Design and synthesis of magnolol-based compounds with potent antiparasitic activity against Ichthyophthirius multifiliis

Ichthyophthirius multifiliis infections constitute a critical challenge demanding urgent resolution in global aquaculture systems. Although over 20 chemicals and natural products have demonstrated efficacy against I. multifiliis in the past two decades, the translation of these findings into clinically viable antiparasitic agents has progressed slowly, with the development of novel potent therapeutics remaining a major challenge. In this study, six novel magnolol-derived compounds were rationally designed through structural optimization of the magnolol scaffold. All synthesized compounds exhibited potent antiparasitic activity against both theront and tomont stages of I. multifiliis . Compound C1 showed superior efficacy to magnolol, with 18-fold and 26-fold lower EC 50 values against theronts (0.035 mg/L) and tomonts (0.024 mg/L) in vitro, respectively. Notably, these activity surpassed the potency of malachite green. Moreover, compound C1 showed lower toxicity than malachite green. ADMET and Density functional theory (DFT) calculations revealed that compound C1 had a favorable electrostatic potential (ESP) distribution and a lower energy gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), indicating good druggability. Integrated in silico strategy identified Mapk3 (mitogen-activated protein kinase 3) as the primary therapeutic target mediating C1's antiparasitic effects. Protein-Protein Interaction (PPI) network analysis, along with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, suggested that compound C1 might disrupt lipid metabolism and lipoxygenases pathways by modulating Mapk3 expression, thereby exerting potent antiparasitic effects. These findings establish compound C1 as the most potent anti- I. multifiliis agent reported to date. Its potential application in aquaculture may provide valuable insights for addressing this critical bottleneck issue.