P066

1:Research Center Borstel Leibniz Lung Center; 2:ETH Zürich

Intra-patient resistance evolution of Mycobacterium tuberculosis complex (MTBC) strains is a major player in tuberculosis (TB) treatment failure. Researchers have hypothesized that due to the complex pathology of TB infection, certain niches throughout the body allow for the low concentration exposure of drug to pathogen, fostering resistance selection and proliferation. Here, we aimed to investigate the consequences of long-term exposure to very low concentrations of the important MDR-TB drugs bedaquiline and moxifloxacin. This work provides a framework for understanding resistance selection by examining the resistance mutant selection window.

We performed several in vitro assays to investigate resistant selection and proliferation at sub-inhibitory concentrations, and found resistant mutants arise at concentrations as low as one-eighth the MIC after only five culture passages. We then utilized mathematical modelling to predict the effect of longer treatment durations, which is not as feasible in the lab. The mathematical model accounted for the population genetics and incorporated the experimentally determined pharmacodynamics of bedaquiline and moxifloxacin. These simulations indicated that one-sixteenth of the MIC was enough to enrich mutant populations, even in mutants with a major competitive fitness loss. Further, we estimated that mutant clones with different resistance mechanisms to the same drug, but even mutants with a single variant at different loci within the same gene, had variable mutant selection windows.

This work gives insights on how little drug is likely needed to select resistant MTBC clones, further showing the importance of high-quality treatment design and using drugs which meet their target at therapeutic concentrations.