Overcoming Resistance Mechanisms to Melanoma Immunotherapy

The advent of immune checkpoint inhibition has revolutionized treatment of advanced melanoma. While most patients derive survival benefit from established immunotherapies, notably monoclonal antibodies blocking cytotoxic T-lymphocyte antigen 4 and programmed cell death protein 1, a subset does not optimally respond due to the manifestation of innate or acquired resistance to these therapies. Combination regimens have proven efficacious relative to single-agent blockade, but also yield high-grade treatment toxicities that are often dose-limiting for patients. In this review, we discuss the significant strides made in the past half-decade toward expanding the melanoma immunotherapy treatment paradigm. These include newly approved therapies, adoption of neoadjuvant immunotherapy, and studies in the clinical trials pipeline targeting alternative immune checkpoints and key immunoregulatory molecules. We then review how developments in molecular and functional diagnostics have furthered our understanding of the tumor-intrinsic and -extrinsic mechanisms driving immunotherapy resistance, as well as highlight novel biomarkers for predicting treatment response. Throughout, we discuss potential approaches for targeting these resistance mechanisms in rational combination with established immunotherapies to improve outcomes for patients with melanoma.