Phytochemicals from Agro-Industrial By-Products for Breast Cancer Prevention and Therapy: Molecular Mechanisms and Circular Bioeconomy Perspective

Breast cancer remains one of the most frequently diagnosed malignancies and a leading cause of cancer-related mortality among women worldwide. The growing interest in natural bioactive compounds has highlighted plant-derived phytochemicals as promising agents for cancer prevention and adjunctive therapy due to their pleiotropic biological activities and relatively low toxicity. In parallel, increasing attention has been directed toward agro-industrial by-products generated during food processing, which represent abundant and sustainable sources of valuable phytochemicals. This review provides a comprehensive overview of recent advances in the identification, extraction, and biological evaluation of phytochemicals derived from plants and agro-industrial residues, using pomegranate (Punica granatum) peels, onion (Allium cepa) skins, and citrus by-products as representative examples of phytochemical-rich agro-industrial residues. These by-products are rich in polyphenols, flavonoids, and other secondary metabolites—including punicalagins, ellagic acid, quercetin, hesperidin, and naringin—that have demonstrated significant antioxidant, anti-inflammatory, and anticancer properties. Recent in vitro and in vivo studies indicate that these compounds can modulate key molecular pathways involved in breast cancer initiation and progression, such as oxidative stress regulation, apoptosis induction, inhibition of cell proliferation, and suppression of signaling cascades including PI3K/Akt, NF-κB, and MAPK pathways. Furthermore, the valorization of agro-industrial waste offers a sustainable strategy to recover high-value bioactive compounds while reducing environmental impact. Overall, phytochemicals obtained from plant materials and food processing by-products represent promising functional agents for breast cancer prevention and therapy, although further studies are required to improve bioavailability, elucidate mechanisms of action, and validate their clinical potential.