Cytotoxic Activity of Propolis Extracts from the Stingless Bee Trigona Sirindhornae Against Primary and Metastatic Head and Neck Cancer Cell Lines

Background: Propolis, a resinous substance produced by the honeybee, has a wide spectrum of potent biological activities. However, anti-cancer activity of propolis obtained from Trigona sirindhornae, a new species of stingless bee, has not yet been reported. This study concerned cytotoxicity of propolis extracts from T. sirindhornae against two head and neck squamous cell carcinoma (HNSCC) cell lines. Materials and Methods: A dichloromethane extract of propolis (DMEP) was prepared generating 3 fractions: DMEP-A, DMEP-B, and DMEP-C. Genetically-matched HNSCC cell lines derived from primary (HN30) and metastatic sites (HN31) in the same patient were used to study cytotoxic effects of the DMEPs by MTT assays. The active compounds in the DMEPs were analyzed by reversephase high performance liquid chromatography. Results: DMEP-A exhibited cytotoxic activity on HN30 cells with significantly decreased viability at 200 μg/ml compared with the control (p<0.05). However, no significant cytotoxic effect was evident in HN31 cells. DMEP-B and DMEP-C significantly decreased the viability of both cell lines from 100–200 μg/ml and 50–200 μg/ml, respectively (p<0.05). Interestingly, HN31 cells were more toxically sensitive compared with the HN30 cells when treated with DMEP-B and DMEP-C. IC50 values for DMEP-B with HN30 and HN31 cells were more than 200 μg/ml and 199.8±1.05 μg/ml, respectively. The IC50 of DMEP-C to HN30 and HN31 cells was found to be 114.3±1.29 and 76.33±1.24 μg/ml, respectively. Notably, apigenin, pinocembrin, p-coumaric acid, and caffeic acid were not detected in our propolis extracts. Conclusion: T. sirindhornae produced propolis displays cytotoxic effects against HNSCC cells s. Moreover, DMEP-B and DMEP-C differentially inhibited the proliferation of a metastatic HNSCC cell line.