Effects of continuous and discontinued exposure to 2-AAF on urinary bladder hyperplasia and neoplasia.

The effect of discontinued feeding of 2-acetylaminofluorine (2-AAF) was compared with the effect of continuous 2-AAF feeding on the urinary bladder of BALB/c female mice. Dietary concentrations of 2-AAF examined in this regard were 60, 75, 100, and 150 ppm. These diets were fed for 9, 12, or 15 months and then replaced with control feed, and the animals were killed at 18 or 24 months. Observations on these animals were contrasted with observations made at the same intervals on animals continuously fed these concentrations of 2-AAF throughout the study. Discontinuing 2-AAF exposure decreased prevalence of bladder tumors below that found in continuously exposed mice and resulted in a decrease in severity and prevalence of hyperplasia. Mild hyperplasia was retained in a portion of each population and the prevalence of this hyperplasia was dependent on both the period of exposure and the dose of 2-AAF. Mild persistent hyperplasia was correlated closely with neoplasia and may be a necessary stage in neoplastic development. Moderate or severe hyperplasia apparently regress when 2-AAF feeding is discontinued and do not appear to be essential steps in the neoplastic process.