Interspecies differences in perfluoroalkyl substances (PFAS) toxicokinetics and application to health-based criteria.

Toxicokinetics are important for extrapolating health effects and effect levels observed in laboratory animals to humans for purposes of establishing health-based criteria. We conducted a comprehensive review of key absorption, distribution, metabolism, and excretion (ADME) parameters across different mammalian species for five perfluoroalkyl substances (PFAS) and discussed how these data can be used to inform human health risk assessment of these substances. Our analysis revealed several notable differences among the different PFAS regarding species- and substance-specific tissue partitioning, half-life, and transfer to developing offspring via the placenta or lactation, as well as highlighted data gaps for certain substances. We incorporated these observations in an analysis of whether health-based values for specific PFAS can be applied to other PFAS of differing chain length or toxicological mode of action. Overall, our analysis provides one of the first syntheses of available empirical PFAS toxicokinetic data to facilitate interpreting human relevance of animal study findings and developing health-based criteria for PFAS from such studies.

Ozone exposure and systemic biomarkers: Evaluation of evidence for adverse cardiovascular health impacts.

The US Environmental Protection Agency (EPA) recently concluded that there is likely to be a causal relationship between short-term (< 30 days) ozone exposure and cardiovascular (CV) effects; however, biological mechanisms to link transient effects with chronic cardiovascular disease (CVD) have not been established. Some studies assessed changes in circulating levels of biomarkers associated with inflammation, oxidative stress, coagulation, vasoreactivity, lipidology, and glucose metabolism after ozone exposure to elucidate a biological mechanism. We conducted a weight-of-evidence (WoE) analysis to determine if there is evidence supporting an association between changes in these biomarkers and short-term ozone exposure that would indicate a biological mechanism for CVD below the ozone National Ambient Air Quality Standard (NAAQS) of 75 parts per billion (ppb). Epidemiology findings were mixed for all biomarker categories, with only a few studies reporting statistically significant changes and with no consistency in the direction of the reported effects. Controlled human exposure studies of 2 to 5 hours conducted at ozone concentrations above 75 ppb reported small elevations in biomarkers for inflammation and oxidative stress that were of uncertain clinical relevance. Experimental animal studies reported more consistent results among certain biomarkers, although these were also conducted at ozone exposures well above 75 ppb and provided limited information on ozone exposure-response relationships. Overall, the current WoE does not provide a convincing case for a causal relationship between short-term ozone exposure below the NAAQS and adverse changes in levels of biomarkers within and across categories, but, because of study limitations, they cannot not provide definitive evidence of a lack of causation.

Diazinon and diazoxon impair the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons.

Evidence from in vivo and epidemiological studies suggests that organophosphorus insecticides (OPs) are developmental neurotoxicants, but possible underlying mechanisms are still unclear. Astrocytes are increasingly recognized for their active role in normal neuronal development. This study sought to investigate whether the widely-used OP diazinon (DZ), and its oxygen metabolite diazoxon (DZO), would affect glial-neuronal interactions as a potential mechanism of developmental neurotoxicity. Specifically, we investigated the effects of DZ and DZO on the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons. The results show that both DZ and DZO adversely affect astrocyte function, resulting in inhibited neurite outgrowth in hippocampal neurons. This effect appears to be mediated by oxidative stress, as indicated by OP-induced increased reactive oxygen species production in astrocytes and prevention of neurite outgrowth inhibition by antioxidants. The concentrations of OPs were devoid of cytotoxicity, and cause limited acetylcholinesterase inhibition in astrocytes (18 and 25% for DZ and DZO, respectively). Among astrocytic neuritogenic factors, the most important one is the extracellular matrix protein fibronectin. DZ and DZO decreased levels of fibronectin in astrocytes, and this effect was also attenuated by antioxidants. Underscoring the importance of fibronectin in this context, adding exogenous fibronectin to the co-culture system successfully prevented inhibition of neurite outgrowth caused by DZ and DZO. These results indicate that DZ and DZO increase oxidative stress in astrocytes, and this in turn modulates astrocytic fibronectin, leading to impaired neurite outgrowth in hippocampal neurons.

Strengthening the foundation of next generation risk assessment.

