Lack of carcinogenicity of chlorpyrifos insecticide in a high-dose, 2-year dietary toxicity study in Fischer 344 rats.

Chlorpyrifos (CPF) was administered daily in the feed to evaluate toxicity and oncogenicity potential in male and female Fischer 344 rats, according to U.S. EPA guidelines. Doses for the 2-year study were based on findings in a 13-week feeding study in which lower body weights, urinary perineal staining, adrenal cortical vacuolization, and inhibition (slightly more than 60%) of brain cholinesterase (ChE) occurred at 15 mg/kg/day. The high dose in the subsequent 2-year study was 10 mg/kg/day, with lower doses of 0, 0.05, 0.1, or 1.0 mg/kg/day chosen to define dose-response patterns. Rats given 10 mg/kg/day for 2 years were healthy and there was no evidence of premature deaths. Mild toxicity occurred only in rats given 10 mg/kg/day and consisted of perineal urine soiling in females and a 6-8% body-weight decrease in males. Males given 10 mg/kg/day also had increased adrenal weights and vacuolation of the adrenal zona fasciculata. ChE was considered a measure of exposure. Plasma, RBC, and brain ChE activities were inhibited in rats given 10 mg/kg/day, and the plasma and RBC ChE activities were inhibited in rats given 1.0 mg/kg/day. Chronic exposure to 0.1 mg/kg/day was considered a threshold exposure level for inhibition of plasma ChE. Rats given 10 mg/kg/day, considered a maximum-tolerated dose, had approximately 60% chronic inhibition of brain ChE. This group had similar numbers and types of neoplasms as control rats. Consequently, CPF was not carcinogenic at dose levels up to 10 mg/kg/day.