Repin, a sesquiterpene lactone from Acroptilon repens possessing exceptional biological activity.

Consumption of Russian knapweed (Acroptilon repens) by horses results in the necrosis of neural cells in the substantia nigra. Repin, one of the sesquiterpene lactones found in Russian knapweed, has been shown to possess high toxicity toward chick embryo sensory neurons. The possible causal relationship between repin and equine nigropallidial encephalomacia disease prompted a more complete structural assignment of repin, which was accomplished by X-ray and 1H-nmr analyses.