Libertellenones A-D: induction of cytotoxic diterpenoid biosynthesis by marine microbial competition.

The induction of biosynthesis of four new diterpenoids was observed following the addition of a marine alpha-proteobacterium to an established culture of the marine-derived fungus Libertella sp. The fungal strain and the marine bacterium, cultured alone, do not produce diterpenoid metabolites. The induced diterpenoids, libertellenones A-D, are cross-conjugated ketones of the pimarane class. The libertellenones show varying levels of cytotoxicity against the HCT-116 human adenocarcinoma cell line with libertellenone D being the most potent (IC50 = 0.76 microM).