Methyl carbamate. Species-dependent variations in metabolism and clearance in rats and mice

Studies of the chronic toxicity and carcinogenicity of methyl carbamate (MC) in F344 rats and B6C3F1 mice indicate that this compound is more toxic to rats than mice. MC was also a carcinogen for rats but not a carcinogen for mice even when administered at a higher dose. The present study of the comparative metabolism and disposition of MC in these two species was conducted to determine possible sources of these varying responses. Results of this study indicate that, although the initial distribution of MC in the two species is similar, the mouse metabolizes and clears MC much more rapidly than does the rat. In the mouse, clearance was primarily by metabolism to CO2 and elimination in exhaled air, which accounted for approximately 70% of the dose in 48 hr. On the other hand, the rat eliminated approximately 18% of the dose as CO2 in 48 hr and a similar amount in urine. The parent compound accounted for approximately 90% of the material excreted in urine of both rats and mice. Only the parent compound was detected in tissues of either species. Less than 4% of the dose was excreted in feces of either species. The lesser ability of the rat to metabolize and eliminate MC as CO2 results in bioaccumulation of this compound on repeat exposure. Therefore, bioaccumulation of MC by the rat on chronic exposure probably results in both greater total exposure and higher peak exposure of most rat tissues vs. those of mice and may thus account for the greater toxicity and possibly carcinogenicity of MC to rats.