Urinary metabolites of diisodecyl phthalate in rats

Diisodecyl phthalate (DiDP) is an isomeric mixture of phthalates with predominantly 10-carbon branched-dialkyl chains, widely used as a plasticizer for polyvinyl chloride. The extent of human exposure to DiDP is unknown in part because adequate biomarkers of exposure to DiDP are not available. We identified several major metabolites of DiDP in urine of adult female Sprague-Dawley rats after a single oral administration of DiDP (300 mg/kg). These metabolites can potentially be used as biomarkers of exposure to DiDP. The metabolites extracted from urine were chromatographically resolved and identified by their chromatographic behavior and full scan negative ion electrospray ionization mass spectrum. The identity of metabolites with similar molecular weights was further examined in accurate mass mode. For some metabolites, unequivocal identification was done using authentic standards. Among these were the hydrolytic monoester of DiDP, monoisodecyl phthalate (MiDP), detected as a minor metabolite, and one omega oxidation product of MiDP, mono(carboxy-isononyl) phthalate (MCiNP), which was the most abundant urinary metabolite. We also tentatively identified other secondary metabolites of MiDP, mono(hydroxy-isodecyl) phthalate, mono(oxo-isodecyl) phthalate, mono(carboxy-isoheptyl) phthalate, mono(carboxy-isohexyl) phthalate, mono(carboxy-isopentyl) phthalate, mono(carboxy-isobutyl) phthalate, and mono(carboxy-ethyl) phthalate. Oxidative metabolites of diisoundecyl phthalate (DiUdP) and diisononyl phthalate (DiNP) were also detected suggesting the presence of DiUdP and DiNP in the DiDP formulation. The urinary concentrations of all these metabolites gradually decreased in the 4 days following the administration of DiDP. MCiNP and other DiDP secondary metabolites are more abundant in urine than MiDP, suggesting that these oxidative products are better biomarkers for DiDP exposure assessment than MiDP. Additional research on the toxicokinetics of these metabolites is needed to understand the extent of human exposure to DiDP from the urinary concentrations of MCiNP and other DiDP secondary metabolites.