Serinolamides and Lyngbyabellins from an <i>Okeania</i> sp. Cyanobacterium Collected from the Red Sea

NMR- and MS-guided fractionation of an extract of an <i>Okeania</i> sp. marine cyanobacterium, collected from the Red Sea, led to the isolation of four new metabolites, including serinolamides C (<b>1</b>) and D (<b>2</b>) and lyngbyabellins O (<b>3</b>) and P (<b>4</b>), together with the three known substances lyngbyabellins F (<b>5</b>) and G (<b>6</b>) and dolastatin 16 (<b>7</b>). The planar structures of the new compounds were determined using NMR and MS analyses. The absolute configurations of <b>1</b> and <b>2</b> were determined by Marfey’s analysis of their hydrolysates. The absolute configuration of <b>3</b> was ascertained by chiral-phase chromatography of degradation products, while that of <b>4</b> was determined by comparison to <b>3</b> and <b>5</b>. The cytotoxic and antifouling activities of these compounds were evaluated using MCF7 breast cancer cells and <i>Amphibalanus amphitrite</i> larvae, respectively. Compounds <b>3</b>, <b>4</b>, and <b>7</b> exhibited strong antifouling activity, and <b>3</b> and <b>7</b> were not cytotoxic. A structure–activity relationship was observed for the cytotoxicity of the lyngbyabellins with the presence of a side chain (<b>4</b> is more active than <b>3</b>) leading to greater activity. For the antifouling activity, the acyclic form without a side chain (<b>3</b>) was the most active