Cysteine-Based Amino Acid Mimetics as First-in-Class Antitubercular Agents: Discovery, SAR, and Identification of a Lead Compound with Intracellular Activity

Tuberculosis (TB), caused byMycobacterium tuberculosis (Mtb), remains a leading global infectious disease killer, and the emergence of drug-resistant strains underscores the urgent need for new antitubercular agents with novel mechanisms of action. Herein, we report the discovery of cysteine-based amino acid mimetics as a first-in-class scaffold with anti-Mtb activity, identified through bioisosteric replacement of a previously reported 1,2,4-triazole-3-thiol series. Systematic structure–activity relationship (SAR) studies across the eastern, western, and amino acid core regions of the scaffold generated a series of analogs with IC50 values ranging from 3 to >200 μM against wild-type Mtb. Lead compound 7, a modified l-cysteine derivative, exhibited an IC50 of 4.029 ± 1.04 μM against Mtb with no mammalian cytotoxicity (CC50 > 100 μM, selectivity index >25). Compound 7 retained full potency against an isoniazid-resistant Mtb strain, indicating activity independent of KatG-mediated resistance. Activity was further confirmed in a murine macrophage infection model, demonstrating efficacy against Mtb in an intracellular environment where the bacteria can reside during infection. Broader profiling revealed that cysteine core analogs are predominantly Mtb-selective, while tryptophan and tyrosine core analogs extended activity to drug-resistant Staphylococcus aureus strains. Initial in vitro ADME characterization of compound 7 demonstrated good human metabolic and plasma stability, with poor aqueous solubility and mouse plasma instability identified as liabilities for future optimization. These findings establish cysteine-based amino acid mimetics as a promising new scaffold for TB drug discovery.