Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment by enhancing antitumor immune responses; however, clinical outcomes remain highly variable across patients and tumor types. While tumor-intrinsic factors such as PD-L1 expression and tumor mutational burden provide some predictive value, they do not fully explain response heterogeneity. Increasing evidence highlights the critical role of host-related determinants in modulating ICI efficacy. This review provides a comprehensive overview of key host factors, including baseline immune competence, T-cell repertoire diversity, cytokine profiles, and peripheral biomarkers. It further examines the impact of germline genetics, HLA genotype, and epigenetic regulation on immune responses. The role of the gut microbiome and its metabolites is explored, alongside the impact of metabolic status, obesity, nutrition, and lifestyle behaviors. Additionally, the effects of co-medications and comorbidities are discussed. Integrating these host-related factors may improve patient stratification and support the development of personalized immunotherapy strategies.

Background: Immune checkpoint inhibitors (ICIs) have transformed cancer treatment but are associated with immune-related adverse events (irAEs) and variable clinical outcomes. Clinical predictors of organ-specific irAEs remain indeterminate, particularly in real-world populations. Methods: We conducted a retrospective single-center cohort study including 751 adult patients with solid tumors treated with ICIs between 2018 and 2025. Clinical, demographic, and treatment-related variables were analyzed. Multivariable logistic regression identified predictors of irAEs, while associations with objective response rate (ORR), progression-free survival (PFS), and overall survival (OS) were assessed using logistic and Cox regression models. Results: The most frequent irAEs were endocrine (9.9%), dermatologic (9.1%), gastrointestinal (7.6%), and pulmonary (4.7%). Female sex independently predicted endocrine (aOR 1.98, p = 0.007) and rheumatologic irAEs (aOR 4.06, p = 0.007). Combination immunotherapy was associated with increased dermatologic (aOR 2.66, p = 0.013) and gastrointestinal irAEs (aOR 2.65, p = 0.016), while concurrent radiotherapy predicted gastrointestinal toxicity (aOR 1.82, p = 0.044). Atezolizumab was associated with higher pulmonary irAE risk (aOR 2.97, p = 0.048). Endocrine, dermatologic, gastrointestinal, and pulmonary irAEs were independently associated with improved ORR (OR range: 2.53–4.30, all p < 0.01). Conclusions: Organ-specific irAEs exhibit distinct clinical predictors and differential prognostic implications in patients receiving ICIs. Select irAEs are associated with improved treatment response and disease control, yet our results should be regarded as hypothesis-generating requiring further investigation.

Cutaneous melanoma remains a global health challenge. Although BRAF/MEK-targeted therapies and immune checkpoint inhibitors (ICIs) have improved outcomes in advanced disease, durable responses remain difficult to achieve for most patients. Therapeutic resistance represents the central barrier to long-term disease control. This review comprehensively examines the molecular and immunologic mechanisms underlying melanoma resistance and integrates these insights with emerging pharmaceutical strategies designed to overcome them. We explore the genetic landscape of melanoma, including oncogenic alterations in BRAF, NRAS, NF1, CDKN2A, and PTEN, and explain how dysregulation of the MAPK and PI3K/AKT/mTOR signaling axes drives therapeutic escape. Phenotypic plasticity is discussed as a critical epigenetic driver of drug tolerance. The tumor microenvironment (TME) is examined as an active co-conspirator in resistance, encompassing immunosuppressive cell populations, cancer-associated fibroblasts, and metabolic competition. Resistance mechanisms to targeted therapy, including MAPK reactivation, bypass signaling, transcriptional reprogramming, and metabolic rewiring, are reviewed alongside tumor-intrinsic and tumor-extrinsic mechanisms of ICI resistance. Emerging therapeutic strategies are surveyed, including next-generation RAF and ERK inhibitors, dual-pathway blockade, and metabolic therapies targeting oxidative phosphorylation. Innovations in molecular imaging, liquid biopsy, and artificial intelligence-driven biomarker discovery are highlighted as pivotal tools for real-time resistance monitoring and adaptive treatment. By linking mechanistic insights with translational advances, this review advocates for combination strategies and adaptive clinical frameworks to achieve more durable disease control in melanoma.