Plant- and Algae-Derived Compounds Enhance the Anticancer Activity of Doxorubicin in Colorectal Cancer Cell Lines

Colorectal cancer (CRC) remains one of the leading causes of cancer-related mortality worldwide, and the efficacy of conventional chemotherapy is frequently limited by systemic toxicity, chemoresistance, and tumor recurrence. Natural products derived from marine algae and plants have attracted increasing interest as multitarget adjuvant agents capable of modulating apoptosis, oxidative stress, and tumor-associated signaling pathways. In the present study, we evaluated the anticancer activity of commercially available formulations enriched in fucoxanthin, fucoidan, tocotrienols, astaxanthin, and apple polyphenols, either alone or in combination with doxorubicin (DOX), using two-dimensional and three-dimensional colorectal cancer models. Initial IC₅₀ screening in ovarian (OVCAR3), prostate (PC3), colorectal (Caco2 and HT-29), and non-tumorigenic colon epithelial cells demonstrated that formulations 2.1 and 10.0 exhibited the most relevant cytotoxic activity, particularly in colorectal cancer cells. Combined treatments with DOX significantly reduced cell viability compared to individual treatments, particularly in Caco2 cells, where viability decreased to approximately 10% under combined exposure conditions. Mechanistically, combined treatments enhanced caspase-3/7 activation in both Caco2 and HT-29 cells, indicating apoptosis-associated effects. These findings were further supported in three-dimensional spheroid models, where supplement combinations impaired spheroid expansion, induced apoptotic AO/EB staining patterns, and reduced HT-29 spheroid growth by approximately 30-35%, reaching inhibitory effects comparable to DOX alone. Collectively, these results suggest that plant- and algae-derived formulations enriched in antioxidant bioactives may enhance chemotherapy-associated antitumor responses through apoptosis-related mechanisms and modulation of tumor-like growth behavior. The present findings support the further exploration of natural-product-based adjuvant strategies in colorectal cancer therapy using more clinically representative chemotherapeutic schemes and in vivo models.