Abstract 4667: A synergistic combination strategy for optimal inhibition of colon cancer stem cells: Simultaneous inhibition of Insulin-like growth factor-1 receptor-AKT-mammalian target of rapamycin axis and the mevalonate-isoprenoid biosynthetic pathway
Authors: Sharon Chetna , Boothello Rio , Vera Alberto , Patel Bhaumik B. 
Published: August 2019

Abstract
Introduction: Targeting cancer stem cells (CSCs) is an attractive new paradigm to achieve long-term remission/cure. The mevalonate-isoprenoid biosynthetic (MIB) pathway is critical in the regulation of colonic CSCs. However, feedback activation of SREBP2 following inhibition of the MIB pathway reduces the efficiency of the latter strategy. We described that Insulin-like growth factor-1 receptor-AKT-mammalian target of rapamycin (InAT) axis inhibits CSCs via inhibition of the MIB pathway at a level distinct from the target of statin - HMGCR, while simultaneously inhibiting SREBP2. Hence, we hypothesized that combining a MIB pathway inhibitor with the InAT axis inhibition will result in optimal inhibition of CSCs.
Methods: Four possible combinations with the MIB pathway inhibitors (Simvastatin or zoledronate) and an InAT axis inhibitors (OSI-906) or Everolimus were tested against spheroids from 12 colorectal cancer cells. Synergistic effect of the combinations was determined using Chou Talalay method (Calcusyn). Most promising combinations were examined in an in vivo model of Dual high CSCs (CD133+/CXCR4+) induced xenografts to assess tumor volume, and ex vivo CSC phenotype (spheroid formation and CSC marker expression) in xenograft-derived cells.
Results: OSI-906 + Simvastatin, as well as Everolimus + simvastatin, showed a robust synergistic effect (Combination index &lt;0.75) in a majority of colon cancer cells which was independent of mutation in PI3K/mTOR or RAS pathways. In fact, both the combinations showed a significantly higher inhibition of Dual high CSCs-induced xenograft volume as well as ex-vivo spheroid formation (2°→4°) and CSC markers (CD133, DCLK1, LGR etc.) than either agents alone or vehicle controls in two model of colon cancer HCT-116 (MSI high, KRAS mut), HT29 (MSS, KRAS mut). Importantly, Simvastatin treatment failed to inhibit SREBP2 levels. However, InAT axis inhibitors caused a significant inhibition of SREBP2 and the MIB pathway genes. In fact, inhibition of the MIB pathway at the level of isopentenyl-diphosphate delta isomerase 1 (IDI1) was critical for simultaneous inhibition of the MIB pathway and InAT axis to attenuate CSC self-renewal as supplementation with DMAPP, and FPP, but not mevalonate or IPP, reversed the effect of the combination on CSCs.
Conclusion: Orally deliverable and combinations of the FDA-approved drugs targeting the MIB pathway and the InAT axis cause optimum inhibition of colonic CSCs through downregulation of SREBP2, achieving maximal inhibition of the MIB pathway.
Note: This abstract was not presented at the meeting.
Citation Format: Chetna Sharon, Rio Boothello, Alberto Vera, Bhaumik B. Patel. A synergistic combination strategy for optimal inhibition of colon cancer stem cells: Simultaneous inhibition of Insulin-like growth factor-1 receptor-AKT-mammalian target of rapamycin axis and the mevalonate-isoprenoid biosynthetic pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4667.