High-Throughput Screening of Five Compound Libraries for Anthelmintic Activity and Toxicity Leads to the Discovery of Two Flavonoid Compounds

Gastrointestinal nematode infections (GINs) in ruminants are a major constraint to efficient livestock production worldwide. Currently, only a limited number of anthelmintic drugs are available for the control of these infections, but their widespread use in preventive deworming campaigns and the incorrect administration of the drugs are responsible for the emergence of resistance. Therefore, new anthelmintic drugs are urgently needed. However, drug discovery methods for new anthelmintics based on GINs isolated from ruminants often have low throughput. In this study, a screening of five commercial collections of chemical compounds, including one collection of anti-infective drugs, three plant-based natural product collections, and one collection from the FDA-approved Chinese <i>Pharmacopoeia</i>, with a total of 2228 molecules, have been carried out in a high-throughput format. In the single slot screen, 32 compounds (1.44% success rate) achieved a >70% motility inhibition rate. Of these, 10 are known anthelmintic drugs, while the remaining 22 were tested against <i>Haemonchus contortus</i> and a resistant strain of <i>Teladorsagia circumcincta</i>. Four compounds (two flavonoids, chalcone and trans-chalcone), and two anti-infectives (octenidine and tolfenpyrad), showed anthelmintic activity with EC₅₀ values below 20 µM, and were further tested for their safety against HepG2 spheroids and mouse intestinal organoids. Trans-chalcone and chalcone emerged as promising candidates for future development, showing selective indexes > 5, while tolfenpyrad and octenidine require careful evaluation due to their toxicity profiles.