Estimation of acute toxicity by fitting a dose-time-response surface

Aquatic Toxicity of Chemical Road Dust Suppressants to Freshwater Organisms

Unpaved roads make up at least 14 million kilometers of the worldwide road network. Although investigations of road runoff often are focused on paved roads, unpaved roads contribute large volumes of runoff to roadside aquatic habitats and introduce unique constituents to runoff, such as chemical dust suppressants. At least 200 products across five chemical categories are commercially available for road dust suppression and are typically applied at rates up to 4.5 L/m². Many of these products are poorly described and are lacking basic information on environmental transport, fate, and potential toxicity to roadside organisms. We characterized the aquatic toxicity of 27 commercially available dust control products, including 13 biobased products from the U.S. Department of Agriculture BioPreferred catalog, using juvenile rainbow trout. Acute toxicity varied by more than 1000-fold among products, with 96-h LC50 values ranging from 1.7 to > 16,000 mg/L. Toxicity was not well-predicted by product category. Testing with selected products after simulated weathering under UV radiation did not provide evidence of photoenhanced toxicity. Additional tests with freshwater mussels, juvenile crayfish, pond snails, and amphibian larvae indicated that juvenile rainbow trout were reasonable surrogates for these organisms for a subset of products. This effort represents one of the first comparative studies of dust suppressant toxicity and provides important information for assessing risk to aquatic resources from a widely used but understudied class of contaminants in road runoff.

A 3D ecotoxi-topological profile: Using concentration-time-response surfaces to show peroxidase activity in Zea mays (L.) exposed to aluminium or arsenic in hydroponic conditions

This study sought to use concentration-time-response surfaces to show the effects of exposure to toxic (semi-)metals on peroxidase activity in higher plants as a function of exposure-concentration and exposure-time. Maize (Zea mays L.) seedlings (i.e., leaves and roots) were exposed to arsenic (as As³⁺) or aluminium (as Al³⁺) under hydroponic conditions, and their biomass and peroxidase enzyme responses were assessed at different concentration-time-exposures. The 3D ecotoxi-profile generated with these data showed two distinct regions: the first region is formed by exposures (i.e., points for time-concentration pairings) that were not statistically different from the results of the control points (i.e., zero toxicant concentration and all exposure-times), whereas the second region is formed by exposure pairings with results that were statistically different to those obtained from control pairings. Overall, the data show that enzyme activity increased over a shorter exposure-time when there was an increase in the exposure-concentration of the toxicant, which can be seen on a 3-D toxicity profile. We propose that quantitative relationship ratios from different assessed endpoints (e.g., biomass and enzyme activity) and enzymatic concentration-time-response surfaces could be helpful in the field of environmental-policy management.

Toxicity of Cadmium, Copper, and Zinc to the Threatened Chiricahua Leopard Frog (Lithobates [Rana] chiricahuensis)

The Chiricahua leopard frog (Lithobates chiricahuensis) is in decline throughout the western United States, and is particularly sensitive to physical, chemical and biotic changes in their habitat. Acute toxicity tests revealed that among the metals detected in Chiricahua leopard frog habitat, copper was toxic at concentrations lower than those observed in the environment. Developing tadpoles were chronically exposed for 60 days to cadmium, copper and zinc because of the potential for long term exposure to these metals during early development. Cadmium was toxic, but at concentrations above observed environmental levels. Copper was especially toxic to this species at concentrations of about 10% of concentrations observed in their habitats. The onset of toxicity occurred within a few days of exposure, thus pulsed exposures from rain events could potentially be acutely toxic to tadpoles of this species. Zinc did not appear to have a negative impact during the acute or chronic exposures.

