Reply to Schade G. Comment on Hess et al. "Assessing Agreement in Exposure Classifications between Proximity-Based Metrics and Air Monitoring Data in Epidemiology Studies of Unconventional Resource Development."

We appreciate the comments by Dr [...].

Response to Buonocore et al. Comments on Wendt Hess et al. "Assessing Agreement in Exposure Classification between Proximity-Based Metrics and Air Monitoring Data in Epidemiology Studies of Unconventional Resource Development." Int. J. Environ. Res. Public Health 2019, 16, 3055.

We appreciate the comments by Buonocore et al [...].

Assessing Agreement in Exposure Classification between Proximity-Based Metrics and Air Monitoring Data in Epidemiology Studies of Unconventional Resource Development.

Recent studies of unconventional resource development (URD) and adverse health effects have been limited by distance-based exposure surrogates. Our study compared exposure classifications between air pollutant concentrations and "well activity" (WA) metrics, which are distance-based exposure proxies used in Marcellus-area studies to reflect variation in time and space of residential URD activity. We compiled Pennsylvania air monitoring data for benzene, carbon monoxide, nitrogen dioxide, ozone, fine particulates and sulfur dioxide, and combined this with data on nearly 9000 Pennsylvania wells. We replicated WA calculations using geo-coordinates of monitors to represent residences and compared exposure categories from air measurements and WA at the site of each monitor. There was little agreement between the two methods for the pollutants included in the analysis, with most weighted kappa coefficients between -0.1 and 0.1. The exposure categories agreed for about 25% of the observations and assigned inverse categories 16%-29% of the time, depending on the pollutant. Our results indicate that WA measures did not adequately distinguish categories of air pollutant exposures and employing them in epidemiology studies can result in misclassification of exposure. This underscores the need for more robust exposure assessment in future analyses and cautious interpretation of these existing studies.

The current status of exposure-driven approaches for chemical safety assessment: A cross-sector perspective.

For the purposes of chemical safety assessment, the value of using non-animal (in silico and in vitro) approaches and generating mechanistic information on toxic effects is being increasingly recognised. For sectors where in vivo toxicity tests continue to be a regulatory requirement, there has been a parallel focus on how to refine studies (i.e. reduce suffering and improve animal welfare) and increase the value that in vivo data adds to the safety assessment process, as well as where to reduce animal numbers where possible. A key element necessary to ensure the transition towards successfully utilising both non-animal and refined safety testing is the better understanding of chemical exposure. This includes approaches such as measuring chemical concentrations within cell-based assays and during in vivo studies, understanding how predicted human exposures relate to levels tested, and using existing information on human exposures to aid in toxicity study design. Such approaches promise to increase the human relevance of safety assessment, and shift the focus from hazard-driven to risk-driven strategies similar to those used in the pharmaceutical sectors. Human exposure-based safety assessment offers scientific and 3Rs benefits across all sectors marketing chemical or medicinal products. The UK's National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs) convened an expert working group of scientists across the agrochemical, industrial chemical and pharmaceutical industries plus a contract research organisation (CRO) to discuss the current status of the utilisation of exposure-driven approaches, and the challenges and potential next steps for wider uptake and acceptance. This paper summarises these discussions, highlights the challenges - particularly those identified by industry - and proposes initial steps for moving the field forward.

In vivo distribution of nanosilver in the rat: The role of ions and de novo-formed secondary particles.

Silver nanoparticles are advertised as antimicrobial agents in a wide range of products. The majority of available studies suggest that silver nanoparticle toxicity is mainly caused by silver ions released from the particles. However, it remains challenging to distinguish between the effect of silver nanoparticles and silver ions. Here we used a combination of a short-term in vivo study in rats and an in silico-based toxicokinetic model to determine tissue distribution of administered ionic and nanoparticulate silver, and to estimate mixture ratios of the different silver species, namely primary nanoparticles, ions and secondary particles. Our data indicate that silver nanoparticles and silver ions are not or only marginally bioavailable after oral ingestion of a single, non-toxic dose. Experimental data on organ distribution after intravenous injection were accurately reflected by the predictions of the in silico model. Toxicokinetic modeling suggests systemic distribution of a major proportion of the injected ionic silver as de novo formed secondary nanoparticles, and the presence of such particles was proven by electron microscopy. The observation that silver ions form secondary particles, underlines the difficulties in distinguishing between particle- and ion-dependent effects of silver nanoparticles.

Biomonitoring Equivalents for interpretation of silver biomonitoring data in a risk assessment context.

Silver is widely used as an antimicrobial agent in both ionic and nanoparticle forms, and general population exposure to silver can occur through the presence of trace levels in foods and dusts, through dermal contact with treated textiles, from use of wound care products, and other sources. Biomonitoring for silver in blood or urine in persons in the general population is being conducted by the Canadian Health Measures Survey (CHMS). Tolerable exposure guidance values for silver designed to prevent adverse effects of excess exposure are available from the United States Environmental Protection Agency (an oral reference dose, or RfD), from the United States Food and Drug Administration (a draft provisional tolerable intake, or TI) and from literature evaluations of recent data on responses to nanoparticle silver (a recommended tolerable daily intake, or TDI). A current physiologically-based pharmacokinetic model is used to estimate Biomonitoring Equivalents (BEs) for silver, which are steady-state biomarker concentrations consistent with the RfD, provisional TI, or recommended TDI (BERfD, BETI, or BETDI, respectively). The BE values based on silver in whole blood range from 0.2 to 0.9µg/L. BE values for silver in urine were not derived due to low confidence in the predicted steady-state urinary silver excretion rates. Comparison of general population biomonitoring data from Canada to the derived BE values indicate that general population exposure levels are generally below levels consistent with current risk assessment-derived exposure guidance values.

