Cancer Stem Cells—Key Players in Tumor Relapse

SIMPLE SUMMARY: Cancer is one of the hardest pathologies to fight, being one of the main causes of death worldwide despite the constant development of novel therapeutic strategies. Therapeutic failure, followed by tumor relapse, might be explained by the existence of a subpopulation of cancer cells called cancer stem cells (CSCs). The survival advantage of CSCs relies on their ability to shape their phenotype against harmful conditions. This Review will summarize the molecular mechanisms exploited by CSCs in order to escape from different kind of therapies, shedding light on the potential novel CSC-specific targets for the development of innovative therapeutic approaches. ABSTRACT: Tumor relapse and treatment failure are unfortunately common events for cancer patients, thus often rendering cancer an uncurable disease. Cancer stem cells (CSCs) are a subset of cancer cells endowed with tumor-initiating and self-renewal capacity, as well as with high adaptive abilities. Altogether, these features contribute to CSC survival after one or multiple therapeutic approaches, thus leading to treatment failure and tumor progression/relapse. Thus, elucidating the molecular mechanisms associated with stemness-driven resistance is crucial for the development of more effective drugs and durable responses. This review will highlight the mechanisms exploited by CSCs to overcome different therapeutic strategies, from chemo- and radiotherapies to targeted therapies and immunotherapies, shedding light on their plasticity as an insidious trait responsible for their adaptation/escape. Finally, novel CSC-specific approaches will be described, providing evidence of their preclinical and clinical applications.