Botanical Adjuvants in Oncology: A Review on Natural Compounds in Synergy with Conventional Therapies as Next-Generation Enhancers of Breast Cancer Treatment

Breast cancer remains a major global health challenge despite advances in chemotherapy, endocrine therapy, targeted therapy, and radiotherapy, which are frequently constrained by therapeutic resistance, cumulative toxicity, and high costs. Accumulating preclinical and translational evidence demonstrates that plant-derived natural compounds can synergistically enhance the efficacy of conventional treatments, improve tumor response, and potentially reduce adverse effects. This review critically synthesizes in vitro, in vivo, and emerging clinical studies from 2015 to 2025, focusing on key phytochemicals, including curcumin, epigallocatechin-3-gallate, resveratrol, kaempferol, genistein, and other bioactive molecules as stand alone agents and as mechanistically validated adjuvants to chemotherapy, hormonal therapy, and radiotherapy. These compounds exert complementary actions, including the inhibition of PI3K/Akt/mTOR and NF-κB signaling, induction of apoptosis and cell-cycle arrest, suppression of epithelial-mesenchymal transition, and modulation of drug resistance pathways. Preclinical studies consistently show that combination strategies enhance tumor inhibition and may permit cytotoxic dose reduction, mitigating systemic and cardiotoxic effects. Nanocarrier-based delivery systems further optimize solubility, bioavailability, and tumor targeting. Despite robust preclinical evidence, clinical translation is limited by variability in raw materials, lack of standardization, regulatory barriers, and scarce large-scale trials. This review emphasizes both the therapeutic promise and translational challenges of integrating natural compounds as synergistic adjuvants in evidence-based breast cancer therapy.