutations conferred different levels of resistance to ofloxacin and moxifloxacin. Consideration of gyr mutation combinations during the interpretation of molecular test results may improve the accuracy of predicting the fluoroquinolone resistance phenotype. Further, the differential effects of gyr mutations on the activity of early- and later-generation fluoroquinolones requires further investigation and could inform the selection of a fluoroquinolone for treatment.

The Molecular Genetics of Fluoroquinolone Resistance in Mycobacterium tuberculosis

The fluoroquinolones (FQs) are synthetic antibiotics effectively used for curing patients with multidrug-resistant tuberculosis (TB). When a multidrug-resistant strain develops resistance to the FQs, as in extensively drug-resistant strains, obtaining a cure is much more difficult, and molecular methods can help by rapidly identifying resistance-causing mutations. The only mutations proven to confer FQ resistance in M. tuberculosis occur in the FQ target, the DNA gyrase, at critical amino acids from both the gyrase A and B subunits that form the FQ binding pocket. GyrA substitutions are much more common and generally confer higher levels of resistance than those in GyrB. Molecular techniques to detect resistance mutations have suboptimal sensitivity because gyrase mutations are not detected in a variable percentage of phenotypically resistant strains. The inability to find gyrase mutations may be explained by heteroresistance: bacilli with a resistance-conferring mutation are present only in a minority of the bacterial population (>1%) and are therefore detected by the proportion method, but not in a sufficient percentage to be reliably detected by molecular techniques. Alternative FQ resistance mechanisms in other bacteria--efflux pumps, pentapeptide proteins, or enzymes that inactivate the FQs--have not yet been demonstrated in FQ-resistant M. tuberculosis but may contribute to intrinsic levels of resistance to the FQs or induced tolerance leading to more frequent gyrase mutations. Moxifloxacin is currently the best anti-TB FQ and is being tested for use with other new drugs in shorter first-line regimens to cure drug-susceptible TB.

Susceptibility testing breakpoints for Mycobacterium tuberculosis categorize isolates with resistance mutations in gyrA as susceptible to fluoroquinolones: implications for MDR-TB treatment and the definition of XDR-TB

OBJECTIVES

Fluoroquinolones (FQs) are important in the treatment of MDR-TB and in the definition of XDR-TB. Our objective was to investigate how discrepancies in the phenotypic and genotypic methods for antimicrobial susceptibility testing could affect the interpretation of antimicrobial susceptibility test results.

METHODS

We analysed MICs of ofloxacin and levofloxacin in Middlebrook 7H10 broth (7H10) as well as sequencing of the quinolone resistance-determining region of the gyrA gene and the MTBDRsl assay in 75 resistant isolates, including MDR and XDR strains of Mycobacterium tuberculosis.

RESULTS

Among 75 resistant isolates, 27 had mutations associated with FQ resistance. Among isolates with resistance mutations in gyrA, 26% (seven of 27) were susceptible to levofloxacin and ofloxacin by phenotypic testing at 1 mg/L and 2 mg/L. The most common mutation was in codon 94 and these isolates had significantly increased MICs of levofloxacin (2-8 mg/L) compared with isolates with mutations in codon 90 (0.25-2 mg/L, P < 0.05). The sensitivity and specificity for the MTBDRsl assay compared with gyrA sequencing were 96% and 98%, respectively.

CONCLUSION

Current critical concentrations may classify up to 26% of isolates with gyrA mutations as susceptible to FQs due to a close relationship between susceptible and resistant populations. These results should be considered while improving clinical breakpoints for M. tuberculosis and may have an impact on the definition of XDR-TB.

Primary Capreomycin Resistance Is Common and Associated With Early Mortality in Patients With Extensively Drug-Resistant Tuberculosis in KwaZulu-Natal, South Africa

BACKGROUND

Capreomycin is a key antimycobacterial drug in treatment of extensively drug-resistant tuberculosis (XDR-TB). Drug-susceptibility testing (DST) for capreomycin is not routinely performed in newly diagnosed XDR-TB in South Africa. We performed this study to assess the prevalence, clinical significance, and molecular epidemiology of capreomycin resistance in newly diagnosed patients with XDR-TB in KwaZulu-Natal, South Africa.

METHODS

Retrospective cohort study of consecutive patients with XDR-TB admitted to a TB referral hospital without previous XDR-TB treatment. A subset of isolates had extended DST (including capreomycin), mutational analysis, and IS6110 restriction fragment length polymorphism assays.

RESULTS

A total of 216 eligible patients with XDR-TB were identified. The majority were treated with capreomycin (72%), were young (median age: 35.5 years), and were female (56%). One hundred five (76%) were HIV+, and 109 (66%) were on antiretroviral therapy. A subset of 52 patients had full DST. A total of 47/52 (90.4%) patients with XDR-TB were capreomycin resistant. Capreomycin-resistant patients experienced worse mortality and culture conversion than capreomycin susceptible, although this difference was not statistically significant. The A1401G mutation in the rrs gene was associated with capreomycin resistance. The majority of capreomycin-resistant strains were F15/LAM4/KZN lineage (80%), and clustering was common in these isolates (92.5%).

CONCLUSIONS

Capreomycin resistance is common in patients with XDR-TB in KwaZulu-Natal, is predominantly because of ongoing province-wide transmission of a highly resistant strain, and is associated with high mortality. Capreomycin should be included in routine DST in all patients with XDR-TB. New drug regimens that do not include injectable agents should be operationally tested as empiric treatment in XDR-TB.

The Race Is On To Shorten the Turnaround Time for Diagnosis of Multidrug-Resistant Tuberculosis

To realize the most benefit from multidrug-resistant tuberculosis (MDR-TB) screening, all nucleic acid amplification test (NAAT)-positive respiratory specimens should be universally tested. Once an MDR-TB diagnosis is established, additional testing is warranted to provide details about the detected mutations. The lab-on-chip technology described by A. M. Cabibbe et al. (J Clin Microbiol 53:3876-3880, 2015, http://dx.doi.org/10.1128/JCM.01824-15) potentially provides this much needed information.

