Promoting Effect of Saccharin and DL-Tryptophan in Urinary Bladder Carcinogenesis1

we have been particularly concerned with the identification of possible morphological markers of neoplastic change in bladder epithelium, using light microscopy and both scan fling and transmission electron microscopy. This required a model in which the sequence of biological and pathological events was predictable. In a previous study, we identified as †̃ †̃subthreshold' †̃ a dose of FANFT, 0.2%, which when fed for 6 weeks produced no bladder tumors if the rats were then fed control diet until 84 weeks, although the same dose produced tumors if fed for 8 weeks followed by control diet (34). The present investigation was based on the concept that this subthreshold dose of FANFT, an †̃ †̃ incom pletecarcinogen― atthe levelofadministration, could serve as an initiator, permitting us to explore the phase of tumor promotion in this model system. This concept, the use of minimal amounts of known chemical carcinogens as initiating agents and the subse quent administration of agents known to be noncarcino genic when administered by themselves, has been used by investigators for many years in their attempts to identify various stages in the carcinogenic process (67), beginning with investigations of 2-stage murine skin carcinogenesis (8-10). A similar process has subsequently been demon strated in other tissues, notably the liver (47). For the urinary bladder (29, 31 , 32), it has been shown by Hicks that instillation into the rat bladder of a subcarcino genic dose of MNU followed by p.o. administration of saccharin or cyclamate resulted in a high incidence of bladder cancer, while saccharin or cyclamate alone in duced a very low incidence of bladder cancer. Cyclophos phamide, known to induce necrosis and then a marked hyperplasia of the bladder epithelium in rats, did not induce bladder tumors in the same experimental system after a subcarcinogenic dose of MNU. In the experiments described in this report, we attempted to determine whether at least 2 stages existed in bladder carcinogenesis, using for initiation an agent would could be given in the diet rather than instilled into the bladder and as promoting agents in this system, saccharin and tryptophan. Saccharin, administered in the diet rather than in the drinking water, was used because of its demonstrated effectiveness as a promoting agent in the MNU model as described above. Tryptophan was chosen as another pos sible promoting agent because of the suggested relation ship between abnormal tryptophan metabolite levels and bladder cancer in humans (14, 48, 68) and because of a variety of studies in experimental animals indicating a possible relationship of tryptophan metabolites with blad der cancer (14, 48). Previous studies in our laboratory on the FANFT bladder cancer model in Fischer rats indicated that an early marker ABSTRACT