EGFR-mediated re-activation of MAPK signaling contributes to insensitivity of BRAF mutant colorectal cancers to RAF inhibition with vemurafenib.

BRAF mutations occur in 10-15% of colorectal cancers (CRCs) and confer adverse outcome. While RAF inhibitors such as vemurafenib (PLX4032) have proven effective in BRAF mutant melanoma, they are surprisingly ineffective in BRAF mutant CRCs, and the reason for this disparity remains unclear. Compared to BRAF mutant melanoma cells, BRAF mutant CRC cells were less sensitive to vemurafenib, and P-ERK suppression was not sustained in response to treatment. Although transient inhibition of phospho-ERK by vemurafenib was observed in CRC, rapid ERK re-activation occurred through EGFR-mediated activation of RAS and CRAF. BRAF mutant CRCs expressed higher levels of phospho-EGFR than BRAF mutant melanomas, suggesting that CRCs are specifically poised for EGFR-mediated resistance. Combined RAF and EGFR inhibition blocked reactivation of MAPK signaling in BRAF mutant CRC cells and markedly improved efficacy in vitro and in vivo. These findings support evaluation of combined RAF and EGFR inhibition in BRAF mutant CRC patients. BRAF valine 600 (V600) mutations occur in 10% to 15% of colorectal cancers, yet these tumors show a surprisingly low clinical response rate (~5%) to selective RAF inhibitors such as vemurafenib, which have produced dramatic response rates (60%–80%) in melanomas harboring the identical BRAF V600 mutation. We found that EGFR-mediated MAPK pathway reactivation leads to resistance to vemurafenib in BRAF-mutant colorectal cancers and that combined RAF and EGFR inhibition can lead to sustained MAPK pathway suppression and improved efficacy in vitro and in tumor xenografts.

Corcoran RB ，Ebi H ，Turke AB ，Coffee EM ，Nishino M ，Cogdill AP ，Brown RD ，Della Pelle P ，Dias-Santagata D ，Hung KE ，Flaherty KT ，Piris A ，Wargo JA ，Settleman J ，Mino-Kenudson M ，Engelman JA ... -  《-》

Antitumor activity of BRAF inhibitor vemurafenib in preclinical models of BRAF-mutant colorectal cancer.

The protein kinase BRAF is a key component of the RAS-RAF signaling pathway which plays an important role in regulating cell proliferation, differentiation, and survival. Mutations in BRAF at codon 600 promote catalytic activity and are associated with 8% of all human (solid) tumors, including 8% to 10% of colorectal cancers (CRC). Here, we report the preclinical characterization of vemurafenib (RG7204; PLX4032; RO5185426), a first-in-class, specific small molecule inhibitor of BRAF(V600E) in BRAF-mutated CRC cell lines and tumor xenograft models. As a single agent, vemurafenib shows dose-dependent inhibition of ERK and MEK phosphorylation, thereby arresting cell proliferation in BRAF(V600)-expressing cell lines and inhibiting tumor growth in BRAF(V600E) bearing xenograft models. Because vemurafenib has shown limited single-agent clinical activity in BRAF(V600E)-mutant metastatic CRC, we therefore explored a range of combination therapies, with both standard agents and targeted inhibitors in preclinical xenograft models. In a BRAF-mutant CRC xenograft model with de novo resistance to vemurafenib (RKO), tumor growth inhibition by vemurafenib was enhanced by combining with an AKT inhibitor (MK-2206). The addition of vemurafenib to capecitabine and/or bevacizumab, cetuximab and/or irinotecan, or erlotinib resulted in increased antitumor activity and improved survival in xenograft models. Together, our findings suggest that the administration of vemurafenib in combination with standard-of-care or novel targeted therapies may lead to enhanced and sustained clinical antitumor efficacy in CRCs harboring the BRAF(V600E) mutation.

Yang H ，Higgins B ，Kolinsky K ，Packman K ，Bradley WD ，Lee RJ ，Schostack K ，Simcox ME ，Kopetz S ，Heimbrook D ，Lestini B ，Bollag G ，Su F ... -  《-》

BGB-283, a Novel RAF Kinase and EGFR Inhibitor, Displays Potent Antitumor Activity in BRAF-Mutated Colorectal Cancers.

