p53 protein expression, apoptosis and growth delay induced by nedaplatin, a novel platinum compound, were investigated in vivo, and compared with those induced by ionizing radiation. A human ependymoblastoma with wild-type p53 was transplanted subcutaneously to the thighs of nude mice. The incidences of p53 protein-positive cells and apoptosis in tumors increased following exposure to ionizing radiation. In tumors treated with nedaplatin, they also increased, but the incidences of p53 protein-positive cells and apoptosis induced by 32 mg/kg nedaplatin, 1/2 LD50, were lower than those induced by 1 Gy irradiation. However, growth-delay assay showed no significant difference between the efficacy of 32 mg/kg nedaplatin and that of 1 Gy irradiation. These results suggest that the main antineoplastic activity caused by nedaplatin may be mediated through different mechanisms than those of the p53-dependent early apoptosis.

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