Mechanisms of resistance to delamanid, a drug for Mycobacterium tuberculosis.

Delamanid, a bicyclic nitroimidazooxazole, is effective against M. tuberculosis. Previous studies have shown that resistance to a bicyclic nitroimidazooxazine, PA-824, is caused by mutations in an F420-dependent bio-activation pathway. We investigated whether the same mechanisms are responsible for resistance to delamanid. Spontaneous resistance frequencies were determined using M. bovis BCG Tokyo (BCG) and M. tuberculosis H37Rv. F420 high-performance liquid chromatography (HPLC) elution patterns of homogenates of delamanid-resistant BCG colonies and two previously identified delamanid-resistant M. tuberculosis clinical isolates were examined, followed by sequencing of genes in the F420-dependent bio-activation pathway. Spontaneous resistance frequencies to delamanid were similar to those of isoniazid and PA-824. Four distinct F420 HPLC elution patterns were observed, corresponding to colonies with mutations on fgd1, fbiA, fbiB, and fbiC with no change in the ddn mutants from the wildtype. Complementation with the wildtype sequence of the mutated gene restored susceptibility. The two delamanid-resistant clinical isolates had ddn mutations and the wildtype F420 HPLC elution pattern. In conclusion, delamanid-resistant bacilli have mutations in one of the 5 genes in the F420-dependent bio-activation pathway with distinct F420 HPLC elution patterns. Both genetic and phenotypic changes may be considered in the development of a rapid susceptibility test for delamanid.