Objectives Xanthorrhizol, a sesquiterpene, and d-δ-tocotrienol, a vitamin E molecule, each suppresses the proliferation of a number of tumor cells. This study aims to examine the potentially synergistic effect of xanthorrhizol and d-δ-tocotrienol in tumor cells. Methods Murine B16 melanoma and human DU145 prostate carcinoma cells were incubated for 48 h (B16) or 72 h (DU145) with xanthorrhizol or d-δ-tocotrienol before cell populations were determined by CellTiter 96â Aqueous One Solution. Cells incubated with the agents for 24 hours were stained with propidium iodide and analyzed for cell cycle using flow cytometry and MultiCycle AV. Isobologram and combination index (CI) were used to demonstrate their synergistic anti-proliferative impacts. Results Xanthorrhizol (0–200 µmol/L) and d-δ-tocotrienol (0–40 µmol/L) each elicited a concentration-dependent suppression of the proliferation of B16 cells. A blend of 16.25 µmol/L xanthorrhizol and 10 µmol/L d-δ-tocotrienol achieved 69% (P < 0.05) growth suppression of B16 cells, exceeding the sum of individual effects. B16 cells incubated with 5 and 10 µmol/L d-δ-tocotrienol for 24-h had a concentration-dependent increase in the percentage of cells in the G1 phase with a concomitant decrease in the percentage of cells in the S phase. The G1/S ratio, an indicator of cell cycle arrest at the G1 phase, increased from 1.73 ± 0.05 (Control) to 2.01 ± 0.10 (5 µmol/L) and 2.73 ± 0.05 (10 µmol/L). A parallel pattern of concentration-dependent increase in the G1/S ratio was induced by xanthorrhizol at concentrations equivalent to 25% (16.25 µmol/L) and 50% (32.5 µmol/L) of its IC50 value. A blend of 5 µmol/L d-δ-tocotrienol and 16.25 µmol/L xanthorrhizol, each at no-effect concentrations, significantly increased the percentage of B16 cells in the G1 phase to 62.6 ± 0.6%. Isobologram and CI confirmed the synergistic effect of xanthorrhizol (50 and 100 μmol/L) and d-δ-tocotrienol (10 and 20 μmol/L) on the proliferation of DU145 cells. Conclusions Xanthorrhizol and d-δ-tocotrienol synergistically suppress tumor cell proliferation by inducing G1 arrest and may have potential in cancer prevention and therapy. Funding Sources American River Nutrition, Inc. and the University Assistantship Program and the Department of Nutrition of Georgia State University.

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