Tumor heterogeneity and plasticity as elusive drivers for resistance to MAPK pathway inhibition in melanoma

Despite the recent success of MAPK signaling-targeted drugs in melanoma, the majority of patients with metastatic melanoma still undergo disease progression after initial tumor shrinkage indicating gradually developing therapy resistance. This review will give an overview on currently suggested concepts of resistance to MAPK pathway inhibitors in melanoma with particular focus on interand intraindividual as well as intratumor heterogeneity. The high plasticity of melanoma cells promotes both the clonal evolution of genetic resistance, for example, because of mutations in the MAPK or PI3K/AKT/PTEN pathways, and the emergence of cell phenotypes that functionally and metabolically overcome MAPK inhibition. Like a 'moving target', melanoma cells are shifting between different metabolic, cell cycle and differentiation states reflecting a highly dynamic potential to adapt to exogenous stressors including drugs. The introduction of MAPK inhibitors into the clinics has tremendously pushed the field of melanoma research not just because of the historic therapeutic success but also by providing a new tool to study human melanoma in its natural microenvironment.