Synthesis and Assessment of the In Vitro and Ex Vivo Activity of Salicylate Synthase (Mbti) Inhibitors as New Candidates for the Treatment of Mycobacterial Infections

Abstract

Tuberculosis (TB) causes millions of deaths every year, ranking as one of the most dangerous infectious diseases worldwide. Because several pathogenic strains of M. tuberculosis (Mtb) have developed resistance against most of the established anti-TB drugs, new therapeutic options are urgently needed. An attractive target for the development of new anti-TB agents is the salicylate synthase MbtI, the first enzyme of the mycobacterial siderophore biochemical machinery, absent in human cells. In this work, a set of analogues of 5-(3-cyanophenyl)furan-2-carboxylic acid (I), the most potent MbtI inhibitor identified to date, was synthesized, characterized, and tested to further elucidate the structural requirements for achieving an efficient MbtI inhibition and potent antitubercular activity. The structure–activity relationships (SAR) discussed herein evidenced the importance of the side chain linked to the phenyl moiety to improve the in vitro antimycobacterial activity. In detail, 1f emerged as the most effective analogue against the pathogen, acting without cytotoxicity issues. To deepen the understanding of its mechanism of action, we established a fluorescence-based screening test to quantify the pathogen infectivity within host cells, using MPI-2 murine cells, a robust surrogate for alveolar macrophages. The set-up of the new assay demonstrates significant potential to accelerate the discovery of new anti-TB drugs.

Targeting Siderophore-Mediated Iron Uptake in M. abscessus: A New Strategy to Limit the Virulence of Non-Tuberculous Mycobacteria

“…Therefore, taking advantage of our expertise in the development of MbtI inhibitors [ 14 , 15 , 16 , 17 , 18 , 19 ], we performed functional and structural studies on the SaS encoded by Mab , investigating, for the first time, its potential role as a pharmacological target. In this work, we identified three enzymatic inhibitors that could represent the starting point for the design of more potent analogs.…”

“…The Salicylate Synthase (SaS) from Mtb (MbtI) is the first enzyme involved in siderophore production; it catalyzes the two-step conversion of chorismate to salicylate, via isochorismate as an intermediate, acting as both an isomerase and a lyase ( Figure 1 ). Therefore, taking advantage of our experience in the inhibition of MbtI [ 14 , 15 , 16 , 17 , 18 , 19 ], we explored the possibility of developing potent enzymatic inhibitors of Mab -SaS.…”

“…Among them, three analogs, namely 5-(2,4-bis(trifluoromethyl)phenyl)furan-2-carboxylic acid ( 1 ), 5-(2-amino-4-nitrophenyl)furan-2-carboxylic acid ( 2 ), and 5-(3,5-bis(trifluoromethyl)phenyl)furan-2-carboxylic acid ( 3 ), emerged as promising candidates ( Table 1 ). Notably, the low toxicity of this class of compounds has been previously assessed against human MRC-5 fibroblasts and blood cells [ 16 , 19 ].…”

Iron Acquisition and Metabolism as a Promising Target for Antimicrobials (Bottlenecks and Opportunities): Where Do We Stand?

“…Similarly, an in silico virtual screening of a commercially available library led to the identification of a 5-phenylfuran-2-carboxylate compound, highly active against MbtI, and showing a moderate antitubercular activity in iron-depleted conditions, associated to a reduction in siderophore production [ 78 ]. Subsequent structure–activity relationship investigations were performed on this structure [ 79 , 80 , 81 , 82 , 83 ], leading to the identification of a series of 3-cyanophenyl derivatives with improved antimycobacterial properties. Most notably, some of them were also active against the homolog enzyme of the non-tuberculous opportunistic pathogen, M. abscessus ( Figure 6 H) [ 82 , 83 ].…”

“…Subsequent structure–activity relationship investigations were performed on this structure [ 79 , 80 , 81 , 82 , 83 ], leading to the identification of a series of 3-cyanophenyl derivatives with improved antimycobacterial properties. Most notably, some of them were also active against the homolog enzyme of the non-tuberculous opportunistic pathogen, M. abscessus ( Figure 6 H) [ 82 , 83 ].…”