Halogenation of neolignan scaffolds reveals stage-specific anthelmintic activity against Haemonchus contortus

Gastrointestinal nematodes, including Haemonchus contortus, represent a major constraint to livestock production globally, and the increasing prevalence of anthelmintic resistance necessitates the discovery of new chemotypes. Natural products provide a rich and underexplored source of bioactive scaffolds for anthelmintic development. However, there remains a need to systematically optimise such scaffolds to define structure-activity relationships in whole-organism systems. Here, we investigated neolignan compounds from the fruits of Styrax suberifolius and assessed the impact of halogenation on their anthelmintic activity. Two known neolignans, suberifolioside A and equiselignan B, were isolated and used to generate a series of 11 halogenated derivatives using N-halosuccinimide chemistry. In addition, a new glycoside, suberifolioside B, together with two known metabolites, 7R,8S-dihydrodehydrodiconiferyl alcohol and 3',4-O-dimethylcedrusin, were characterised by spectroscopic and spectrometric methods, and the absolute configuration of equiselignan B was confirmed by X-ray crystallography. All compounds were evaluated for activity against larval stages of H. contortus using an established in vitro phenotypic assay. While no compounds significantly affected exsheathed third-stage larvae, multiple derivatives displayed activity against fourth-stage larvae, indicating pronounced stage-specific susceptibility. Notably, halogenation of the neolignan scaffold modulated biological activity, with several derivatives inducing substantial reductions in larval motility and distinct abnormal phenotypes. This modulation was non-linear, with both mono- and poly-halogenated derivatives displaying activity. A monobrominated ether analogue exhibited maximal motility inhibition approaching that observed for moxidectin under the assay conditions, highlighting the potential of this scaffold for optimisation. These findings demonstrate that halogenated neolignans represent a tractable chemical class for anthelmintic discovery and reveal stage-specific vulnerabilities in H. contortus. This work provides a foundation for further optimisation and prioritisation of neolignan derivatives for downstream studies of mechanism of action and translational potential.