Experiments conducted on triple-negative breast cancer have shown that fucoidan from Lessonia trabeculata (FLt) exhibits cytotoxic and antitumor properties. However, further research is necessary to gain a complete understanding of its bioactivity and level of cytotoxicity. The cytotoxic effect of FLt was determined by the 2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Apoptosis was analyzed using annexin V and caspase 3/7 staining kit and DNA fragmentation. In addition, transcriptional expression of antiapoptotic (Bcl-2 and XIAP) and proapoptotic (caspase 8, caspase 9, and AIF) genes were analyzed in TNBC 4T1 cells. After 72 h of culture, the IC₅₀ for FLt was 561 μg/mL, while doxorubicin (Dox) had an IC₅₀ of 0.04 μg/mL. In addition, assays for FLt + Dox were performed. Annexin V and caspase 3/7 revealed that FLt induces early and late-stage apoptosis. DNA fragmentation results support necrotic death of 4T1 cells. Similarly, transcripts that prevent cell death were decreased, while transcripts that promote cell death were increased. This study showed that FLt induces apoptosis by both caspase-dependent and caspase-independent mechanisms. These findings suggest that FLt may have potential applications in breast cancer treatment. Further research will provide more information to elucidate the mechanism of action of FLt.

Triple-negative breast cancer (TNBC) is known for being aggressive and potentially resistant to chemotherapy. This means that new ways to improve cancer treatments are a priority. So, the anticancer effect of binary combinations of fucoidan (FuLt) and the chemotherapeutic agents doxorubicin, paclitaxel, and 5-fluorouracil was evaluated. The Chou-Talalay combination index method was used to do this. This method allows the assessment of interactions between products by determining synergism, additive effect and antagonism with combination index 1, respectively. Synergistic indices (SIs) were selected and applied to 4T1 homotypic spheroids. Oxidative stress caused by SIs was determined after 72 h by measuring the production of ROS and NO in both homotypic and heterotypic spheroids. Three SIs with an inhibitory effect of at least ≥ 0.50 and a dose reduction index > 1 were selected. Considering the experimental and simulated SI, twelve, nineteen, and seven SIs were found for FuLt with doxorubicin, paclitaxel, and 5-fluorouracil, respectively. The highest levels of ROS and NO occurred at 12 and 72 h, respectively, in homotypic and heterotypic spheroids, indicating an immunomodulatory effect in heterotypic spheroids. These results suggest that the synergistic combination of FuLt with chemotherapeutic agents improves drug efficacy and modulates redox dynamics in 4T1 spheroids. Furthermore, FuLt alone exhibits cytotoxic properties.