In a recent draft report, Next Generation Risk Assessment: Incorporation of Recent Advances in Molecular, Computational, and Systems Biology, the US Environmental Protection Agency presents valuable contributions to understanding the roles that evolving toxicity testing methods and associated interpretative techniques can play in assessing the risks associated with chemical exposures. However, the evaluations presented in the NexGen report would benefit from more thorough consideration of several essential components of a critical review of toxicity data, e.g., data quality, data relevance, and the extent to which the test endpoints reflect adverse effects. Such considerations are necessary to ensure that the NexGen report evaluations--and the resulting conclusions and recommendations--are grounded in scientifically sound, representative data reviews. We illustrate these concerns with a critique of the report's prototype ozone evaluation. Although substantial additional research is needed before new toxicity data types can be used reliably in rigorous risk assessment applications, they clearly offer exciting opportunities for advancing toxicological science and risk assessment. By explicitly identifying limitations still to be addressed and providing stronger guideposts for future research needs, the NexGen report could serve an influential role in achieving the promise of these new research approaches.

Quantitative assessment of lung and bladder cancer risk and oral exposure to inorganic arsenic: Meta-regression analyses of epidemiological data.

Inorganic arsenic (iAs) in drinking water varies geographically and is prevalent worldwide. While exposures in the US are generally low, there are some areas with higher levels of naturally occurring iAs (potentially >100µg/L) where residents rely on unregulated drinking water wells. Much of the evidence on the association between iAs and cancer comes from epidemiological studies conducted in South American and Asian populations. These populations have generally been exposed to much higher levels of iAs and have differing underlying characteristics, both of which make comparing them to Western populations difficult. A key question is whether and how one should extrapolate from these high exposure studies to estimate cancer risk at lower exposures. We conducted an independent analysis to determine the most appropriate cancer endpoints, studies, and models to support an oral carcinogenicity assessment of iAs, taking into consideration factors that affect the apparent potency of iAs across geographically and culturally distinct populations. We identified bladder and lung cancer as high-priority endpoints and used meta-regression to pool data across studies from different regions of the world to derive oral cancer slope factors (CSFs) and unit risks (excess risk per µg/L) for iAs based on the background risks of bladder and lung cancer in the US. We also calculated concentrations of iAs in water that are not likely to result in cancer risk above what is considered acceptable by the United States Environmental Protection Agency (US EPA). While we derived these factors assuming a linear, no-threshold relationship between iAs and cancer risk, we also evaluated the shape of the dose-response curves and assessed the evidence for overall nonlinearity. Overall, we found that the incremental risks of bladder and lung cancer associated with iAs were relatively low. The sensitivity analyses we conducted suggested that populations with relatively high iAs exposures appeared to drive the pooled cancer risk estimates, but many of our other tested assumptions did not substantially alter these estimates. Finally, we found that the mode of action evidence supports there being a threshold, but making a robust quantitative demonstration of a threshold using epidemiological data is difficult. When considered in the context of typical exposure levels in the US, our potency estimates indicate that iAs-induced cancer risk is much lower than observed bladder and lung cancer incidences. This suggests that the low iAs levels to which much of the general US population is exposed likely do not result in substantial additional cancer risk.

Astrocytes protect against diazinon- and diazoxon-induced inhibition of neurite outgrowth by regulating neuronal glutathione.

Evidence demonstrating that human exposure to various organophosphorus insecticides (OPs) is associated with neurobehavioral deficits in children continues to emerge. The present study focused on diazinon (DZ) and its active oxygen metabolite, diazoxon (DZO), and explored their ability to impair neurite outgrowth in rat primary hippocampal neurons as a mechanism of developmental neurotoxicity. Both DZ and DZO (0.5-10 µM) significantly inhibited neurite outgrowth in hippocampal neurons, at concentrations devoid of any cyototoxicity. These effects appeared to be mediated by oxidative stress, as they were prevented by antioxidants (melatonin, N-t-butyl-alpha-phenylnitrone, and glutathione ethyl ester). Inhibition of neurite outgrowth was observed at concentrations below those required to inhibit the catalytic activity of acetylcholinesterase. The presence of astrocytes in the culture was able to provide protection against inhibition of neurite outgrowth by DZ and DZO. Astrocytes increased neuronal glutathione (GSH) in neurons, to levels comparable to those of GSH ethyl ester. Astrocytes depleted of GSH by L-buthionine-(S,R)-sulfoximine no longer conferred protection against DZ- and DZO-induced inhibition of neurite outgrowth. The findings indicate that DZ and DZO inhibit neurite outgrowth in hippocampal neurons by mechanisms involving oxidative stress, and that these effects can be modulated by astrocytes and astrocyte-derived GSH. Oxidative stress from other chemical exposures, as well as genetic abnormalities that result in deficiencies in GSH synthesis and regulation, may render individuals more susceptible to these developmental neurotoxic effects of OPs.