Novel derivatives of 1-phenyl-1H-pyrazolo[3,4-d]pyrimidine substituted at the 4-position of the pyrimidine ring are provided. A typical example is 1-phenyl-1H-pyrazolo[3,4-d]pyrimidine-4-carbonitrile. The compounds exhibit excellent anti-carcinogenic activities as evidenced by the animal test with mice.
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