Antifungal cyclic peptides from the marine sponge Microscleroderma herdmani.

Screening natural product extracts from the National Cancer Institute Open Repository for antifungal discovery afforded hits for bioassay-guided fractionation. Using LC-MS analysis to generate chemical structure information on potentially active compounds, two new cyclic hexapeptides, microsclerodermins J (1) and K (2), were isolated from the deep-water sponge Microscleroderma herdmani, along with microsclerodermins A (3) and B (4), previously isolated from an unidentified Microscleroderma species. The structures of the new compounds were elucidated by spectroscopic analysis and chemical methods. In vitro antifungal testing showed that the four compounds possessed strong activities against the opportunistic fungal pathogens Candida albicans, Candida glabrata, Candida krusei, Cryptococcus neoformans, and Aspergillus fumigatus.