Synergistic anti-proliferative effects of gambogic acid with docetaxel in gastrointestinal cancer cell lines

Background: Gambogic acid has a marked anti-tumor effect for gastric and colorectal cancers in vitro and in vivo. However, recent investigations on gambogic acid have focused mainly on mono-drug therapy, and its potential role in cancer therapy has not been comprehensively illustrated. This study aimed to assess the interaction between gambogic acid and docetaxel on human gastrointestinal cancer cells and to investigate the mechanism of gambogic acid plus docetaxel treatment-induced apoptotic cell death. Methods: MTT assay was used to determine IC50 values in BGC-823, MKN-28, LOVO and SW-116 cells after gambogic acid and docetaxel administration. Median effect analysis was applied for determination of synergism and antagonism. Synergistic interaction between gambogic acid and docetaxel was evaluated using the combination index (CI) method. Furthermore, cellular apoptosis was analyzed by Annexin-V and propidium iodide (PI) double staining. Additionally, mRNA expression of drug-associated genes, i.e., beta-tublin III and tau, and the apoptosis-related gene survivin, were measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Results: Gambogic acid provided a synergistic effect on the cytotoxicity induced by docetaxel in all four cell lines. The combined application of gambogic acid and docetaxel enhanced apoptosis in gastrointestinal cancer cells. Moreover, gambogic acid markedly decreased the mRNA expression of docetaxel-related genes, including beta-tubulin III, tau and survivin, in BGC-823 cells. Conclusions: Gambogic acid plus docetaxel produced a synergistic anti-tumor effect in gastrointestinal cancer cells, suggesting that the drug combination may offer a novel treatment option for patients with gastric and colorectal cancers.