A long-term study of reversible and progressive urinary bladder cancer lesions in rats fed N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide.

Hyperplasia and, ultimately, neoplasia of bladder epithelium were produced by feeding 0.2% N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (FANFT) to weaning male Fischer rats. Hyperplasia induced by FANFT feeding for 2, 4, or 6 weeks, followed by feeding control diet until the end of the 84-week experiment, was reversible, and at the end of the experiment bladder epithelium in these animals was normal by light and scanning electron microscopy. Hyperplasia produced by 8 or more weeks of FANFT feeding was irreversible and by 84 weeks had resulted in bladder tumors in all animals fed FANFT for 12 or more weeks and in 4 of 5 and 6 of 7 animals fed FANFT for 8 and 10 weeks, respectively. The epithelial changes after 6 and 8 weeks of FANFT were similar by light microscopy but different by scanning electron microscopy. Pleomorphic microvilli seen with scanning electron microscopy are the hallmark of irreversible, and possibly progressive, epithelial proliferative change.