The development of a satisfactory rodent model for cancer of the large intestine began with the discovery by Laqueur and associates in 1962 that the plant product, cycasin (methylazoxymethanol glycoside), is a potent carcinogen for rodents. Soon after that, DMH, AOM, and MAM were found to be even more efficient intestinal carcinogens in rats. These three compounds, plus two direct acting carcinogens (MNNG, MNU) are used almost exclusively in current animal investigations. Although all these chemicals have some degree of activity in all rodents, they are most effective in rats. Various rat strains differ somewhat in susceptibility, Sprague-Dawley being the most sensitive to these carcinogens. Cancers of the large intestine in the animal model resemble adenocarcinomas in humans, and they spread in a similar manner except that metastases to the liver and lung are very uncommon in animals. Animal studies support epidemiological and human experimental observations of dietary factors involved in colorectal cancer formation. Most physicians believe that the majority of colorectal cancers develop from preexisting adenomas. Morson has shown that large adenomas and villous adenomas have a greater risk of developing cancer than small adenomas. Hill has theorized that there are different factors responsible for the formation of small adenomas from normal mucosa, for the growth of small to large adenomas, and for the development of cancer from large adenomas. Animal studies provide some support for this concept. Weak intestinal carcinogens tend to induce more benign adenomas than carcinomas. Very small doses of strong carcinogens also induce some adenomas and a few early polypoid intestinal cancers after a long latent period. Moderate to large amounts of DHM, for example, induce only malignant lesions even when these lesions are as small as 1 mm. These observations suggest a relationship between adenomas and carcinomas. There is also biochemical evidence to support the staged progression of carcinogenesis. An example is the graded increases in ODC activity that occur in tissues undergoing tumorigenesis.
Adenoma Animals