Di(2-ethylhexyl) phthalate (DEHP) induces testicular toxicity in rats but not in monkeys. To clarify the cause of this species difference and evaluate the risk of DEHP-induced toxicity in humans, the metabolite profiles of rats, monkeys, and humans were compared. Primary hepatocytes from rats, cynomolgus monkeys, and humans were incubated with mono(2-ethylhexyl) phthalate (MEHP), a primary metabolite of DEHP in the intestine. Eighteen radioactive peaks were found to be shared among rats, monkeys, and humans. MEHP was rapidly metabolized in humans compared to that in rats and monkeys. In humans, the metabolic pathway of MEHP was primarily oxidation followed by conjugation with glucuronic acid. In monkeys, the direct glucuronidation of MEHP was predominant. In contrast, in rat hepatocytes, various oxidized forms of MEHP were observed, but the glucuronide forms were minor, indicating that glucuronidation activity was inferior in rats compared to other species. For comparison, monoisononyl phthalate (MINP), a primary metabolite of diisononyl phthalate (DINP), another type of phthalate esters, was applied to the same test system. The metabolic profile of MINP is similar to that of MEHP. Glucuronide-conjugated metabolites were predominantly observed in humans and monkeys, whereas oxidative metabolites were mainly detected in rats. These results suggest that the difference in hepatic glucuronidation activity is the cause of species differences in DEHP-induced testicular toxicity between rats and monkeys. Together with the results of the DINP study, our findings highlight that species differences in metabolism must be carefully considered when extrapolating phthalate ester toxicity data for human risk assessments.
Tasaki et al. (2026) studied this question.