BACKGROUND: Gallbladder carcinoma is a lethal malignant neoplasm with dismal surgical results. Unfortunately, the adjuvant therapies for gallbladder carcinoma such as chemotherapy and radiotherapy are also disappointing. We reported that norcantharidin (NCTD), a demethylated form of cantharidin, which is an active ingredient of the Chinese medicine Mylabris, was used against human gallbladder carcinoma GBC-SD cells. In the present study, we further studied the mechanism underlying the inhibitory effect of NCTD on growth of human gallbladder carcinoma GBC-SD cells in vitro. METHODS:Human gallbladder carcinoma GBC-SD cells were grown in cell culture and divided into a NCTD group and a control group. The inhibitory effect of NCTD on growth of GBC-SD cells was investigated by evaluation of proliferation, cell cycle, apoptosis and morphological changes of the cells. Cell proliferation was assessed by tetrazolium-based colorimetric assay. The induction of cell cycle arrest and apoptosis was measured by lfow cytometry. The morphological changes of the cells were observed by light-and electron-microscopy. To elucidate the anticancer mechanism of NCTD, expression of the proliferation-related gene proteins PCNA, Ki-67, cyclin-D1 and p27 and the apoptosis-related gene proteins Bcl-2, Bax and Survivin were determined by the streptavidin-biotin complex method and RT-PCR. RESULTS: NCTD inhibited the proliferation of GBC-SD cells in a dose- and time-dependent manner, with an IC50 of 56.18 μg/ml at 48 hours. The lfow cytometric proifles revealed that NCTD (at the IC50 for 48 hours) signiifcantly increased the proportion of cells in G2/Mphase and signiifcantly decreased the proportion of cells in S phase, with a signiifcantly increased rate of cell apoptosis. After treatment with the 48-hour IC50 dose of NCTD, cell shrinkage, vacuolar cytoplasm, membrane budding, karyorrhexis, karyolysis, chromosome condensation and chromatin aggregation in some GBC-SD cells were observed by light-microscopy; decreased microvilli, Golgiosome atrophy, mitochondrial swelling, nuclear shrinkage, chromosome condensation and typical apoptosis bodies were seen by electron-microscopy, and the morphological changes of apoptosis occurred in GBC-SD cells. The expression of PCNA, Ki-67 and Bcl-2 proteins decreased signiifcantly;the Pix or relative levels of PCNA mRNA, cyclin-D1 mRNA, Bcl-2 mRNA and Survivin mRNA decreased signiifcantly, whereas the Pix or relative levels of p27 mRNA and Bax mRNA increased signiifcantly. CONCLUSIONS: NCTD inhibits the growth of human gallbladder carcinoma GBC-SD cells in vitro. Its anticancer mechanism may correlate with inhibition of cell proliferation, arrest of the cell cycle, blockage of DNA synthesis, inlfuence on cell metabolism, induction of cell apoptosis and inlfuence on expression of the proliferation-related genes PCNA, Ki-67, cyclin-D1 and p27, and the apoptosis-related genes Bcl-2, Bax and Survivin in human gallbladder carcinoma GBC-SD cells.