Early onset of spontaneous renal preneoplastic and neoplastic lesions in young conventional rats in toxicity studies.

Although occurring in aged laboratory rodents, spontaneous renal tumour are unknown in animals younger than 18 weeks. A survey on renal preneoplastic and neoplastic lesions has been performed on Sprague-Dawley rats from general toxicology studies over the period January 2004 - May 2006. Data from 2249 rats necropsied and 1206 rats (688 males and 518 females) examined microscopically from 52 studies, are reported. The age range at necropsy was between 12 and 18 weeks and all the animals were obtained from the same supplier. Three cases of tubular carcinoma, 1 tubular adenoma, and 4 cases of atypical tubular hyperplasia were observed in 5 females and 3 males from both control and treated groups from 6 studies with unrelated test compounds. In treated rats, the lesions were considered spontaneous in nature, rather than related to treatment, because of: (1) their sporadic incidence, (2) the short duration of the studies, and (3) the absence of similar lesions in other rats given the same test compound. These lesions are considered a recently occurring spontaneous finding, and the similarities with the familial renal cancer models, namely the Eker and the Nihon models, strongly suggest genetic factors as responsible for the lesions.