Synthesis and evaluation of triazole congeners of nitro-benzothiazinones potentially active against drug resistant Mycobacterium tuberculosis demonstrating bactericidal efficacy

Abstract

With growing concerns regarding target residue mutation hovering over established anti-TB pharmacophores, it is imperative to have reserve chemotypes at our disposal to curb unrestrained spread of tuberculosis. In this context, we herein present the synthesis and bio-evaluation of a library of new nitrobenzothiazinone (BTZ) congeners comprising 2-mercapto/amino-benzothiazinone tethered 1,2,3-triazole hybrids as antitubercular agents. In preliminary screening, 10 out of 37 compounds displayed substantial in vitro potency against Mtb H37Rv (MIC 0.5–8 μg mL⁻¹). Structural optimization of the initial hit 5o (MIC 0.5 μg mL⁻¹) led to identification of linker variants 9a, 9b, 9c, and 9d exhibiting potent anti-TB activity (MIC 0.03–0.12 μg mL⁻¹). When tested against Vero cells to determine their selectivity index (SI), these compounds displayed no appreciable cytotoxicity (SI >80). Further studies on activity against drug resistant (DR) Mtb indicated these compounds to be equally potent (MIC 0.03–0.25 μg mL⁻¹). The in silico covalent docking study suggested a similar polar interaction to that of PBTZ169 with an additional and contrasting side chain interaction at the active site of Mtb DprE1 target protein. Further, the time kill kinetic study found compounds 9a and 9d to be demonstrating bactericidal efficacy, completely eliminating bacilli in 7 days at 10× MIC. The most promising compound 9d, considering its potent anti-TB activity (MIC 0.06 μg mL⁻¹ against drug susceptible Mtb and MIC 0.06–0.25 μg mL⁻¹ against DR Mtb) along with a broad therapeutic index (SI >640) demonstrating a comparable concentration dependent bactericidal efficacy to that of RIF, holds a significant edge to be translated into a potent anti-Mtb agent.