Disorazole C1 is a potent, cytotoxic natural product, with IC50 values of < 20 nM in a variety of cancer cell lines. The addition of disorazole C1 disrupted microtubules in cells as early as 24 h after treatment, causing multinucleation and disorganized microtubules at interphase and misaligned chromosomes at the metaphase plate during mitosis. A549 cells that survived a 7 day treatment of disorazole C1 displayed an enlarged, flattened morphology similar to senescent cells treated with doxorubicin. These surviving cells displayed the characteristic β-galactosidase staining at pH 6.0 seen in senescent cells and changes in cell cycle markers indicative of premature or hypermitogenic senescence. A549 cells treated with the IC50 concentration of disorazole C1 (2 nM) for 7 days had increased protein levels of cyclin D, p53 and cyclin-dependent kinase inhibitor p21 as detected by Western blot. This resulted in decreased phosphorylation of Rb (Ser780) and a decrease in total Rb protein levels. A dramatic decrease in BrdU incorporation was also observed in disorazole C1 treated cells, further implicating the induction of senescence. Finally, DC3F/VCRd-5L cells, which are >2000-fold resistant to vincristine as compared to DC3F cells, were only 5-fold resistant to disorazole C1. These results provide the first evidence of a microtubule disrupter that induces premature senescence.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA