There is an unmet medical need for effective treatments
against Mycobacterium abscessus pulmonary infections,
to which cystic
fibrosis (CF) patients are particularly vulnerable. Recent studies
showed that the antitubercular drug isoniazid is inactive against M. abscessus due to the incapacity of the catalase-peroxidase
to convert the pro-drug into a reactive metabolite that inhibits the
enoyl-ACP reductase InhA. To validate InhAMAB as a druggable
target in M. abscessus, we assayed the activity
of NITD-916, a 4-hydroxy-2-pyridone lead candidate initially described
as a direct inhibitor of InhA that bypasses KatG bioactivation in Mycobacterium tuberculosis. The compound displayed low MIC
values against rough and smooth clinical isolates in vitro and significantly
reduced the bacterial burden inside human macrophages. Moreover, treatment
with NITD-916 reduced the number and size of intracellular mycobacterial
cords, regarded as markers of the severity of the infection. Importantly,
NITD-916 significantly lowered the M. abscessus burden in CF-derived lung airway organoids. From a mechanistic perspective,
NITD-916 abrogated de novo synthesis of mycolic acids and NITD-916-resistant
spontaneous mutants harbored point mutations in InhAMAB at residue 96. That NITD-916 targets InhAMAB directly
without activation requirements was confirmed genetically and by resolving
the crystal structure of the protein in complex with NADH and NITD-916.
These findings collectively indicate that InhAMAB is an
attractive target to be exploited for future chemotherapeutic developments
against this difficult-to-treat mycobacterium and highlight the potential
of NITD-916 derivatives for further evaluation in preclinical settings.