P25

Drug resistant Mycobacterium tuberculosis complex strains, the causative agent of tuberculosis (TB), account for 1/3^rd of all drug resistance related deaths reported annually worldwide. The increasing number of multi-drug resistance (MDR) cases is particularly concerning. Therefore, understanding conditions promoting resistance selection is of upmost importance to design effective MDR-TB regimens. The mutant selection window (MSW), i.e. the drug concentration range in which resistant populations are selected, is of particular importance, but has not been well studied for major TB drugs.

In this study, the MSW was investigated in vitro for moxifloxacin, bedaquiline and clofazimine, key MDR-TB drugs. First, the selection of mutants was investigated in a long-term evolution model employing concentrations below the minimum inhibitory concentration (MIC). Indeed, the minimum selection concentration was as low as 1:8 the MIC, allowing the selection subpopulations with a diverse set of variants. Growth of selected clones with mutations in either gyrA, gyrB, atpE, or Rv0678 were then measured in the automated MGIT system at various sub-inhibitory concentrations.  These experiments showed that clones with resistance mutations have a selective advantage to the wild-type under weak selection pressure far below the MIC pointing to a wide MSW.

In conclusion, the MSW for MDR-TB drugs is wider than previously anticipated, indicating that low MIC concentrations e.g. caused by structural constraints in infected organs, can lead to resistance selection and treatment failure. These findings make clear the importance of well-designed drug combinations and dosages, in order to avoid favourable conditions for the selection of resistant variants.