Targeting iron acquisition in M. abscessus: a new strategy for the development of anti-virulence agents for cystic fibrosis patients

M. abscessus (Mab), an opportunistic pathogen belonging to the class of non-tuberculous mycobacteria (NTM), causes chronic pulmonary infections in sensitive individuals, especially cystic fibrosis (CF) patients. Despite its incidence is on the rise, the available therapeutic strategies have only a limited efficacy and are very burdensome for the patients. Hence, due to the intrinsic resilience of NTM to treatments and the dramatic spread of resistant strains, new pharmacological approaches are urgently needed. In this context, iron acquisition has been recently proposed as an interesting target in the phylogenetically close M. tuberculosis (Mtb), due to the involvement of this metal in the pathogenesis and survival of the microorganism in the host. Therefore, we decided to investigate the targeting of mycobacterium-specific iron chelators as a possible anti-virulence approach against Mab. In detail, we focused on the first enzyme involved in the siderophore biosynthetic pathway, a salicylate synthase (SaS) that has been successfully studied to identify antitubercular agents against Mtb. Due to the structural conservation of the active site in the two congeners, we approached this study by screening an in-house library of Mtb-SaS inhibitors. This initial investigation led to the identification of the first hit compounds against Mab-SaS. Their activity was rationalized using a homology model, based on the PDB coordinates of the Mtb protein. However, considering the high plasticity of the active site and the flexibility of two loops adjacent to the binding cavity, we performed crystallization studies to solve the structure of Mab-SaS, thus providing a solid base to structure-based drug design efforts. The obtaining of the first 3D-model of this enzyme allowed us to characterize its orthosteric site and study the peculiar area around it. Our promising results will be presented, along with preliminary data on the use of the new structure for the development of optimized inhibitors.