Abstract. Baicalein is a purified flavonoid extracted from the roots of Scutellaria baicalensis or Scutellaria radix. Although previous studies have suggested that Baicalein possesses an in vitro anti-hepatocellular carcinoma activity, its in vivo effects and mechanisms of action are still not completely understood. In this study, Baicalein at concentrations of 40-120 µM exhibited significant cytotoxicity to three hepatocellular carcinoma (HCC) cell lines but marginal cytotoxicity to a normal liver cell line in vitro. Compared to a standard chemotherapy drug, 5-fluorouracil (5-FU), Baicalein had greater effect on HCC cells but less toxicity on normal liver cells. Treatment with Baicalein dramatically reduced mitochondrial transmembrane potential, and activated caspase-9 and caspase-3. Blockade of Baicaleininduced apoptosis with a pan-caspase inhibitor partially attenuated Baicalein-induced growth inhibition in HCC. Baicalein treatment significantly inhibited tumor growth of HCC xenografts in mice. Induction of apoptosis was demonstrated in Baicalein-treated xenograft tumors by the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Furthermore, Baicalein treatment dramatically decreased the levels of phosphorylation of MEK1, ERK1/2 and Bad in vitro and in vivo. Overexpression of human MEK1 partially blocked Baicalein-induced growth inhibition. Consequently, these findings suggest that Baicalein preferentially inhibits HCC tumor growth through inhibition of MEK-ERK signaling and by inducing intrinsic apoptosis. IntroductionHepatocellular carcinoma (HCC) is one of the common cancers in Asia and Africa. The incidence of HCC is increasing in Europe and the United States (1). Although HCC can be cured at the early stage by surgical resection, most patients can not be diagnosed at the early stage since tumors are asymptomatic (2). Current treatment options for HCC patients at the late stage include chemotherapy, chemoembolization, ablation, and proton beam therapy. These treatment options remain disappointed in clinic. HCC patients will relapse and rapidly progress to the advanced stages with vascular invasion and multiple metastases, which lead to a low 5-year survival rate of less than 7% (3). HCC patients who have surgically resectable localized tumors show a better prognosis. However, even these patients have a dismal 5-year survival rate of 15 to 39% (4). Clearly, there is an urgent need to search for new therapies for this lethal disease.We have reported that chrysanthemum indicum extract (CIE), a Chinese herbal extraction, exerts a significantly inhibitory effect on HCC cells (MHCC97H) in previous studies (5,6). One particular point to stress is that CIE appeares to have no cytotoxic effect on normal liver cells, highlighting an advantage of the herbal treatment. Herbal medicine flavonoids have recently received increasing attention because of the beneficial effects of anti-tumor and