Resistance to BRAF Inhibition in BRAF-Mutant Colon Cancer Can Be Overcome with PI3K Inhibition or Demethylating Agents

Abstract

Purpose Vemurafenib, a selective inhibitor of BRAFV600 has shown significant activity in BRAFV600 melanoma, but not in the <10% of metastatic BRAFV600 colorectal cancers (CRC), suggesting that studies of the unique hypermethylated phenotype and concurrent oncogenic activation of BRAFmut CRC may provide combinatorial strategies Experimental Design We performed comparative proteomic analysis of BRAFV600E melanoma and CRC cell lines, followed by correlation of PI3K pathway activation and sensitivity to the vemurafenib-analog PLX-4720. Pharmacologic inhibitors and siRNA were used in combination with PLX4720 to inhibit PI3K and methyltrasnferase in cell lines and murine models. Results Compared to melanoma, CRC lines demonstrate higher levels of PI3K/AKT pathway activation. CRC cell lines with mutations in PTEN or PIK3CA were less sensitive to growth inhibition by PLX4720 (P=0.03), and knockdown of PTEN expression in sensitive CRC cells reduced growth inhibition by the drug. Combined treatment of PLX4720 with PI3K inhibitors caused synergistic growth inhibition in BRAF-mutant CRC cells with both primary and secondary resistance. In addition, methyltransferase inhibition was synergistic with PLX4720 and decreased AKT activation. In vivo, PLX4720 combined with either inhibitors of AKT or methyltransferase demonstrated greater tumor growth inhibition than PLX4720 alone. Clones with acquired resistance to PLX4720 in vitro demonstrated PI3K/AKT activation with EGFR or KRAS amplification. Conclusions We demonstrate that activation of the PI3K/AKT pathway is a mechanism of both innate and acquired resistance to BRAF inhibitors in BRAFV600E CRC, and suggest combinatorial approaches to improve outcomes in this poor prognosis subset of patients.

Bcl-2 stabilization by paxillin confers 5-fluorouracil resistance in colorectal cancer

“…K-ras and BRAF mutations were linked with activation of AKT and ERK signaling pathway. [19][20][21][22][23][24] Therefore, the involvement of AKT and ERK activation in the increase of Bcl-2 expression can be expected in colorectal cancer. All cell lines used in this study harbored either K-ras mutation or BRAF mutation (Supplementary Figure 7a).…”

Assessment of BRAF V600E Status in Colorectal Carcinoma: Tissue-Specific Discordances between Immunohistochemistry and Sequencing

“…In vitro and in vivo xenograft models have shown that colorectal carcinoma that harbor BRAF V600E may be sensitive to proteasome inhibitors (14). Although colorectal carcinomas have very limited response to BRAF V600E inhibitor monotherapy (15,16), combinatorial strategies are currently under investigation to improve response, and outcome for these patients (17,18).…”

RAS interaction with PI3K p110α is required for tumor-induced angiogenesis

“…Drugs available against other RAS-regulated signaling proteins, such as BRAF and MEK, have also shown moderate efficacy, although this is limited due to rapid development of acquired resistance, in many cases due to aberrant activation of parallel signaling pathways that in turn stimulate the PI3K pathway (8)(9)(10). Moreover, RAS itself has proved impossible to target effectively to date (11,12).…”