The systemic toxicity of a trimethylbenzene isomer and constituent of C9 aromatic solvents (1,3,5-trimethylbenzene, 135-TMB) was studied in Sprague-Dawley rats following a 90-day oral gavage exposure to 0, 50, 200 and 600 mg/kg/day. No statistically significant effects on body weight, body weight gain or food consumption were observed at study termination. Treatment-related changes in clinical chemistry parameters at the end of the 90-day dosing period were limited to small, but statistically significant, increases in phosphorus levels in high dose males and females. Liver enlargement in high dose male/female rats was considered an adaptive response as this was reversible and was not associated with histopathological lesions or increased liver enzyme markers indicative of liver damage. Kidney weight changes were limited to a small, but statistically significant, increase in relative weights in high dose males. This was not associated with histopathological lesions and thus not considered toxicologically relevant. Overall, the No-Observed-Adverse-Effect-Level (NOAEL) was the highest concentration tested (600 mg/kg/day). The results of the present study are relevant for assessing the risk of trimethylbenzenes through the oral route of exposure and provide a basis for the development of provisional screening values for trimethylbenzene isomers while avoiding the uncertainty associated with route-to-route extrapolation.
