Neurotoxic meroditerpenoids from the tropical marine brown alga Stypopodium flabelliforme

Brine shrimp toxicity and TLC analysis guided the isolation of five new and biologically active meroditerpenoids [2β,3α-epitaondiol (1), flabellinol (2), flabellinone (3), stypotriolaldehyde (4), and stypohydroperoxide (5)] along with five known compounds from the marine brown alga Stypopodium flabelliforme collected in Papua New Guinea. The planar structures of compounds 1-5 were determined by extensive spectroscopic analysis (1D and 2D NMR, LRMS, HRMS, IR, and UV), while relative configuration was determined by ID and 2D NOE experiments. X-ray crystallography confirmed the relative configuration of 2β,3α-epitaondiol (1), and the modified Mosher's ester method was used to establish its absolute configuration. All of the new metabolites were moderately toxic to murine neuro-2a cells (LC ₅₀ 2-25 μM), and three [2β,3α-epitaondiol (1), flabellinol (2), and flabellinone (3)] possessed potent sodium channel blocking activity. Stypotriolaldehyde (4) had a biphasic effect on the concentration of intracellular Ca²⁺ in rat cerebellar granule neurons (CGN). The previously known compound, stypoldione (6), also modulated intracellular calcium concentration and was cytotoxic in CGN. Metabolites 2β,3α- epitaondiol (1), flabellinol (2), and flabellinone (3) displayed moderate cytotoxicity to the NCI-H460 human lung cancer cell line.