Pyrroloacridine alkaloids from Plakortis quasiamphiaster: structures and bioactivity.

A re-collection of Plakortis quasiamphiaster from Vanuatu in 2003 resulted in the isolation of three known compounds, plakinidine A (1) and amphiasterins B1 (6) and B2 (7). Also isolated was a new bis-oxygenated pyrroloacridine alkaloid, plakinidine E (8), with a unique O-substitution versus N-substitution at position C-12 in 1. The biological evaluation of the active compounds in two assays provided complementary data. Plakinidine A (1) exhibited cytotoxicity against human colon H-116 cells with an IC50 of 0.23 microg/mL, but there were no effects against the yeast Saccharomyces cerevisiae diploid homozygous deletion strain of topoisomerase I (top1Delta). By contrast, 8 was inactive against H-116 cells but was potent in the yeast halo screen.