Nontuberculous mycobacteria in cystic fibrosis: scouting molecules against M. abscessus iron acquisition pathways

The resistance of the most relevant nontuberculous mycobacteria (NTM), M. abscessus (Mab), against conventional therapies is globally rising, especially among cystic fibrosis (CF) patients. Therefore, new strategies are urgently needed. Iron acquisition is an interesting target because it is critical for pathogen growth and virulence. To get this essential cofactor, mycobacteria produce iron chelators, whose biosynthesis was widely investigated in M. tuberculosis (Mtb), leading to inhibitors with bacteriostatic activity. The aim of the project is the identification of new, safe compounds targeting iron uptake in Mab. To this purpose we will focus on the first enzyme in the siderophore biosynthetic pathway, the salicylate synthase (SaS) that was successfully studied to identify antitubercular agents. A combined experimental/theoretical approach will be employed to identify new Mab SaS inhibitors, by fragment-based and ligand-based drug design. Considering the structural conservation of SaS active site in mycobacteria, we will perform a virtual screening using a homology model. Parallelly, we will build a fragment library and test it by computational and biochemical assays. We will perform crystallization trials to obtain complexes with the most promising fragments to define their mode of interaction. Based on the outcomes of these studies, a collection of optimized compounds will be designed, synthesized, and screened. Inhibitors active in the low micromolar range will be assayed for their antimicrobial activity and toxicity against human cells. The recombinant Mab SaS have been already produced and used to screen a library of Mtb SaS furan-based inhibitors, developed by our groups. Several non-toxic compounds displayed encouraging activity, supporting the possibility to develop specific inhibitors. The results deriving from these complementary approaches will lead to the selection of Mab SaS inhibitors, potentially active against NTM infections. Our panel of validated drug candidates will provide a basis for the development of innovative therapeutic treatments against resistant Mab infections in CF patients.