Though there are hardly any restrictions to choosing a target, pharmacological receptors are investigated most frequently. The nature of the receptor, whether it is membrane-bound or soluble, determines which separation technique is used to terminate the binding assay. For the former, filtration or centrifugation are favored as separation steps, while for the latter suitable methods are gel filtration, equilibrium dialysis, precipitation or adsorption of the nonbound marker by charcoal [23, 24].
The marker should display both an affinity to and a selectivity for the target that is as high as possible while generating as little nonspecific binding as possible. The Kd-values of the suitable markers are typically in the range between 100 pM and 10 nM, enabling the separation of the target-marker complex from the non-bound marker and the subsequent washing steps without noticeable dissociation [7, 16, 17, 21].
From the three basic radioligand binding assay types, competition experiments are no doubt most frequently applied in the drug discovery process, since they allow the characterization of the affinity of any kind of test compound to a defined target [7, 16, 17, 21]. To meet the increasing challenge of throughput in this area, filtration assays and later on assays based on gel filtration as the separation step were established already in the 1990s, using a 96-well plate format [6, 9, 25]. By now filtration assays can be accomplished in a completely automated form . Next to these technical improvements, new ''homogeneous'' techniques have been developed since the 1980s that omit the separation step obligatory in conventional radioligand binding assays [27-30]. These methods, based on the scintillation proximity principle, differ from conventional radioligand binding assays in that the target is first bound on beads or on the surface of microtiter plates which are impregnated with scintillator molecules. After being incubated with the radioligand, the sample can be measured without separating bound from nonbound marker, since only those radioligands in the immediate vicinity of the scintillator molecules, i.e. those bound to the immobilized target, cause emissions of light. This technique, despite the advantage of a high throughput, also has some drawbacks in comparison to conventional radioligand binding assays, e.g. a higher degree of nonspecific binding, the necessity of higher radioligand concentrations and occasionally a reduced affinity of the marker for the target due to the immobilization of the latter [26, 31].
Modern high-throughput screening knows a number of very different rivalling strategies and test systems for the detection and optimization of new lead structures [32-34]. Homogeneous ''mix and measure assays'' avoiding a separation step are generally preferred. Conventional radioligand binding assays, however, are an indispensable tool that represent the ''gold standard'' for the characterization of new test compounds and for the investigation of structure activity relationships [1, 25]. As a simple, robust, flexible, fast, and comparatively inexpensive technology, they are still of great importance for the pharmaceutical industry as well as for companies like Novoscreen and CEREP that specialize in receptor screening [35, 36].
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