Application of a combined aggregate exposure pathway and adverse outcome pathway (AEP-AOP) approach to inform a cumulative risk assessment: A case study with phthalates.

Advancements in measurement and modeling capabilities are providing unprecedented access to estimates of chemical exposure and bioactivity. With this influx of new data, there is a need for frameworks that help organize and disseminate information on chemical hazard and exposure in a manner that is accessible and transparent. A case study approach was used to demonstrate integration of the Adverse Outcome Pathway (AOP) and Aggregate Exposure Pathway (AEP) frameworks to support cumulative risk assessment of co-exposure to two phthalate esters that are ubiquitous in the environment and that are associated with disruption of male sexual development in the rat: di(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DnBP). A putative AOP was developed to guide selection of an in vitro assay for derivation of bioactivity values for DEHP and DnBP and their metabolites. AEPs for DEHP and DnBP were used to extract key exposure data as inputs for a physiologically based pharmacokinetic (PBPK) model to predict internal metabolite concentrations. These metabolite concentrations were then combined using in vitro-based relative potency factors for comparison with an internal dose metric, resulting in an estimated margin of safety of ~13,000. This case study provides an adaptable workflow for integrating exposure and toxicity data by coupling AEP and AOP frameworks and using in vitro and in silico methodologies for cumulative risk assessment.

Development of a human physiologically based pharmacokinetic (PBPK) model for phthalate (DEHP) and its metabolites: A bottom up modeling approach.

DEHP exposure to human comes from different sources such as food, diet, cosmetics, toys, medical products, and food wraps. Recently, DEHP was categorized as non-persistent endocrine disrupting compounds (EDCs) by the world health organization (WHO). Rat experimental studies showed that phthalate and its metabolite(s) can cause hepatic, developmental and reproductive toxicity. In human, DEHP rapidly metabolizes into a toxic metabolite MEHP. This MEHP further metabolizes into the different chemical forms of 5OH-MEHP, 5oxo-MEHP, 5cx-MEPP and phthalic acid. A simple DEHP pharmacokinetics model has been developed, but with a limited number of metabolites. A chemical like DEHP which extensively metabolised indicate the need of a detail metabolic kinetics study. A physiological based pharmacokinetics (PBPK) model of the DEHP considering all the major metabolites in human, has not been developed yet. The objective of this study is to develop a detailed human PBPK model for the DEHP and its major metabolites by using a bottom-up modelling approach with the integration of a in vitro metabolic data. This approach uses an in-vitro-in-vivo extrapolation (IVIVE) and a quantitative structure-activity relationship (QSAR) method for the parameterization of the model. Monte Carlo simulations were performed to estimate the impact of parametric uncertainty onto the model predictions. First, the model was calibrated using the control human kinetic study that represents the time course of DEHP metabolites concentration in both the blood and the urine. Then, the model was evaluated against the published independent data on different dosing scenarios. The results of model predictions for the DEHP metabolites in both the blood and the urine were well within the range of experimentally observed data. The model also captured the similar trend of time course profile to the observed data, shows model good predictability power. The current developed PBPK model can futher be used for the prediction of the time course of chemical concentrations for the different exposure scenarios not only in the blood and the urine but also in the other compartments. Moreover, this model can also be used to explore different biomonitoring studies for the human health risk assessment and might be useful for integrative toxicological study in improving exposure-target tissue dose-response relationship.

Excretion of Di-2-ethylhexyl phthalate (DEHP) metabolites in urine is related to body mass index because of higher energy intake in the overweight and obese.

A higher body mass index (BMI) has been positively associated with the rate of excretion of di-2-ethylhexyl phthalate (DEHP) metabolites in urine in data from the National Health and Nutrition Examination Survey (NHANES), suggesting an association between DEHP exposure and BMI. The association, however, may be due to the association between body mass maintenance and higher energy intake, with higher energy intake being accompanied by a higher intake of DEHP. To examine this hypothesis, we ran a Monte Carlo simulation with a DEHP physiologically-based pharmacokinetic (PBPK) model for adult humans. A realistic exposure sub-model was used, which included the relation of body weight to energy intake and of energy intake to DEHP intake. The model simulation output, when compared with urinary metabolite data from NHANES, supported good model validity. The distribution of BMI in the simulated population closely resembled that in the NHANES population. This indicated that the simulated subjects and DEHP exposure model were closely aligned with the NHANES population of interest. In the simulated population, the ordinary least squares regression coefficient for log(BMI) as a function of log(DEHP nmol/min) was 0.048 (SE 0.001), as compared with the reported value of 0.019 (SE 0.005). In other words, given our model structure, the higher energy intake in the overweight and obese, and the concomitant higher DEHP exposure, describes the reported relationship between BMI and DEHP.

Assessment of di (2-ethylhexyl) phthalate exposure by urinary metabolites as a function of sampling time.

