Abstract 3597: Pazopanib potentiates paclitaxel-induced mitotic catastrophe in association with inhibition of aurora kinases

Abstract

Abstract Background: We recently reported a 49% RECIST response rate in conjunction with a phase 2 clinical trial of the multi-targeted kinase inhibitor pazopanib in differentiated thyroid cancers (DTC; Lancet Oncology 2010, 11:962-72), prompting a search for synergistic pazopanib-containing doublets in thyroid cancer cell lines. Methods and Results: Among all explored pazopanib-containing combinations, paclitaxel and pazopanib produced the greatest combined cytotoxic effects (continuous exposure pazopanib, 24 h paclitaxel; Combination Index, CI, at IC90=0.3). Interestingly, analogous synergy resulted when combining pazopanib with other antimicrotubule agents including other taxanes (docetaxel), Vinca alkyloids (vincristine) and epothilones (ixabepilone) – suggesting a class effect. Further, we found this combination to be synergistic in multiple cancer cell lines including DTC, medullary and anaplastic thyroid cancer lines, and in the A549 lung cancer line. In pursuit of underlying mechanism(s), we noted that the combination produced a greater fraction of cells in G2/M phase of the cell cycle and increased apoptosis in comparison to paclitaxel alone, prompting us to examine whether pazopanib might potentiate the effects of paclitaxel on mitosis. Analysis of cell fate resulting from pazopanib and paclitaxel alone and in combination using time-lapsed video microscopy demonstrated that as low as 2.5 μM pazopanib augmented paclitaxel-induce mitotic catastrophe by 2.4 fold, while inducing no appreciable cell death alone. As pazopanib is somewhat promiscuous in inhibition of kinases, we hypothesized that observed combined effects with antimicrotubule agents might be attributed to its inhibition of cell cycle by targeting CDKs or mitosis by targeting aurora kinases. Inhibition of CDKs 1 and 2 required pazopanib concentrations 1000-times those required to inhibit its primary kinase targets, VEGF-Rs; whereas aurora kinases A and B were inhibited at much lower concentrations (IC50s, 500-1000 nM). Hence, we pursued shRNA aurora A knockdown experiments: potentiation of pazopanib effects were observed in knockdown clones, suggesting that pazopanib single-agent effects are at least in part mediated via aurora A inhibition. Moreover, augmentation of paclitaxel effects were achieved in knockdown clones that rivaled those observed for the pazopanib/paclitaxel combination. Additionally, pazopanib/paclitaxel synergy was reduced in aurora A knockdown clones (CI at IC80: vector<0.5, knockdown clones >1.0), indicating that aurora A inhibition by pazopanib is contributory to paclitaxel/pazopanib synergy. Conclusions: Pazopanib combines with antimicrotubule agents to produce cytotoxic synergy associated with augmentation of mitotic catastrophe that appears at least in part mediated by its ability to inhibit aurora kinases. Supported in part by CA125750. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3597. doi:10.1158/1538-7445.AM2011-3597