Inhibition of the Growth of an Amethopterin-refractory Tumor by Dietary Restriction of Folic Acid^*

The comparative effects of treatment with the folic acid antimetabolite, amethopterin, or dietary depletion of folic acid on the growth of Walker carcinosarcoma 256 were studied.

Treatment with amethopterin (0.25, 0.5, and 1.0 mg/kg on alternate days) did not significantly depress the growth of this tumor except at doses which induced toxic effects. In contrast, marked inhibition of tumor growth was observed in rats maintained on a folic acid-deficient purified diet. When this diet was fed starting 2 weeks before transplantation or on the same day, tumor growth was inhibited by more than 95 per cent on a weight basis on the 28th day after implantation. These effects were observed in animals which were not severely deficient in folic acid, since growth was not impaired and only a slight reduction in leukocyte count was observed. Although tumor growth was inhibited for as long as 6 weeks in deficient rats, the tumor remained viable and resumed rapid growth when the animals were transferred to a complete diet. Analysis of the tissue levels of folic acid indicated that the content of this vitamin was 12 times higher in livers from rats fed a diet containing 1.25 mg folic acid/kg than in those from animals fed a purified diet that was deficient in folic acid; on the other hand, the difference in folic acid level in the tumors was less than twofold. These observations indicate that the Walker tumor is not capable of parasitizing the host or of competing with other tissues to gain the folic acid that it requires for optimum growth.