Soybean Isoflavones Reduce Experimental Metastasis in Mice

We investigated the effect of dietary supplementation with isoflavones on pulmonary metastasis of B16BL6 murine melanoma cells in C57BL/6 mice. Mice were fed a basal AIN-93G diet or the basal diet supplemented with the isoflavones genistein and daidzein at 113 micromol/kg, 225 micromol/kg, 450 micromol/kg, or 900 micromol/kg for 2 wk before and after the intravenous injection of 0.5 x 10(5) melanoma cells. At necropsy, the number and size of tumors that formed in the lungs were determined. The number of mice that had >15 lung tumors was 17 in the control group, and 16, 15, 13, and 10 in the groups fed isoflavones at 113 micromol/kg, 225 micromol/kg, 450 micromol/kg and 900 micromol/kg, respectively. The latter two were significantly different from the control (P </= 0.05). The median number of tumors in the control group was 67, and those in the isoflavone-supplemented groups were 57, 33, 32, and 17, respectively. The last was significantly different from the control (P </= 0.05). Dietary supplementation with isoflavones at 225 micromol/kg, 450 micromol/kg, and 900 micromol/kg also significantly decreased tumor size (median cross-sectional area and volume) compared to the control values. We conclude that dietary supplementation with isoflavones reduces experimental metastasis of melanoma cells in mice.