It is important for the assessment of toxicological effects of chemicals to know what kinds of neoplasms naturally occur in the early life of experimental animals. In the present study, we demonstrated spontaneous neoplasms in Sprague-Dawley rats used in 4-, 13- and 26-week toxicity studies conducted at Bozo Research Center in the last decade. The tumors, which were first observed in 19-week-old animals, included anterior adenoma of the pituitary, follicular cell adenocarcinoma and C cell adenoma of the thyroids, nephroblastoma of the kidneys, basal cell tumor of the skin and malignant lymphoma. Thereafter, hemangiosarcoma of the tongue, adenocarcinoma of the submandibular glands, histiocytic sarcoma of the spleen, oligodendroglioma of the brain and adenocarcinoma and fibroadenoma of the mammary glands were detected in 32-week-old animals. The incidences of mammary adenocarcinoma and pituitary anterior adenoma were higher than those of other tumors. The present results showed that the same tumors as reported in aged rats could also develop in younger rats.

The chemopreventive effects of phenethyl isothiocyanate (PEITC) were investigated in N-nitrosobis(2-oxopropyl)-amine (BOP)-treated hamsters. Female 5-week-old Syrian golden hamsters were divided into six groups. Animals in groups 1-3, each consisting of 30 hamsters, were given BOP by two subcutaneous injections 7 days apart at a dose of 20 mg/kg body weight, plus either 100, 10 or 0 micromol of PEITC in corn oil by gavage 2 h prior to each BOP treatment, respectively per group. Animals in groups 4 and 5, each consisting of 10 hamsters, were given 100 and 10 micromol of PEITC alone in corn oil, and 10 animals in group 6 served as a vehicle control. Animals were sacrificed 52 weeks after the first BOP injection. Both the incidences and multiplicities of lung adenomas and/or adenocarcinomas were significantly decreased in a dose-dependent manner by PEITC treatments (P < 0.01 or 0.05). The lung tumor incidences were inhibited by 100% with 100 micromol PEITC and by 82% with the 10 micromol dosage. In addition, the high dose of PEITC also significantly inhibited pancreatic carcinogenesis (P < 0.05) and showed a tendency to lower the incidences of liver and renal tumors, although these effects were not statistically significant. Under the present experimental conditions, PEITC itself did not cause any apparent toxicity. Our results thus indicate that PEITC is a remarkably effective chemopreventive agent for the BOP-induced lung and pancreatic tumors in hamsters.

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