Correlating clinical breakpoint concentration of moxifloxacin with gyrA mutations using the GenoType MTBDRsl assay Version 2.0.

Introduction: Widespread use of Fluoroquinolones (FQs) has led to the development of its resistance in clinical isolates of Mycobacterium tuberculosis. However, in Mycobacterium tuberculosis, phenotypic resistance to FQs has been shown to be heterogeneous, ranging from low-level resistance to high-level resistance. This stratification in resistance has important implications for the inclusion of moxifloxacin (Mfx) in the treatment regimen. The World Health Organization recommends the use of GenoType MTBDRsl assay as the initial test for detecting resistance conferring mutations (both high and low) to FQs in patients with confirmed MDR-RR TB. The present study was conducted to explore the relationship of MTBDRsl Version 2.0 detected mutations in gyrA gene and genotypic DST of Mfx at WHO defined Clinical Breakpoint (CB).
Materials and Methods: A total of 200 sputum samples from Confirmed MDR/RR TB patients were included in this study. All of these samples had mutations conferring resistance to FQ confirmed by GenoType MTBDRsl assay. These samples were further subjected to Phenotypic DST against moxifloxacin using the Bactec MGIT-960 system.
Results: All of the 200 representative FQ resistant isolates had mutations in gyrA gene only with no detectable mutation in gyrB gene. 109 (54.5%) of the isolates had mutations associated with high-level increase in MIC while 91 (45.5%) isolates had mutations associated with low-level increase in MIC. Phenotypic DST of these 200 isolates against Mfx at CB (1.0μg/ml) revealed that of the 109 isolates with mutations associated with high-level increase in MIC and expected to be resistant at CB, only 34 (31.2%) were resistant and the remaining 75 (68.8%) were sensitive.
Conclusion: Moxifloxacin is an important drug in the regimen for treating Drug-resistant TB and the decision to exclude this drug from the regimen should not be taken merely on the basis of mutational patterns. It should rather be taken after considering the combined results of mutational analysis and phenotypic DST.
(Copyright © 2022 Tuberculosis Association of India. Published by Elsevier B.V. All rights reserved.)