Primary Clofazimine and Bedaquiline Resistance among Isolates from Patients with Multidrug-Resistant Tuberculosis.

Clofazimine has been repurposed for the treatment of tuberculosis, especially for multidrug-resistant tuberculosis (MDR-TB). To test the susceptibility to clofazimine of clinical isolates, MICs of clofazimine were determined using the microplate alamarBlue assay (MABA) method for 80 drug-resistant isolates and 10 drug-susceptible isolates for comparison. For five clofazimine-resistant strains isolated from previously treated pre-extensively drug-resistant TB (pre-XDR-TB) and XDR-TB patients without prior exposure to clofazimine or bedaquiline, clofazimine MICs were ≥1.2 μg/ml. Four isolates with cross-resistance to bedaquiline had mutations. The other isolate with no resistance to bedaquiline had an mutation. This study adds to a recent study showing that 6.3% of MDR-TB patients without prior clofazimine or bedaquiline exposure harbored isolates with mutations, which raises concern that preexisting resistance to these drugs may be associated with prior TB treatment. Furthermore, we propose a tentative breakpoint of 1.2 μg/ml for clofazimine resistance using the MABA method. More-widespread surveillance and individualized testing for clofazimine and bedaquiline resistance, together with assessment of their clinical usage, especially among previously treated and MDR-TB patients, are warranted.