ONCOGENICITY STUDIES OF PIPERONYL BUTOXIDE IN RATS AND MICE

1 The oncogenicity of Piperonyl butoxide (PBO) has been studied in the mouse and rat. CD-1 mice were administered PBO in the diet at target doses of 0, 30, 100 and 300 mg/kg/day for 79 weeks and Sprague-Dawley rats 0, 30, 100 and 500 mg/kg/day for 104/105 weeks. 2 At termination of the study in the mouse there was evidence of increased liver weight s and an increased incidence of eosinophilic adenomas at 100 and 300 m g/kg/day in males and 300 mg/kg/day in females. 3 In rats there was in creased liver weights at 100 and 500 mg/kg/day associated with hepatoc yte hypertrophy in both male and female rats. There was no increased i ncidence of neoplasia at any site. Hypertrophy and hyperplasia of thyr oid follicles was observed at 500 mg/kg/day in both sexes. 4 The obser vations reflect the expected changes related to the induction of the m ixed function oxygenase group of enzymes. In the mouse the increased i ncidence of eosinophilic adenomas is not considered relevant for human risk evaluation.