Human risk assessment of dermal and inhalation exposures to chemicals assessed by route-to-route extrapolation: the necessity of kinetic data.

In toxicity testing the oral route is in general the first choice. Often, appropriate inhalation and dermal toxicity data are absent. Risk assessment for these latter routes usually has to rely on route-to-route extrapolation starting from oral toxicity data. Although it is generally recognized that the uncertainties involved are (too) large, route-to-route extrapolation is applied in many cases because of a strong need of an assessment of risks linked to a given exposure scenario. For an adequate route-to-route extrapolation the availability of at least some basic toxicokinetic data is a pre-requisite. These toxicokinetic data include all phases of kinetics, from absorption (both absorbed fraction and absorption rate for both the starting route and route of interest) via distribution and biotransformation to excretion. However, in practice only differences in absorption between the different routes are accounted for. The present paper demonstrates the necessity of route-specific absorption data by showing the impact of its absence on the uncertainty of the human health risk assessment using route-to-route extrapolation. Quantification of the absorption (by in vivo, in vitro or in silico methods), particularly for the starting route, is considered essential.

Proper knowledge on toxicokinetics improves human hazard testing and subsequent health risk characterisation. A case study approach.

In the current EU legislative frameworks on chemicals safety, the requirements with respect to information on general kinetic parameters (absorption, distribution, metabolism and excretion or ADME) or integrated toxicokinetic parameters (TK, i.e. plasma concentration-time curve, area under the curve etcetera) in humans and experimental animals vary widely. For agrochemicals and cosmetics, there are regulatory requirements whereas for other frameworks, such as food ingredients, biocides, consumer products and high production volume chemicals (REACH) there are very little or no requirements. This paper presents case studies that illustrate the importance of ADME and TK data in regulatory risk characterisations. The examples were collected by interviewing regulatory risk assessors from various chemicals (non-pharmaceutical) frameworks. The case studies illustrate how (1) applying ADME/TK in an early phase of toxicity testing can be used to improve study design and support the 3R-goals and how (2) increased use of ADME/TK data can improve the final risk assessment.

Comparison of the mouse Embryonic Stem cell Test, the rat Whole Embryo Culture and the Zebrafish Embryotoxicity Test as alternative methods for developmental toxicity testing of six 1,2,4-triazoles.

The relatively high experimental animal use in developmental toxicity testing has stimulated the search for alternatives that are less animal intensive. Three widely studied alternative assays are the mouse Embryonic Stem cell Test (EST), the Zebrafish Embryotoxicity Test (ZET) and the rat postimplantation Whole Embryo Culture (WEC). The goal of this study was to determine their efficacy in assessing the relative developmental toxicity of six 1,2,4-triazole compounds,(1) flusilazole, hexaconazole, cyproconazole, triadimefon, myclobutanil and triticonazole. For this purpose, we analyzed effects and relative potencies of the compounds in and among the alternative assays and compared the findings to their known in vivo developmental toxicity. Triazoles are antifungal agents used in agriculture and medicine, some of which are known to induce craniofacial and limb abnormalities in rodents. The WEC showed a general pattern of teratogenic effects, typical of exposure to triazoles, mainly consisting of reduction and fusion of the first and second branchial arches, which are in accordance with the craniofacial malformations reported after in vivo exposure. In the EST all triazole compounds inhibited cardiomyocyte differentiation concentration-dependently. Overall, the ZET gave the best correlation with the relative in vivo developmental toxicities of the tested compounds, closely followed by the EST. The relative potencies observed in the WEC showed the lowest correlation with the in vivo developmental toxicity data. These differences in the efficacy between the test systems might be due to differences in compound kinetics, in developmental stages represented and in the relative complexity of the alternative assays.

Application of toxicogenomics in hepatic systems toxicology for risk assessment: acetaminophen as a case study.

