Correlation between GyrA Substitutions and Ofloxacin, Levofloxacin, and Moxifloxacin Cross-Resistance in Mycobacterium tuberculosis

Abstract

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 g/ml and MXF MICs of >0.5 g/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 g/ml), while other mutations led to MXF MICs of >2 g/ml. Based on these results, a critical concentration of 1 g/ml is suggested for LVX and 0.5 g/ml for MXF drug susceptibility testing by agar proportion with reflex testing for MXF at 2 g/ml.I t is estimated that one-third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), and 5% to 10% of infected persons will become ill with active disease necessitating a complicated treatment regimen. Patients with fully susceptible TB are generally treated with a combination of first-line drugs, including rifampin, isoniazid, ethambutol, and pyrazinamide for 6 months (1). Intolerance or acquisition of resistance to one or more of the first-line drugs requires deviation from the standard treatment regimen, and second-line drugs, including the injectables amikacin (AMK), kanamycin (KAN), and capreomycin (CAP) and one of a number of fluoroquinolone (FQ) derivatives, may be needed to successfully treat the patient.FQs are a class of broad-spectrum, synthetic antibiotics effective against many different bacterial infections such as those of the urinary and respiratory tracts, including infections by M. tuberculosis. Early FQs had a limited spectrum of activity; however, two of the newest FQ derivatives, moxifloxacin (MXF) and levofloxacin (LVX) (the levo isomer of ofloxacin [OFX]), exhibit superior bactericidal activity against a wide range of organisms, have longer half-lives, and are less toxic than older FQs (2, 3). Studies have demonstrated the effectiveness of these new FQs against M. tuberculosis (4, 5), with comparatively low MICs (MXF MIC Ͻ 0.5 g/ml) (1, 6-9). Although clinical resistance to FQs has been reported, estimating the global burden of FQ-resistant TB is difficult given that drug susceptibility testing (DST) is not performed in many areas and testing for FQ resistance is typically undertaken only when resistance to first-line drugs is detected. However, in 2013, the World Health Organization (WHO) estima...

Role of gyrB Mutations in Pre-extensively and Extensively Drug-Resistant Tuberculosis in Thai Clinical Isolates

“…Our findings underscore the possibility of evaluating later-generation quinolones for the treatment of ofloxacin-resistant TB (33,34) and as a salvage therapy in patients with MDR-TB (35). In addition, our results imply that susceptibility to OFX may not necessarily indicate susceptibility to all quinolones when OFX is used as a representative quinolone in susceptibility testing (17). We thus suggest that the susceptibility to each quinolone should be tested as recommended by WHO (36) because the quinolones are not completely cross-resistant.…”

“…Similarly, other studies have reported that clinical isolates carrying D94G exhibit higher levels of resistance to OFX, MXF, and GAT than isolates carrying A90V or D94A (16,17,38). Recently, Rigouts et al showed strong evidence for a higher rate of failure/relapse in the presence of the D94G mutation (70.0%) than in that of the D94A mutation (11.1%) when patients were treated with GAT (38).…”

“…We demonstrate that mutations within not only QRDR-A but also QRDR-B should be considered potential determinants of quinolone resistance (14,18,19), while mutations outside the QRDRs are likely linked to the genotypic background of M. tuberculosis (31,32). Furthermore, knowing the gyrase mutations not only is relevant for the prediction of quinolone resistance but also may be useful for estimating levels of resistance to various quinolones (16,17,40), which can aid in the selection of the appropriate quinolone and its dosing.…”

“…Resistance to quinolones in M. tuberculosis results primarily from mutations in gyrA and gyrB, particularly in the quinolone resistance-determining regions (QRDRs) of GyrA (QRDR-A, residues 74 to 113) and GyrB (QRDR-B, residues 500 to 540), which cause alterations in the amino acids of the QBP (14,15). A wide variety of mutations within the QRDRs have been reported; however, different amino acid substitutions in gyrase may cause different levels of resistance to quinolones (16,17).…”

Wild-Type and Non-Wild-Type Mycobacterium tuberculosis MIC Distributions for the Novel Fluoroquinolone Antofloxacin Compared with Those for Ofloxacin, Levofloxacin, and Moxifloxacin

“…We usually sequenced isolates recovered from the drug-free LJ slopes, but where no resistance mutations were found in phenotypically resistant strains, sequencing was repeated with isolates recovered from the OFX-containing LJ slope to detect low-frequency mutations (8, 9). Strains belonging to the East Asian lineage were identified on the basis of RD105 (10).…”