Outbred male Sprague-Dawley CD rats were fed a complete semisynthetic diet and were given supplemental low doses (2 ppm) of
selenium as H2SeO3 in their
drinking water or 50 mg
13-cis-retinoic acid (13-cis-RA) and 2 g
beta-sitosterol/kg diet either singly, in combinations of two, or in combinations of all three. Intestinal
tumors were induced with eight weekly sc
injections of 8 mg
azoxymethane (AOM)/kg
body weight, and inhibition of
tumor formation was determined by
tumor counts after 26 weeks. Noncarcinogen controls for each dietary group received eight
injections of sterile water.
Tumor inhibition was statistically significant in 2 groups of animals: Dietary control animals had a
tumor frequency of 5.07
tumors/rat, rats receiving
selenium- plus 13-cis-RA supplementation had a
tumor frequency of 3.77, and those being given the combination of all three inhibitors had 2.75
tumors/rat. Analysis of fecal
steroids from 3 AOM groups (dietary controls, the
beta-sitosterol plus 13-cis-RA-supplemented group, and the group receiving all three additives) after 4 months of supplementation showed that the addition of
beta-sitosterol to the diet had no effect on acidic or neutral
steroids, regardless of the observed difference in
tumor frequency. These results suggest that subpharmacologic doses of inhibitors, particularly those that inhibit the process by different mechanisms, while ineffective alone, may provide significant inhibition of
tumorigenesis when used in combination.