Indirect endpoints

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

It is furthermore noted that the TTC concepts, including the structure-based approaches, are derived from databases covering substances used as direct and indirect food additives, pesticides, and industrial chemicals, and cover toxic effects related to systemic exposure to these chemicals. In addition, it is underlined that TTC has not been developed for endpoints associated with direct contact such as irritation or sensitization. 

It is important when choosing a particular measurement scale to answer a number of questions. Is the choice that is made of clinical relevance How is the endpoint to be measured Can we measure the clinical endpoint directly, or must we choose an indirect approach Is the choice that is made sensitive enough to measure real treatment effects Having collected the information how are we to analyse it Some of these issues are illustrated in the following sections. 

The assessment endpoint should be not only measurable (at least potentially) but also modelable. Defining a modelable endpoint is likely to require close discussion between an assessor (who knows what they can model) and a risk manager (who knows what they want to protect). Sometimes the assessment endpoint is only indirectly related to the management goal, for example, if the assessment endpoint is a risk to individuals, but the aim is to protect population sustainability. In such cases, qualitative inference will be required to interpret the assessment result. This inference will need to be done jointly by the risk assessor and risk manager. It is likely to involve substantial uncertainty, which will have to be taken into account qualitatively when producing a narrative description of the assessment outcome. This step should be identified as part of the conceptual model. 

Indirect comparisons are potentially subject to methodological flaws, mainly because, outside the confines of a single randomized controlled clinical trial, one cannot be sure that the patients enrolled in the various trials are equivalent in terms of baseline risk, the settings for the trials are comparable, and that endpoints are measured in the same way. Therefore, an apparent superiority for one therapy over another, in an indirect comparison, may be as much due to differences in the trials as to differences between the therapies themselves. 

Among these methods, multicalibration (multivariate Calibration) is important. Multicalibration is the final development of indirect analytical methods. The analytical method has been previously defined as the whole of operational steps (reactions, separations,.. .) that lead to a highly selective endpoint where one measured physical variable is univocally related to one chemical variable (quantity, concentration,... ) this correlation is shown by the calibration curve (a straight line, generally). Multicalibration brinp a complete change of this definition the analytical method is the whole of chemical and mathematical operations that enable us to reach a multivariate selective system where several measured physical quantities are univocally related to several chemical quantities the correlation is shown by the calibration hypersurface. Multicalibration is surely destined to used with great effect in many areas in the future. 

Since QSAR models for narcosis toxicity based on Kn/W are available for many endpoints and species, it has become a popular approach applied for screening the ecological risk posed by substances for which no data are available. ECOSAR itself, with 150 relationships defined for over 50 chemical classes, has been used to predict toxicity and estimate hazards for chemical warfare agents in marine environments [96], pharmaceuticals [102-104], direct and indirect food additives, and industrial chemicals [105]. Although there are several QSAR and other predictive tools currently available, this section focuses on ECOSAR as it is one of the most widely and easily used. 

The Microplate algal toxicity test, another popular small-scale bioassay, is equally employed for WASTOXHAS applications (see Chapter 3, volume 1 of this book). In our case, rapid endpoint determinations of growth (72h-IC50s) are made with a microplate fluorescence reader, where algal biomass is indirectly measured via chlorophyll a fluorescence (excitation filter 440 nm - emission filter 640 nm). 

Because the safety profiles defined by safety pharmacology studies can have a significant impact on the successful development of new therapeutic agents, it is important that the techniques and assays used in safety pharmacology studies minimize the occurrence of false negative and false positive results. For this reason, techniques that provide direct measures of respiratory parameters should be used. A direct measure is one that provides the endpoint of interest, in contrast to an indirect measure that provides a surrogate endpoint,... 

If the chemical composition of the samples is known or at least partly known (in a stepwise TIE approach) or existing data allow for QSAR calculation, the samples can be ranked by TUs. Arts et al. (2006) studied, in 12 outdoor ditch mesocosms, the effects of sequential contamination with 5 pesticides in a regression design. They applied dosages equivalent with 0.2%, 1%, and 5% of the predicted environmental concentration (PEC) subsequently over 17 weeks. Endpoints recorded over 30 weeks included community composition of macroinvertebrates, plankton, and macrophytes, and leaf litter decomposition as functional ecosystem parameters. TUs were calculated in relation to acute toxicity data for the most sensitive standard species Daphnia magna and Lemna minor. Principal response curves (PRCs), a special form of constrained PCA, and Williams test (NOEC, class 2 LOEC) were used to identify the most sensitive taxa. Next to direct effects on certain species, also indirect effects, for example, how the change in abundance of a sensitive species affects the abundance of another, more tolerant species, can be detected only in mesocosm or in situ experiments. All observed effects were summarized in effect classes in a descriptive manner. 

Direct effect An effect resulting from an agent acting on the assessment endpoint or other ecological component of interest itself, not through effects on other components of the ecosystem. See also indirect effect. 

Make a conceptual model that ties the stressors to the receptors and to the assessment endpoints. The conceptual model delineates the potential connections between sources, stressors, habitat, and endpoints that will be used in each risk region. The conceptual model is an extension of the basic framework for a regional risk assessment with sources providing stressors identified by particular habitats. In this instance, the habitats are broadly defined as terrestrial and aquatic to capture the exposure pathways and location within the region of our endpoints. In this instance, there are numerous interconnected endpoints to show both the valued ecosystem components and the interdependence and potential indirect effects. 

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