In Vitro and Ultrastructural Evaluation of the Cytotoxic and Antileishmanial Activities of Thiosemicarbazone Compounds Against Promastigotes and Axenic Amastigotes of Leishmania infantum

Leishmaniasis remains a global health challenge, and the search for effective and selective therapeutic agents is crucial. This study evaluated the in vitro antileishmanial and cytotoxic activities of thiosemicarbazone compounds (LT-70, LT-73, LT-75, and LT-89) against <i>Leishmania infantum</i> promastigote and axenic amastigote forms. The compounds demonstrated strong leishmanicidal activity, with IC₅₀ values ranging from 10.5 to 14 µM. At the lowest tested concentration (20 µM) the compounds produced percent inhibitions of 100% (LT-70), 100% (LT-73), 100% (LT-75) and 100% (LT-89). Cytotoxicity assays on J774.A1 macrophages revealed CC₅₀ values from 60 µM to >75 µM, with LT-73 and LT-75 showing low toxicity (CC₅₀ > 75µM). Selectivity index (SI) ranged from 7.1 for LT-75 and 5.8 for LT-73, indicating potential for further development. Ultrastructural analysis by SEM and TEM revealed cellular and organelle damage, including membrane rupture and mitochondrial swelling, especially after LT-73 and LT-75 treatment. Immunomodulatory assays indicated induction of TNF-α and IFN-γ production, with significant IL-6 reduction. Flow cytometry data suggest mitochondrial dysfunction and apoptosis-like features, particularly for LT-73. Membrane potential assays suggested mitochondrial depolarization by LT-73. LT thiosemicarbazone derivatives present specific structural modifications that enhance antileishmanial selectivity and reveal a dual mechanism of action combining mitochondrial dysfunction and immunomodulatory effects. These findings support the potential of thiosemicarbazone derivatives as promising antileishmanial agents with selective cytotoxicity and immunomodulatory effects.