Importance of the duration of inhibition on intestinal carcinogenesis by difluoromethylornithine in rats.

The effect of the duration and sequence of inhibition of intestinal tumor formation in rats was studied to determine whether part time inhibition has any value. Four groups of male Sprague-Dawley rats were given 8 weekly s.c. injections of azoxymethane (AOM) 8 mg/rat. Three groups were given the inhibitor, difluoromethylornithine (DFMO) in the drinking water; one for the entire 26 weeks of the study, one for the first 13 weeks only, and one for the last 13 weeks. A control group was not given the inhibitor. While the continuous treatment group developed the least number of tumors per rat (1.5 vs. 5 for controls), still both groups given the inhibitor for just 13 weeks also developed fewer tumors than controls 5 vs. 3.2 (early treatment) and 5 vs. 2.8 (late treatment). These results show that part time inhibition, including its late application, does reduce intestinal tumor formation in rats.