Oncogenic BRAF, which drives cell transformation and proliferation, has been detected in approximately 50% of human malignant melanomas and 5% to 15% of colorectal cancers. Despite the remarkable clinical activities achieved by vemurafenib and dabrafenib in treating BRAF(V600E) metastatic melanoma, their clinical efficacy in BRAF(V600E) colorectal cancer is far less impressive. Prior studies suggested that feedback activation of EGFR and MAPK signaling upon BRAF inhibition might contribute to the relative unresponsiveness of colorectal cancer to the first-generation BRAF inhibitors. Here, we report characterization of a dual RAF kinase/EGFR inhibitor, BGB-283, which is currently under clinical investigation. In vitro, BGB-283 potently inhibits BRAF(V600E)-activated ERK phosphorylation and cell proliferation. It demonstrates selective cytotoxicity and preferentially inhibits proliferation of cancer cells harboring BRAF(V600E) and EGFR mutation/amplification. In BRAF(V600E) colorectal cancer cell lines, BGB-283 effectively inhibits the reactivation of EGFR and EGFR-mediated cell proliferation. In vivo, BGB-283 treatment leads to dose-dependent tumor growth inhibition accompanied by partial and complete tumor regressions in both cell line-derived and primary human colorectal tumor xenografts bearing BRAF(V600E) mutation. These findings support BGB-283 as a potent antitumor drug candidate with clinical potential for treating colorectal cancer harboring BRAF(V600E) mutation.

Tang Z ，Yuan X ，Du R ，Cheung SH ，Zhang G ，Wei J ，Zhao Y ，Feng Y ，Peng H ，Zhang Y ，Du Y ，Hu X ，Gong W ，Liu Y ，Gao Y ，Liu Y ，Hao R ，Li S ，Wang S ，Ji J ，Zhang L ，Li S ，Sutton D ，Wei M ，Zhou C ，Wang L ，Luo L ... -  《-》

Unresponsiveness of colon cancer to BRAF(V600E) inhibition through feedback activation of EGFR.

Prahallad A ，Sun C ，Huang S ，Di Nicolantonio F ，Salazar R ，Zecchin D ，Beijersbergen RL ，Bardelli A ，Bernards R ... -  《-》

Discovery of a novel pan-RAF inhibitor with potent anti-tumor activity in preclinical models of BRAF(V600E) mutant cancer.

BRAF mutations, especially BRAF V600E, are a frequent occurrence in malignant melanomas. The BRAF inhibitors are used as the care standard for BRAF-mutant metastatic melanomas. However, melanomas rapidly develop resistance to BRAF inhibitors after a median response duration of 6months, and the subsequent rapid development of cutaneous toxicity is enhanced by the paradoxical activation of CRAF. In this study, we discovered a potent and selective pan-RAF inhibitor: INU-152. The goal of this study was to investigate whether the inhibition of pan-RAF with INU-152 completely disrupts the MAPK pathway in cancer cells bearing BRAF or RAS mutations. Using a structure-based molecular modeling, we discovered INU-152, which is a potent and selective pan-RAF inhibitor. In kinase assays against RAF proteins, INU-152 exhibited a potent effect against RAF isoforms. INU-152 was tested for its inhibitory effect on the growth of human cancer cells bearing BRAFV600E. To study in vivo effects, INU-152 was administered using human melanoma and colorectal cancer xenograft models. To explore INU-152's potential as a prospective drug candidate, pharmacokinetic studies and toxicity tests were performed using mice. To inhibit and suppress paradoxical activation in mutant RAS cancer cells completely, it is important for RAF inhibitors to exhibit potent inhibitory activities against RAF isoforms. INU-152 inhibits all RAF isoforms and inhibits MAPK pathways in mutant BRAF cells. More importantly, INU-152 exhibits minimal paradoxical pathway activation in melanoma cells with mutant RAS. INU-152 exhibits anti-tumor activities in xenograft models carrying BRAF mutations.

Hong SP ，Ahn SK 《-》