Abstract B52: Dual mTORC1/2 and PI3K inhibitor VS-5584 preferentially targets cancer stem cells.

Abstract

Abstract Cancer stem cells (CSCs) constitute a subpopulation of cancer cells with tumor-initiating capability. Because CSCs are generally resistant to chemotherapy and drive tumor metastasis and recurrence, they represent an important challenge in the clinical treatment of cancer. In a high-throughput screen for compounds that preferentially target CSCs, several inhibitors of the PI3K/mTOR pathway were identified, suggesting the importance of this signaling pathway for CSC biology. VS-5584 is a highly potent and selective mTORC1/mTORC2/PI3K inhibitor with equipotency against all four human Class I PI3K isoforms and the mTOR kinase. We demonstrate that VS-5584 preferentially targets CSCs in vitro and in vivo in multiple orthogonal assays based on known characteristics of CSCs. Cancer stem cells express higher levels of aldehyde dehydrogenase (ALDH) than bulk tumor cells, thus an Aldefluor assay which measures ALDH enzymatic activity is used to quantify CSCs. VS-5584 decreased the percentage of Aldefluor-positive cells across multiple breast cancer cell lines. In SUM159 triple negative breast cancer cells, VS-5584 dose-dependently induced apoptosis preferentially in Aldefluor-positive cells as measured by both Annexin V and Caspase 3/7 assays. In contrast, paclitaxel induced more apoptosis in Aldefluor-negative cells and as a consequence increased the proportion of CSCs. Another characteristic of CSCs is enhanced drug efflux compared to non-CSCs, thus a Hoechst 33342 dye-exclusion assay, also known as a side population (SP) assay, is used to identify CSCs. In the H69 small cell lung cancer model, VS-5584 effectively eliminated the CSC side population, while the cytotoxic agents cisplatin and etoposide increased the CSC side population. To further evaluate the effect of VS-5584 on CSCs, we utilized ex vivo culture of surgically excised primary cancer tissue. Ex vivo treatment of breast tumors with VS-5584 decreased the proportion of Aldefluor-positive and CD44hi/CD24lo CSCs. Similarly, VS-5584 reduced the proportion of Aldefluor-positive and CD44hi/CD117hi CSCs in primary ovarian cancer tumor specimens. Significantly, oral dosing with VS-5584 was found to substantially reduce CSCs in vivo in the MDA-MB-231 triple negative and MCF-7 ER+ breast cancer xenograft models as evidenced by decreases in the percentage of Aldefluor-positive cells, tumorsphere-forming efficiency, and tumor-initiating capability in an in vivo limiting dilution re-implantation assay. Consistent with its strong suppression of CSCs, VS-5584 effectively impeded tumor regrowth following cisplatin treatment in a H69 small cell lung cancer xenograft model. The preferential targeting of CSCs thus provides a strong rationale for the clinical development of VS-5584 with the goal of achieving a more durable response in cancer patients. Phase 1 testing of VS-5584 is planned to initiate in the fourth quarter of 2013. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B52. Citation Format: Vihren N. Kolev, Quentin G, Wright, Christian M. Vidal, Jennifer E. Ring, Irina M. Shapiro, David T. Weaver, Mahesh V. Pavdal, Jonathan A. Pachter, Qunli Xu. Dual mTORC1/2 and PI3K inhibitor VS-5584 preferentially targets cancer stem cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B52.