Isolation, Characterization and Antiproliferative Activity of New Metabolites from the South African Endemic Red Algal Species Laurencia alfredensis

Abstract

The marine red algae of the genus <i>Laurencia</i> have been widely studied for their structurally diverse and biologically active secondary metabolites. We report here the natural product investigation of the organic extract of a newly identified South African endemic species, <i>Laurencia alfredensis</i>. A sequence of column chromatography, preparative TLC and normal phase HPLC resulted in the isolation of eleven compounds comprising three labdane-type diterpenes (<b>1</b>-<b>3</b>), four polyether triterpenes (<b>4</b>-<b>7</b>), three cholestane-type ecdysteroids (<b>8</b>-<b>1</b><b>0</b>) and a glycolipid (<b>11</b>). Compounds <b>1</b>-<b>3</b>, <b>5</b>-<b>8</b> and <b>10</b> have not previously been reported, while compound <b>9</b> is reported here for the first time from a natural source and the known compound <b>11</b> isolated for the first time from the genus <i>Laurencia</i>. The structural elucidation and the relative configuration assignments of the compounds were accomplished by extensive use of 1D- and 2D-NMR, HR-ESI-MS, UV and IR spectroscopic techniques, while the absolute configuration of compound <b>1</b> was determined by single-crystal X-ray diffraction analysis. All compounds were evaluated against the MDA-MB-231 breast and HeLa cervical cancer cell lines. Compound <b>2</b> exhibited low micromolar antiproliferative activity (IC₅₀ = 9.3 µM) against the triple negative breast carcinoma and compound <b>7</b> was similarly active (IC₅₀ = 8.8 µM) against the cervical cancer cell line.