Pharmacokinetic modeling of perfluorooctanoic acid during gestation and lactation in the mouse

Why is elevation of serum cholesterol associated with exposure to perfluoroalkyl substances (PFAS) in humans? A workshop report on potential mechanisms

Serum concentrations of cholesterol are positively correlated with exposure to perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in humans. The associated change in cholesterol is small across a broad range of exposure to PFOA and PFOS. Animal studies generally have not indicated a mechanism that would account for the association in humans. The extent to which the relationship is causal is an open question. Nonetheless, the association is of particular importance because increased serum cholesterol has been considered as an endpoint to derive a point of departure in at least one recent risk assessment. To gain insight into potential mechanisms for the association, both causal and non-causal, an expert workshop was held Oct 31 and Nov 1, 2019 to discuss relevant data and propose new studies. In this report, we summarize the relevant background data, the discussion among the attendees, and their recommendations for further research.

Per- and polyfluoroalkyl substances in human breast milk and current analytical methods

Per- and polyfluoroalkyl substances (PFASs) have since become a major health concern as they have been reportedly found in human tissues, blood and breast milk. The main aim of the study was to review the current data on PFASs in human breast milk, including the challenges of analysis as well as the possible modes of transfer from maternal blood. In this paper, previously published data on the concentrations of PFASs in human breast milk from around the world were reviewed and summarised. Eligible studies with reference lists published before 1 June 2017 were included by searching several databases (including Scopus, ScienceOpen and SciFinder). From this search, studies with the number of participants in each study ranging from 2 to 1237 were identified. The review indicated that based on the structural profiles and concentration levels, there was variation in the geographical distribution of these compounds in breast milk. Although there are no recorded investigations on the modes of transfer from maternal blood to breast milk, literature suggests that the PFASs tend to be transferred through binding to various proteins. The review also examined the different sample preparation and analytical methods employed to measure the concentrations of PFASs in human breast milk. This showed that solid phase extraction was the most common extraction method. After extraction, liquid chromatography coupled with tandem mass spectrometry was the most common analysis method. Since several of these methods were initially dedicated to monitoring PFASs in food and water, they demonstrate some limitations with regard to specificity and sensitivity to human fluids. Additionally, there are currently no published records of certified reference materials and/or proficiency scheme devoted to standardising PFAS concentrations in breast milk.

Incorporation of fetal and child PFOA dosimetry in the derivation of health-based toxicity values

BACKGROUND

Multiple agencies have developed health-based toxicity values for exposure to perfluorooctanoic acid (PFOA). Although PFOA exposure occurs in utero and through breastfeeding, current health-based toxicity values have not been derived using fetal or child dosimetry. Therefore, current values may underestimate the potential risks to fetuses and nursing infants.

OBJECTIVE

Using fetal and child dosimetry, we aimed to calculate PFOA maternal human equivalent doses (HEDs), corresponding to a developmental mouse study lowest observed adverse effect level (LOAEL, 1mg/kg/day). Further, we investigated the impact of breastfeeding duration and PFOA half-life on the estimated HEDs.

METHODS

First, a pharmacokinetic model of pregnancy and lactation in mice was used to estimate plasma PFOA levels in pups following a maternal exposure to 1mg PFOA/kg/day for gestational days 1-17. Four plasma PFOA concentration metrics were estimated in pups: i) average prenatal; ii) average postnatal; iii) average overall (prenatal and postnatal); and iv) maximum. Then, Monte Carlo simulations were performed using a pharmacokinetic model of pregnancy and lactation in humans to generate distributions of maternal HEDs that would result in fetal/child plasma levels equivalent to those estimated in pups using the mouse model. Median (HED₅₀) and 1st percentile (HED₀₁) of calculated HEDs were calculated.

RESULTS

Estimated PFOA maternal HED₅₀s ranged from 3.0×10^-4 to 1.1×10^-3mg/kg/day and HED₀₁s ranged from 4.7×10^-5 to 2.1×10^-4mg/kg/day. All calculated HEDs were lower than the HED based on adult dosimetry derived by the Environmental Protection Agency (EPA) (5.3×10^-3mg/kg/day).

CONCLUSION

Our results suggest that fetal/child dosimetry should be considered when deriving health-based toxicity values for potential developmental toxicants.

