Comparison of <i>In Vitro</i> Activity and MIC Distributions between the Novel Oxazolidinone Delpazolid and Linezolid against Multidrug-Resistant and Extensively Drug-Resistant Mycobacterium tuberculosis in China

Abstract

Oxazolidinones are efficacious in treating mycobacterial infections, including tuberculosis (TB) caused by drug-resistant <i>Mycobacterium tuberculosis</i> In this study, we compared the <i>in vitro</i> activities and MIC distributions of delpazolid, a novel oxazolidinone, and linezolid against multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) in China. Additionally, genetic mutations in 23S rRNA, <i>rplC</i>, and <i>rplD</i> genes were analyzed to reveal potential mechanisms underlying the observed oxazolidinone resistance. A total of 240 <i>M. tuberculosis</i> isolates were included in this study, including 120 MDR-TB isolates and 120 XDR-TB isolates. Overall, linezolid and delpazolid MIC₉₀ values for <i>M. tuberculosis</i> isolates were 0.25 mg/liter and 0.5 mg/liter, respectively. Based on visual inspection, we tentatively set epidemiological cutoff (ECOFF) values for MIC determinations for linezolid and delpazolid at 1.0 mg/liter and 2.0 mg/liter, respectively. Although no significant difference in resistance rates was observed between linezolid and delpazolid among XDR-TB isolates (<i>P</i> > 0.05), statistical analysis revealed a significantly greater proportion of linezolid-resistant isolates than delpazolid-resistant isolates within the MDR-TB group (<i>P</i> = 0.036). Seven (53.85%) of 13 linezolid-resistant isolates were found to harbor mutations within the three target genes. Additionally, 1 isolate exhibited an amino acid substitution (Arg126His) within the protein encoded by <i>rplD</i> that contributed to high-level resistance to linezolid (MIC of >16 mg/liter), compared to a delpazolid MIC of 0.25. In conclusion, <i>in vitro</i> susceptibility testing revealed that delpazolid antibacterial activity was comparable to that of linezolid. A novel mutation within <i>rplD</i> that endowed <i>M. tuberculosis</i> with linezolid, but not delpazolid, resistance was identified.