Targeting the Extracellular Signal-Regulated Kinase Pathway in Cancer Therapy

The extracellular signal-regulated kinase (ERK) pathway is a major determinant in the control of diverse cellular processes such as proliferation, survival, and motility. This pathway is often upregulated in human cancers and as such represents an attractive target for mechanism-based approaches to cancer treatment. However, specific blockade of the ERK pathway alone induces mostly cytostatic rather than proapoptotic effects, resulting in limited therapeutic efficacy. Blockade of the constitutively activated ERK pathway by an ERK kinase (MEK) inhibitor sensitizes tumor cells to apoptotic cell death induced by several cytotoxic anticancer agents including microtubule-destabilizing agents and histone deacetylase inhibitors, not only in vitro but also in tumor zenografts in vivo. Thus, low concentrations of these anticancer drugs that by themselves show little cytotoxicity effectively kill tumor cells in which the ERK pathway is constitutively activated when co-administrated with a MEK inhibitor. The combination of a cytostatic signaling pathway inhibitor (MEK inhibitors) and conventional anticancer drugs (microtubule-destabilizing agents or histone deacetylase inhibitors) provides an excellent basis for the development of safer anticancer chemotherapies with enhanced efficacy through lowering the required dose of the latter cytotoxic drugs.