Genistein potentiates inhibition of tumor growth by radiation in a prostate cancer orthotopic model.

OBJECTIVE We have shown previously that pretreatment with genistein potentiated cell killing induced by radiation in human PC-3 prostate carcinoma cell line in vitro. We tested this approach in vivo using an orthotopic prostate carcinoma model of PC-3 cells in nude mice. METHODS Established prostate tumors were pretreated with p.o. genistein at a dose of 5 mg/d for 2 days followed by tumor irradiation with 5 Gy photons. One day after radiation, genistein was resumed and given every other day for 4 weeks. RESULTS Genistein combined with radiation caused a significantly greater inhibition of primary tumor growth (87%) compared with genistein (30%) or radiation (73%) alone. The number of metastatic lymph nodes was also significantly decreased following genistein and radiation. Paradoxically, genistein alone increased the size of lymph nodes associated with heavy tumor infiltration. Genistein-treated prostate tumors were large with necrosis, apoptotic cells, and giant cells and have a lower proliferation index than in control tumors. Following radiation, areas of tumor destruction replaced by fibrotic tissue and inflammatory cells as well as giant cells were observed, which are typical of radiation effect. After radiation and genistein treatment, an increase in giant cells, apoptosis, inflammatory cells, and fibrosis was observed with decreased tumor cell proliferation consistent with increased tumor cell destruction. Long-term therapy with genistein after prostate tumor irradiation significantly increased survival. CONCLUSIONS Genistein combined with prostate tumor irradiation led to a greater control of the growth of the primary tumor and metastasis to lymph nodes than genistein or radiation alone, resulting in greater survival.