Correlation between GyrA substitutions and ofloxacin, levofloxacin, and moxifloxacin cross-resistance in Mycobacterium tuberculosis

The newer fluoroquinolones moxifloxacin (MXF) and levofloxacin (LVX) are becoming more common components of tuberculosis (TB) treatment regimens. However, the critical concentrations for testing susceptibility of Mycobacterium tuberculosis to MXF and LVX are not yet well established. Additionally, the degree of cross-resistance between ofloxacin (OFX) and these newer fluoroquinolones has not been thoroughly investigated. In this study, the MICs for MXF and LVX and susceptibility to the critical concentration of OFX were determined using the agar proportion method for 133 isolates of M. tuberculosis. Most isolates resistant to OFX had LVX MICs of >1 μg/ml and MXF MICs of >0.5 μg/ml. The presence of mutations within the gyrA quinolone resistance-determining regions (QRDR) correlated well with increased MICs, and the level of LVX and MXF resistance was dependent on the specific gyrA mutation present. Substitutions Ala90Val, Asp94Ala, and Asp94Tyr resulted in low-level MXF resistance (MICs were >0.5 but ≤2 μg/ml), while other mutations led to MXF MICs of >2 μg/ml. Based on these results, a critical concentration of 1 μg/ml is suggested for LVX and 0.5 μg/ml for MXF drug susceptibility testing by agar proportion with reflex testing for MXF at 2 μg/ml.

Pyrosequencing for rapid detection of Mycobacterium tuberculosis second-line drugs and ethambutol resistance

The aim of this work was to study the diagnostic accuracy of pyrosequencing to detect resistance to fluoroquinolones, kanamycin, amikacin, capreomycin, and ethambutol (EMB) in Mycobacterium tuberculosis clinical strains. One hundred four clinical isolates previously characterized by BACTEC 460TB/MGIT 960 were included. Specific mutations were targeted in gyrA, rrs, eis promoter, and embB. When there was a discordant result between BACTEC and pyrosequencing, Genotype MTBDRsl (Hain Lifescience, Nehren, Germany) was performed. Sensitivity and specificity of pyrosequencing were 70.6% and 100%, respectively, for fluoroquinolones; 93.3% and 81.7%, respectively, for kanamycin; 94.1% and 95.9%, respectively, for amikacin; 90.0% and 100%, respectively, for capreomycin; and 64.8% and 87.8%, respectively, for EMB. This study shows that pyrosequencing may be a useful tool for making early decisions regarding second-line drugs and EMB resistance. However, for a correct management of patients with suspected extensively drug-resistant tuberculosis, susceptibility results obtained by molecular methods should be confirmed by a phenotypic method.

Second-line anti-tuberculosis drug resistance and its genetic determinants in multidrug-resistant Mycobacterium tuberculosis clinical isolates

BACKGROUND

Mutations in several genetic loci have been implicated in the development of resistance to second-line anti-tuberculosis (TB) drugs (SLDs). The purpose of this study was to investigate the prevalence of resistance to SLDs and its association with specific mutations in multidrug-resistant (MDR) Mycobacterium tuberculosis clinical isolates.

MATERIALS AND METHODS

The study included 46 MDR-TB isolates. Mutation profiling was performed by amplifying and sequencing the following six genes: gyrA/gyrB, rrs, tlyA, and ethA/ethR, in which mutations are implicated in resistance of tubercle bacilli to ofloxacin (OFX), amikacin (AMK), capreomycin, and ethionamide (ETH), respectively.

RESULTS

Of the strains analyzed, 14 (30.4%) showed resistance to at least one of the four SLDs tested. Mutations in the gyrA gene occurred in 34 (73.9%) strains, with the most common amino acid change being Ser95Thr. The Asp94Asn and Ala90Val substitutions in the gyrA were present exclusively in OFX-resistant strains, yet represented only 40% of all OFX-resistant strains. The only mutation in the gyrB gene was substitution Ser447Phe, detected in one OFX-resistant isolate. None of the AMK-resistant strains carried a mutation in the rrs gene. Mutations in the ethA/ethR loci were found in one ETH-resistant and 11 ETH-susceptible strains.

CONCLUSIONS

The results of this study challenge the usefulness of sequence analyses of tested genes (except gyrA) for the prediction of SLD resistance patterns and highlight the need for searching other genetic loci for detection of mutations conferring resistance to SLDs in M. tuberculosis.

Concordance of Mycobacterium tuberculosis fluoroquinolone resistance testing: implications for treatment

Fluoroquinolone (FQ) drug susceptibility testing (DST) is an important step in the design of effective treatment regimens for multidrug-resistant tuberculosis. Here we compare ciprofloxacin, ofloxacin and moxifloxacin (MFX) resistance results from 226 multidrug-resistant samples. The low level of concordance observed suggests that DST should be performed for the specific FQ planned for clinical use. The results also support the new World Health Organization recommendation for testing MFX at a critical concentration of 2.0 μg/ml.

Whole genome sequencing based characterization of extensively drug-resistant Mycobacterium tuberculosis isolates from Pakistan

Improved molecular diagnostic methods for detection drug resistance in Mycobacterium tuberculosis (MTB) strains are required. Resistance to first- and second- line anti-tuberculous drugs has been associated with single nucleotide polymorphisms (SNPs) in particular genes. However, these SNPs can vary between MTB lineages therefore local data is required to describe different strain populations. We used whole genome sequencing (WGS) to characterize 37 extensively drug-resistant (XDR) MTB isolates from Pakistan and investigated 40 genes associated with drug resistance. Rifampicin resistance was attributable to SNPs in the rpoB hot-spot region. Isoniazid resistance was most commonly associated with the katG codon 315 (92%) mutation followed by inhA S94A (8%) however, one strain did not have SNPs in katG, inhA or oxyR-ahpC. All strains were pyrazimamide resistant but only 43% had pncA SNPs. Ethambutol resistant strains predominantly had embB codon 306 (62%) mutations, but additional SNPs at embB codons 406, 378 and 328 were also present. Fluoroquinolone resistance was associated with gyrA 91-94 codons in 81% of strains; four strains had only gyrB mutations, while others did not have SNPs in either gyrA or gyrB. Streptomycin resistant strains had mutations in ribosomal RNA genes; rpsL codon 43 (42%); rrs 500 region (16%), and gidB (34%) while six strains did not have mutations in any of these genes. Amikacin/kanamycin/capreomycin resistance was associated with SNPs in rrs at nt1401 (78%) and nt1484 (3%), except in seven (19%) strains. We estimate that if only the common hot-spot region targets of current commercial assays were used, the concordance between phenotypic and genotypic testing for these XDR strains would vary between rifampicin (100%), isoniazid (92%), flouroquinolones (81%), aminoglycoside (78%) and ethambutol (62%); while pncA sequencing would provide genotypic resistance in less than half the isolates. This work highlights the importance of expanded targets for drug resistance detection in MTB isolates.