Hepatic systems toxicology is the integrative analysis of toxicogenomic technologies, e.g., transcriptomics, proteomics, and metabolomics, in combination with traditional toxicology measures to improve the understanding of mechanisms of hepatotoxic action. Hepatic toxicology studies that have employed toxicogenomic technologies to date have already provided a proof of principle for the value of hepatic systems toxicology in hazard identification. In the present review, acetaminophen is used as a model compound to discuss the application of toxicogenomics in hepatic systems toxicology for its potential role in the risk assessment process, to progress from hazard identification towards hazard characterization. The toxicogenomics-based parallelogram is used to identify current achievements and limitations of acetaminophen toxicogenomic in vivo and in vitro studies for in vitro-to-in vivo and interspecies comparisons, with the ultimate aim to extrapolate animal studies to humans in vivo. This article provides a model for comparison of more species and more in vitro models enhancing the robustness of common toxicogenomic responses and their relevance to human risk assessment. To progress to quantitative dose-response analysis needed for hazard characterization, in hepatic systems toxicology studies, generation of toxicogenomic data of multiple doses/concentrations and time points is required. Newly developed bioinformatics tools for quantitative analysis of toxicogenomic data can aid in the elucidation of dose-responsive effects. The challenge herein is to assess which toxicogenomic responses are relevant for induction of the apical effect and whether perturbations are sufficient for the induction of downstream events, eventually causing toxicity.

Quantitative extrapolation of in vitro whole embryo culture embryotoxicity data to developmental toxicity in vivo using the benchmark dose approach.

If in vitro data are to be used as a basis for hazard characterization, a translation of an in vitro concentration toward an in vivo dose must be made. In this study we examined the correlation between dose descriptors from the in vitro Whole Embryo Culture (WEC) test and in vivo developmental toxicity tests. We applied the Benchmark Dose (BMD) approach to estimate equipotent in vitro concentrations (Benchmark Concentrations [BMCs]) and equipotent in vivo doses (BMDs). Using the data generated in an European Center for the Validation of Alternative Methods validation study we found that the BMCs were highly reproducible among laboratories. The three endpoints analyzed (head length, crown-rump length, and total morphological score) were strongly correlated. A clear in vitro-in vivo correlation was found between BMCs and BMDs. However, a considerable uncertainty would remain if the BMDs were estimated from the BMC using this correlation: the confidence interval of such an in vivo dose estimate would span various orders of magnitude. Differences in toxicokinetic properties among the compounds explained at least part of the scatter of the in vitro-in vivo correlation. But also heterogeneity in the design of the available in vivo studies underlies much of the scatter, and this puts a limit on validating in vitro data as predictors of in vivo data. Further analysis of the in vitro-in vivo correlation would therefore require high-quality in vivo data, generated by appropriate (and similar) study designs.

The margin of internal exposure (MOIE) concept for dermal risk assessment based on oral toxicity data - A case study with caffeine.

Route-to-route extrapolation is a common part of human risk assessment. Data from oral animal toxicity studies are commonly used to assess the safety of various but specific human dermal exposure scenarios. Using theoretical examples of various user scenarios, it was concluded that delineation of a generally applicable human dermal limit value is not a practicable approach, due to the wide variety of possible human exposure scenarios, including its consequences for internal exposure. This paper uses physiologically based kinetic (PBK) modelling approaches to predict animal as well as human internal exposure dose metrics and for the first time, introduces the concept of Margin of Internal Exposure (MOIE) based on these internal dose metrics. Caffeine was chosen to illustrate this approach. It is a substance that is often found in cosmetics and for which oral repeated dose toxicity data were available. A rat PBK model was constructed in order to convert the oral NOAEL to rat internal exposure dose metrics, i.e. the area under the curve (AUC) and the maximum concentration (Cmax), both in plasma. A human oral PBK model was constructed and calibrated using human volunteer data and adapted to accommodate dermal absorption following human dermal exposure. Use of the MOIE approach based on internal dose metrics predictions provides excellent opportunities to investigate the consequences of variations in human dermal exposure scenarios. It can accommodate within-day variation in plasma concentrations and is scientifically more robust than assuming just an exposure in mg/kg bw/day.

Neurobehavioural evaluation and kinetics of inhalation of constant or fluctuating toluene concentrations in human volunteers.

