Thiolactomycin-based morpholino and piperidino conjugates: synthesis, antimalarial evaluation, and PfFAS-II inhibition

Abstract

Background and purpose: Malaria remains a formidable global healthcare challenge due to emerging drug-resistant Plasmodium spp., underscoring the urgent need for novel antimalarial scaffolds. Thiolactomycin, a FAS-II inhibitor, is a promising lead for structural optimization. This study aims to design, synthesize, and evaluate a series of thiomorpholino and piperidino conjugates bearing a thiolactone moiety as target-specific antimalarial agents. Experimental approach: A series of thiolactomycin-based conjugates was synthesized, including intermediate 7b and conjugate 4g. Antimalarial activity was evaluated in vitro against the Plasmodium falciparum D10 strain. To elucidate the mechanism, multi-target docking studies of 7b and 4g were performed against key FAS-II enzymes, followed by 200 ns explicit-solvent MD simulations and MM-PBSA binding free energy calculations targeting PfFabI. In silico pharmacokinetics and drug-likeness profiling of potent molecules were also carried out. Findings/Results: Biological evaluation identified 7b and 4g as potent compounds with IC50 values of 0.2 and 1.81 μM, respectively. However, the higher phenotypic potency of 7b was accompanied by mutagenic liability (Ames positive) revealed by toxicological profiling. Conversely, 4g exhibited superior binding affinity (-7.2 kcal/mol) against PfFabI. MD simulations revealed an induced-fit stabilization of the 4g-PfFabI complex and the complex achieved superior binding free energy (ΔGTotal = -22.14 kcal/mol) compared to thiolactomycin. Conclusion and implications: Conjugate 4g was identified as a safer potent compound from the series. MD simulation of the 4g-PfFabI complex revealed an induced fit stabilization involving persistent H-bonds that were absent in docking. These findings suggest 4g as a promising molecule for further optimization.