Re-evaluation of current critical concentrations of fourthgeneration anti-TB fluoroquinolones in the MGIT 960 system

Moxifloxacin (Mfx) and gatifloxacin (Gfx) are fourth-generation fluoroquinolones (FQ) that prevent DNA synthesis of Mycobacterium tuberculosis complex (MTBc). The gold standard for FQ resistance testing is phenotypic drug-susceptibility testing (pDST) using drug-containing media at a critical concentration (CC) to differentiate phenotypically wild type and phenotypically non-wild type MTBC, and at a clinical breakpoint to differentiate strains that will likely respond to treatment at a higher dose from those who will likely not respond to treatment at all. Due to the widespread use of novel/re-purposed anti-TB drugs, the level of FQ resistance may have become less relevant but detecting FQ resistance in a sensitive manner is still important.

We re-evaluated the current WHO-recommended CCs of Mfx (0.25 µg/ml) and Gfx (0.25 µg/ml) for MGIT, using 147 MTBc isolates with known gyrA and gyrB sequences. We tested two-fold dilutions around the CCs and some intermediate concentrations to challenge provisional breakpoints in MGIT960. The CCs of both FQs classified 100% of WT isolates as “susceptible”. However, CCs of Mfx and Gfx misclassified 14.3% (15/105) and 17.1% (18/105) of gyrA/B mutants as “susceptible” respectively. Lowering the CC of Mfx to 0.125 µg/ml would
allow to correctly classify all wildtype and mutant isolates while lowering the CC of Gfx to 0.125 µg/ml would still misclassify some gyrAB mutants.

Based on our findings, a MIC 0.125 µg/ml in MGIT medium may be an appropriate critical concentration for Mfx and probably also for Gfx. Testing of more WT isolates is needed to correctly define the epidemiological cut-off.