Evaluation of pyrosequencing for detecting extensively drug-resistant Mycobacterium tuberculosis among clinical isolates from four high-burden countries

Reliable molecular diagnostics, which detect specific mutations associated with drug resistance, are promising technologies for the rapid identification and monitoring of drug resistance in Mycobacterium tuberculosis isolates. Pyrosequencing (PSQ) has the ability to detect mutations associated with first- and second-line anti-tuberculosis (TB) drugs, with the additional advantage of being rapidly adaptable for the identification of new mutations. The aim of this project was to evaluate the performance of PSQ in predicting phenotypic drug resistance in multidrug- and extensively drug-resistant tuberculosis (M/XDR-TB) clinical isolates from India, South Africa, Moldova, and the Philippines. A total of 187 archived isolates were run through a PSQ assay in order to identify M. tuberculosis (via the IS6110 marker), and to detect mutations associated with M/XDR-TB within small stretches of nucleotides in selected loci. The molecular targets included katG, the inhA promoter and the ahpC-oxyR intergenic region for isoniazid (INH) resistance; the rpoB core region for rifampin (RIF) resistance; gyrA for fluoroquinolone (FQ) resistance; and rrs for amikacin (AMK), capreomycin (CAP), and kanamycin (KAN) resistance. PSQ data were compared to phenotypic mycobacterial growth indicator tube (MGIT) 960 drug susceptibility testing results for performance analysis. The PSQ assay illustrated good sensitivity for the detection of resistance to INH (94%), RIF (96%), FQ (93%), AMK (84%), CAP (88%), and KAN (68%). The specificities of the assay were 96% for INH, 100% for RIF, FQ, AMK, and KAN, and 97% for CAP. PSQ is a highly efficient diagnostic tool that reveals specific nucleotide changes associated with resistance to the first- and second-line anti-TB drug medications. This methodology has the potential to be linked to mutation-specific clinical interpretation algorithms for rapid treatment decisions.

Molecular diagnosis of fluoroquinolone resistance in Mycobacterium tuberculosis

As a consequence of the use of fluoroquinolones (FQ), resistance to FQ has emerged, leading to cases of nearly untreatable and extensively drug-resistant tuberculosis. Mutations in DNA gyrase represent the main mechanism of FQ resistance. A full understanding of the pattern of mutations found in FQ-resistant (FQ(r)) clinical isolates, and of their proportions, is crucial for improving molecular methods for the detection of FQ resistance in Mycobacterium tuberculosis. In this study, we reviewed the detection of FQ resistance in isolates addressed to the French National Reference Center for Mycobacteria from 2007 to 2012, with the aim of evaluating the performance of PCR sequencing in a real-life context. gyrA and gyrB sequencing, performed prospectively on M. tuberculosis clinical isolates, was compared for FQ susceptibility to 2 mg/liter ofloxacin by the reference proportion method. A total of 605 isolates, of which 50% were multidrug resistant, were analyzed. The increase in FQ(r) strains among multidrug-resistant (MDR) strains during the time of the study was alarming (8% to 30%). The majority (78%) of the isolates with gyrA mutations were FQ(r), whereas only 36% of those with gyrB mutations were FQ(r). Only 12% of the FQ(r) isolates had a single mutation in gyrB. Combined gyrA and gyrB sequencing led to >93% sensitivity for detecting resistance. The analysis of the four false-positive and the five false-negative results of gyrA and gyrB sequencing illustrated the actual limitations of the reference proportion method. Our data emphasize the need for combined gyrA and gyrB sequencing in the investigation of FQ susceptibility in M. tuberculosis and challenge the validity of the current phenotype-based approach as the diagnostic gold standard for determining FQ resistance.

Correlating Minimum Inhibitory Concentrations of ofloxacin and moxifloxacin with gyrA mutations using the genotype MTBDRsl assay

OBJECTIVE

To correlate gyrA mutations found on the Genotype MTBDRsl assay in Mycobacterium tuberculosis (MTB) isolates with Minimum Inhibitory Concentrations (MICs) to the fluoroquinolones compounds ofloxacin (OFX) and moxifloxacin (MXF).

METHODS

MICs for OFX and MXF were ascertained for 93 archived clinical MTB isolates that showed gyrA mutations at Ala90Val, Ser91Pro, Asp94Ala, Asn/Tyr, Gly and His. Thirty fluoroquinolones susceptible isolates as determined by presence of all wild-type gyrA bands on the Genotype MTBDRsl assay were also included.

RESULTS

gyrA mutations at Ala90Val (n = 25), Ser91Pro (n = 6), Asp94Ala (n = 4), Asp94Asn/Tyr (n = 13), Asp94Gly (n = 42) and Asp94His (n = 3) were observed. Isolates with mutations at Ala90Val or Ser91Pro had MIC90 of 4.0 μg/ml and 1.0 μg/ml for OFX and MXF, respectively, and isolates with mutations at Asp 94Ala, Asn/Tyr, Gly and His had MIC90 of 8.0 μg/ml, and 2.5 μg/ml for OFX and MXF, respectively.

CONCLUSIONS

MTB MICs were found to be consistently lower for MXF than for OFX among isolates with the same gyrA mutation (e.g. Ala90Val). The majority of MTB isolates containing mutations at Asp94Ala, Asn/Tyr, Gly and His in gyrA were associated with a moderate level of resistance to MXF (MIC = 2.5 μg/ml), although 3 isolates with the mutations Asp94Asn/Tyr/Gly were associated with a high level of resistance to both fluoroquinolones (MXF MICs = 5.0-8.0 μg/ml, OFX MICs = ≥10.0 μg/ml).

Pharmacokinetics and safety of moxifloxacin in children with multidrug-resistant tuberculosis

BACKGROUND

Moxifloxacin is currently recommended at a dose of 7.5-10 mg/kg for children with multidrug-resistant (MDR) tuberculosis, but pharmacokinetic and long-term safety data of moxifloxacin in children with tuberculosis are lacking. An area under the curve (AUC) of 40-60 µg × h/mL following an oral moxifloxacin dose of 400 mg has been reported in adults.

METHODS

In a prospective pharmacokinetic and safety study, children 7-15 years of age routinely receiving moxifloxacin 10 mg/kg daily as part of multidrug treatment for MDR tuberculosis in Cape Town, South Africa, for at least 2 weeks, underwent intensive pharmacokinetic sampling (predose and 1, 2, 4, 8, and either 6 or 11 hours) and were followed for safety. Assays were performed using liquid chromatography-tandem mass spectrometry, and pharmacokinetic measures calculated using noncompartmental analysis.

