Braf Mutation Status, But Not Erk Activation, Is Predictive Of Response To The Mek Inhibitor Azd6244 In Colorectal Cancer

Introduction: KRAS mutations are detected in 50% and BRAF mutations in 10% of colorectal cancers (CRC). Although considerable effort has been made to target Ras for cancer treatment, anti-Ras therapies remain elusive. Treatment strategies have focused on inhibition of the Ras effector pathway, the Raf-MEK-ERK mitogen-activated protein kinase cascade, with inhibitors of MEK currently under clinical evaluation. AZD6244, currently in phase II clinical trials, is one such inhibitor. Lessons learned from the success of targeted therapies such as anti-EGFR and imatinib demonstrate the importance of identifying biomarkers that will identify patients who will best respond to specific therapies. Therefore, the aims of this study were to determine if 1) KRAS or BRAF mutation status or 2) Raf-MEK-ERK pathway activation is predictive of response to AZD6244. Methods: We examined a panel of 13 CRC cell lines for KRAS and BRAF mutation status. Cell lines were treated for 24 h with increasing doses of AZD6244 or vehicle control and analyzed for inhibition of ERK activation. In order to determine whether AZD6244 inhibited the transformed growth properties of CRC cell lines, 5×103 cells per 6 well plate were suspended in soft agar and treated with increasing doses of AZD6244. Colonies >30 cells were scored after 4 weeks. Results: Variable levels of ERK activation was found in 12/13 CRC lines in the absence of treatment. Surprisingly however, the ability of AZD6244 to inhibit ERK activation was not predictive of its ability to inhibit anchorage-independent growth. Significant growth inhibition was seen in all BRAF mutant lines (4/4) and 2/3 KRAS/BRAF wild-type lines. No inhibition of growth by AZD6244 was seen in 4/4 KRAS mutant lines, despite effective inhibition of ERK activation. Conclusions: Our findings demonstrate that BRAF mutation status may be a biomarker for AZD6244 response. However, neither KRAS mutation status or ERK activation will be feasible biomarkers, suggesting that in non-BRAF mutant cell lines, AZD6244 acts via an ERK-independent pathway. Therefore, we suggest that determining gene expression profiles correlating with AZD6244 sensitivity will not only elucidate this AZD6244-sensitive, but ERK-independent pathway, but also will be critical for further selecting patients with CRC who may be candidates for AZD6244 therapy.