Objective: Recent experimental data have shown that dietary soy isoflavones such as genistein can significantly suppress invasiveness and growth of a number of human malignancies. In this study we examined whether genistein, at a concentration typical of plasma levels following soy formula intake, in combination with cisplatin or vincristine exhibited an additive or synergistic inhibitory effect on the growth of medulloblastoma cells. Methods: Three human medulloblastomas cell lines (HTB-186, CRL-8805 and MED-1) were treated with genistein at 6 μM, the maximum reported dietary plasma level in children, combined with cisplatin (0–10 μM) or vincristine (0–1 μM). Monolayer cell growth and cytotoxicity, as measured by colonigenic survival in soft agarose, were then compared in control and drug-treated cultures. Presence of apoptosis, using the DNA ladder assay and laser scanning cytometry, was investigated in all cell lines at those concentrations at which an enhancement of antiproliferative effect of cisplatin and vincristine in presence of genistein was observed. Results: Genistein at 6 μM led to a 2.8-fold increase in the monolayer growth inhibitory effect of cisplatin (0.05 μM) in HTB-186 cells (p = 4.5 × 10–4 by one-tailed t test). Genistein increased colonigenic survival inhibition of HTB-186 2.6-fold at the same cisplatin concentration (p = 1.5 × 10–4). Genistein caused a 1.3-fold increase in antiproliferative effect of cisplatin (0.5 μM) in CRL-8805 cells (p = 3.1 × 10–4). Similarly the inhibition of colonigenic survival was enhanced 2.0-fold in CRL-8805 (p = 1.22 × 10–5). The addition of genistein to 0.5 μM cisplatin led to a 1.7-fold increase in monolayer growth inhibition and 2.4-fold increase in colonigenic survival inhibition of MED-1 cells (p = 8.3 × 10–4 and p = 1.1 × 10–4 respectively). These effects were primarily synergistic but also additive in nature. The combination of genistein and vincristine, as compared to vincristine alone, caused a minimal-to-modest increase in antiproliferative effect on medulloblastoma cells studied here. We were unable to detect apoptosis by two methodologies in any of the medulloblastoma lines when genistein was combined with cisplatin or vincristine. Conclusion: These results indicate that genistein at typical dietary plasma levels can significantly enhance the antiproliferative and cytotoxic action of cisplatin and, to a lesser extent, vincristine. The implication for treatment of medulloblastomas of early childhood may be a reduction in the chemotherapeutic dose recommendations of these agents and subsequently a decrease in the risk of treatment sequelae for these patients.

Keywords:
Synergism, Genistein, Cisplatin, Vincristine, Medulloblastoma
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2000
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