Novel thiosemicarbazones of coumarin incorporated isatins: synthesis, structural characterization and antileishmanial activity

Leishmaniasis, a neglected tropical disease affecting 0.7 to 1.3 million people annually, has only a few toxic therapeutic options. This study describes the synthesis, structural characterization, <i>in silico</i> and <i>in vitro</i> assessment of novel thiosemicarbazones of coumarin incorporated isatins (<b>6a-6m</b>) as highly as potent and safe antileishmanial agents. Molecular docking was initially used to determine the binding of these compounds to the active cavity of the target protein (Leishmanolysin gp63) of <i>Leishmania</i> (L.) <i>tropica.</i> Among all the docked compounds, three <b>6d, 6f</b> and <b>6l</b> showed high binding affinities due to strong H-bonds and hydrophobic π-interactions. Importantly, the <i>in vitro</i> investigations of thirteen synthesized compounds for antileishmanial activity against <i>L. tropica</i> promastigotes and axenic amastigotes, complemented the docking results. The compound <b>6d</b> was found to be the most active of the series at micromolar concentrations both against promastigotes (IC₅₀ = 2.985 μmol/L) and axenic amastigotes (IC₅₀ = 13.46 μmol/L) in comparison to the tarter emetic (IC₅₀ = 12.56 μmol/L) and amphotericin B (IC₅₀ = 1.826 μmol/L), respectively. Significantly, all active compounds are much less toxic as compared to the positive control (Triton X-100) and, tartar emetic (TA) and amphotericin B when screened for their toxicity against human erythrocytes. To gain further insight into the interaction dynamics of our target protein on binding with compound <b>6d</b>, molecular dynamic simulation was performed for a course of 100 ns for both the apo-protein and the protein-ligand complex. The results revealed consistent structural stability for the protein-ligand complex, aligning with characteristics seen in the apo-proteins.