Abstract A36: Combined inhibition of PI3K isoforms and mTOR kinase is critical for cancer stem cell inhibition by VS-5584

Abstract

Abstract Cancer stem cells (CSCs) have been implicated in resistance to anticancer therapies, disease recurrence, and metastasis, which represent major challenges in the clinical management of cancer. Therapeutic targeting of CSCs thus holds promise in addressing these challenges. VS-5584 is a selective dual PI3K/mTOR kinase inhibitor that potently inhibits mTORC1, mTORC2 and all four Class I PI3K isoforms. We report here that VS-5584 is up to 30-fold more potent at inhibiting the proliferation and survival of CSCs than non-CSCs in breast cancer cell lines using multiple orthogonal CSC assays. Moreover, VS-5584 preferentially induced apoptosis in Aldefluor-positive CSCs relative to Aldefluor-negative non-CSCs as measured by Annexin V and Caspase 3/7 assays. In contrast, paclitaxel induced more apoptosis in non-CSCs than CSCs cells. VS-5584 also preferentially diminished CSCs in human breast and small cell lung cancer xenograft models in vivo, as evidenced by marked reduction of tumor-initiating capacity in an in vivo limiting dilution re-implantation assay. Likewise, ex vivo VS-5584 treatment of surgically removed breast and ovarian patient tumors preferentially reduced CSC populations. In contrast, cytotoxic chemotherapeutic agents such as paclitaxel and cisplatin preferentially eliminate non-CSCs and thus enrich the CSC population. Interestingly, preferential targeting of CSCs requires inhibition of multiple components of the PI3K/mTOR pathway; simultaneous knockdown of PI3Kα, PI3Kβ, and mTOR by siRNA phenocopied the effect of VS-5584 and exhibited the strongest preferential targeting of CSCs, while knocking down of individual PI3K isoform or mTOR failed to do so. Consistent with its strong suppression of CSCs, VS-5584 effectively delayed tumor regrowth following chemotherapy in small cell lung cancer xenograft models including a patient-derived primary tumor model. The preferential targeting of CSCs thus prompts a new paradigm for testing VS-5584 in the clinic: clinical trials designed with CSC-directed endpoints may facilitate demonstration of the therapeutic benefit of VS-5584 at well-tolerated doses and thus enable an optimal therapeutic window. Furthermore, suppression of CSCs by VS-5584 in combination with other agents that target the bulk tumor may represent an appealing strategy to achieve more durable responses for cancer patients. Citation Format: Vihren N. Kolev, Quentin G. Wright, David T. Weaver, Mahesh V. Padval, Jonathan A. Pachter, Qunli Xu. Combined inhibition of PI3K isoforms and mTOR kinase is critical for cancer stem cell inhibition by VS-5584. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr A36.