Figure 5: Ophiobolin A (OPA) and its cytotoxic adduct with PE (OPA-PE). PE is abundant in the inner leaflet of most cell membranes. However, by unknown mechanisms, some cancer cells flip PE from the inner leaflet to the outer leaflet. OPA has been proposed to react with the PE in the outer leaflet generating a cytotoxic species that causes leaky membranes and that eventually kills the cells.