The central role of the BRAF-MEK-ERK pathway in controlling cell fate has made this pathway a primary target for deregulated activation in cancer. BRaf is activated by Ras proteins allowing Ras oncogenes to constitutively activate the pathway. Activating BRaf mutations are also frequent in several cancers, being the most common oncogenic mutation in thyroid carcinoma and melanoma. There are currently two inhibitors, vemurafenib and dabrafenib, approved for treatment of malignant melanoma having activating BRaf mutations. Concurrent administration of BRAF inhibitor and MEK inhibitor (trametinib) is significantly more active in patients with BRAF mutant melanoma than either single agent alone, but progression to resistance ultimately occurs by different mechanisms that increase the activation of ERK. Such adaptive changes in tumor cell signaling networks allows bypass of targeted oncoprotein inhibition. This is true with targeted inhibitors for BRaf and MEK as well as specific inhibitors for AKT, mTOR and many receptor tyrosine kinases such as EGFR and HER2. It is this adaptive response to targeted kinase inhibitors that contributes to the failure of single agent kinase inhibitors to have durable responses. This failure is seen in virtually all cancers treated with single agent kinase inhibitors, most of which are not as dependent on a single signaling pathway such as BRaf-MEK-ERK in melanoma. Thus, understanding the breadth of adaptive reprogramming responses to specific targeted kinase inhibition will be critical to develop appropriate combination therapies for durable clinical responses
