Plant-Derived Bioactive Compounds, Cancer Stemness and Cellular Senescence: A Narrative Review of Preclinical Evidence and Translational Relevance to Cancer Recurrence and Drug Resistance

Background/Objectives: Cancer recurrence and drug resistance arise from tumour-cell plasticity and microenvironmental adaptations that may persist after treatment. Cancer stem cells (CSCs) are associated with self-renewal, tumour initiation, residual disease, and treatment tolerance, whereas cellular senescence has context-dependent effects. Although therapy-induced senescence may restrict tumour-cell proliferation, persistent senescent tumour or stromal cells and their senescence-associated secretory phenotype (SASP) can, in selected models, promote inflammatory signalling, stemness-associated phenotypes and therapeutic resistance. This narrative review critically evaluates preclinical and translational evidence concerning curcumin, resveratrol, epigallocatechin gallate, berberine, piperine and shikonin as potential adjunctive modulators of CSCs and senescence-associated processes.