The great escape: How metastases of melanoma, and other carcinomas, avoid elimination

Abstract

Cancer mortality ensues from metastatic growths. Cancers use two strategies to allow for this unrelenting expansion. The first way is that early metastases are often cryptic or dormant, being invisible to both innate suppressive actions and undetected clinically. Second, both the micrometastases and later clinically lethal growths are resistant to therapies, whether standard chemotherapies, targeted biologics, or even immunotherapies. These two modes of resistance necessitate new approaches to treatments if we are to eliminate mortality from solid tumors. However, to develop such therapeutic strategies, we first need to better understand the cellular behaviors and molecular events that enable the resistances. Herein, we present a comprehensive model of changing methods of avoidance and resistance that occur during tumor progression, and doubly confound treatment by mixing survival strategies throughout the continuum creating moving targets. Melanoma is presented as the model cancer, as it is being targeted by all three types of agents for disseminated disease, with breast and prostate cancer as two other key carcinomas. Impact statement: Cancers kill mainly because metastatic disease is resistant to systemic therapies. It was hoped that newer targeted and immunomodulatory interventions could overcome these issues. However, recent findings point to a generalized resistance to elimination imparted by both cancer-intrinsic and -extrinsic changes to provide survival advantages to the disseminated tumor cells. Here, we present a novel conceptual framework for the microenvironmental inputs and changes that contribute to this generalized therapeutic resistance. In addition we address the issues of experimental systems in terms of studying this phenomenon with their advantages and limitations. This is meant to spur studies into this critical aspect of tumor progression that directly leads to cancer mortality.