Chronic studies evaluating the carcinogenicity of monomethylarsonic acid in rats and mice.

Monomethylarsonic acid (MMA) was administered in the diet of male and female Fischer F344 rats and B6C3F1 mice in 2-year feeding studies according to US EPA guidelines. Rats were treated with 50, 400, or 1300 ppm MMA and mice were treated with 10, 50, 200, or 400 ppm MMA based on preliminary short-term studies. The highest dose in the male and female rat groups was reduced to 1000 ppm during week 53 and then further reduced to 800 ppm during week 60 due to high mortality in the male rats. There was no treatment-related mortality in the mice. The primary target organ for MMA-induced toxicity in rats and mice was the large intestine. Toxicity was more severe in rats compared to mice and in male rats compared to female rats. The maximum tolerated dose for chronic dietary administration of MMA in rats and mice was assessed as 400 ppm, and the no effect level with regard to intestinal toxicity was assessed as 50 ppm for rats and female mice and 200 ppm for male mice. There were no treatment-related neoplastic effects detected in either the rat or the mouse.