Introduction: Marine seaweeds has received increased attention in the protection or treatment of cancer, because of their bioactive compounds. The aim of the present study was to evaluate the anticancer capacity of two green seaweeds, Ulva fasciata and Ulva lactuca. Methods: The phenolic and flavonoid content of the hydro-methanolic extracts was measured, respectively by Folin-Ciocateu and aluminum chloride methods. The antioxidant activity of the extracts was evaluated by FRAP and DPPH assay and were compared to ascorbic acid. Cytotoxic effect of the extracts on the MCF-7 (ER + ) and MDA-MB-231 (ER -) breast cancer cell lines was also evaluated by MTT assay after 48 and 72 hours of incubation. Results:The phenolic and flavonoid content of Ulva fasciata was respectively 14.92±1.38 µgGAE/mg and 72.15±15.4 µgQE/mg which was significantly higher than Ulva lactuca. The reducing power and radical scavenging activity of Ulva fasciata was also higher. The cytotoxic effects of Ulva fasciata on the MCF-7 and MDA-MB-231 cell lines was more than Ulva lactuca, in concentration and time dependent manner. The cytotoxic effects of the both seaweeds were more potent on the MDA-MB-231 compared to the MCF-7 cell line and indicated an estrogen and progesterone receptor independent manner of cellular growth inhibition. Conclusion: It is appeared that the Ulva fasciata extract was a better drug candidate in treatment of triple negative breast cancer due to higher antioxidant activity, phenolic and flavonoid content. Further studies in fractionation and bioactive extraction of Ulva fasciata are recommended.

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