Advances in the development of cancer therapeutics directed against the RAS-mitogen-activated protein kinase pathway.

Among mammalian mitogen-activated protein kinase (MAPK) signaling cascades, the extracellular signal-related kinase (ERK) pathway has received the most attention in the oncology drug discovery arena. By virtue of its central role in promoting proliferation, survival, and metastasis, this pathway directly affects both the formation and progression of human tumors. The identification of non-ATP-competitive inhibitors of the MAPK kinase MAPK/ERK kinase (MEK) resulted in the first demonstration that the ERK pathway could be effectively shut down in a highly selective fashion. Subsequent discovery of the oncogenic nature of B-raf kinase led to the escalation of drug discovery efforts revolving around MEK and RAF. The emergence of multiple drug candidates targeting these downstream kinases provides us with the means for validating the importance of the RAS-RAF-MEK-ERK signaling cascade in human tumors. This article highlights the lessons learned in the clinical evaluation of MAPK pathway inhibitors as anticancer agents and the complexities surrounding optimization of their therapeutic potential in light of the challenges posed by genetic heterogeneity within patient populations.