Bioaccumulation of perfluoroalkyl acids in dairy cows in a naturally contaminated environment

Beef and dairy products may be important vectors of human exposure to perfluoroalkyl acids (PFAAs), but the understanding of how PFAAs are accumulated and transferred through agricultural food chains is very limited. Here, the bioaccumulation of PFAAs in dairy cows receiving naturally contaminated feed and drinking water was investigated by conducting a mass balance of PFAAs for a herd of dairy cows in a barn on a typical Swedish dairy farm. It was assumed that the cows were able to reach steady state with their dietary intake of PFAAs. Perfluorooctane sulfonic acid (PFOS) and perfluoroalkyl carboxylic acids (PFCAs) with 8 to 12 carbons were detected in cow tissue samples (liver, muscle, and blood) at concentrations up to 130 ng kg(-1). Mass balance calculations demonstrated an agreement between total intake and excretion within a factor of 1.5 and consumption of silage was identified as the dominant intake pathway for all PFAAs. Biomagnification factors (BMFs) were highly tissue and homologue specific. While BMFs of PFOS and PFCAs with 9 and 10 fluorinated carbons in liver ranged from 10 to 20, perfluorooctanoic acid (PFOA) was not biomagnified (BMF<1) in any of the investigated tissues. Biotransfer factors (BTFs; defined as the concentration in tissue divided by the total daily intake) were calculated for muscle and milk. Log BTFs ranged from -1.95 to -1.15 day kg(-1) with the highest BTF observed for PFOS in muscle. Overall, the results of this study suggest that long-chain PFAAs have a relatively high potential for transfer to milk and beef from the diet of dairy cows. However, a low input of PFAAs to terrestrial systems via atmospheric deposition and low bioavailability of PFAAs in soil limits the amount of PFAAs that enter terrestrial agricultural food chains in background contaminated environments and makes this pathway less important than aquatic exposure pathways. The BTFs estimated here provide a useful tool for predicting human exposure to PFAAs via milk and beef under different contamination scenarios.

Dosimetric anchoring of in vivo and in vitro studies for perfluorooctanoate and perfluorooctanesulfonate

In order to compare between in vivo toxicity studies, dosimetry is needed to translate study-specific dose regimens into dose metrics such as tissue concentration. These tissue concentrations may then be compared with in vitro bioactivity assays to perhaps identify mechanisms relevant to the lowest observed effect level (LOEL) dose group and the onset of the observed in vivo toxicity. Here, we examine the perfluorinated compounds (PFCs) perfluorooctanoate (PFOA) and perfluorooctanesulfonate (PFOS). We analyzed 9 in vivo toxicity studies for PFOA and 13 in vivo toxicity studies for PFOS. Both PFCs caused multiple effects in various test species, strains, and genders. We used a Bayesian pharmacokinetic (PK) modeling framework to incorporate data from 6 PFOA PK studies and 2 PFOS PK studies (conducted in 3 species) to predict dose metrics for the in vivo LOELs and no observed effect levels (NOELs). We estimated PK parameters for 11 combinations of chemical, species, strain, and gender. Despite divergent study designs and species-specific PK, for a given effect, we found that the predicted dose metrics corresponding to the LOELs (and NOELs where available) occur at similar concentrations. In vitro assay results for PFOA and PFOS from EPA's ToxCast project were then examined. We found that most in vitro bioactivity occurs at concentrations lower than the predicted concentrations for the in vivo LOELs and higher than the predicted concentrations for the in vivo NOELs (where available), for a variety of nonimmunological effects. These results indicate that given sufficient PK data, the in vivo LOELs dose regimens, but not necessarily the effects, could have been predicted from in vitro studies for these 2 PFCs.

Development of PBPK models for PFOA and PFOS for human pregnancy and lactation life stages

Perfluoroalkyl acid carboxylates and sulfonates (PFAA) have many consumer and industrial applications. Developmental toxicity studies in animals have raised concern about potential reproductive/developmental effects of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS); however, in humans conflicting results have been reported for associations between maternal PFAA levels and these outcomes. Risk assessments and interpretation of available human data during gestation and lactation are hindered due to lack of a framework for understanding and estimating maternal, fetal, and neonatal pharmacokinetics (PK). Physiologically based pharmacokinetic (PBPK) models were developed for PFOA and PFOS for the gestation and lactation life stages in humans to understand how the physiological changes associated with development affect pharmacokinetics of these compounds in the mother, fetus, and infant. These models were derived from PBPK models for PFOA/PFOS that were previously developed for adult humans and rats during gestation and lactation and from existing human pregnancy and lactation models developed for other chemicals. The models simulated PFOA and PFOS concentrations in fetal, infant, and maternal plasma and milk, were compared to available data in humans, and also were used to estimate maternal exposure. The models reported here identified several research needs, which include (1) the identification of transporters involved in renal resorption to explain the multiyear half-lives of these compounds in humans, (2) factors affecting clearance of PFOA/PFOS during gestation and lactation, and (3) data to estimate clearance of PFOA/PFOS in infants. These models may help address concerns regarding possible adverse health effects due to PFOA/PFOS exposure in the fetus and infant and may be useful in comparing pharmacokinetics across life stages.

