Leishmaniasis is a highly prevalent, yet neglected disease caused by protozoan parasites of the genus Leishmania. In the search for newer, safer, and more effective antileishmanial compounds, we herein present a study of the mode of action in addition to a detailed structural and biological characterization of LQOF-G6 [N-benzoyl-N′-benzyl-N″-(4-tertbutylphenyl)guanidine]. X-ray crystallography and extensive NMR experiments revealed that LQOF-G6 nearly exclusively adopts the Z conformation stabilized by an intramolecular hydrogen bond. The investigated guanidine showed selective inhibitory activity on Leishmania major cysteine protease LmCPB2.8ΔCTE (CPB) with ~73% inhibition and an IC_50-CPB of 6.0 µM. This compound did not show any activity against the mammalian homologues cathepsin L and B. LQOF-G6 has been found to be nontoxic toward both organs and several cell lines, and no signs of hepatotoxicity or nephrotoxicity were observed from the analysis of biochemical clinical plasma markers in the treated mice. Docking simulations and experimental NMR measurements showed a clear contribution of the conformational parameters to the strength of the binding in the active site of the enzyme, and thus fit the differences in the inhibition values of LQOF-G6 compared to the other guanidines. Furthermore, the resulting data render LQOF-G6 suitable for further development as an antileishmanial drug.

Leishmaniasis and Chagas disease are neglected tropical diseases that continue to pose significant public health challenges, highlighting the need for new therapeutic strategies. Brazilian snake venoms represent a valuable and underexplored source of bioactive molecules with diverse pharmacological activities and promising therapeutic potential. In this context, the present study aimed to investigate the in vitro antiprotozoal potential of venoms from Bothrops and Crotalus species, including Bothrops jararacussu, Bothrops pauloensis, Bothrops alternatus, Bothrops leucurus, Bothrops atrox, Bothrops jararaca, Crotalus durissus terrificus, and Crotalus durissus ruruima, metalloprotease (MTL)- and crotoxin (CTX)-enriched, against Leishmania spp. and Trypanosoma cruzi. Crude venom protein profiles were analyzed by SDS-PAGE, and cytotoxicity was assessed in differentiated C2C12 myotubes and murine peritoneal macrophages. Antiprotozoal activity was evaluated against extracellular parasite forms, including promastigotes of Leishmania amazonensis and Leishmania infantum and epimastigotes of T. cruzi. Venoms exhibiting the highest selectivity indices (SIs), calculated using murine macrophage cytotoxicity data, were further tested against intracellular amastigotes of L. amazonensis. Among the tested samples, B. jararacussu venom showed activity against promastigotes of both L. amazonensis (IC₅₀ = 2.9 µg mL⁻¹) and L. infantum (IC₅₀ = 2.8 µg mL⁻¹), as well as in epimastigotes of T. cruzi (IC₅₀ = 15.5 µg mL⁻¹). The MTLand CTX pools from C. d. ruruima venom exhibited the highest activity against promastigotes of L. amazonensis, with IC₅₀ values of 1.6 and 3.8 µg mL⁻¹, respectively. Against intracellular amastigotes, MTL and CTX showed IC₅₀ values of 4.1 and 2.5 µg mL⁻¹, respectively, with selectivity indices (SIs) of 73.1 and 120, respectively. Overall, these findings provide preliminary evidence for the antiparasitic potential of snake venom-derived molecules, supporting further characterization and in vivo evaluation.