Biomarker-Driven Insights into Immune Checkpoint Inhibitor Therapy in Small-Cell Lung Cancer.

Small cell lung cancer (SCLC) is an aggressive malignancy with limited treatment options, and the integration of immune checkpoint inhibitors (ICIs) into treatment regimens has reshaped the therapeutic landscape. However, challenges such as rapid resistance and lack of effective predictive biomarkers remain. This review highlights the pivotal roles of biomarkers in SCLC, underscoring their contributions to treatment resistance across tumor-intrinsic mechanisms, immune landscape of the tumor microenvironment (TME) and systemic host factors. Tumor-intrinsic features, including molecular subtypes, genetic alterations such as TP53 and RB1 mutations, and tumor mutation burden (TMB), have shown varying associations with ICI efficacy. Notably, the SCLC-I (inflamed) molecular subtype appears more responsive to immunotherapy. Within the TME, programmed cell death ligand 1 expression, tumor-infiltrating lymphocytes (TILs), regulatory T cells, myeloid-derived suppressor cells, and tissue-associated cytokines and chemokines contribute to immune modulation. High CD8+ TILs are linked to better outcomes, while increased immunosuppressive populations often suppress anti-tumor response. Systemic factors, encompassing both tumor-derived (ctDNA, CTCs, tumor-derived EVs) and host-derived (circulating immune cell phenotypes, MHC expression, immune profile, baseline clinical characteristics) components, provide valuable insights into treatment response and prognosis. Further research is needed to validate biomarkers and investigate combination approaches to overcome resistance, offering hope for improved patient outcomes in SCLC.