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Curcuma aromatica Salisb. (C. aromatica) is one of the traditional herbs used to treat microbial infection, skin eruption, coronary heart disease, and other diseases, including cancer. However, the inhibitory effects and underlying mechanisms of action of C. aromatica on gastric cancer cells have not yet been fully elucidated. Our study aimed to examine the possible molecular mechanisms underlying the cytotoxic effects attributed to C. aromatica rhizome standardized extract against gastric cancer cells. The components of two major active compounds in C. aromatica rhizome extract were quantitatively analyzed using a simple and validated HPLC method. Cytotoxicity was determined in different gastric cancer and non-cancer cell lines. The biological activities of the extract targeting apoptosis and cell cycle-related genes on gastric cancer AGS cells were also investigated to elucidate the mechanisms relating to the anti-proliferative effect of C. aromatica rhizomes. The two major active compounds curdione and germacrone, in the C. aromatica extract were standardized to 0.64% and 1.12% w/w, respectively. The standardized extract (CAE) exerted cytotoxic effects on various cancer cells, whereas minimal effects at equivalent doses were noted for normal cells. CAE concentration-dependently suppressed growth of gastric cancer AGS cells via induction of apoptosis. Further studies revealed that CAE treatment disrupted mitochondrial membrane potential (ΔΨm), increased Bax/Bcl-2 ratio, and cytochrome c release, resulting in activation of caspase-9/-3 and subsequent cleavage of PARP. Further, the inhibitory effects of caspase-9/-3 expression by a synthetic pan-caspase inhibitor partially protected cells against apoptosis following CAE treatment. In addition, CAE significantly promoted cell death in AGS cells via an accumulation of cells in the G0/G1 phase. This effect was associated with upregulation of the CDK inhibitor p21 and downregulation of cyclin D1, cyclin E, CDK4, and CDK2 expression. Our data indicated that CAE exerted anti-proliferative activity by activating the mitochondria-mediated caspase-dependent apoptotic pathway and arresting the p21-mediated G0/G1 cell cycle on human gastric cancer AGS cells. [ABSTRACT FROM AUTHOR]