Thiosemicarbazone Derivatives Exhibit Potential Leishmanicidal Effects against L. infantum

Visceral leishmaniasis remains a significant global health burden, with current therapeutic options hampered by high toxicity and the emergence of drug-resistant strains. Thiosemicarbazones, a class of compounds with well-established antimicrobial and antiparasitic properties, represent a promising avenue for novel therapeutics. This study screened 18 thiosemicarbazone derivatives against Leishmania infantum and identified two lead candidates, TBD and TBB, with potent antileishmanial activity. TBD and TBB inhibited promastigotes (IC50 = 5.43 μM and 3.14 μM, respectively) and axenic amastigotes (EC50 = 8.22 μM and 16.11 μM), with favorable selectivity indices in human peripheral blood mononuclear cells (SI = 51.23 for TBD; SI = 9.58 for TBB). Cell death assays revealed that TBD induced dose-dependent apoptosis in promastigotes, with late-apoptotic populations increasing from 5.75% at 1× IC50 to 33.6% at 4× IC50. In axenic amastigotes, TBD at 4× EC50 triggered early apoptosis in 31.0% of cells, significantly surpassing amphotericin B (20.1%, p < 0.01). Furthermore, TBD induced mitochondrial membrane depolarization in L. infantum promastigotes, with Variation Index values of -0.451, -0.444, and -0.805 at 1×, 2×, and 4× IC₅₀, respectively. Importantly, TBD exhibited no detectable cytotoxicity in human PBMCs. These findings position TBD as a compelling candidate for further preclinical development as a novel treatment for visceral leishmaniasis.