Induction of reversible urothelial cell hyperplasia in rats by clorsulon, a flukicide with weak carbonic anhydrase inhibitory activity.

Clorsulon, a flukicide registered for use in treating Fasciola hepatica infections in cattle, has induced urinary bladder urothelial cell hyperplasia in rats at oral doses of 30 mg/kg/day or more. Despite previous testing at doses above this threshold, this lesion had not been found in subchronic or chronic toxicity studies in rats. After ruling out the presence of a contaminant as the causative factor in producing this lesion, a study was conducted in which clorsulon increased the pH and altered the electrolyte composition of urine, consistent with its weak carbonic anhydrase inhibitory activity. The acid/base balance of the diet markedly affected the threshold for induction of the urothelial cell hyperplasia: acidification by addition of 5% ammonium chloride to the diet reduced the incidence and severity. In additional studies it was found that the urothelial cell hyperplasia was most pronounced after 1 week of treatment compared with daily exposure for either 5 or 15 wk. The reversibility of the hyperplasia despite continued treatment confirms that the hyperplasia is not a preneoplastic lesion, a conclusion supported by negative studies of carcinogenicity in rodent bioassays of clorsulon and other drugs with carbonic anhydrase inhibitory activity.