IMMUNE CHECKPOINT INHIBITORS IN SOLID TUMORS: MECHANISMS, CLINICAL EFFICACY, BIOMARKER-GUIDED PATIENT SELECTION, AND IMMUNE-RELATED ADVERSE EVENTS

Background: Immune checkpoint inhibitors (ICIs)—monoclonal antibodies blocking the PD-1/PD-L1 and CTLA-4 inhibitory pathways—have transformed the management of multiple advanced solid tumors, producing durable responses in a subset of patients who previously had no effective treatment options. Their mechanisms exploit the physiological immune tolerance pathways co-opted by tumors to evade cytotoxic T-cell killing. Objective: To review the molecular mechanisms of immune checkpoint blockade, clinical efficacy data across major tumor types, validated biomarkers for patient selection (PD-L1 expression, TMB, MSI/dMMR), and the spectrum and management of immune-related adverse events (irAEs). Methods: A systematic review of eight primary peer-reviewed sources was conducted, including pivotal randomized clinical trials, meta-analyses, and authoritative guidelines published between 2010 and 2024. Results: Pembrolizumab (anti-PD-1) achieved 5-year overall survival of 31.9% in advanced NSCLC versus 16.3% with chemotherapy (KEYNOTE-024). Nivolumab + ipilimumab dual checkpoint blockade produced objective responses in 58% of advanced melanoma patients with 5-year OS of 52% (CheckMate 067). High TMB (≥10 mut/Mb), MSI-H/dMMR status, and PD-L1 TPS ≥50% are the strongest predictive biomarkers. Grade 3–4 irAEs occur in 10–15% of anti-PD-1 monotherapy and 30–55% of anti-CTLA-4 or combination therapy patients. Conclusion: Immune checkpoint inhibitors represent a paradigm shift in oncology, converting previously incurable solid tumors into potentially long-term manageable diseases. Biomarker-guided patient selection and systematic irAE surveillance are essential for maximizing therapeutic benefit and minimizing toxicity.