Tumor heterogeneity as a driver of drug resistance and its implications for personalized therapy.

Tumors are highly dynamic diseases characterized by significant heterogeneity. They consist of multiple cellular populations with distinct properties that respond differently to therapeutic pressure. This heterogeneity may arise from spatial variation across tumor regions (spatial heterogeneity) as well as from temporal changes during tumor evolution and treatment (temporal heterogeneity). As a consequence, drug-resistant subclones often emerge under therapy and contribute to treatment failure. Advances in single-cell and spatial multi-omics technologies enable precise quantification of tumor heterogeneity, supporting detailed investigation of how heterogeneity contributes to chemoresistance and informing the development of personalized therapeutic strategies. In this review, we summarize the evolutionary dynamics underlying the emergence of tumor drug resistance and examine the molecular mechanisms responsible for failure of targeted therapies. We highlight how advances in single-cell and spatial multi-omics have significantly improved our ability to elucidate these processes. We further suggest that addressing tumor drug resistance may require a shift from static, single-target approaches toward dynamic, biology-informed personalized strategies. Integrating high-resolution multi-omics monitoring with functional validation could enable identification of subclonal vulnerabilities, support adaptive treatment adjustment, and contribute to more durable clinical responses.