The metabolism of di(2-ethylhexyl) phthalate (DEHP) and mono-(2-ethylhexyl) phthalate (MEHP) in rats: In vivo and in vitro dose and time dependency of metabolism

This study investigated the in vivo metabolism of di(2-ethylhexyl) phthalate (DEHP) and mono(2-ethylhexyl) phthalate (MEHP) in rats after multiple dosing, the metabolism of MEHP in primary rat hepatocyte cultures for periods of up to 3 days, and the biotransformation of some major metabolites of MEHP. Rats were orally administered [ 14C]DEHP or [ 14C]MEHP at doses of 50 and 500 mg/kg body wt for three consecutive days. Urine was collected at 24-hr intervals, and metabolite profiles were determined. After a single dose of either compound, urinary metabolite profiles were similar to those previously reported. However, after multiple administration of both DEHP and MEHP at 500 mg/kg, increases in ω-/β-oxidation products [metabolites I and V, mono(3-carboxy-2-ethylpropyl) phthalate and mono(5-carboxy-2-ethylpentyl) phthalate, respectively] and decreases in ω — 1-oxidation products [metabolites VI and IX, mono(2-ethyl-5-oxohexyl) phthalate and mono(2-ethyl-5-hydroxyhexyl) phthalate, respectively] were seen. At the low dose of 50 mg/kg little or no alteration in urinary metabolite profiles was observed. At 500 mg/kg of MEHP a 4-fold stimulation of CN −-insensitive palmitoyl-CoA oxidation (a peroxisomal β-oxidation marker) was seen after three consecutive daily doses. At the low dose of 50 mg/kg only a 1.8-fold increase was noted. Similar observations were made with rat hepatocyte cultures. MEHP at concentrations of 50 and 500 μ m was extensively metabolized in the rat hepatocyte cultures. Similar metabolic profiles to those seen after in vivo administration of MEHP were observed. At the high (500 μ m) concentration of MEHP, changes in the relative proportions of ω- and ω — 1-oxidized metabolites were seen. Over the 3-day experimental period, ω-/β-oxidation products increased in a time-dependent manner at the expense of ω — 1-oxidation products. At a concentration of 500 μ m MEHP, a 12-fold increase of CN −-insensitive palmitoyl CoA oxidation (a peroxisomal β-oxidation marker) was observed. At the low concentration of MEHP (50 μ m) only a 3-fold increase in CN −-insensitive palmitoyl-CoA oxidation was noted and little alteration in the metabolite profile of MEHP was observed with time. Biotransformation studies of the metabolites of MEHP confirmed the postulated metabolic pathways. Metabolites I and VI appeared to be endpoints of metabolism, while metabolite V was converted to metabolite I, and metabolite IX to metabolite VI. It was also possible to deduce the tr