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Increased Sensitivity of the Antiestrogen‐Resistant MCF‐7/LY2 Human Breast Carcinoma Cell Line to Apoptosis Induced by the Novel Microtubule Stabilizing Agent (+)‐Discodermolide

Abstract: (+)‐Discodermolide is a sponge‐derived natural product with the most potent microtubule stabilizing activity yet discovered. Its actions parallel that of the promising antibreast cancer agent paclitaxel despite the lack of any apparent similarities in the drugs' structures. To complement our previous studies on human breast cancer cells, we compared the effects of the two drugs against the estrogen receptor positive but tamoxifen‐resistant MCF‐7/LY2 line. Growth inhibition, cell, and nuclear morphological, electrophoretic, and flow cytometric analyses were performed. (+)‐Discodermolide potently inhibited the growth of the cells (e.g., 48‐hours IC50 of 1.5 nM) at concentrations similar to those observed with paclitaxel, and somewhat lower than the values observed previously with estrogen responsive MCF‐7 cells and estrogen nonresponsive MDA‐MB231 cells. (+)‐Discodermolide‐treated MCF‐7/LY2 cells had condensed and highly fragmented nuclei, as well as micronuclei, suggesting mitotic block and the induction of apoptosis. Flow cytometric comparison of cells treated with either drug at 10 nM showed both caused accumulation into the G2/M portion of the cell cycle as well as induction of a pronounced hypodiploid cell population, with (+)‐discodermolide yielding a greater effect. The timing and type of high molecular weight DNA fragmentation induced by the two agents was fully consistent with induction of apoptosis, again with (+)‐discodermolide showing an advantage over paclitaxel in this regard. More extensive DNA fragmentation was noted in MCF‐7/LY2 than has been observed in MCF‐7 and MDA‐MB231 cells. These in vitro results, coupled with those obtained previously, suggest that (+)‐discodermolide might have promise as a new chemotherpeutic agent against breast cancers. In addition, its novel and synthetically approachable structure make (+)‐discodermolide a promising lead compound for design and discovery of new microtubule stabilizing agents as alternatives to taxoids.