NBT-287, a third generation taxane analog, and paclitaxel resistance due to MDR-1 and mutant tubulin

3114 Background: The therapeutic utility of taxanes, the most widely used class of anticancer agents, is hampered by resistance due to over-expression of MDR-1 and tubulin mutations. By modifying the paclitaxel (TX) molecule, we have prepared a third-generation taxane designed to circumvent MDR-1. We tested this taxane in human tumor cell lines, including an ovarian cancer cell line characterized by mutant tubulin. Methods: For initial screening, we selected tumors exhibiting various levels of MDR-1 expression. Twenty-four hours following inoculation of tumor cells into 96-well plates, different concentrations of analogs solubilized in DMSO / PBS were added to six replicate wells for 1-hour and 24-hour periods. After five additional days of incubation in drug-free media, we determined cell survival using the MTT or WST-1 assay, and compared the mean O.D.s of sextuplicate wells containing taxane analog treated, untreated, and paclitaxel treated controls. Results: NBT-287 was 4- to 200-fold more potent than paclitaxel in the ovarian mutant tubulin MDR-1 negative tumor, in MDR-1 positive cell lines, and equivalent to paclitaxel in other MDR-1 negative cell lines. Exposure of NCI-AR paclitaxel-resistant breast tumor cells to NBT-287 at a 25% inhibitory concentration did not sensitize the cells to paclitaxel, suggesting that this analog was circumventing rather than reversing MDR-1. The cytotoxic ratios of the ED50 comparing TX and NBT-287 in representative cell lines are tabulated below. Conclusions: The cytotoxic advantage of NBT-287 over paclitaxel in tumor cells is independent of the expression of MDR-1 and of mutant tubulin-associated resistance. The latter mechanism of action may be due to different binding sites on beta-tubulin for paclitaxel and NBT-287. As a result, this taxane may be an appropriate compound for further efficacy and safety studies in mouse xenotransplants and in patients. Author Disclosure Employment or Leadership Consultant or Advisory Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration NaPro BioTherapeutics, Inc. Bruce Myers & Associates; NaPro BioTherapeutics, Inc. NaPro BioTherapeutics, Inc.