Chemical investigation of the organic extract of a Red Sea strain of the cyanobacterium Moorea producens has afforded 2,3-seco-2,3-dioxo-lyngbyatoxin A (1). Five known compounds including lyngbyatoxin A (2), majusculamides A and B (3 and 4), aplysiatoxin (5) and debromoaplysiatoxin (6) were also isolated. Their structures were elucidated by using HR-FAB-MS, 1D and 2D NMR analyses. The compounds were evaluated for antiproliferative activity against HeLa cancer cells. Lyngbyatoxin A (2)showed potent activity, with an IC50 of 9.2 nM, while 5 and 6 displayed modest activity with IC50 values of 13.3 and 3.03 mM, respectively. In contrast, compounds 1, 3 and 4
were inactive, with IC50 values greater than 50 mM. The lack of cytotoxicity for 2,3-seco-2,3-dioxo-lyngbyatoxin A (1) demonstrates that the indole moiety in lyngbyatoxin (2) is essential for its cytotoxicity, and suggests that detoxification of 2
may be carried out by biological oxidation of the indole moiety to yield 1.