Molecular docking, design, synthesis and in vitro analysis identify [1,2,4]triazolo[4,3-b]pyridazine derivatives as inhibitors of Leishmania donovani sterol methyltransferase

The current therapeutic agents for the treatments of visceral leishmaniasis are ineffective, and cytotoxic. Therefore, there is the urgent need for new chemotypes for the treatment of the disease with novel mechanisms of action. In our previous investigation, we identified the triazolopyridazine, STOCK6S-84928 as a potential inhibitor of Leishmania donovani sterol methyltransferase (LdSMT) with IC₅₀ value of 118.3 µM. To improve the biological activity of the initial hit compound, we hereby describe the results of structural-activity relationship studies on STOCK6S-84928 via chemical modifications on the scaffold and virtually screening of 250 compounds obtained against the Modeller generated LdSMT structure. A total of 21 compounds were found to have binding energies ranging from − 7.0 to − 9.2 kcal/mol lower or comparable to the 22,26-azasterol (− 7.6 kcal/mol), the known inhibitor of the target. Molecular docking and molecular dynamics simulations revealed Ile272 and Tyr275 to be pivotal for ligand binding. The compounds were predicted to possess leishmanicidal activities with good drug-like properties. Significantly, the compounds 8, 9, 21, and 23 were predicted to possess antineoplastic, anti-inflammatory, analgesics, protein and MAP kinase inhibitory activities with probability of activity (Pa) > 0.2 and probability of inactivity (Pi) < 0.16. Through the applications of cyclization, amination, Williamson’s ether synthesis, and Suzuki cross-coupling reactions, the selected analogues of STOCK6S-84928 were synthesised in moderate to high yields and characterized by FTIR, LC–MS, and NMR spectroscopy methods. In vitro antileishmanial evaluation of the synthesized compounds identified 23 as the most potent, exhibiting L. donovani promastigotes inhibitory activities with IC₅₀ value of (1.9 ± 0.1) µM. The ortho-difluoropheneyl group as well as the triazolopyridine moieties were suspected to be responsible for the observed activity. Similarly, the compounds 10 and 16 required (0.8 ± 0.1) µM and (0.6 ± 0.0) µM, respectively to eliminate 50% of Trypanosoma brucei.