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 mu mol/kg, 225 mu mol/kg, 450 mu mol/kg, or 900 mu mol/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 mu mol/kg, 225 mu mol/kg, 450 mu mol/kg and 900 mu mol/kg, respectively. The latter two were significantly different from the control (P less than or equal to 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 less than or equal to 0.05). Dietary supplementation with isoflavones at 225 mu mol/kg, 450 mu mol/kg, and 900 mu mol/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.