Unintentional weakness of cancers: the MEK-ERK pathway as a double-edged sword

Recent advances in molecular targeted therapies have greatly improved treatment outcomes for cancers driven by oncogenic mutations. Despite initial and dramatic clinical responses, tumors eventually acquire resistance to these targeted therapies, showing flexible and diverse responses. Interestingly, cancer cells sometimes overadapt to the drug treatment environment, leading to a state in which cancer cells cannot survive without the drug. This interesting phenomenon (often called "drug dependency" or "drug addiction") is exemplified in preclinical acquired resistance models of BRAF-mutated melanoma treated with vemurafenib and EGFR-mutated lung cancer treated with EGFR tyrosine kinase inhibitors. A number of intriguing parallels in drug-addicted cancers became apparent in a comparison of the two models: (i) overexpression of driver oncogenes as causes of acquired resistance; (ii) overexpression of driver oncogenes causing MEK-ERK hyperactivation under drug-free conditions; (iii) hyperactivation of the MEK-ERK pathway as critical to this drug addiction phenomenon; (iv) ongoing dependence on the oncogenic driver; and (v) morphologic changes in resistant cells under drug-free conditions. This Perspective article not only focuses on this interesting and peculiar phenomenon but also discusses weapon strategies to exploit this unintentional weakness of cancers.