Oxidative stress prediction: A preliminary approach using a response surface based technique

A response surface was built to predict the lipid peroxidation level, generated in an iron-ascorbate in vitro model, of any organ, which is correlated with the oxidative stress injury in biological membranes. Oxidative stress studies are numerous, usually performed on laboratory animals. However, ethical concerns require validated methods to reduce the use of laboratory animals. The response surface described here is a validated method to replace animals. Tissue samples of rabbit liver, kidney, heart, skeletal muscle and brain were oxidized with different concentrations of FeCl₃ (0.1 to 8mM) and ascorbate (0.1mM), during different periods of time (0 to 90min) at 37°C. Experimental data obtained, with lipid content and antioxidant activity of each organ, allowed constructing a multidimensional surface capable of predicting, by interpolation, the lipid peroxidation level of any organ defined by its antioxidant activity and fat content, when exposed to different oxidant conditions. To check the predictive potential of the technique, two more experiments were carried out. First, in vitro oxidation data from lung tissue were collected. Second, the antioxidant capacity of kidney homogenates was modified by adding melatonin. Then, the response surface generated could predict lipid peroxidation levels produced in these new situations. The potential of this technique could be reinforced using collaborative databases to reduce the number of animals in experimental procedures.

A New Stochastic Kriging Method for Modeling Multi-Source Exposure-Response Data in Toxicology Studies

One of the most fundamental steps in risk assessment is to quantify the exposure-response relationship for the material/chemical of interest. This work develops a new statistical method, referred to as SKQ (stochastic kriging with qualitative factors), to synergistically model exposure-response data, which often arise from multiple sources (e.g., laboratories, animal providers, and shapes of nanomaterials) in toxicology studies. Compared to the existing methods, SKQ has several distinct features. First, SKQ integrates data across multiple sources and allows for the derivation of more accurate information from limited data. Second, SKQ is highly flexible and able to model practically any continuous response surfaces (e.g., dose-time-response surface). Third, SKQ is able to accommodate variance heterogeneity across experimental conditions and to provide valid statistical inference (i.e., quantify uncertainties of the model estimates). Through empirical studies, we have demonstrated SKQ's ability to efficiently model exposure-response surfaces by pooling information across multiple data sources. SKQ fits into the mosaic of efficient decision-making methods for assessing the risk of a tremendously large variety of nanomaterials and helps to alleviate safety concerns regarding the enormous amount of new nanomaterials.

Concentration-time-response modeling for acute and short-term exposures

Risk of health effects from acute and short-term exposure depends on exposure time as well as exposure concentration. A general approach to extending a concentration-response model to include time as a variable is described using mortality of rats exposed to hydrogen sulfide (H(2)S) as an example. This particular example resulted in a logit model with concentration-time (c-t) relationship linear in time and log-concentration. It provided an improved statistical fit, based on the Akaike information criterion in the observed time range, 30 m-360 m, over implementing the c-t relationship of [ten Berge, W.F., Zwart, A., Appelman, L.M., 1986. Concentration-time mortality response relationship of irritant and systemically acting vapours and gases. J. Hazard. Mater. 13, 301--309] as a default in the logit model. This approach also indicated that there might be a fundamental difference in the relationship between concentration, time, and response at short exposure times, somewhere less than 30 m, a hypothesis for further consideration from a biological perspective. In general, the proposed approach provides flexibility to develop a concentration-time-response model, and the associated concentration-time relationship, from the data. Interpretation and potential implications, however, need to be considered within the context of biological plausibility as well. Implementation of the proposed approach requires adequate data for separate concentration-response modeling at each of several exposure durations.

Stochastic modelling of apoptosis kinetics

Robust quantitative estimation of average whole cell mitochondrial dysfunction is a useful tool for assessing sensitivity to apoptotic stimuli induced either by novel agents, or following manipulation of apoptotic threshold by pharmacological or functional genomics approaches. We have mathematically modelled the kinetics of whole cell mitochondrial membrane potential depolarisation within a population of cells as a Bernouli transition. An exponential distribution enables the median latency preceding mitochondrial membrane potential dissipation to be derived. The kinetic model can be fitted to in vitro single cell resolution data derived from kinetic flow cytometric studies by non-linear regression. We propose that kinetic determination of cumulative frequency distributions provides a useful approach for estimating apoptosis sensitivity across cell populations over short time-frames.