New Okadaic Acid Analogues from the Marine Sponge Merriamum oxeato and Their Effect on Mitosis

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Abstract
Inhibitors of the G2 DNA damage checkpoint can selectively sensitize cancer cells with impaired p53 tumor suppressor activity to killing by DNA-damaging drugs or ionizing radiation and have been proposed as a promising therapeutic strategy. An extract from the Northeastern Pacific marine sponge Merriamum oxeato showed G2 checkpoint inhibitory activity, and fractionation identified the known dinoflagellate toxin dinophysistoxin 1 (1) and the two novel analogues 27-O-acetylokadaic acid (2) and 27-O-acetyldinophysistoxin 1 (3) as the active compounds. The mixture of 1, 2, and 3 was extremely potent at inhibiting the G2 checkpoint (IC50 = 1 ng/mL) and cellular protein Ser/Thr phosphatases (IC50 = 1 ng/mL), and it radiosensitized MCF-7 breast cancer cells expressing mutated p53 at all concentrations tested. However, the mixture of 1, 2, and 3 was also very toxic to cells not exposed to DNA damage (IC50 = 1 ng/mL), making these compounds poor candidates for therapeutic agents to augment the effectiveness of DNA-damaging therapies.