Antiproliferative effects of halichondrin B analog eribulin mesylate (E7389) against paclitaxel-resistant human cancer cells in vitro

C58 Eribulin (E7389, ER-086526, NSC-707389), a structurally-simplified synthetic analog of the biologically active macrocyclic portion of the marine natural product halichondrin B, is currently undergoing Phase 3 clinical trials for metastatic breast cancer. Preclinical studies have shown that eribulin exerts potent anticancer activities in vitro and in vivo by inhibiting microtubule dynamics via a mechanism distinct from other tubulin-targeted agents. Eribulin inhibits proliferation of a wide spectrum of human cancer cell types at sub-nanomolar concentrations and shows impressive anticancer activity against a broad range of human cancer xenograft models in mice, including complete tumor regressions. To further evaluate eribulin as an anticancer agent, its activity against drug-resistant human cancer cells was studied. Cancer cells can acquire resistance to chemotherapy agents by several different mechanisms. One mechanism with particular relevance to some tubulin-targeted drugs involves mutations in β-tubulin that result in decreased responsiveness to tubulin-targeted agents such as the taxanes. In this study, antiproliferative effects of eribulin were evaluated in an in vitro cell-based model of resistance based on mutated β-tubulin. Three ovarian cancer cell lines, A2780/1A9, 1A9PTX10 and 1A9PTX22, were obtained from the laboratory of Dr. Tito Fojo, US National Cancer Institute. The two paclitaxel resistant sublines of A2780/1A9, 1A9PTX10 and 1A9PTX22, were originally isolated as individual clones in a single step selection following exposure of A2780/1A9 cells to paclitaxel. Both sublines harbor different mutations in the M40 β-tubulin isotype and have been shown to have significant resistance to paclitaxel. To verify that paclitaxel resistance of 1A9PTX10 and 1A9PTX22 cells is not based on expression of the multidrug resistance efflux pump P-glycoprotein (PgP), cells were stained with fluorescently-labeled anti-PgP antibody and analyzed by flow cytometry. Neither of the 2 resistant lines, 1A9PTX10 or 1A9PTX22, nor parental A2780/1A9 cells, expressed detectable PgP, indicating that paclitaxel resistance seen in 1A9PTX10 and 1A9PTX22 cells is not PgP-mediated. Antiproliferative effects of eribulin, paclitaxel and vinblastine were then measured in A2780/1A9, 1A9PTX10 and 1A9PTX22 cells. Consistent with literature reports, 1A9PTX10 and 1A9PTX22 cells showed 15- to 19-fold resistance to paclitaxel. In contrast, 1A9PTX10 and 1A9PTX22 cells showed almost identical sensitivities to eribulin and vinblastine compared with parental A2780/1A9 cells (IC 50 ratios ~1.3). These results indicate that eribulin and vinblastine retain essentially full in vitro potency in cells harboring β-tubulin mutations that lead to substantial resistance to paclitaxel.