Synthesis and evaluation of conformationally constrained analogues of SAL-AMS, a potent bisubstrate inhibitor of the mycobacterial aryl acid adenylating enzyme MbtA involved in mycobactin biosynthesis

Abstract

Tuberculosis, caused primarily by the bacillus Mycobacterium tuberculosis (Mtb), is the leading cause of bacterial infectious disease mortality. Mtb scavenges the essential micronutrient iron from its host via the synthesis, secretion, and reuptake of smallmolecule iron chelators known as siderophores. Siderophores in Mtb, termed mycobactins, have been linked to virulence through targeted genetic disruption of the mycobactin biosynthetic pathway, validating inhibition of mycobactin biosynthesis for the development of novel antitubercular agents. Mycobactins are synthesized by mixed nonribosomal peptide synthetiase/polyketide synthase (NRPS/PKS) machinery in Mtb. The NRPS assembly line is primed by an aryl acid adenylating enzyme (AAAE) from Mtb known as MbtA. MbtA is an attractive therapeutic target due to lack of a human homologue, availability of structural information from homologous AAAEs, and extensive knowledge of the enzymatic mechanism of a functionally- and structurallysimilar AAAE. The MbtA mechanism and a potent MbtA bisubstrate inhibitor, 5′-O-[N- (salicyl)sulfamoyl]adenosine (Sal-AMS), are shown in the figure below. While demonstrating some adequate pharmacokinetic parameters, Sal-AMS is ultimately plagued by poor oral bioavailability. Previous studies in our lab indicate that an internal hydrogen bond is formed between the phenol and charged sulfamate nitrogen atom (estimated pKa around 2) of Sal-AMS, enforcing a coplanar arrangement of the salicyl group when bound to the MbtA active site. We thus proposed the synthesis of conformationally constrained analogues 1–3 to mimic the bound conformation of Sal- AMS and potentially improve the oral bioavailability of the parent Sal-AMS compound by removing two rotatable bonds and the charged sulfamate moiety. Oral bioavailability studies are dependent on the proposed analogues’ relative biochemical and antitubercular potencies versus those of Sal-AMS. Herein is reported the synthesis, biochemical and antitubercular evaluation of conformationally constrained analogues of Sal-AMS 1–3, as well as its 2,3-dihydroxybenzoyl variant 4.