The appearance of chemically induced mammary gland carcinomas in virgin female Sprague-Dawley rats was blocked by the administration of d,l-2-difluoromethylomithine (DFMO) in drinking water during the stage of tumor promotion. Rats were given injections s.c. at 50 days of age with either 35 mg of 1-methyl-1-nitrosourea (MNU) per kg of body weight or the 0.9% NaCl solution in which the carcinogen was dissolved. At 57 days of age, the rats were each randomly allocated to one of 14 treatment groups. Ten groups (five solvent treated and five MNU treated) were assigned to treatments consisting of 0.00, 0.0625, 0.125, 0.25, or 0.50% (w/v) solution of DFMO in their drinking water; two MNU-treated groups were placed on or removed from DFMO treatment (0.5%; w/v) at 90 days post-carcinogen exposure; and two carcinogen-treated groups received either putrescine (0.5-g/kg diet) or putrescine and DFMO (0.5%; w/v) throughout the experiment.

The study was terminated 183 days after carcinogen treatment. All doses of DFMO exerted a protective effect against the induction of mammary cancer; however, only the feeding of the 0.125% and the 0.5% solutions of DFMO resulted in a significant reduction in cancer incidence. The average number of cancers per rat was reduced, and cancer-free time was extended at all concentrations of DFMO. The protective effect of DFMO was sustained following withdrawal of treatment at 90 days post-MNU injection. Feeding putrescine in conjunction with DFMO treatment partially blocked the inhibitory activity of DFMO. DFMO treatment did not affect food or water intake; body weight gain; the weight of ovaries, uterus, adrenal glands, liver, kidney, or spleen; or the periodicity of the estrous cycle. These data provide evidence of an inhibitory effect of DFMO against mammary cancer induced by MNU which cannot be attributed to a systemic toxic effect of this compound.

1

Supported by USPHS Grant CA 32465 from the National Cancer Institute.

2

Scientific Contribution No. 1332 from the New Hampshire Agricultural Experiment Station.

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