The growth-suppressive effect of d-δ-tocotrienol and geranylgeraniol is at least partially attributed to their impact on 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme in the mevalonate pathway that provides essential intermediates for the posttranslational modification of growth-related proteins including RAS. We hypothesize that these agents synergistically impact cell growth based on their complementary mechanisms of action with HMG CoA reductase. d-δ-tocotrienol (0-40 µmol/L; half maximal inhibitory concentration [IC50] = 15 µmol/L) and geranylgeraniol (0-100 µmol/L; IC50 = 60 µmol/L) each induced concentration-dependent suppression of the growth of human DU145 prostate carcinoma cells. Blends of the two agents synergistically suppressed the growth of DU145 cells, with combination index values ranging 0.67-0.75. While 7.5 µmol/L d-δ-tocotrienol and 30 µmol/L geranylgeraniol individually had no impact on cell cycle distribution in DU145 cells, a blend of the agents induced cell cycle arrest at the G1 phase. The synergistic downregulation of the expression of HMG CoA reductase by 7.5 µmol/L d-δ-tocotrienol and 30 µmol/L geranylgeraniol was accompanied by a reduction in membrane K-RAS protein. Our finding supports the cancer chemopreventive action of plant-based diets and their isoprenoid constituents. Properly formulated isoprenoids and derivatives may provide novel approaches in prostate cancer prevention and therapy.