Carcinogenicity of metamifop, a novel herbicide, in Wistar rats following oral administration for 104 weeks

Metamifop is a novel herbicide with as yet undetermined properties. To assess its carcinogenicity, metamifop was mixed into standard rodent chow and fed to male and female Wistar rats at doses of 10, 100 and 750 ppm for 104 weeks. The viability/mortality of these rats was not affected by treatment with metamifop. Treatment had no significant effects on clinical parameters, and food consumption. Males and females fed 750 ppm of metamifop for 104 weeks showed decreases in body weight and body weight gain. Histopathological examination revealed that treatment with metamifop reduced non-neoplastic findings (chronic progressive nephropathy, tubular basophilia, tubular casts, glomerulosclerosis, basophilic and clear cell foci, senile atrophy, and mesothelial hyperplasia) and reduced neoplastic findings (thymoma, pituitary adenoma, and mammary fibroadenoma and adenocarcinoma in females, and mesenteric lymph node hemangioma in males) compared with control groups. Benign granulosa cell tumors were increased in a dose-dependent manner. As metamifop did not show any genotoxic potential, and there was no correlation between ovarian cancer and increased gonadal hormone levels in humans, the granulosa cell tumors observed in female rats fed a high dose of metamifop were considered not relevant to humans.