Background: Skin cancer has become the most commonly diagnosed cancer in the United States, of which melanoma is the most lethal form. Conventional chemotherapy treatments come with a host of side effects. d-δ-Tocotrienol and xanthorrhizol have shown potential in treatment of cancers. Tocotrienol, an isomer of vitamin E, has demonstrated apoptotic and cell cycle arrest effects on cancer cells. Xanthorrhizol, a sesquiterpenoid, has demonstrated anticancer effects through both hormonal and nonhormonal pathways. It is unknown whether combinations of d-δ-tocotrienol and xanthorrhizol provide greater growth-suppressive activity than these agents alone.

Methods: The proliferation of murine B16 melanoma cells was measured using MTT assay with CellTiter 96 Aqueous One Solution (Progmega, Madison WI, USA). Cell cycle distribution was examined by flow cytometry. Protein expression was detected by Western blot analysis with chemiluminescent imaging.

Results: Xanthorrhizol inhibited cell proliferation in a dose-response manner. The IC₅₀ of xanthorrhizol, or the concentration of xanthorrhizol required to suppress the cell proliferation by 50%, was estimated to be 65 μM. A combination of 16.25 μM xanthorrhizol and 10 μM d-δ-tocotrienol inhibited cell proliferation to a greater extent than those by individual compounds (P

Conclusion: Xanthorrhizol and d-δ-tocotrienol demonstrate additive and possibly synergistic effects on the viability of murine B16 melanoma cells. The underlying mechanisms of action for this finding, including apoptosis and cell cycle, warrant further investigation.