Benzothiazole Derivative as a Novel <i>Mycobacterium tuberculosis</i> Shikimate Kinase Inhibitor: Identification and Elucidation of Its Allosteric Mode of Inhibition

<i>Mycobacterium tuberculosis</i> shikimate kinase (Mtb-SK) is a key enzyme involved in the biosynthesis of aromatic amino acids through the shikimate pathway. Since it is proven to be essential for the survival of the microbe and is absent from mammals, it is a promising target for anti-TB drug discovery. In this study, a combined approach of <i>in silico</i> similarity search and pharmacophore building using already reported inhibitors was used to screen a procured library of 20,000 compounds of the commercially available ChemBridge database. From the <i>in silico</i> screening, 15 hits were identified, and these hits were evaluated <i>in vitro</i> for Mtb-SK enzyme inhibition. Two compounds presented significant enzyme inhibition with IC₅₀ values of 10.69 ± 0.9 and 46.22 ± 1.2 μM. The best hit was then evaluated for the <i>in vitro</i> mode of inhibition where it came out to be an uncompetitive and noncompetitive inhibitor with respect to shikimate (SKM) and ATP, respectively, suggesting its binding at an allosteric site. Potential binding sites of Mtb-SK were identified which confirmed the presence of an allosteric binding pocket apart from the ATP and SKM binding sites. The docking simulations were performed at this pocket in order to find the mode of binding of the best hit in the presence of substrates and the products of the enzymatic reaction. Molecular dynamics (MD) simulations elucidated the probability of inhibitor binding at the allosteric site in the presence of ADP and shikimate-3-phosphate (S-3-P), that is, after the formation of products of the reaction. The inhibitor binding may prevent the release of the product from Mtb-SK, thereby inhibiting its activity. The binding stability and the key residue interactions of the inhibitor to this product complex were also revealed by the MD simulations. Residues ARG43, ILE45, and PHE57 were identified as crucial that were involved in interactions with the best hit. This is the first report of an allosteric binding site of Mtb-SK, which could largely address the selectivity issue associated with kinase inhibitors

Synthesis and Biological Evaluation of Polar Functionalities Containing Nitrodihydroimidazooxazoles as Anti-TB Agents

Novel polar functionalities containing 6-nitro-2,3-dihydroimidazooxazole (NHIO) analogues were synthesized to produce a compound with enhanced solubility. Polar functionalities including sulfonyl, uridyl, and thiouridyl-bearing NHIO analogues were synthesized and evaluated against <i>Mycobacterium tuberculosis</i> (MTB) H₃₇Rv. The aqueous solubility of compounds with MIC values ≤0.5 μg/mL were tested, and six compounds showed enhanced aqueous solubility. The best six compounds were further tested against resistant (Rif^R and MDR) and dormant strains of MTB and tested for cytotoxicity in HepG2 cell line. Based on its overall <i>in vitro</i> characteristics and solubility profile, compound <b>6d</b> was further shown to possess high microsomal stability, solubility under all tested biological conditions (PBS, SGF and SIF), and favorable oral <i>in vivo</i> pharmacokinetics and <i>in vivo</i> efficacy