Two isoquinoline alkaloids, including the new compound 1, were isolated fro
m the cytotoxic fractions of an aqueous extract of the marine sponge Halicl
ona sp. The structures of these compounds were established as 1-hydroxymeth
yl-7-methoxyisoquinolin-6-ol (1) and mimosamycin (2) by conventional spectr
oscopic methods and by comparison with related compounds. Mimosamycin (2) w
as the principal cytotoxin with an IC50 of approximately 10 mug/mL against
melanoma and ovarian human tumor cell lines.