Quantitative investigation of the urinary excretion of three specific monoester metabolites of the plasticizer diisononyl adipate (DINA)

Abstract

Diisononyl adipate (DINA) is a plasticizer used in PVC products as an alternative for restricted phthalate plasticizers. With this study, we provide first data on human DINA metabolism and excretion. We postulated mono(hydroxy-isononyl) adipate (OH-MINA), mono(oxo-isononyl) adipate (oxo-MINA), and mono(carboxy-isooctyl) adipate (cx-MIOA) as specific DINA metabolites based on the known human metabolism of structurally similar adipates and phthalates. Urinary excretion was quantitatively investigated after a single oral dose (113 to 145 µg/kg body weight) to three healthy volunteers using a newly developed online-SPE-LC-MS/MS method with isotope dilution and LOQs between 0.3 - 0.6 µg/L. OH-MINA turned out to be the major of the three metabolites with consistent urinary excretion fractions (F_UEs) of 0.020-0.023 % among all volunteers. Oxo-MINA and cx-MIOA were excreted with lower shares (mean: 0.003 % and 0.009 %, respectively). For all three metabolites, urinary concentrations peaked quickly between 1.4 and 2.3 h post dose with maximum concentrations of 23.1 (OH-MINA), 2.87 (oxo-MINA) and 9.83 µg/L (cx-MIOA). Thus, F_UEs and urinary concentrations were rather low for these specific metabolites, with the major share of the dose presumably being excreted as non-specific metabolites such as adipic acid. In a pilot population (n=35) of German adults without known DINA exposure, we could not detect any of the three metabolites, contrary to the dosage study, indicating to population exposures lower than 50 µg/kg body weight/day. The new HBM method in conjunction with the new F_UEs can be used for objective DINA exposure and risk assessment especially in populations with potentially higher DINA exposures.

Keywords: DINA; diisononyl adipate; human biomonitoring; metabolism; oral dose; plasticizer.

Table 1. Elimination kinetics for OH-MINA, oxo-MINA, and cx-MIOA in three volunteers after a single oral dose. Peak concentrations (c_max - first maximum) after oral dose and their respective time points (t_max) (mean values; ranges in parentheses). Given the limited number of data points, elimination half-lives were not calculated

Table 2. Urinary excretion fractions (F_UEs) of the DINA metabolites OH-MINA, oxo-MINA, and cx-MIOA after a single oral dose in three healthy volunteers (mean values; ranges in parentheses)

Figure 1. Human metabolism pathways of DINA. The side chain oxidized monoester metabolites OH-MINA, oxo-MINA and cx-MIOA (only isomers based on the 4-methyloctyl side chain shown for simplification) were confirmed and quantitatively investigated in the current study. Furthermore, in analogy to human DEHA and DnBA metabolism, adipic acid (AA) is assumed as the ultimate breakdown product (not investigated in this study). For simplification, phase two metabolites (e.g. glucuronic acid conjugates) are not shown.

Figure 2. Chromatograms of (a) a calibration standard in water; (b) a urine sample collected before dose (OH-MINA, oxo-MINA, and cx-MIOA below the LOQ; only the internal standard peaks can be seen); and (c) a urine sample collected 2 h after oral dose. Native DINA metabolite traces shown in black (quantifier transitions as continuous lines and qualifier transitions in dotted lines) and labeled internal standard traces in gray. The time frame of elution of the dose related metabolite isomers in (c) is indicated by open brackets

Figure 3. Elimination kinetics of OH-MINA (top), oxo-MINA (middle), and cx-MIOA (bottom) for all three volunteers after oral DINA dose. Unadjusted concentrations in μg/L with respective LOQs shown with gray dotted lines are presented in column (a), creatinine-adjusted concentrations in μg/g creatinine in column (b), and excretion rates in μg/h in column (c).