en Biochemistry Biochemistry پژوهشي Orginal research papers <span lang="TR" style="font-size:11.0pt"><span calibri="" style="font-family:"><span style="color:black">Marine seaweeds are sources of bioactive compounds such as novel anticancer components. This study investigated the phytochemical composition, antioxidant, and anticancer activities of aqueous and methanolic extracts of three brown algae (<i>Sargassum obtusifolium</i>, <i>Padina gymnospora</i>, and <i>Cystoseira indica</i>) from the Persian Gulf. The contents of total polyphenols, total flavonoids, and total carbohydrates were determined. The antioxidant activity of the extracts was determined by DPPH method. The cytotoxicity of extracts was evaluated using the MTT assay on breast cancer (MCF7) and lung cancer (A549) cell lines. The aqueous extracts had higher levels of polyphenols, flavonoids, and carbohydrates (ranging from 125&plusmn;11 to 202&plusmn;19 mg/g) than methanolic extracts (ranging from 43.8&plusmn;3.4 to 76.6&plusmn;6.9 mg/g). At 50 &micro;g/mL, the inhibition of DPPH radical ranging from 40.9&plusmn;0.6 to 62.3&plusmn;1.4% with an IC₅₀ ranged from 60.9&plusmn;0.5 to 30.4&plusmn;0.4 </span></span></span><span lang="TR" style="font-size:11.0pt"><span calibri="" style="font-family:"><span style="color:black">&micro;g/mL</span></span></span><span lang="TR" style="font-size:11.0pt"><span calibri="" style="font-family:"><span style="color:black">. The MTT cell proliferation assay confirmed a significant reduction in MCF7 cell line viability in the methanolic extracts at 390 &micro;g/mL and 190 &micro;g/mL compared to the control group. The methanolic extract of <i>S. obtusifolium</i> showed the highest selectivity index on MCF7 and A549 cells (1.16&plusmn;0.19 and 1.77&plusmn;0.16 respectively), despite containing lower levels of phytochemicals than the aqueous extracts. The findings suggest that the methanolic extract of <i>S. obtusifolium</i> exhibits selective cytotoxicity against MCF7 cells. Also, the aqueous extract of <i>C. indica</i> shows an adequate antioxidant index, usable in nutritional science.</span></span></span> Polyphenol, Flavonoid, Cytotoxicity, Selectivity index, MTT assay, Breast cancer 385 403 http://jifro.ir/browse.php?a_code=A-10-4965-1&slc_lang=en&sid=1 A. Zarei-Ahmady zarei-a@ajums.ac.ir 100319475328460055175 100319475328460055175 Yes 1Marine Pharmaceutical Science Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran M. zare MD.zare7@gmail.com 100319475328460055176 100319475328460055176 No 1Marine Pharmaceutical Science Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran M. Rashidi ms_rashidi60@yahoo.com 100319475328460055177 100319475328460055177 No 3Department of Clinical Biochemistry, Faculty of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran A. Siahoosh siahpoosh-a@ajums.ac.ir 100319475328460055178 100319475328460055178 No 4Medicinal Plant Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran I. Nabiour nabipourpg@bpums.ac.ir 100319475328460055179 100319475328460055179 No 5The Persian Gulf Marine Biotechnology Research Center, the Persian Gulf Biomedical Sciences Research Institute, Bushehr University of Medical Sciences, Bushehr, Iran