Abstract 1666: Increased sensitivity of βIII tubulin-containing microtubules to cabazitaxel compared with docetaxel: A possible role of βIII tubulin in specifying good efficacy of cabazitaxel for docetaxel-resistant tumors

Abstract

Abstract Tumor progression and resistance to taxane-based therapies have been shown to correlate with overexpression of the class III β-tubulin isotype (βIII tubulin). Elevated expression of βIII tubulin is often associated with tumor aggressiveness in patients with metastatic docetaxel-resistant prostate cancer. Cabazitaxel is a novel taxane recently approved for the treatment of metastatic prostate cancer that failed docetaxel therapy. To understand the mechanistic distinctions between cabazitaxel and docetaxel, we analyzed the effects of both drugs on bovine brain microtubules consisting of unfractionated tubulin, which contains 25% βIII tubulin, and with microtubules depleted of βIII tubulin. BetaIII tubulin was depleted using immunoaffinity chromatography purification technique. Both cabazitaxel and docetaxel enhanced polymerization of unfractionated tubulin to similar extents. The stoichiometry of 14C-cabazitaxel and 14C-docetaxel binding to microtubules composed of unfractionated tubulin were similar at very low concentrations (≤ 10 μM). The Kds for binding were 8.4 μM and 7.5 μM for 14C-cabazitaxel and 14C-docetaxel, respectively. However, at higher concentrations (10 - 40 μM) significantly more 14C-cabazitaxel bound to the microtubules than 14C-docetaxel, with an apparent higher affinity. We also analyzed the effects of cabazitaxel and docetaxel on in vitro dynamic instability of microtubules assembled from unfractionated and βIII-depleted tubulin. We found that 100 nM cabazitaxel or docetaxel stabilized the dynamics of microtubules assembled from unfractionated tubulin similarly. However, cabazitaxel suppressed the dynamics of microtubules containing βIII tubulin more strongly than docetaxel. For example, 100 nM cabazitaxel suppressed the shortening rate of unfractionated microtubules by 49% and the shortening rate of βIII-depleted microtubules by only 31%. The overall suppression of dynamicity of microtubules assembled from unfractionated βIII-rich tubulin was 49% vs. 33% for βIII-depleted microtubules. In contrast with cabazitaxel, 100 nM docetaxel suppressed the shortening rate of microtubules assembled from unfractionated and βIII-depleted tubulin by 48% and 52%, respectively. The overall suppression of dynamicity of microtubules assembled from unfractionated βIII-rich tubulin was 56% vs. 50% for βIII-depleted microtubules. Experiments to examine the binding of cabazitaxel and docetaxel to microtubules assembled from βIII-depleted tubulin are ongoing. Our results indicate that the more potent activity of cabazitaxel for microtubules containing βIII tubulin may contribute to its superior efficacy in prostate tumors overexpressing βIII tubulin as compared with that of docetaxel. Supported by a research award from Sanofi Oncology. Citation Format: Leslie Wilson, Olga Azarenko, Gregoriy Smiyun, Herbert Miller, Mary Ann Jordan. Increased sensitivity of βIII tubulin-containing microtubules to cabazitaxel compared with docetaxel: A possible role of βIII tubulin in specifying good efficacy of cabazitaxel for docetaxel-resistant tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1666. doi:10.1158/1538-7445.AM2015-1666