RESULTS

Twenty-three children were included (median age, 11.1 years; interquartile range [IQR], 9.2-12.0 years); 6 of 23 (26.1%) were human immunodeficiency virus (HIV)-infected. The median maximum serum concentration (Cmax), area under the curve from 0-8 hours (AUC0-8), time until Cmax (Tmax), and half-life for moxifloxacin were 3.08 (IQR, 2.85-3.82) µg/mL, 17.24 (IQR, 14.47-21.99) µg × h/mL, 2.0 (IQR, 1.0-8.0) h, and 4.14 (IQR, 3.45-6.11), respectively. Three children, all HIV-infected, were underweight for age. AUC0-8 was reduced by 6.85 µg × h/mL (95% confidence interval, -11.15 to -2.56) in HIV-infected children. Tmax was shorter with crushed vs whole tablets (P = .047). Except in 1 child with hepatotoxicity, all adverse effects were mild and nonpersistent. Mean corrected QT interval was 403 (standard deviation, 30) ms, and no prolongation >450 ms occurred.

CONCLUSIONS

Children 7-15 years of age receiving moxifloxacin 10 mg/kg/day as part of MDR tuberculosis treatment have low serum concentrations compared with adults receiving 400 mg moxifloxacin daily. Higher moxifloxacin dosages may be required in children. Moxifloxacin was well tolerated in children treated for MDR tuberculosis.

The diagnostic accuracy of the GenoType(®) MTBDRsl assay for the detection of resistance to second-line anti-tuberculosis drugs

BACKGROUND

Accurate and rapid tests for tuberculosis (TB) drug resistance are critical for improving patient care and decreasing the transmission of drug-resistant TB. Genotype(®)MTBDRsl (MTBDRsl) is the only commercially-available molecular test for detecting resistance in TB to the fluoroquinolones (FQs; ofloxacin, moxifloxacin and levofloxacin) and the second-line injectable drugs (SLIDs; amikacin, kanamycin and capreomycin), which are used to treat patients with multidrug-resistant (MDR-)TB.

OBJECTIVES

To obtain summary estimates of the diagnostic accuracy of MTBDRsl for FQ resistance, SLID resistance and extensively drug-resistant TB (XDR-TB; defined as MDR-TB plus resistance to a FQ and a SLID) when performed (1) indirectly (ie on culture isolates confirmed as TB positive) and (2) directly (ie on smear-positive sputum specimens).To compare summary estimates of the diagnostic accuracy of MTBDRsl for FQ resistance, SLID resistance and XDR-TB by type of testing (indirect versus direct testing).The populations of interest were adults with drug-susceptible TB or drug-resistant TB. The settings of interest were intermediate and central laboratories.

SEARCH METHODS

We searched the following databases without any language restriction up to 30 January 2014: Cochrane Infectious Diseases Group Specialized Register; MEDLINE; EMBASE; ISI Web of Knowledge; MEDION; LILACS; BIOSIS; SCOPUS; the metaRegister of Controlled Trials; the search portal of the World Health Organization International Clinical Trials Registry Platform; and ProQuest Dissertations & Theses A&I.

SELECTION CRITERIA

We included all studies that determined MTBDRsl accuracy against a defined reference standard (culture-based drug susceptibility testing (DST), genetic testing or both). We included cross-sectional and diagnostic case-control studies. We excluded unpublished data and conference proceedings.

DATA COLLECTION AND ANALYSIS

For each study, two review authors independently extracted data using a standardized form and assessed study quality using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. We performed meta-analyses to estimate the pooled sensitivity and specificity of MTBDRsl for FQ resistance, SLID resistance, and XDR-TB. We explored the influence of different reference standards. We performed the majority of analyses using a bivariate random-effects model against culture-based DST as the reference standard.

MAIN RESULTS

We included 21 unique studies: 14 studies reported the accuracy of MTBDRsl when done directly, five studies when done indirectly and two studies that did both. Of the 21 studies, 15 studies (71%) were cross-sectional and 11 studies (58%) were located in low-income or middle-income countries. All studies but two were written in English. Nine (43%) of the 21 included studies had a high risk of bias for patient selection. At least half of the studies had low risk of bias for the other QUADAS-2 domains.As a test for FQ resistance measured against culture-based DST, the pooled sensitivity of MTBDRsl when performed indirectly was 83.1% (95% confidence interval (CI) 78.7% to 86.7%) and the pooled specificity was 97.7% (95% CI 94.3% to 99.1%), respectively (16 studies, 1766 participants; 610 confirmed cases of FQ-resistant TB; moderate quality evidence). When performed directly, the pooled sensitivity was 85.1% (95% CI 71.9% to 92.7%) and the pooled specificity was 98.2% (95% CI 96.8% to 99.0%), respectively (seven studies, 1033 participants; 230 confirmed cases of FQ-resistant TB; moderate quality evidence). For indirect testing for FQ resistance, four (0.2%) of 1766 MTBDRsl results were indeterminate, whereas for direct testing 20 (1.9%) of 1033 were MTBDRsl indeterminate (P < 0.001).As a test for SLID resistance measured against culture-based DST, the pooled sensitivity of MTBDRsl when performed indirectly was 76.9% (95% CI 61.1% to 87.6%) and the pooled specificity was 99.5% (95% CI 97.1% to 99.9%), respectively (14 studies, 1637 participants; 414 confirmed cases of SLID-resistant TB; moderate quality evidence). For amikacin resistance, the pooled sensitivity and specificity were 87.9% (95% CI 82.1% to 92.0%) and 99.5% (95% CI 97.5% to 99.9%), respectively. For kanamycin resistance, the pooled sensitivity and specificity were 66.9% (95% CI 44.1% to 83.8%) and 98.6% (95% CI 96.1% to 99.5%), respectively. For capreomycin resistance, the pooled sensitivity and specificity were 79.5% (95% CI 58.3% to 91.4%) and 95.8% (95% CI 93.4% to 97.3%), respectively. When performed directly, the pooled sensitivity for SLID resistance was 94.4% (95% CI 25.2% to 99.9%) and the pooled specificity was 98.2% (95% CI 88.9% to 99.7%), respectively (six studies, 947 participants; 207 confirmed cases of SLID-resistant TB, 740 SLID susceptible cases of TB; very low quality evidence). For indirect testing for SLID resistance, three (0.4%) of 774 MTBDRsl results were indeterminate, whereas for direct testing 53 (6.1%) of 873 were MTBDRsl indeterminate (P < 0.001).As a test for XDR-TB measured against culture-based DST, the pooled sensitivity of MTBDRsl when performed indirectly was 70.9% (95% CI 42.9% to 88.8%) and the pooled specificity was 98.8% (95% CI 96.1% to 99.6%), respectively (eight studies, 880 participants; 173 confirmed cases of XDR-TB; low quality evidence).

