Drug exposure and susceptibility of second-line drugs correlate with treatment response in patients with multidrug-resistant tuberculosis: a multi-centre prospective cohort study in China.

Background Understanding the impact of drug exposure and susceptibility on treatment response of multidrug-resistant tuberculosis (MDR-TB) will help to optimize treatment. This study aimed to investigate the association between drug exposure, susceptibility and response to MDR-TB treatment. Methods Drug exposure and susceptibility for second-line drugs were measured for patients with MDR-TB. Multivariate analysis was applied to investigate the impact of drug exposure and susceptibility on sputum culture conversion and treatment outcome. Probability of target attainment was evaluated. Random Forest and classification and regression tree (CART) analysis was used to identify key predictors and their clinical targets among patients on WHO-recommended regimens. Results Drug exposure and corresponding susceptibility were available for 197 patients with MDR-TB. Target attainment was highly variable ranging from 0% for ethambutol to 97% for linezolid, while patients with fluoroquinolones above targets had higher probability of two-month culture conversion (56.3% versus 28.6%, OR 2.91, 95% CI 1.42–5.94) and favourable outcome (88.8% versus 68.8%, OR 2.89, 95% CI 1.16–7.17). Higher exposure values of fluoroquinolones, linezolid and pyrazinamide were associated with earlier sputum culture conversion. CART analysis selected moxifloxacin AUC/MIC of 231 and linezolid AUC/MIC of 287 as best predictors for six-month culture conversion in patients receiving identical Group A-based regimen. These association were confirmed in multivariate analysis. Conclusions Our findings indicated that target attainment of TB drugs is associated with response to treatment. The CART-derived thresholds may serve as targets for early dose adjustment in a future randomized controlled study to improve the MDR-TB treatment outcome.