Toxicity estimation methods

A new in vitro genotoxicity assay has been developed by the Microbics Corp. This assay, known by the trade name of Mutatox assay, is a convenient method to detect DNA-damaging substances (genotoxins) by measuring light emissions from an isolated dark mutant strain of Photobacterium phosphoreum. The Mutatox assay compares favorably in sensitivity with the Ames test, and it is easier and more rapid to perform [15]. As a toxicity estimation method, one convenient assay method was developed and uses cultured fish cells this is introduced in section 7.4. 

Basak SC, Gute BD, Mills D, Hawkins DM. Quantitative molecular similarity methods in the property/toxicity estimation of chemicals a comparison of arbitrary versus tailored similarity spaces. J Mol Struct (Theochem) 2003 622 127-45. 

There is a discrepancy between the cyanide criteria for both aquatic and drinking water standards and the current analytical technology. The criteria are stated for free cyanide (which Includes hydrocyanic acid and the cyanide ion), but the EPA approved analytical methodology for total cyanide measures the free and combined forms (11). This test probably overestimates the potential toxicity. An alternative method (cyanides amenable to chlorination) measures those cyanide complexes which are readily dissociated, but does not measure the iron cyanide complexes which dissociate in sunlight. This method probably tends to underestimate the potential toxicity. Other methods have been proposed, but similar problems exist (12). The Department of Ecology used the EPA-approved APHA procedure which includes a distillation step for the quantification of total cyanide (13,14). A modification of the procedure which omits the distillation step was used for estimation of free cyanide. Later in the study, the Company used a microdiffusion method for free cyanide (15). 

A final optical application deals with the measurement of intracellular nicotinamide adenine dinucleotide (NADH) by fluorescence [77], giving information about the physiological status of wastewater treatment plant biomass. This indirect method could be envisaged for toxicity estimation. 

To fill the data gaps for determining potential hazards to human health and the environment, the EPA often relies on internally developed estimation methods. These include empirical data available for structural analogs and computational methods for the estimation of physicochemical properties, which in turn are used to estimate environmental fate, bioavailability, toxicity in humans and aquatic organisms, and exposure [17]. 

The PMN review process has evolved over time within the constraints set by TSCA. An important constraint is that submitters are required to furnish only test data already in their possession (if any) and are not required to conduct a battery of tests as a precondition for approval. This generalization holds true for basic chemical property data as well as toxicity data, and it is the main reason why TSCA has been such a powerful impetus for developing estimation methods for many of the parameters needed in environmental assessment. To illustrate how extreme the situation is, in one study of more than 8,000 PMNs for class 1 chemical substances (i.e., those for which a specific chemical structure can be drawn) that were received from 1979 through 1990,Lynch et al. (1991) found only 300 that contained any of the property data noted earlier as needed for environmental assessment. The U.S. is unique among industrialized nations in requiring its assessors to work in the virtual absence of test data. 

This handbook makes no attempt to describe environmental transport properties because these phenomena are specific to environmental conditions such as wind speed and water current velocity. A notable exception is the diffusion or permeation rate of a substance through biological membranes, which is the key process controlling dermal absorption and therefore a key determinant of the dose actually available to exert a toxic effect. Chapter 11 treats this topic. The reader seeking estimation methods for molecular diffusiv-ities in air and water should consult the text by Reed et al. (1987). 

Zeeman M (1995) Ecotoxicity testing and estimation methods developed under Sect. 5 of the Toxic Substances Control Act (TSCA). Chap. 23, In Rand G (ed) Pimdamentals of Aquatic Toxicology Effects, Environmental Fate, and Risk Assessment, 2nd edn. Taylor Francis, Washington, D.C., pp 703 - 715... 