The health risks of inhalation exposure to volatile organic solvents may not only depend on the total external dose, but also on the pattern of exposure. It has been suggested that exposure to regularly occurring peak concentrations may have a stronger impact on the brain than constant exposure at the same average level. Recent animal experimental studies conducted in our laboratory using relatively high concentrations of toluene have shown different effects on discrimination performance and motor activity during and after exposure, depending on the exposure scenario. Relevance of these findings for man was evaluated in a volunteer study in which 11 healthy men (age 20-49 years) were exposed by inhalation for 4h to either a constant concentration of 40ppm toluene or to three 30-min exposure peaks at 110ppm during this 4h period. Selected tests from the Neurobehavioural Evaluation System (NES) were performed repeatedly during and after exposure. Blood concentrations of toluene as well as urinary o-cresol excretion were measured at relevant time points. The results show that toluene concentration in blood increased during constant exposure and fluctuated during occupationally relevant peak exposures. Presumably, brain concentrations showed similar qualitative patterns. No clear changes were observed on neurobehavioural measures of motor performance, attention, perceptual coding and memory, or on measures of mood and affect. The exposure conditions do not seem to induce significant acute changes in central nervous system function similar to those observed at much higher concentrations in animals, although a statistical correlation was found between one motor performance test (Finger Tapping Test with alternating hands) and blood toluene concentrations. Urinary o-cresol excretion appeared to be significantly higher during the first 2h after exposure.

Subacute (28-day) toxicity of furfural in Fischer 344 rats: a comparison of the oral and inhalation route.

The subacute oral and inhalation toxicity of furfural vapour was studied in Fischer 344 rats to investigate whether route-to-route extrapolation could be employed to derive the limit value for inhalation exposure from oral toxicity data. Groups of 5 rats per sex were treated by gavage daily for 28 days at dose levels of 6-192 mg/kg bw/day, or exposed by inhalation to concentrations of 20-1280 mg/m3 (6 h/day, 5 days/week) or 160-1280 mg/m3 (3 h/day, 5 days/week) for 28 days. Controls received vehicle (corn oil) or were exposed to clean air. Daily oral treatment with the highest dose of furfural (initially 192 mg/kg bw/day, later reduced to 144 mg/kg bw/day and finally to 120 mg/kg bw/day) resulted in mortality, and in increases in absolute and relative kidney and liver weight in surviving females of this group. Exposure of rats by inhalation for 6 h/day, 5 days/week for 28 days induced mortality at concentrations of 640 mg/m3 and above within 1-8 days. At 640 mg/m3 (3 h/day) and at 320 mg/m3 (3 and 6 h/day) and below, however, exposure was tolerated without serious clinical effects. In contrast, histopathological nasal changes were seen even at the lowest concentration of 20 mg/m3. With increasing exposure concentration, the nasal effects increased in incidence and severity and also expanded from the anterior part to the posterior part, including the olfactory epithelium. It was concluded that the no-observed-adverse-effect level (NOAEL) for oral toxicity was 96 mg/kg bw/day. The NOAEL for systemic inhalation toxicity was comparable, i.e. 92 mg/kg bw/day (corresponding to 320 mg/m3 (6 h/day) or 640 mg/m3 (3 h/day)) assuming 100% absorption. The presence of the histopathological nasal changes at the lowest tested concentration of 20 mg/m3 (corresponding to 6 mg/kg bw/day) proves that for locally acting substances like furfural extrapolation from the oral to the inhalation route is not valid.

Oral-to-inhalation route extrapolation in occupational health risk assessment: a critical assessment.

Due to a lack of route-specific toxicity data, the health risks resulting from occupational exposure are frequently assessed by route-to-route (RtR) extrapolation based on oral toxicity data. Insight into the conditions for and the uncertainties connected with the application of RtR extrapolation has not been clearly described in a systematic manner. In our opinion, for a reliable occupational health risk assessment, it is necessary to have insight into the accuracy of the routinely applied RtR extrapolation and, if possible, to give a (semi-)quantitative estimate of the possible error introduced. Therefore, experimentally established no-observed-adverse-effect-levels for inhalation studies were compared to no-adverse-effect-levels predicted from oral toxicity studies by RtR extrapolation. From our database analysis it can be concluded that the widely used RtR extrapolation methodology based on correction for differences in (estimates of) absorption is not generally reliable and certainly not valid for substances inducing local effects. More experimental data are required (from unpublished data or new experiments) to get insight into the reliability of RtR extrapolation and the possibility to derive an assessment factor to account for the uncertainties. Moreover, validated screening methods to predict/exclude the occurrence of local effects after repeated exposure are warranted. Especially, in cases where chemical exposure by inhalation or skin contact cannot be excluded route-specific toxicity studies should be considered to prevent from inadequate estimates of human health risks.