AUTHORS' CONCLUSIONS

In adults with TB, a positive MTBDRsl result for FQ resistance, SLID resistance, or XDR-TB can be treated with confidence. However, MTBDRsl does not detect approximately one in five cases of FQ-resistant TB, and does not detect approximately one in four cases of SLID-resistant TB. Of the three SLIDs, MTBDRsl has the poorest sensitivity for kanamycin resistance. MTBDRsl will miss between one in four and one in three cases of XDR-TB. The diagnostic accuracy of MTBDRsl is similar when done using either culture isolates or smear-positive sputum. As the location of the resistance causing mutations can vary on a strain-by-strain basis, further research is required on test accuracy in different settings and, if genetic sequencing is used as a reference standard, it should examine all resistance-determining regions. Given the confidence one can have in a positive result, and the ability of the test to provide results within a matter of days, MTBDRsl may be used as an initial test for second-line drug resistance. However, when the test reports a negative result, clinicians may still wish to carry out conventional testing.

Moxifloxacin resistance in the F15/LAM4/KZN extensively drug-resistant strain of Mycobacterium tuberculosis

Objectives

Moxifloxacin (MXF) has been advocated for the treatment of extensively drug-resistant (XDR) tuberculosis despite resistance to older-generation fluoroquinolones. We investigated the relationship between the minimum inhibitory concentration (MIC) of MXF and mutations in the gyrA and gyrB genes in Mycobacterium tuberculosis (MTB) isolates from KwaZulu-Natal (KZN) Province of South Africa.

Materials and methods

MICs of 56 MTB isolates were compared to the mutations in the quinolone resistance-determining region known to confer fluoroquinolone resistance. Isolates were genotyped by IS6110 restriction fragment length polymorphism analysis.

Results

The circulating F15/LAM4/KZN XDR strain circulating in KZN Province harbored the A90V mutation and displayed high-level resistance with MICs of 8 mg/L for ciprofloxacin and ofloxacin and ≥1 mg/L for MXF.

Conclusion

The inclusion of MXF in XDR-TB treatment regimens requires careful consideration in our setting, where clinical outcome data in MXF-containing regimens are unavailable.

Long-term outcomes of patients with extensively drug-resistant tuberculosis in South Africa: a cohort study

BACKGROUND

Long-term treatment-related outcomes in patients with extensively drug-resistant (XDR) tuberculosis are unknown. We followed up a cohort of patients to address knowledge gaps.

METHODS

Between March, 2008, and August, 2012, we prospectively followed up a cohort of 107 patients from three provinces in South Africa, who had been diagnosed with XDR tuberculosis between August 2002, and February, 2008. Available isolates from 56 patients were genotyped to establish strain type and used for extended susceptibility testing.

FINDINGS

All patients were treated empirically as inpatients with a median of eight drugs (IQR six to ten). 44 patients (41%) had HIV. 36 (64%) of 56 isolates were resistant to at least eight drugs, and resistance to an increasing number of drugs was associated with the Beijing genotype (p=0·01). After 24 months of follow-up, 17 patients (16%) had a favourable outcome (ie, treatment cure or completion), 49 (46%) had died, seven (7%) had defaulted (interruption of treatment for at least 2 consecutive months), and 25 (23%) had failed treatment. At 60 months, 12 patients (11%) had a favourable outcome, 78 (73%) had died, four (4%) had defaulted, and 11 (10%) had failed treatment. 45 patients were discharged from hospital, of whom 26 (58%) had achieved sputum culture conversion and 19 (42%) had failed treatment. Median survival of patients who had failed treatment from time of discharge was 19·84 months (IQR 4·16-26·04). Clustering of cases and transmission within families containing a patient who had failed treatment and been discharged were shown with genotypic methods. Net sputum culture conversion occurred in 22 patients (21%) and median time to net culture conversion was 8·7 months (IQR 5·6-26·4). Independent predictors of probability of net culture conversion were no history of multidrug-resistant tuberculosis (p=0·0007) and use of clofazamine (p=0·0069). Independent overall predictors of survival were net culture conversion (p<0·0001) and treatment with clofazamine (p=0·021). Antiretroviral therapy was also a predictor of survival in patients with HIV (p=0·003).

INTERPRETATION

In South Africa, long-term outcomes in patients with XDR tuberculosis are poor, irrespective of HIV status. Because appropriate long-stay or palliative care facilities are scarce, substantial numbers of patients with XDR tuberculosis who have failed treatment and have positive sputum cultures are being discharged from hospital and are likely to transmit disease into the wider community. Testing of new combined regimens is needed urgently and policy makers should implement interventions to minimise disease spread by patients who fail treatment.

FUNDING

European and Developing Countries Clinical Trials Partnership, South African National Research Foundation (SARChI), and the South African Medical Research Council.

Association of gyrA/B mutations and resistance levels to fluoroquinolones in clinical isolates of Mycobacterium tuberculosis

To evaluate the association between mutations in the genes gyrA/B and resistance levels to fluoroquinolones in clinical isolates of Mycobacterium tuberculosis, a total of 80 ofloxacin-resistant isolates collected in 2009 by the Shanghai Municipal Centers for Disease Control and Prevention were studied. The minimum inhibitory concentration (MIC) of ofloxacin, moxifloxacin and gatifloxacin for each isolate was determined using the microscopic observation drug susceptibility assay. Sequencing was used to identify mutations in the quinolone resistance-determining region (QRDR) of the gyrA and gyrB genes. In total, 68 isolates had mutations in gyrA, three isolates had mutations in gyrB, six isolates had mutations in both gyrA and gyrB, and three isolates had no mutations. Two common mutations in gyrA, the D94G and D94N mutations, were associated with higher-level resistance to all three fluoroquinolones than two other common mutations (A90V and D94A). Understanding the relationship between MICs and mutations in ofloxacin-resistant isolates will facilitate the optimization of the use of new-generation fluoroquinolones to treat patients with ofloxacin-resistant tuberculosis (TB).