OBJECTIVES: In most DEHP exposure assessment studies, single spot urine sample was used. It could not compare the exposure level among studies. Therefore, we are going to represent the necessity of selection of proper sampling time of spot urine for assessing the environmental DEHP exposure, and the association urinary DEHP metabolites with steroid hormones. METHODS: We collected urine and plasma from 25 men. The urine sampling times were at the end of the shift (post-shift) and the next morning before the beginning of the shift (pre-shift). Three metabolites of DEHP {mono(2-ethylhexyl) phthalate [MEHP], mono-(2-ethyl-5-hydroxyhexyl)phthalate [MEHHP], and mono(2-ethyl-5-oxohexyl)phthalate [MEOHP]} in urine were analyzed by HPLC/MS/MS. Plasma luteinzing hormone, follicle stimulating hormone, testosterone, and 17ß-estradiol were measured at pre-shift using a ELISA kit. A log-transformed creatinine-adjusted urinary MEHP, MEHHP, and MEOHP concentration were compared between the post- and pre-shift. The Pearson's correlation was calculated to assess the relationships between log-transformed urinary MEHP concentrations in pre-shift urine and hormone levels. RESULTS: The three urinary metabolite concentrations at post-shift were significantly higher than the concentrations in the pre-shift (p<0.0001). The plasma hormones were not significantly correlated with log-transformed creatinine - adjusted DEHP metabolites. CONCLUSIONS: To assess the environmental DEHP exposure, it is necessary to select the urine sampling time according to the study object. There were no correlation between the concentration of urinary DEHP metabolites and serum hormone levels.

A simple pharmacokinetic model to characterize exposure of Americans to di-2-ethylhexyl phthalate.

A simple pharmacokinetic model to predict concentrations of metabolites of di-2-ethylhexyl phthalate, DEHP, in humans starting from intakes of DEHP was developed and applied. This model predicts serum and urine concentrations of five DEHP metabolites: MEHP, 5oxo-MEHP, 5OH-MEHP, 5cx-MEPP, and 2cx-MMHP. The model was calibrated using data from an individual who dosed himself with 48.5 mg DEHP, and then took blood and urine samples over a 44-h period. The calibrated model was then used in two applications: one on a second set of individuals whose exposure to DEHP was through PVC medical devices in a blood platelet donation procedure, and one on background exposures in the United States (US). Based on 2001/02 NHANES data, median US background urine concentrations of MEHP, 5OH-MEHP, and 5oxo-MEHP are 4.1, 20.1, and 14.0 microg/l, respectively. Creatine and urine volume-correction approaches were used to backcalculate an average daily dose of DEHP in the range of 0.6-2.2 microg/kg per day. A "background cohort" including 8 individuals and 57 complete days of urination were assumed to be exposed to1.5 microg/kg per day, spread out in equal doses of 0.3 microg/kg per day at 0900, 1200, 1500, 1800, and 2100 h. The average predicted urine concentrations were 4.6, 15.9, and 9.4 microg/l for MEHP, 5OH-MEHP, and 5oxo-MEHP. These are similar, but the two secondary metabolites are slightly lower than medians found in NHANES. This slight difference between the NHANES results and the background simulations could have been due to differences in metabolism between the individual who provided the calibration data (61-year-old Caucasian male) and the general US population. Another explanation evaluated was that urine concentrations further from the time of exposure may have larger disparities between MEHP and the two secondary metabolites as compared with concentrations measured closer to the time of exposure.

Risk assessment of di(2-ethylhexyl)phthalate released from PVC blood circuits during hemodialysis and pump-oxygenation therapy.

This study deals with in vitro investigation of the release of di(2-ethylhexyl)phthalate (DEHP) during hemodialysis and pump-oxygenation therapy using medical grade PVC tubing. High resolution GC-MS analysis showed that the release of DEHP was time-dependently increased by circulation of bovine blood into a major system for the hemodialysis that is used in Japan, and the amount of DEHP released into the blood had reached 7.3 mg by 4 h of circulation. No significant difference was observed in the release patterns of DEHP under the conditions with and without fluid removal treatment during hemodialysis, indicating that the treatment seems not to be effective for eliminating DEHP from the blood through the hemodialysis membrane. Mono(2-ethylhexyl)phthalate (MEHP) analysis revealed that a small amount of DEHP (3-4%) was converted to MEHP by hydrolysis during the circulation of blood. A considerable amount of DEHP was also released from the PVC circuit mimicking the pump-oxygenation system, and 7.5-12.1 mg of DEHP had migrated into bovine blood from the circuit by 6 h. It was noticed, however, that the release was obviously suppressed by covalently coating the inner surface of the PVC tubing with heparin, though this effect was not observed with ionic bond type-heparin coating. Covalent bond type-heparin coating of PVC tubing seems to offer the advantage of decreasing the amount of DEHP exposure to patients during treatment using a PVC circuit.