A9.6.4.7 The Nordic Council of Ministers issued a report (Pederson et al, 1995) entitled Environmental Hazard Classification, that includes information on data collection and interpretation, as well as a section (5.2.8) entitled QSAR estimates of water solubility and acute aquatic toxicity . This section also discusses the estimation of physicochemical properties, including log Kow For the sake of classification purposes, estimation methods are recommended for prediction of minimum acute aquatic toxicity, for ...neutral, organic, non-reactive and non-ionizable compounds such as alcohols, ketones, ethers, alkyl, and aryl halides, and can also be used for aromatic hydrocarbons, halogenated aromatic and aliphatic hydrocarbons as well as sulphides and disulphides, as cited in an earlier OECD Guidance Document (OECD, 1995). The Nordic document also includes diskettes for a computerized application of some of these methods. 

In addition to the biochemistry introduced in this chapter, a great deal of emphasis is placed on the determination of the activity of a compound by an analysis of its structure. Quantitative structure-activity relationships (QSAR), used judiciously, have the ability to help set testing priorities and identify potentially toxic materials in mixtures. Heavily reliant upon the quality of the toxicity data discussed in Chapter 4, these methods use sophisticated statistical techniques or analysis of interaction of a toxicant with the receptor to estimate toxicity. A method that uses structure-activity relationships coupled with availability and an assumed additive model for toxicity is presented to estimate the risk due to polyaromatic hydrocarbons (PAHs). 

ASTM-PROBIT — Another study written by C. Stephan as part of an ASTM Committee E-47 effort to produce a standard method of calculating toxicity estimates. 

From this brief overview of toxicity tests, their uses and methods of assessing their responses to toxic substances, it can be seen that this is a very complex topic with many unknowns and no single best way of addressing the problem of bioavailable toxicant estimation. In this chapter, an attempt will be made to describe some applications of toxicological data, how they could be analysed and how toxicity results could be integrated in ecological risk assessment frameworks. 

Quantitative Structure - Activity Relationships (QSARs) are estimation methods developed and used to predict certain effects or properties of chemical substances, which are primarily based on the structure of the chemicals. The development of QSARs often relies on the application of statistical methods such as multiple linear regression (MLR) or partial least squares regression (PLS). However, since toxicity data often include uncertainties and measurements errors, when the aim is to point out the more toxic and thus hazardous chemicals and to set priorities, order models can be used as alternative to statistical methods such as multiple linear regression. 

Furthermore, for some cationic surfactants, quantitative structure-activity relationships have been established for daphnids and fish. Increments used in the ECOSAR toxicity estimation software (Syracuse Research Center, freely available from the US Environmental Protection Agency as part of the software EPI Suite) are -0.13 and -0.37 for each carbon atom in monoalkyl quaternary ammonium surfactants in daphnids and fish, respectively. These increments are much smaller than the ones found here. Our increments for the n-alkyl chain in Rj position are more in accordance with increments for CH2 groups in n-octanol/water partitioning constant estimation procedures, which are, e.g., 0.66 and 0.49 in Hansch and Leo s method and Meylan and Howard s method, respectively [20]. This suggests that uptake into the cells is governed by lipophilicity, or maybe more exactly by membrane/water partitioning [21, 22]. 

For the reliable application of taxonomic correlations, therefore, the same conceptions about the compounds potential modes of action are required, as for QSARs. Subjected to similar limitations, QSARs and interspecies correlations use similar principles and methods and may complement each other when used to obtain toxicity estimates if experimental results are unavailable. 

Meylan WM, Howard PH (1994a) Upgrade of PCGEMS water solubility estimation method (draft). US Environmental Protection Agency, Office of Pollution Prevention and Toxics, Washington, DC... 

The risk characterization step involves two components risk estimation and risk description. The risk estimation component is similar to the hnman health risk characterization conducted for non-cancer effects of chemicals in that it qnantifies potential effects from chemical exposure. Depending on the methods nsed to estimate exposure and toxicity, the methods nsed in risk estimation for ecological receptors may differ from those used for humans. One method that can be used, which is similar to the method used for humans, is the toxicity quotient method. In this method, the estimated exposure is divided by a safe level of exposure developed in the characterization of effects component. The resulting value is compared to a threshold level of one. Below this level, no effects are expected (regardless of what the impact might be). Above this level, there may be effects. 

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