Pharmacokinetics of ofloxacin and levofloxacin for prevention and treatment of multidrug-resistant tuberculosis in children

Limited data on fluoroquinolone pharmacokinetics and cardiac effects in children exist. Among 22 children receiving drug-resistant tuberculosis prophylaxis or treatment, serum concentrations following oral doses of levofloxacin (15 mg/kg of body weight) and ofloxacin (20 mg/kg) were lower than those expected from existing pediatric data, possibly due to differences in the formulations (crushed tablets). Drug exposures were lower than those in adults following standard doses and below the proposed pharmacodynamic targets, likely due to more rapid elimination in children. No QT prolongation was observed.

Moxifloxacin retains antimycobacterial activity in the presence of gyrA mutations

Moxifloxacin-resistant Mycobacterium tuberculosis mutants were selected in vitro using different concentrations of moxifloxacin. gyrA mutations at codons 88 and 94 were associated with resistance (defined as an MIC of ≥2 μg/ml) (P < 0.0001 and P = 0.0053, respectively). Despite the presence of gyrA mutations, moxifloxacin significantly impedes bacterial growth, supporting its use for the treatment of ofloxacin-resistant M. tuberculosis.

Medical treatment of pulmonary multidrug-resistant tuberculosis

Treatment of multidrug-resistant tuberculosis (MDR-TB) is challenging because of the high toxicity of second-line drugs and the longer treatment duration required compared with drug-susceptible TB. The efficacy of treatment for MDR-TB is poorer than that for drug-susceptible TB. The selection of drugs in MDR-TB is based on previous treatment history, drug susceptibility results, and TB drug resistance patterns in the each region. Recent World Health Organization guidelines recommend the use of least 4 second-line drugs (a newer fluoroquinolone, an injectable agent, prothionamide, and cycloserine or para-aminosalicylic acid) in addition to pyrazinamide. The kanamycin is the initial choice of injectable durgs, and newer fluoroquinolones include levofloxacin and moxifloxacin. For MDR-TB, especially cases that are extensively drug-resistant, group 5 drugs such as linezolid, clofazimine, and amoxicillin/clavulanate need to be included. New agents with novel mechanisms of action that can be given for shorter durations (9-12 months) for MDR-TB are under investigation.

Current status and future trends in the diagnosis and treatment of drug-susceptible and multidrug-resistant tuberculosis

The global burden of tuberculosis (TB) is still large. The increasing incidence of drug-resistant, multidrug-resistant (MDR) (resistant to at least rifampicin and isoniazid), and extensively drug-resistant (XDR) (additionally resistant to a fluoroquinolone and kanamycin/amikacin/capreomycin) strains of Mycobacterium tuberculosis and the association of active disease with human immunodeficiency virus coinfection pose a major threat to TB control efforts. The rapid detection of M. tuberculosis strains and drug susceptibility testing (DST) for anti-TB drugs ensure the provision of effective treatment. Rapid molecular diagnostic and DST methods have been developed recently. Treatment of drug-susceptible TB is effective in ≥95% of disease cases; however, supervised therapy for ≥6 months is challenging. Non-adherence to treatment often results in the evolution of drug-resistant strains of M. tuberculosis due to mutations in the genes encoding drug targets. Sequential accumulation of mutations results in the evolution of MDR and XDR strains of M. tuberculosis. Effective treatment of MDR-TB involves therapy with 5-7 less effective, expensive, and toxic second-line and third-line drugs for ≥24 months and is difficult in most developing countries. XDR-TB is generally an untreatable disease in developing countries. Some currently existing drugs and several new drugs with novel modes of action are in various stages of development to shorten the treatment duration of drug-susceptible TB and to improve the outcome of MDR-TB and XDR-TB.

Moxifloxacin population pharmacokinetics and model-based comparison of efficacy between moxifloxacin and ofloxacin in African patients

Pharmacokinetic exposure and the MIC of fluoroquinolones are important determinants of their efficacy against Mycobacterium tuberculosis. Population modeling was used to describe the steady-state plasma pharmacokinetics of moxifloxacin in 241 tuberculosis (TB) patients in southern Africa. Monte Carlo simulations were applied to obtain the area under the unbound concentration-time curve from 0 to 24 h (fAUC0-24) after daily doses of 400 mg or 800 mg moxifloxacin and 800 mg ofloxacin. The MIC distributions of ofloxacin and moxifloxacin were determined for 197 drug-resistant clinical isolates of Mycobacterium tuberculosis. For a specific MIC, the probability of target attainment (PTA) was determined for target fAUC0-24/MIC ratios of ≥53 and ≥100. The PTAs were combined with the MIC distributions to calculate the cumulative fraction of response (CFR) for multidrug-resistant (MDR) Mycobacterium tuberculosis strains. Even with the less stringent target ratio of ≥53, moxifloxacin at 400 mg and ofloxacin at 800 mg achieved CFRs of only 84% and 58% for multidrug-resistant isolates with resistance to an injectable drug, while the 800-mg moxifloxacin dose achieved a CFR of 98%. Using a target ratio of ≥100 for multidrug-resistant strains (without resistance to injectable agents or fluoroquinolones), the CFR was 88% for moxifloxacin and only 43% for ofloxacin, and the higher dose of 800 mg moxifloxacin was needed to achieve a CFR target of >90%. Our results indicate that moxifloxacin is more efficacious than ofloxacin in the treatment of MDR-TB. Further studies should determine the optimal pharmacodynamic target for moxifloxacin in a multidrug regimen and clarify safety issues when it is administered at higher doses.

Exploration of fluoroquinolone resistance in Streptococcus pyogenes: comparative structure analysis of wild-type and mutant DNA gyrase

Quinolone resistance-determining region is known to be the druggability site of the target protein that undergoes frequent mutation and thus renders quinolone resistance. In the present study, ligands were tested for their inhibitory activity against DNA gyrase of Streptococcus pyogenes involved in DNA replication. In silico mutational analysis on modelled gyrase A revealed that GLU85 had the most possible interactions with all the ligands used for the study. The amino acid residue GLU85 had also been predicted with an essential role of maintaining the three-dimensional structure of the protein. When introduced with a mutation (GLU 85 LYS) on this particular residue, it had readily denatured the whole α-helix (from 80 to 90 amino acids). This was confirmed through the molecular dynamics simulation and revealed that this single mutation can cause many functional and structural changes. Furthermore, LYS85 mutation has altered the original secondary structure of the protein, which in turn led to the steric hindrance during the ligand-receptor interaction. The results based on the G-score revealed that ligands have reduced interaction with the mutant protein. The semisynthetic fluoroquinolone 6d, which is an exception, forms a strong interaction with the mutant protein and was experimentally verified using the antimicrobial test. Hence, the present study unravels the fact that mutation at the drug binding site is the major cause for different level of resistance by the S. pyogenes when exposed against the varying concentrations of the fluoroquinolones. Furthermore, a comparative assessment of quinolone derivative with the older generation fluoroquinolones will be of great impact for S. pyogenes-related infections.

