Cytotoxic lissoclimide-type diterpenes from the molluscs Pleurobranchus albiguttatus and Pleurobranchus forskalii

Three new chlorinated diterpenes, 6-8, along with five known ones, 1-5, were isolated from the molluscs Pleurobranchus albiguttatus and P. forskalii collected in the Philippines. These diterpenes are presumably metabolites of a Lissoclinum species of ascidian on which the molluscs have fed. The structures of the new compounds were determined by interpretation of their spectral data. Compounds 1 and 2 were found to be potent cytotoxins in the National Cancer Institute's screening panel of 60 tumor cell lines and showed some selectivity for melanomas. Two other samples exhibited solid tumor selectivity in a soft agar disk diffusion assay.