Why tumors outsmart immunotherapy: the hidden role of intratumoral heterogeneity

Introduction. Intratumoral heterogeneity (ITH) represents one of the most formidable challenges in modern oncology, directly influencing the efficacy of immune checkpoint inhibitors and other immunotherapeutic strategies. Understanding the clinical implications of ITH is essential for optimizing patient outcomes. Aim. This systematic review aims to synthesize the current evidence on the relationship between intratumoral heterogeneity and immunotherapy resistance, with particular emphasis on clinical biomarkers, therapeutic strategies to overcome resistance, and emerging predictive tools. Material and methods. A systematic literature search was conducted in PubMed, Scopus, Web of Science, and the Cochrane Library from January 2015 to December 2025, following referred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. Studies examining the association between ITH and immunotherapy response, resistance mechanisms, or clinical outcomes were included. Quality assessment was performed using the Newcastle-Ottawa Scale for observational studies and the Cochrane risk-of-bias tool for clinical trials. The outcomes of the quality appraisal were integrated into the synthesis qualitatively, with findings from high-quality studies given greater emphasis and results from lower-quality studies reported alongside explicit cautionary remarks rather than being excluded. Results. A total of 42 studies met the inclusion criteria. The evidence consistently demonstrates that high ITH is associated with reduced response rates to immune checkpoint inhibitors across multiple tumor types. Key mechanisms linking ITH to resistance include neoantigen heterogeneity, clonal evolution under immune selection pressure, dynamic remodeling of the tumor microenvironment, and spatial variation in immune cell infiltration. Multi-region sequencing studies reveal that single-biopsy sampling substantially underestimates true ITH, potentially leading to inaccurate biomarker assessment. Emerging strategies, including combination immunotherapy, adaptive trial designs, and liquid biopsy-based monitoring show promise in addressing ITH-driven resistance. Conclusions. Intratumoral heterogeneity is a critical determinant of immunotherapy efficacy and represents both a prognostic and a predictive biomarker. Clinical integration of multi-region sampling, longitudinal liquid biopsy monitoring, and combination therapeutic approaches is essential for overcoming ITH-mediated resistance and improving patient survival.