Extracts of the seeds of the leguminous tree Millettia thonningii are molluscicidal and schistosomicidal and contain a mixture of isoflavonoids and 3-phenylcoumarins. Robustic acid and a pure mixture of alpinumisoflavone and dimethylalpinumisoflavone, as characterized by nuclear magnetic resonance, were isolated from a dichloromethane extract. The bioactivity of these compounds against Schistosoma mansoni miracidia, cercariae, and adult worms was tested in vitro. Both robustic acid and the alpinumisoflavone mixture killed cercariae and adult schistosomes in vitro. At sublethal concentrations of the isoflavonoids, reductions in miracidial motility were measured using a video-imaging technique. The inhibition of mitochondrial electron transport at site I by the isolated isoflavonoids was also investigated. The results suggest that robustic acid and at least 1 alpinumisoflavone compound from the dichloromethane extract of the seeds of M. thonningii are responsible for some of the observed bioactivity of this extract against schistosomes.

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