Laxaphycins B5 and B6 from the cultured cyanobacterium UIC 10484

Two laxaphycin type-B cyclic dodecapeptides, laxaphycins B5 and B6, were obtained from UIC 10484, a freshwater cf. Phormidium sp. Analysis using the 16S rRNA sequence found UIC 10484 to clade with UIC 10045, a known laxaphycin type-A and -B producer, and MS/MS analysis revealed the presence of two novel laxaphycin type-B compounds. The structures of the metabolites were elucidated using 2D NMR and MS/MS. The absolute configurations of the amino acids were determined by advanced Marfey's analysis. Both metabolites were evaluated against the same three cancer cell lines. The IC₅₀ of both laxaphycins B5 and B6 was near 1 μM against breast cancer MDA-MB-231, melanoma MDA-MB-435, and ovarian cancer OVCAR3 cell lines.