Lophocladines, bioactive alkaloids from the red alga Lophocladia sp.

Lophocladines A (1) and B (2), two 2,7-naphthyridine alkaloids, were isolated from the marine red alga Lophocladia sp. collected in the Fijian Islands. Their structures were deduced on the basis of high-resolution mass spectra and one- and two-dimensional NMR spectroscopy. Lophocladine A (1) displayed affinity for NMDA receptors and was found to be a delta-opioid receptor antagonist, whereas lophocladine B (2) exhibited cytotoxicity to NCI-H460 human lung tumor and MDA-MB-435 breast cancer cell lines. Immunofluorescence studies indicated that the cytotoxicity of lophocladine B (2) was correlated with microtubule inhibition. This is the first reported occurrence of alkaloids based on a 2,7-naphthyridine skeleton from red algae.