Anti-Fertilization Activity of a Spirocyclic Sesquiterpene Isocyanide Isolated from the Marine Sponge Geodia exigua and Related Compounds

Abstract

(−)-10-epi-Axisonitrile-3, a spirocyclic sesquiterpene isocyanide obtained from the marine sponge Geodia exigua, immobilized sperm of sea urchin and starfish to block fertilization at the minimum effective concentration of 0.4 μg/ml. On the other hand, fertilized eggs developed normally to the gastrula stage in the presence of a 250-times higher concentration of the isocyanide. Analysis by ³¹P NMR revealed an accumulation of phosphocreatine and a depletion of inorganic phosphate in the isocyanide-treated sperm, suggesting that (−)-10-epi-axisonitrile-3 inhibited the phosphocreatine shuttle participating in the high-energy phosphate metabolism, thereby immobilizing sperm to block fertilization. No analogs of (−)-10-epi-axisonitrile-3 containing different functionalities or isocyanides with different carbon skeleton exhibited such activity.