Abstract 239: The dual PI3K/mTOR inhibitor VS-5584 preferentially targets cancer stem cells in vitro and in vivo.
Authors: Xu Qunli , Kolev Vihren N. , Wright Quentin G. , Weaver David T. , Padval Mahesh V. , Pachter Jonathan A. 
Published: August 2014

The PI3 kinase/mTOR pathway plays a central role in cancer cell proliferation and survival. Depending on the mode of pathway activation, different PI3K isoforms and mTOR complexes have been shown to play essential roles in oncogenesis. VS-5584 potently and selectively inhibits PI3K/mTOR signaling with equipotency against all four human Class I PI3K isoforms and the mTOR kinase. VS-5584 also displays potent anticancer activities in a number of human cancer xenograft models in vivo either as a single agent or in combination.

In a high-throughput screen designed to identify compounds that selectively target cancer stem cells (CSCs), the PI3K/mTOR pathway was found to be an important target. The effect of the PI3K/mTOR inhibitor VS-5584 was evaluated in multiple CSC assays. Using HMLE immortalized human mammary cells driven through epithelial-mesenchymal transition (EMT) by knock down of E-cadherin, the PI3K/mTOR inhibitor VS-5584 selectively inhibited proliferation of CSC-like mesenchymal HMLEs. Accordingly, VS-5584 preferentially decreased the proportion of CD44Hi/CD24Lo CSC-like cells in Ras-transformed HMLER cells in contrast to paclitaxel that increased the proportion of CD44Hi/CD24Lo CSCs in the same model. CSCs express higher levels of aldehyde dehydrogenase (ALDH), thus an ALDEFLUOR assay that measures the enzymatic activity of ALDH was used to assess CSCs. In SUM159 triple negative breast cancer cells, VS-5584 dose-dependently decreased the percentage of ALDEFLUOR-positive cells, in contrast to paclitaxel which again enriched CSCs in the same assay. Another characteristic of CSCs is the enhanced ability to exclude cytotoxic agents. This drug-resistant CSC population, termed the side population, can be identified by virtue of exclusion of Hoechst 33342 dye. In SUM159 cells, VS-5584 effectively eliminated the CSC side population, while paclitaxel increased this population. The effect of VS-5584 on cancer stem cells was further evaluated in the MDA-MB-231 triple negative breast cancer model in vivo. After one week of oral dosing, VS-5584 reduced the proportion of cancer stem cells in tumors 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. Lastly, VS-5584 caused tumor regression in a primary triple negative breast cancer model, likely as a result of potent inhibitory activities against both cancer stem cells and cancer cells in general.

These data taken together provide strong rationale for the clinical development of VS-5584 for breast and other cancers to achieve durable clinical responses.

Citation Format: Qunli Xu, Vihren N. Kolev, Quentin G. Wright, David T. Weaver, Mahesh V. Padval, Jonathan A. Pachter. The dual PI3K/mTOR inhibitor VS-5584 preferentially targets cancer stem cells in vitro and in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 239. doi:10.1158/1538-7445.AM2013-239