Lessons learned from a decade of immune checkpoint inhibition: The good, the bad, and the ugly

Discovering the brakes/checkpoints that cancer places on the immune system to prevent being eradicated led to the 2018 Nobel Prize and the development of multiple Food and Drug Administration-approved immune checkpoint inhibitors (ICIs). ICIs have transformed the treatment of numerous cancer types and, remarkably, some patients with end-stage metastatic disease can achieve durable, complete remissions - cures. Still, ICIs cause significant immune-related toxicities, and most tumors are resistant. Unusual progression patterns such as pseudo-progression and hyper-progression (accelerated progression) can occur. Biomarkers for ICI response/resistance include microsatellite instability, high tumor mutational burden, and PD-L1 immunohistochemistry positivity; but they are imperfect, perhaps because of immune system complexity. Herein, we explore the good, the bad, and the ugly of ICIs in cancer treatment.