Abstract LB-228: Resistance to BRAF inhibition in BRAF V600E colorectal cancer is associated with PI3K/AKT activation and hypermethylation

Abstract

Abstract BRAF V600E mutation has been detected in approximately 8% of metastatic colorectal cancers (CRC) and results in constitutive activation of BRAF and the downstream MEK-ERK1/2 pathway. PLX4032, a selective inhibitor of BRAF V600E, has shown meaningful activity in melanoma (greater than 70% response) but only modest activity in BRAF V600E CRC (5%). The resistance of BRAF V600E CRC to BRAF inhibitors has been poorly understood. To explore these differences, BRAF V600E CRC cell lines were compared to BRAF V600E melanoma cell lines by reverse-phase proteomic array, which demonstrated higher levels of phospho-AKT and associated downstream pathway activation in CRC cell lines. Both colorectal cancer cells inherently resistant to PLX4720, an analog of PLX4032, and resistant subclones that were derived from BRAF V600E positive cells, showed increased expression of phospho-AKT and phospho-GSK3β, despite the sustained ability of PLX4720 to effectively inhibit MEK signaling. Silencing of PTEN, which in turn induced AKT activation, desensitized cells to the BRAF inhibitor. Furthermore, combined treatment of PLX4720 with two different PI3K inhibitors, LY294002 and GDC-0941, synergistically inhibited growth in a panel of BRAF V600E cells through both apoptosis and G1 phase arrest. It has been previously shown that BRAF V600E colorectal cancers are commonly associated with the CpG island methylation phenotype (CIMP), which is associated with epigenetically diminished PTEN expression. Combination treatment with the demethylating agent azacitadine and PLX4032 was also synergistic in cell lines with intact but suppressed PTEN, and was associated with PTEN re-expression. This was not seen in non-CIMP or PTEN BRAF V600E cell lines. Thus, colorectal cancer cells appear to escape BRAF targeting through a mechanism of increased PI3K/AKT activity, providing evidence to support investigation of new combination therapies with PI3K/AKT pathway inhibitors or demethylating agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-228. doi:10.1158/1538-7445.AM2011-LB-228