Several types of analogs of the natural pyrimidines have been tested for their ability to restrain the growth of mammary Adenocarcinoma 755 in the C57BL mouse host. Several 6-azapyrimidines and two 5-azapyrimidines as well as a number of pyrimidines were found active. The most active single member of this group was 6-azacytosine, which produced a significant inhibition and regression rate at dosages of less than one-tenth the LD50 level.

The pyrimidine analogs were tested together with urethan, which in itself was active. Most of the combinations showed little or no synergism, but with 6-azauracil a striking potentiation was observed. The combination of 6-azauracil and urethan was found capable of producing resorption of all tumors of Strain 755 at dose levels which had no activity when the individual compounds were used singly.

By means of reversal studies and investigations of the incorporation of uracil-2-C14, the attempt has been made to relate the activity of the inhibitors to their effects on pyrimidine metabolism.

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