Wild-type MIC Distribution and Evaluation of Epidemiological Cut-Offs of Second-line TB-drugs in Susceptible and MDR-TB Clinical Isolates from Chennai, India

Abstract

Introduction: The rise of drug-resistant tuberculosis poses a significant challenge in patient management. Epidemiological cut-off values define drug resistance in Mycobacterium tuberculosis. In our previous study, we reported deviations from the WHO-recommended epidemiological cut-off values and identified subtherapeutic concentrations of rifampicin in clinical Mycobacterium tuberculosis isolates. Building on these findings, the present study systematically evaluated the epidemiological cut-off values and pharmacodynamic profiles of newer and repurposed second-line antiTB drugs - Bedaquiline, Delamanid, Moxifloxacin, Linezolid, Clofazimine, Levofloxacin, and Pretomanid against the first-line drug-sensitive and isolates that are resistant to Rifampicin and Isoniazid from tuberculosis patients in and around Chennai. Method: The Broth microdilution-based Microscopic Observation Drug Susceptibility assay was employed to determine the minimum inhibitory concentration of the drugs against well-characterized wild-type and drug-resistant clinical Mycobacterium tuberculosis clinical isolates. The resulting MIC profiles were subsequently analyzed to delineate pharmacodynamic relationships underlying therapeutic efficacy and resistance development. Results and Discussion: Deviations from the World Health Organization–recommended epidemiological cut-off values were observed, with lower thresholds for delamanid and levofloxacin and higher concentrations for clofazimine and bedaquiline. These shifts indicate region-specific susceptibility patterns in Mycobacterium tuberculosis that have direct implications for the effective treatment of multidrug-resistant tuberculosis. Inaccurate cut-off values may lead to misclassification of resistance, inappropriate regimen selection, and exposure to suboptimal drug concentrations, thereby compromising treatment efficacy and amplifying the risk of acquired resistance. Concordantly, pharmacodynamic analyses revealed sub-therapeutic exposure ranges for several newer and repurposed anti-TB drugs, underscoring the potential for treatment failure even in strains classified as susceptible. Collectively, these findings highlight the urgent need for regionally calibrated epidemiological cut-off values to optimize drug dosing, improve MDR-TB treatment outcomes, and strengthen resistance surveillance frameworks.