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The receptor concept, now more than century old, remains the core concept in understanding the mechanisms of disease causality and drug action. Originally formulated in the early 1900s, receptor theory has evolved in both detail and complexity as the tools of molecular biology and increasingly sophisticated research technologies have facilitated the study of drug/ligand receptor interactions at the molecular level. The result has been a more detailed and nuanced understanding of the parameters of drug action that together with knowledge of the pleotropic nature of cellular targets and associated signaling pathways has reshaped the receptor concept. Added to the basic agonist/antagonist view of drug/ligand function, concepts like allosterism, inverse agonism, biased signaling and compound residence time have provided a broader understanding of compound-target interactions to better inform drug discovery efforts. The iteration of descriptive data from pharmacological experiments to provide generic indices of affinity and efficacy can now be used to predict expected drug/compound effects in other cell/organ systems where the sensitivity of the latter to drug effects (either via expression levels of cell surface receptors or variances in the efficiency with which those receptors are coupled to cytosolic metabolic processes) can be accommodated., (Copyright © 2013. Published by Elsevier Inc.)

A pharmacological experiment is typically conducted to: i) test or expand a hypothesis regarding the potential role of a target in the mechanism(s) underlying a disease state using an existing drug or tool compound in normal and/or diseased tissue or animals; or ii) characterize and optimize a new chemical entity (NCE) targeted to modulate a specific disease-associated target to restore homeostasis as a potential drug candidate. Hypothesis testing necessitates an intellectually rigorous, null hypothesis approach that is distinct from a high throughput fishing expedition in search of a hypothesis. In conducting an experiment, the protocol should be transparently defined along with its powering, design, appropriate statistical analysis and consideration of the anticipated outcome (s) before it is initiated. Compound-target interactions often involve the direct study of phenotype(s) unique to the target at the cell, tissue or animal/human level. However, in vivo studies are often compromised by a lack of sufficient information on the compound pharmacokinetics necessary to ensure target engagement and also by the context-free analysis of ubiquitous cellular signaling pathways downstream from the target. The use of single tool compounds/drugs at one concentration in engineered cell lines frequently results in reductionistic data that have no physiologically relevance. This overview, focused on trends in the peer-reviewed literature, discusses the execution and reporting of experiments and the criteria recommended for the physiologically-relevant assessment of target engagement to identify viable new drug targets and facilitate the advancement of translational studies., (Copyright © 2013 Elsevier Inc. All rights reserved.)

This review addresses key pharmacology and virology issues relevant in discovery and development of CCR5 antagonists as anti-HIV drugs, such as target validation, receptor internalization, allosterism, viral resistance and tropism. Recent progress in the discovery and development of CCR5 antagonists, SAR and clinical status are reviewed. Finally, modeling-based structure of CCR5 is discussed in the context of a small-molecule antagonism of the CCR5 receptor.

Carbachol has been shown to produce a biphasic response in rat left atria. At low concentrations, carbachol depresses basal inotropy, while at high doses a positive inotropic effect is observed. The negative inotropic response can be selectively eliminated by pretreatment of rats with pertussis toxin. The aim of these studies was to determine whether or not evidence could be obtained to show that different muscarinic receptors produced these different biochemical responses to the agonist carbachol. Schild analysis was used to measure the equilibrium dissociation constant of the antagonist-receptor complex for antagonism of the negative inotropy to carbachol by atropine, scopolamine 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and AF-DX 116. The antagonism of the positive inotropic response to carbachol by these antagonists was studied in atria from rats pretreated with pertussis toxin where the negative inotropy was nearly completely abolished. In general, it was found that the antagonists did not produce simple competitive blockade of the positive inotropy but rather a nominal shift to the right of the dose-response curves followed by a depression of maximal responses. However, it was found that when pA2 or pKb values could be calculated, they coincided with those determined for the antagonism of the negative inotropy to carbachol. The conclusion drawn from these experiments was that no evidence was obtained to disprove the null hypothesis that a common receptor, interacting with two G-proteins, mediates these two effects of carbachol in rat left atria. The implications of these data for the classification of drug receptors with agonists is discussed.

