Design, Synthesis, and Antitubercular Activity of Thiolutin–Cycloserine Hybrids: Reducing Cytotoxicity

The development of dual-acting hybrid antibiotics is a promising strategy to combat the ongoing spread of drug-resistant tuberculosis. Inspired by the structure of the natural dithiolopyrrolone hybrid antibiotic thiomarinol and based on the synergistic effects confirmed by checkerboard assays, we employed a molecular hybridization strategy. The dithiolopyrrolone natural product thiolutin was covalently linked to the pharmacophores of four clinical antitubercular drugs—cycloserine, linezolid, isoniazid, and pyrazinamide—through alkyl linkers of 7–10 carbon atoms via amide condensation, leading to the design and synthesis of 15 novel hybrids. In vitro antitubercular activity evaluation revealed that the cycloserine series exhibited the best activity, with MIC values as low as 1 μg/mL, followed by the isoniazid series. Cytotoxicity assays showed that all cycloserine hybrids had IC50 values > 40 μg/mL against RAW 264.7 mouse macrophages, markedly lower than that of thiolutin alone. Among them, T1-CS and T4-CS displayed the best selectivity indices, achieving an effective reduction in cytotoxicity. This study successfully constructed a class of thiolutin–cycloserine hybrids with low cytotoxicity and high selectivity, providing a valuable molecular template for the discovery of novel antitubercular lead compounds.