Abstract 5753: RAF inhibitors prime wild-type RAF to activate the MAPK pathway and enhance growth

Abstract

Abstract Activating mutations in KRAS or B-RAF are found in over 30% of all human tumors and targeting this pathway could have broad therapeutic impact. Small molecule ATP-competitive RAF kinase inhibitors have potent anti-tumor effects on B-RAFV600E tumors but, in contrast to MEK inhibitors, are not potent against RAS mutant tumor models, despite RAF functioning as a key effector downstream of RAS and upstream of MEK. In this study we show that ATP-competitive RAF inhibitors have two opposing mechanisms of action depending on cellular context. In B-RAFV600E tumors, RAF inhibitors effectively block the MAPK signaling pathway and decrease tumor growth. Surprisingly, in KRAS mutant and RAS/RAF wildtype tumors, RAF inhibitors activate the RAF/MEK/ERK pathway in a RAS-dependent manner, thus enhancing tumor growth in some xenograft models. Inhibitor binding activates wildtype RAF isoforms by inducing dimerization, membrane localization and interaction with RAS-GTP. These events occur independently of kinase inhibition and are, rather, linked to direct conformational effects of inhibitors on the RAF kinase domain. Based on these findings, we demonstrate that ATP-competitive kinase inhibitors can have opposing functions as inhibitors or activators of signaling pathways, depending upon cellular context. In addition, this work provides new insights into the therapeutic use of ATP-competitive RAF inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5753.