Carbachol produces both negative and positive inotropy in rat left atria. It is not clear whether these two effects are mediated by two separate cell surface muscarinic receptors or a single receptor interacting with two coupling proteins in the cell membrane. Pirenzepine, known to selectively block some biochemical muscarinic responses, was used in this study to block the biphasic response to carbachol in rat left atria. The negative inotropy to carbachol was blocked by pirenzepine, and Schild analysis indicated a -log dissociation constant (pKb) for the pirenzepine-receptor complex of 6.2. However, the Schild analysis may have been complicated by positive inotropy observed with pirenzepine. This positive inotropic effect was sensitive to blockade by other muscarinic antagonists. In atria from rats pretreated with pertussis toxin, carbachol produced a positive inotropic effect. Schild analysis with pirenzepine for antagonism of this response indicated a -log equilibrium dissociation constant for the pirenzepine-receptor complex of 6.7, significantly different from that for antagonism of negative inotropy. This ostensibly suggested a difference in the receptors mediating these responses. In view of the possible complicating effects of the positive inotropic effects of pirenzepine in this assay, an alternative method for the measurement of pirenzepine affinity was utilized. Resultant analysis was used to measure the pKb for pirenzepine antagonism of negative inotropy to carbachol. This method had the advantage of cancelling the positive inotropy to pirenzepine. Under these circumstances, pirenzepine had a pKb of 6.9, a value not significantly different from for antagonism of the positive inotropy to carbachol. The relevance of these findings is discussed in terms of a single promiscuous muscarinic receptor or heterogeneous receptors in this tissue. These data do not support the hypothesis that two separate receptors mediate these two effects.

Doses of dobutamine (steady-state infusions) required to increase inotropy (as measured by isovolumic indices of contractility) were lower than those required to produce tachycardia in the anaesthetized cat. When compared to isoprenaline, dobutamine produced less tachycardia for common increases in inotropy and thus demonstrated inotropic selectivity in this model. Dobutamine also produced mild pressor effects which were potentiated by beta-adrenoceptor blockade with propranolol. In efforts to define the role of the partial agonist activity of dobutamine for alpha-adrenoceptors in the production of selective inotropy the effects of dobutamine infusions were observed in cats pretreated with the alpha-adrenoceptor antagonist phentolamine. In phentolamine-treated cats dobutamine did not demonstrate inotropic selectivity and showed a relationship between increased inotropy and tachycardia which was not significantly different from that obtained with isoprenaline. In contrast, phentolamine did not change the relationship between isoprenaline-induced tachycardia and increased inotropy. These data suggest that the agonist activity of dobutamine for alpha-adrenoceptors could be responsible for selective inotropy in the anaesthetized cat probably by baroreceptor-mediated reflex modulation of heart rate and/or possible stimulation of inotropic cardiac alpha-adrenoceptors.

Some agonists act indirectly in the sense that they cause the release of a second substance that brings about the response finally observed. An antagonist which competitively inhibits the action of the intermediate substance will also reduce the response to the indirectly acting agonist, provided that the receptors are freely accessible. A simple mass-law model for indirect antagonism of this kind is presented, and its predictions are compared with the results obtained in an experimental study of the influence of propranolol and sotalol on the inotropic response of isolated rat atria to tyramine. While there is reasonable qualitative agreement, the fit is not exact and reasons for this are discussed.

N,N-Diethyl-2-(1-pyridyl)-ethylamine (E-2-P) has been shown previously to behave as a simple partial agonist at the histamine H1-receptor of guinea-pig ileum. When isolated longitudinal muscle strips from this preparation were tested with E-2-P before and after blockade with 2-haloalkylamines, it was found that these agents produced an irreversible shift to the right in the dose-response curve without significant depression of the maximum response even at very high antagonist concentrations. Under these circumstances the maximum response to the partial agonist may exceed the maximum response to histamine itself since the latter shows a much diminished maximum response at a very high concentrations of antagonist. These findings are not readily explicable in terms the usual "receptor-reserve' model of the histamine receptor system in ileum. A tentative explanation is provided, involving interaction with the antagonist at more than one site.