Assay basic concepts

B22. Brodie, B. B., Basic principles in development of methods for drug assay. In Concepts in Biochemical Pharmacology (B. B. Brodie and J. R. Gillette, eds.). Handbook of Experimental Pharmacology, Vol. 28, Part 2, pp. 1-8. Springer-Verlag, Berlin and New York, 1971. 

Analogous to cell viability measurements, many of the same basic concepts apply to the development of cell-based assays for apoptosis. The length of incubation of cells with the test compound is among the most important issues to address and optimize. The length of incubation is important because the markers of apoptosis may be present for relatively brief transient periods and subsequently disappear as the population of cells undergoes secondary necrosis. The induction of measurable caspase activity can occur in only a few minutes or can take days, depending on the model cell line, type of inducer, and effective concentration inside the cells. 

Now, although the quantity being measured is a property of the particular sample under study, the degree of expected fluctuation from one measurement to another depends most fundamentally on the measurement process itself—that is, how the assay is conducted—rather than on the particular sample. Since, depending on the circumstances, the amount of fluctuation among attempts to measure the same quantity may be trivial or crucially important, we now consider briefly some basic concepts that help the biochemist deal with variability among measurements. 

The basic concepts of competitive-binding solid phase EIA have been described elsewhere.The required separation of antibody from the assay mixture can be accomplished in a variety of ways. Double-antibody techniques are quite popular and involve the use of a second antibody, an antibody to the principal antibody, to induce separation. Cross-linking the second antibody serum - with ethyl chloroformate forms an insoluble suspension of small particles, which still maintain a high degree of im-munoreactivity toward the first antibody. Addition of such particles to an EIA assay mixture pulls the first antibody from solution along with enzyme-labeled molecules bound to the antibody. Such a process is depicted in Fig. 1 and employed for all separations in this work. Measurement of enzyme activity in the bound solid phase is desirable because complete purification of the labeled substance is not necessary. 

Bakers yeast (Saccharomyces cerviseae) has also been used in combination with mass spectrometry thereby assaying volatile products from cell metabolism (39). The basic concept here is that the living cells are kept in a flow system continuously fed with buffer or substrate and that any volatile reactant passes through a semi permeable membrane into the evacuated room containing an ionizer and a quadrepole mass spectrometer (Fig. 12)... 

Scheme 5 Basic concept of an indicator displacement assay (IDA) the optical properties of an indicator (color, fluorescence) change upon replacement hy an analyte...

Many of the more important cytokines are listed in Table 22-1. Most of these have multiple effects as demonstrated by the multiplicity and diversity of effects that they elicit. These effects depend on the responding cell type, biological network, and assay system in use. Also the same molecule produced from different cells may exert a completely different biological action that may be opposite or even unrelated. In fact, the biological activity originally attributed to one cytokine may now be shown to be shared by several different molecules (redundancy), leading to the concept that very few individual cytokines are essential for basic... 

One of the major aims of this review has been to examine the current status of immunological assays used to measure the pituitary gonadotropic hormones. Since all immunoassays basically depend on the interaction of an antigen of unique identity (in this case FSH or LH) and its specific antibody, it was necessary also to discuss the chemical nature of the various preparations of FSH and LH which are currently available and to consider the concept of antibody specificity as it applies in this field. 

So far in this chapter, the chemical biology reader has been introduced to examples of biocatalysts, kinetics assays, steady state kinetic analysis as a means to probe basic mechanisms and pre-steady-state kinetic analysis as a means to measure rates of on-catalyst events. In order to complete this survey of biocatalysis, we now need to consider those factors that make biocatalysis possible. In other words, how do biocatalysts achieve the catalytic rate enhancements that they do This is a simple question but in reality needs to be answered in many different ways according to the biocatalyst concerned. For certain, there are general principles that underpin the operation of all biocatalysts, but there again other principles are employed more selectively. Several classical theories of catalysis have been developed over time, which include the concepts of intramolecular catalysis, orbital steering , general acid-base catalysis, electrophilic catalysis and nucleophilic catalysis. Such classical theories are useful starting points in our quest to understand how biocatalysts are able to effect biocatalysis with such efficiency. 

The majority of recently described methods and applications have used a primer extension concept to determine the genotype present in a particular SNP posihon and sample. One of the drivers for these assay concepts was the basic limitation in mass resolution and mass accuracy, which did not allow for a simple mass determination of PCR products to accurately define genotypes. 

Elucidation of how the general principles underlying the concept of validation should be expressed in practice is an evolving process, as exemplified by the ongoing evolution of validation requirements for bioanalytical assays in the pharmaceutical industry (Shah 1992, 2000 FDA 2001 Viswanathan 2007). The complementary principle of fitness for purpose (Section 9.2) applies not only to the assay method but also to the validation process itself. Procedures that are considered to be fit for purpose in validation of an analytical method to be used in drug development, for example, need not necessarily apply to, e.g., methods used to screen pesticide residues in foodstuffs. As noted in Section 9.2, this point of view appears to be consistent with the definition of validation applied to all measurements (ISO 1994) Validation Confirmation by examination and provision of objective evidence that the particular requirements for a specified intended use are fulfilled. Of course, some basic principles are common to all validation schemes. 

One very prominent area, which is only rather loosely connected to pharmaceutical fields, but which is nevertheless an essential aspect of health-related research, and in which cellulose-based materials, namely different paper types, play a fundamental role, is the development of paper-based disposable point-of-care diagnostics tools and low-cost diagnostics tools [49-51], Although the concept of using cellulose-based materials as an essential part is not new as such—basically every commercially available lateral flow immunoassay (e.g., pregnancy tests) contains a specialized paper as an integral part—the development of devices completely made out of paper-like materials has revolutionized the field, and surprisingly simple but powerful devices made out of paper have been developed and tested rmder real conditions in field trials various assay formats have been realized, and it is expected that these devices will have an impact on health care systems due to their cost-effectiveness. 

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