Programmatically selected multidrug-resistant strains drive the emergence of extensively drug-resistant tuberculosis in South Africa

Background

South Africa shows one of the highest global burdens of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB). Since 2002, MDR-TB in South Africa has been treated by a standardized combination therapy, which until 2010 included ofloxacin, kanamycin, ethionamide, ethambutol and pyrazinamide. Since 2010, ethambutol has been replaced by cycloserine or terizidone. The effect of standardized treatment on the acquisition of XDR-TB is not currently known.

Methods

We genetically characterized a random sample of 4,667 patient isolates of drug-sensitive, MDR and XDR-TB cases collected from three South African provinces, namely, the Western Cape, Eastern Cape and KwaZulu-Natal. Drug resistance patterns of a subset of isolates were analyzed for the presence of commonly observed resistance mutations.

Results

Our analyses revealed a strong association between distinct strain genotypes and the emergence of XDR-TB in three neighbouring provinces of South Africa. Strains predominant in XDR-TB increased in proportion by more than 20-fold from drug-sensitive to XDR-TB and accounted for up to 95% of the XDR-TB cases. A high degree of clustering for drug resistance mutation patterns was detected. For example, the largest cluster of XDR-TB associated strains in the Eastern Cape, affecting more than 40% of all MDR patients in this province, harboured identical mutations concurrently conferring resistance to isoniazid, rifampicin, pyrazinamide, ethambutol, streptomycin, ethionamide, kanamycin, amikacin and capreomycin.

Conclusions

XDR-TB associated genotypes in South Africa probably were programmatically selected as a result of the standard treatment regimen being ineffective in preventing their transmission. Our findings call for an immediate adaptation of standard treatment regimens for M/XDR-TB in South Africa.

Impact of fluoroquinolone resistance on bactericidal and sterilizing activity of a moxifloxacin-containing regimen in murine tuberculosis

It has been shown previously that fluoroquinolone resistance (defined by resistance to at least 2 mg/liter ofloxacin) has a different impact on moxifloxacin monotherapy depending on the mutation in the sole fluoroquinolone target in Mycobacterium tuberculosis, i.e., DNA gyrase. Since tuberculosis treatment relies on multidrug therapy, we wished to determine the impact of fluoroquinolone resistance on the bactericidal and sterilizing activity of a second-line antituberculous regimen containing moxifloxacin. A total of 280 mice were inoculated with the wild-type Mycobacterium tuberculosis H37Rv strain or one of 3 isogenic fluoroquinolone-resistant mutant strains with increasing moxifloxacin resistance (the GyrB D500N, GyrA A90V, and GyrA D94G strains) and then treated for 6 months with a second-line regimen containing moxifloxacin, pyrazinamide, and ethionamide supplemented with amikacin during the first 2 months. Mice were sacrificed during treatment for measurement of bactericidal activity and 3 months after treatment completion for measurement of relapse rates (sterilizing activity). The CFU counts decreased faster in mice inoculated with the wild type than in mice inoculated with the mutant strains. The relapse rate after treatment completion was different among mice inoculated with mutant strains in relation to the drug resistance level: wild type, 0%; GyrB D500N strain, 33%; GyrA A90V strain, 50%; and GyrA D94G strain, 86%. The relapse rate observed with the GyrB D500N strain was the only one not statistically different from that observed with the wild-type strain. We demonstrated that the impact on sterilizing activity of the most active second-line drug regimen containing moxifloxacin depends on the MIC of moxifloxacin. We suggest that the precise level of moxifloxacin resistance be determined for all strains resistant to 2 mg/liter ofloxacin.

Resistance to fluoroquinolones and second-line injectable drugs: impact on multidrug-resistant TB outcomes

A meta-analysis for response to treatment was undertaken using individual data of multidrug-resistant tuberculosis (MDR-TB) (resistance to isoniazid and rifampicin) patients from 26 centres. The analysis assessed the impact of additional resistance to fluoroquinolones and/or second-line injectable drugs on treatment outcome. Compared with treatment failure, relapse and death, treatment success was higher in MDR-TB patients infected with strains without additional resistance (n=4763; 64%, 95% CI 57-72%) or with resistance to second-line injectable drugs only (n=1130; 56%, 95% CI 45-66%), than in those having resistance to fluoroquinolones alone (n=426; 48%, 95% CI 36-60%) or to fluoroquinolones plus second-line injectable drugs (extensively drug resistant (XDR)-TB) (n=405; 40%, 95% CI 27-53%). In XDR-TB patients, treatment success was highest if at least six drugs were used in the intensive phase (adjusted OR 4.9, 95% CI 1.4-16.6; reference fewer than three drugs) and four in the continuation phase (OR 6.1, 95% CI 1.4-26.3). The odds of success in XDR-TB patients was maximised when the intensive phase reached 6.6-9.0 months duration and the total duration of treatment 20.1-25.0 months. In XDR-TB patients, regimens containing more drugs than those recommended in MDR-TB but given for a similar duration were associated with the highest odds of success. All data were from observational studies and methodologies varied between centres, therefore, the bias may be substantial. Better quality evidence is needed to optimise regimens.

Fluoroquinolone susceptibility in Mycobacterium tuberculosis after pre-diagnosis exposure to older- versus newer-generation fluoroquinolones

Fluoroquinolone exposure before tuberculosis (TB) diagnosis is common. We anticipated that exposure to older-generation fluoroquinolones is associated with greater fluoroquinolone MICs in Mycobacterium tuberculosis than exposure to newer agents. A nested case-control study was performed among newly diagnosed TB patients reported to the Tennessee Department of Health (January 2002-December 2009). Each fluoroquinolone-resistant case (n=25) was matched to two fluoroquinolone-susceptible controls (n=50). Ciprofloxacin and ofloxacin were classified as older-generation fluoroquinolones; levofloxacin, moxifloxacin and gatifloxacin were considered newer agents. There was no difference between median ofloxacin MIC for isolates from 9 patients exposed only to older fluoroquinolones, 25 exposed only to newer fluoroquinolones, 6 exposed to both and 35 fluoroquinolone-unexposed patients (Kruskal-Wallis, P=0.35). Using multivariate proportional odds logistic regression adjusting for age and sex, duration of exposure to newer fluoroquinolones was independently associated with higher MIC (OR=1.79, 95% CI 1.22-2.64), but duration of exposure to older fluoroquinolones was not (OR=0.94, 95% CI 0.50-1.78). Isolates from patients exposed only to newer fluoroquinolones tended to have mutations at gyrA codons 90, 91 or 94 more frequently than those exposed only to older fluoroquinolones (44% vs. 11%). We were surprised to find that duration of exposure to newer fluoroquinolones, but not older ones, was independently associated with higher ofloxacin MIC. This suggests that the mutant selection window lower boundary is likely to have clinical relevance; caution is warranted when newer fluoroquinolones are prescribed to patients with TB risk factors.

