Abstract P5-03-01: Therapeutic targeting of Rac GTPases in ER+ and HER2+ breast cancer

Abstract

Abstract As crucial regulators of cell motility, survival, and proliferation, Rac GTPases (Rac1/2/3) have been implicated in cancer. We previously found that P-REX1, a guanine nucleotide exchange factor that activates Rac GTPases, forms a PI3K-driven positive feedback loop to promote activation of Rac enzymes, receptor tyrosine kinases, PI3K/AKT/mTOR, and MEK/ERK in ER+ breast cancer cells. Importantly, inhibition of Rac GTPases with a small molecule (EHT1864) simultaneously suppressed activation of both the PI3K/AKT/mTOR and MEK/ERK pathways. While co-targeting of the PI3K/AKT/mTOR and MEK/ERK pathways with drug combinations has anti-tumor activity in preclinical models and is being tested in ongoing clinical trials, targeting Rac as a common upstream signaling node may be a more efficient means of simultaneously targeting these two oncogenic pathways. In breast cancer cell lines, treatment with EHT1864 decreased activation of AKT, mTOR, p70S6K, and S6 in a dose-dependent manner. Pulldown of activated Rac from cell lysates revealed that GTP-bound Rac1 and/or Rac3 bind MEK1/2, ERK1/2, p70S6K, S6, Raptor, Rictor, and mTOR. Temporal analysis indicated that EHT1864 inhibits phosphorylation of p70S6K (an mTORC1 substrate) before AKT is inhibited, suggesting that Rac may directly activate p70S6K and/or mTORC1. Mining of sensitivity data from ∼700 cell lines to a panel of 138 drugs (COSMIC/GDSC database) revealed that the growth-suppressive effects of EHT1864 correlate most strongly with growth-suppressive patterns induced by a p70S6K inhibitor, supporting the notion that Rac directly activates p70S6K. Additionally, EHT1864 treatment dose-dependently decreased phosphorylation of ERK1/2 and MEK1/2, and induced apoptosis in breast cancer cell lines. In a panel of 16 breast cancer lines, cells with activating mutations in PIK3CA (encodes the p110-alpha subunit of PI3K) exhibit increased EHT1864 sensitivity. Pharmacokinetic analysis of EHT1864 (100 mg/kg, i.p.) in plasma in NSG mice revealed that drug was present at >50 uM for 1 h after administration. In mice bearing s.c. ER+/HER2+ BT-474 breast cancer xenografts, EHT1864 (100 mg/kg BID) significantly slowed tumor growth compared to vehicle control. Thus, therapeutically targeting Rac could be an effective means of reducing cancer cell proliferation and survival by simultaneously suppressing both the PI3K/AKT/mTOR and MEK/ERK signaling pathways in ER+ and HER2+ breast cancers. Citation Format: Hampsch RA, Shee K, Miller TW. Therapeutic targeting of Rac GTPases in ER+ and HER2+ breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-03-01.