Evaluation of placental and lactational pharmacokinetics of PFOA and PFOS in the pregnant, lactating, fetal and neonatal rat using a physiologically based pharmacokinetic model

Perfluoroalkyl carboxylates and sulfonates (PFAAs) have many consumer and industrial applications. Developmental toxicity studies in animals have raised concern about potential developmental effects of PFOA and PFOS in humans. We have developed PBPK models for PFAAs in the rat to help define a relationship between external dose, internal tissue concentrations, and observed adverse effects, and to understand how physiological changes that occur during gestation and lactation affect tissue distribution of PFAAs in the mother, fetus, and neonate. The models developed here expand upon a PBPK model for PFAAs in the adult female rat, and are consistent with available PK data. These models, along with the adult rat PFAA models, published in the companion paper, will help address concerns about possible health effects due to PFAA exposure in the fetus and neonate and will be useful in comparing PK across life stages.

Prenatal perfluorooctanoic acid exposure in CD-1 mice: low-dose developmental effects and internal dosimetry

Perfluorooctanoic acid (PFOA) is an environmental contaminant that causes adverse developmental effects in laboratory animals. To investigate the low-dose effects of PFOA on offspring, timed-pregnant CD-1 mice were gavage dosed with PFOA for all or half of gestation. In the full-gestation study, mice were administered 0, 0.3, 1.0, and 3.0 mg PFOA/kg body weight (BW)/day from gestation days (GD) 1-17. In the late-gestation study, mice were administered 0, 0.01, 0.1, and 1.0 mg PFOA/kg BW/day from GD 10-17. Exposure to PFOA significantly (p < 0.05) increased offspring relative liver weights in all treatment groups in the full-gestation study and in the 1.0 mg PFOA/kg group in the late-gestation study. In both studies, the offspring of all PFOA-treated dams exhibited significantly stunted mammary epithelial growth as assessed by developmental scoring. At postnatal day 21, mammary glands from the 1.0 mg/kg GD 10-17 group had significantly less longitudinal epithelial growth and fewer terminal end buds compared with controls (p < 0.05). Evaluation of internal dosimetry in offspring revealed that PFOA concentrations remained elevated in liver and serum for up to 6 weeks and that brain concentrations were low and undetectable after 4 weeks. These data indicate that PFOA-induced effects on mammary tissue (1) occur at lower doses than effects on liver weight in CD-1 mice, an observation that may be strain specific, and (2) persist until 12 weeks of age following full-gestational exposure. Due to the low-dose sensitivity of mammary glands to PFOA in CD-1 mice, a no observable adverse effect level for mammary developmental delays was not identified in these studies.

The occurrence of perfluorinated alkyl compounds in human milk from different regions of China

Perfluorinated compounds (PFCs), especially perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA), are known to occur throughout the environment and in the human population (Houde et al., 2006). The occurrence of PFCs in human umbilical cord blood and human milk, coupled with the potential developmental toxicity of PFCs, suggests the need for determining the exposure sources and magnitudes of PFCs in infants. In this study, 10 PFCs were measured in 24 pooled samples consisting of 1237 individual human milk samples. The samples were collected from 12 provinces of China in 2007. PFOS and PFOA were the predominant PFCs found in all the samples tested. The geometric mean (GM) and median of the concentrations were 46 pg/mL and 49 pg/mL for PFOS, 46 pg/mL and 34.5 pg/mL for PFOA respectively. A large variation in geographical distribution was observed for PFCs in human milk. High concentrations of PFOA (814 pg/ml for the rural samples and 616 pg/ml for the urban samples) were found in human milk from Shanghai. Estimated dietary intakes (EDI) were established and the median, GM and the highest EDI of the total PFCs were 17.2 ng/kg/d, 17.8 ng/kg/d and 129.1 ng/kg/d respectively. The EDI for PFOA (88.4 ng/kg/d) for Shanghai was close to the tolerable daily intake (100 ng/kg/d) proposed by the German Federal Institute for Risk Assessment and the Drinking Water Commission. The results suggest both mothers and infants have a high exposure to PFCs in the Shanghai region. The potential health impact of postnatal exposure through breastfeeding to infants should therefore be comprehensively evaluated.