Advances in the development of new tuberculosis drugs and treatment regimens

Despite the introduction 40 years ago of the inexpensive and effective four-drug (isoniazid, rifampicin, pyrazinamide and ethambutol) treatment regimen, tuberculosis (TB) continues to cause considerable morbidity and mortality worldwide. For the first time since the 1960s, new and novel drugs and regimens for all forms of TB are emerging. Such regimens are likely to utilize both repurposed drugs and new chemical entities, and several of these regimens are now progressing through clinical trials. This article covers current concepts and recent advances in TB drug discovery and development, including an update of ongoing TB treatment trials, newer clinical trial designs, TB biomarkers and adjunct host-directed therapies.

The rationale for using rifabutin in the treatment of MDR and XDR tuberculosis outbreaks

Genetically related Mycobacterium tuberculosis strains with alterations at codon 516 in the rpoB gene were observed amongst a substantial number of patients with drug resistant tuberculosis in the Eastern Cape Province (ECP) of South Africa. Mutations at codon 516 are usually associated with lower level rifampicin (RIF) resistance, while susceptibility to rifabutin (RFB) remains intact. This study was conducted to assess the rationale for using RFB as a substitution for RIF in the treatment of MDR and XDR tuberculosis outbreaks. Minimum inhibitory concentrations (MICs) of 34 drug resistant clinical isolates of M tuberculosis were determined by MGIT 960 and correlated with rpoB mutations. RFB MICs ranged from 0.125 to 0.25 µg/ml in the 34 test isolates thereby confirming phenotypic susceptibility as per critical concentration (CC) of 0.5 µg/ml. The corresponding RIF MICs ranged between 5 and 15 µg/ml, which is well above the CC of 1.0 µg/ml. Molecular-based drug susceptibility testing provides important pharmacogenetic insight by demonstrating a direct correlation between defined rpoB mutation and the level of RFB susceptibility. We suggest that isolates with marginally reduced susceptibility as compared to the epidemiological cut-off for wild-type strains (0.064 µg/ml), but lower than the current CC (≤0.5 µg/ml), are categorised as intermediate. Two breakpoints (0.064 µg/ml and 0.5 µg/ml) are recommended to distinguish between susceptible, intermediate and RFB resistant strains. This concept may assist clinicians and policy makers to make objective therapeutic decisions, especially in situations where therapeutic options are limited. The use of RFB in the ECP may improve therapeutic success and consequently minimise the risk of ongoing transmission of drug resistant M. tuberculosis strains.

Activities of drug combinations against Mycobacterium tuberculosis grown in aerobic and hypoxic acidic conditions

Mycobacterium tuberculosis is exposed to hypoxia and acidity within granulomatous lesions. In this study, an acidic culture model of M. tuberculosis was used to test drug activity against aerobic 5-day-old (A5) and hypoxic 5-, 12-, and 19-day-old (H5, H12, and H19, respectively) bacilli after 7, 14, and 21 days of exposure. In A cultures, CFU and pH rapidly increased, while in H cultures growth stopped and pH increased slightly. Ten drugs were tested: rifampin (R), isoniazid (I), pyrazinamide (Z), ethambutol (E), moxifloxacin (MX), amikacin (AK), metronidazole (MZ), nitazoxanide (NZ), niclosamide (NC), and PA-824 (PA). Rifampin was the most active against A5, H5, H12, and H19 bacilli. Moxifloxacin and AK efficiently killed A5 and H5 cells, I was active mostly against A5 cells, Z was most active against H12 and H19 cells, and E showed low activity. Among nitrocompounds, NZ, NC, and PA were effective against A5, H5, H12, and H19 cells, while MZ was active against H12 and H19 cells. To kill all A and H cells, A5- and H5-active agents R, MX, and AK were used in combination with MZ, NZ, NC, or PA, in comparison with R-I-Z-E, currently used for human therapy. Mycobacterial viability was determined by CFU and a sensitive test in broth (day to positivity, MGIT 960 system). As shown by lack of regrowth in MGIT, the most potent combination was R-MX-AK-PA, which killed all A5, H5, H12, and H19 cells in 14 days. These observations demonstrate the sterilizing effect of drug combinations against cells of different M. tuberculosis stages grown in aerobic and hypoxic acidic conditions.

Population pharmacokinetics and pharmacodynamics of ofloxacin in South African patients with multidrug-resistant tuberculosis

Despite the important role of fluoroquinolones and the predominant use of ofloxacin for treating multidrug-resistant tuberculosis in South Africa, there are limited data on ofloxacin pharmacokinetics in patients with multidrug-resistant tuberculosis, no ofloxacin pharmacokinetic data from South African patients, and no direct assessment of the relationship between ofloxacin pharmacokinetics and the MIC of ofloxacin of patient isolates. Our objectives are to describe ofloxacin pharmacokinetics in South African patients being treated for multidrug-resistant tuberculosis and assess the adequacy of ofloxacin drug exposure with respect to the probability of pharmacodynamic target attainment (area under the time curve/MIC ratio of at least 100). Sixty-five patients with multidrug-resistant tuberculosis were recruited from 2 hospitals in South Africa. We determined the ofloxacin MICs for the Mycobacterium tuberculosis isolates from baseline sputum specimens. Patients received daily doses of 800 mg ofloxacin, in addition to other antitubercular drugs. Patients underwent pharmacokinetic sampling at steady state. NONMEM was used for data analysis. The population pharmacokinetics of ofloxacin in this study has been adequately described. The probability of target attainment expectation in the study population was 0.45. Doubling the dose to 1,600 mg could increase this to only 0.77. The currently recommended ofloxacin dose appeared inadequate for the majority of this study population. Studies to assess the tolerability of higher doses are warranted. Alternatively, ofloxacin should be replaced with more potent fluoroquinolones.