Translocation of gold nanoparticles across the lung epithelial tissue barrier: Combining in vitro and in silico methods to substitute in vivo experiments.

BACKGROUND: The lung epithelial tissue barrier represents the main portal for entry of inhaled nanoparticles (NPs) into the systemic circulation. Thus great efforts are currently being made to determine adverse health effects associated with inhalation of NPs. However, to date very little is known about factors that determine the pulmonary translocation of NPs and their subsequent distribution to secondary organs. METHODS: A novel two-step approach to assess the biokinetics of inhaled NPs is presented. In a first step, alveolar epithelial cellular monolayers (CMLs) at the air-liquid interface (ALI) were exposed to aerosolized NPs to determine their translocation kinetics across the epithelial tissue barrier. Then, in a second step, the distribution to secondary organs was predicted with a physiologically based pharmacokinetic (PBPK) model. Monodisperse, spherical, well-characterized, negatively charged gold nanoparticles (AuNP) were used as model NPs. Furthermore, to obtain a comprehensive picture of the translocation kinetics in different species, human (A549) and mouse (MLE-12) alveolar epithelial CMLs were exposed to ionic gold and to various doses (i.e., 25, 50, 100, 150, 200 ng/cm(2)) and sizes (i.e., 2, 7, 18, 46, 80 nm) of AuNP, and incubated post-exposure for different time periods (i.e., 0, 2, 8, 24, 48, 72 h). RESULTS: The translocation kinetics of the AuNP across A549 and MLE-12 CMLs was similar. The translocated fraction was (1) inversely proportional to the particle size, and (2) independent of the applied dose (up to 100 ng/cm(2)). Furthermore, supplementing the A549 CML with two immune cells, i.e., macrophages and dendritic cells, did not significantly change the amount of translocated AuNP. Comparison of the measured translocation kinetics and modeled biodistribution with in vivo data from literature showed that the combination of in vitro and in silico methods can accurately predict the in vivo biokinetics of inhaled/instilled AuNP. CONCLUSION: Our approach to combine in vitro and in silico methods for assessing the pulmonary translocation and biodistribution of NPs has the potential to replace short-term animal studies which aim to assess the pulmonary absorption and biodistribution of NPs, and to serve as a screening tool to identify NPs of special concern.

Using physiologically based pharmacokinetic (PBPK) modeling for dietary risk assessment of titanium dioxide (TiO2) nanoparticles.

Nano-sized titanium dioxide particles (nano-TiO2) can be found in a large number of foods and consumer products, such as cosmetics and toothpaste, thus, consumer exposure occurs via multiple sources, possibly involving different exposure routes. In order to determine the disposition of nano-TiO2 particles that are taken up, a physiologically based pharmacokinetic (PBPK) model was developed. High priority was placed on limiting the number of parameters to match the number of underlying data points (hence to avoid overparameterization), but still reflecting available mechanistic information on the toxicokinetics of nano-TiO2. To this end, the biodistribution of nano-TiO2 was modeled based on their ability to cross the capillary wall of the organs and to be phagocytosed in the mononuclear phagocyte system (MPS). The model's predictive power was evaluated by comparing simulated organ levels to experimentally assessed organ levels of independent in vivo studies. The results of our PBPK model indicate that: (1) within the application domain of the PBPK model from 15 to 150 nm, the size and crystalline structure of the particles had a minor influence on the biodistribution; and (2) at high internal exposure the particles agglomerate in vivo and are subsequently taken up by macrophages in the MPS. Furthermore, we also give an example on how the PBPK model may be used for risk assessment. For this purpose, the daily dietary intake of nano-TiO2 was calculated for the German population. The PBPK model was then used to convert this chronic external exposure into internal titanium levels for each organ.

A physiologically based pharmacokinetic model for ionic silver and silver nanoparticles.

Silver is a strong antibiotic that is increasingly incorporated into consumer products as a bulk, salt, or nanosilver, thus potentially causing side-effects related to human exposure. However, the fate and behavior of (nano)silver in the human body is presently not well understood. In order to aggregate the existing experimental information, a physiologically based pharmacokinetic model (PBPK) was developed in this study for ionic silver and nanosilver. The structure of the model was established on the basis of toxicokinetic data from intravenous studies. The number of calibrated parameters was minimized in order to enhance the predictive capability of the model. We validated the model structure for both silver forms by reproducing exposure conditions (dermal, oral, and inhalation) of in vivo experiments and comparing simulated and experimentally assessed organ concentrations. Therefore, the percutaneous, intestinal, or pulmonary absorption fraction was estimated based on the blood silver concentration of the respective experimental data set. In all of the cases examined, the model could successfully predict the biodistribution of ionic silver and 15-150 nm silver nanoparticles, which were not coated with substances designed to prolong the circulatory time (eg, polyethylene glycol). Furthermore, the results of our model indicate that: (1) within the application domain of our model, the particle size and coating had a minor influence on the biodistribution; (2) in vivo, it is more likely that silver nanoparticles are directly stored as insoluble salt particles than dissolve into Ag⁺; and (3) compartments of the mononuclear phagocytic system play a minor role in exposure levels that are relevant for human consumers. We also give an example of how the model can be used in exposure and risk assessments based on five different exposure scenarios, namely dietary intake, use of three separate consumer products, and occupational exposure.