Optimizing the drug discovery process for the development of MbtI inhibitors as promising anti-virulence agents against TB

In 2021, tuberculosis (TB) was still the second cause of death from a single infectious agent in the world, surpassed only by COVID-19. The global impact of the pandemic has been extensive, substantially reversing more than twenty years of progress in the diagnosis and control of the disease. This alarming trend has been worsened by the growing emergence of resistant mycobacteria, which pose a serious threat to the public’s health. Therefore, the identification of new antitubercular agents remains a priority for the scientific community. In this context, anti-virulence therapy is an innovative approach that aims to trigger a host-mediated response, rather than directly killing the pathogen. This strategy does not exert selective pressure on the microorganisms, thus preventing resistance. Among the possible targets, the acquisition of iron has been recently proposed as a viable option, due to its involvement in the pathogenesis and survival of M. tuberculosis in the host. In detail, the salicylate synthase MbtI, a Menaquinone, Siderophore and Tryptophan (MST) enzyme that catalyzes the first step of the siderophore biosynthetic pathway, has been validated as a pharmacological target. Our efforts in the discovery of antitubercular candidates acting on MbtI have led over the years to the identification of a series of promising compounds, acting as potent enzymatic inhibitors, and exhibiting a good antimycobacterial activity. The design of derivatives has been supported by the obtaining of the co-crystal structure of MbtI with one of the best compounds. Although this important result allowed us to shed light on the binding mode of our molecules and infer critical information regarding the catalytic activity of the enzyme, it was not optimal for structure-based drug design. Therefore, we performed additional crystallographic studies to obtain new, more suitable structures. In the meantime, considering the well-known conservation of the active sites of MST enzymes, we expanded our structural investigations to the homolog protein from M. abscessus (Mab), a non-tuberculous mycobacterium (NTM) that causes severe, resistant pulmonary infections in sensitive individuals, especially cystic fibrosis patients. Our latest results will be discussed, along with perspectives on future developments.