New Steroids and a New Alkaloid from the Gorgonian Isis minorbrachyblasta: Structures, Cytotoxicity, and Antilarval Activity

Two new polyoxygenated steroids, (1α,3β,7α,11α,12β)‐gorgost‐5‐ene‐1,3,7,11,12‐pentol 12‐acetate (1) and 11‐O‐acetyl‐22‐epihippuristanol (2), and a new alkaloid, 2,3,5,6,11,11b‐hexahydro‐2‐hydroxy‐1H‐indolizino[8,7‐b]indole‐2‐carboxylic acid (3), together with three known compounds, 22‐epihippuristanol (4), hippuristanol (5), and tryptamine (6), were isolated from the EtOH/CH2Cl2 extracts of the South China Sea gorgonian Isis minorbrachyblasta. The structures of the new compounds were determined by spectroscopic methods. Compound 1 showed weak cytotoxicity against A549, HONE1, and HeLa cancer cell lines and strong antilarval activity towards Bugula neritina larvae with an EC50 value of 5.8 μg/ml. Compound 5 showed moderate cytotoxicity against A549, HONE1, and HeLa cell lines, and the epimer mixture 4/5 (weight ratio 3 : 2) exhibited potent cytotoxicity against A549 and HONE1 cell lines with IC50 values of 4.2 and 4.8 μg/ml, which indicated that epimers 4 and 5 might have a synergistic effect on their cytotoxicity against A549 and HONE1 cell lines.