Repin is a sesquiterpene lactone isolated from Centaurea repens (Russian knapweed). Its cytotoxic activity is driven by the alpha-methylenebutyrolactone group, which conjugates with glutathione to induce neurotoxicity, a mechanism confirmed by the complete loss of toxicity upon selective reduction of this reactive double bond. In PC12 cells, repin reduces cellular dopamine levels, while glutathione conjugation abolishes this effect entirely. The compound is widely used in neurotoxicity research to study oxidative stress pathways and dopamine-related cellular damage.