Immune Checkpoint Inhibitors: Mechanisms and Emerging Challenges

Immune checkpoint inhibitors (ICIs) have revolutionized cancer immunotherapy by harnessing the body’s immune system to target and eliminate tumor cells. This chapter provides a comprehensive overview of the mechanistic basis, clinical applications, and emerging challenges associated with ICIs. It includes the role of immune system in cancer surveillance and the rationale for immunotherapy, and importance of key immune checkpoint pathways, including CTLA-4 and PD-1/PD-L1, as well as emerging targets like TIM-3, LAG-3, TIGIT, and BTLA. The molecular mechanisms of these pathways are elucidated to explain how ICIs reactivate T-cell function and enhance antitumor immunity. The chapter highlights the transformative clinical impact of ICIs, underscoring their FDA approval for over 20 cancer types and their use in combination therapies with chemotherapy, radiotherapy, and targeted agents. Despite their success, significant challenges remain, including immune-related adverse events (irAEs), which affect a majority of patients and range from mild to life-threatening, and resistance mechanisms—both primary and acquired—that limit long-term efficacy. Looking forward, the chapter discusses strategies to overcome these limitations, such as dual checkpoint blockade, novel inhibitory targets, and the integration of artificial intelligence for biomarker discovery and treatment personalization. The future of ICI therapy lies in multidisciplinary approaches that combine advanced technologies, predictive biomarkers, and combination regimens to optimize efficacy and minimize toxicity.