The measurement of the relative efficacy of agonists by selective potentiation of tissue responses: studies with isoprenaline and prenalterol in cardiac tissue

Detection of the secondary, low-affinity β1 -adrenoceptor site in living cells using the fluorescent CGP 12177 derivative BODIPY-TMR-CGP

Background and Purpose

CGP 12177 not only inhibits agonist effects mediated through the catecholamine site of the β₁-adrenoceptor with high affinity, but also exhibits agonist effects of its own at higher concentrations through a secondary, low-affinity β₁-adrenoceptor site or conformation. β-blocker affinities for this ‘CGP 12177’ site of the human β₁-adrenoceptor have thus far only been characterized in functional studies. Here, we used the fluorescent CGP 12177 analogue BODIPY-TMR-CGP to directly investigate receptor–ligand interactions at the secondary binding site of the β₁-adrenoceptor.

Experimental Approach

The human β₁-adrenoceptor was stably expressed in CHO cells containing a cAMP response element (CRE)-secreted placental alkaline phosphatase (SPAP) reporter gene construct. Functional responses of BODIPY-TMR-CGP were determined in the CRE-SPAP reporter gene assay, and manual and automated confocal microscopy platforms used to investigate the binding properties of BODIPY-TMR-CGP.

Key Results

BODIPY-TMR-CGP displayed a pharmacological profile similar to that of CGP 12177, retaining agonist activity at the secondary β₁-adrenoceptor site. In confocal microscopy studies, specific BODIPY-TMR-CGP binding allowed clear visualization of β₁-adrenoceptors in live cells. Using a wider concentration range of labelled ligand in a high-content fluorescence-based binding assay than is possible in radioligand binding assays, two-site inhibition binding curves of β-adrenoceptor antagonists were revealed in CHO cells expressing the human β₁-adrenoceptor, but not the β₂-adrenoceptor.

Conclusions and Implications

The fluorescent CGP 12177 analogue allowed the detection of the β₁-adrenoceptor secondary site in both functional and binding studies. This suggests that BODIPY-TMR-CGP presents an important and novel fluorescent tool to investigate the nature of the secondary β₁-adrenoceptor site.

Quantifying biological activity in chemical terms: a pharmacology primer to describe drug effect

Drugs can initiate, inhibit, modulate, or potentiate basal activity in cells to produce physiological effects. The interplay between the fundamental affinity and efficacy of drugs with the functional texture imposed on the receptor by the cell (e.g., variation in basal set points or cytosolic signal proteins) generates behaviors for drugs in different tissues that can cause apparently capricious variation between tissues under various physiological conditions. This poses a problem for pharmacologists studying drugs in test systems to predict effects in therapeutic ones. De-emphasis of tissue-specific drug behaviors by reducing drug effects to chemical terms can, to a large extent, reduce the effects of variances in biological systems (changing basal set points, genetic and biochemical variability, etc.). This Perspective discusses the application of four major pharmacodynamic parameters (affinity, efficacy, orthosteric vs allosteric binding, and rate of dissociation of drug from the biological target) to the quantification of biological activity to furnish chemical structure-activity relationships (SARs). These four parameters can be used to quantify effects in test systems and predict subsequent activity in a therapeutic setting. Because at least three different SARs are involved in the drug discovery process (primary therapeutic activity, pharmacokinetics, and safety), with more possible if target selectivity is required, some simple statistical approaches to multivariate structure-activity studies (i.e., primary activity plus selectivity data) also are considered. In total, these data can provide system-independent data to characterize biological activity of molecules in chemical terms that can greatly reduce biologically induced variability.

Side effects of beta-blocker treatments as related to the central nervous system

During the last decade beta-adrenoceptor antagonists have become one of the first-line treatments for hypertension. Generally, they have been shown to be safe with a low frequency of serious side effects. However, minor subjective symptoms, usually considered to be CNS-related, have been reported for all beta-blockers used. Thus, all beta-blockers on the market seem to have a high benefit:risk ratio; independent of their physicochemical properties and pharmacodynamic profile, however, they seem to cause CNS-related side effects to about the same extent. These minor side effects, the mechanisms of which are unclear, consist of subtle effects on general well being, decreased initiative, a depressed frame of mind, and disturbed sleep. Generally, however, beta-blockers in therapeutic dosages do not affect the qualitative functions of the brain. The results so far available have been obtained primarily by using objective methods. Further comparison has now been initiated using documented subjective methods to investigate whether the objectively documented differences are of any clinical relevance to the patient's quality of life. Although it cannot be claimed with certainty, nonselective beta-blockers seem to cause CNS-related side effects to a greater extent than beta 1-selective blockers. Differences in the degree of hydrophilicity of the beta-blocker are apparently of no clinical relevance in this respect. Rather, the plasma concentration of the beta-blocking drug (degree of beta-blockade) seems to be the major determinant of whether or not CNS-related symptoms appear in susceptible patients.

A comparison of the properties of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors in vitro

1. The affinities and efficacies (relative to isoprenaline) of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors, have been determined in isolated cardiac and vascular tissues respectively. 2. Prenalterol and corwin have similar affinities for cardiac beta 1-adrenoreceptors. The affinity of prenalterol for beta 2-adrenoreceptors is approximately 10 times lower than for beta 1-adrenoreceptors; that for corwin is approximately 100 times lower than for beta 1-adrenoreceptors. 3. The efficacies of prenalterol and corwin, relative to isoprenaline, at beta 1 and beta 2-adrenoreceptors, are similar. 4. The greater selectivity of corwin compared with prenalterol, as an agonist at beta 1-adrenoreceptors, is a reflection of its lower affinity for beta 2-adrenoreceptors.

Beta 2-adrenoceptor selectivity in four series of beta-adrenoceptor agonists

We studied the effect of two catechol and two resorcinol series of beta-adrenoceptor agonists on isolated papillary (beta 1-adrenoceptors) and soleus (beta 2-adrenoceptors) muscles preparations from the guinea-pig. One of the catechol and one of the resorcinol series were substituted by branched alkyl groups on the amino nitrogen, and the other two series were substituted by cycloalkyl groups. It was found that: the catechol derivatives were more potent than their corresponding resorcinol derivatives on both the papillary and the soleus muscle preparations, the resorcinol derivatives were more selective for the soleus muscle than their corresponding catechol derivatives, the substitution on the amino nitrogen gave compounds that were highly selective for the soleus muscle, both in the catechol and the resorcinol series. Greatest selectivity was obtained with a tertiary butyl group. the branched alkyl substitution on the amino nitrogen favoured selectivity for the soleus muscle more than cycloalkyl substitution did.

The beta 1- and beta 2-adrenoceptor stimulatory effects of alprenolol, oxprenolol and pindolol: a study in the isolated right atrium and uterus of the rat

The rat isolated right atrium (frequency response) and progesterone-treated rat uterus (relaxation) were used to examine the beta 1- and beta 2-adrenoceptor stimulatory effects of alprenolol, oxprenolol and pindolol. In addition, the beta 1-adrenoceptor stimulatory effect of practolol was studied in the right atrium. All the compounds studied caused a concentration-dependent increase in atrial frequency and relaxation of the uterus. The atrial response to pindolol was competitively inhibited by the beta 1-selective blocker pafenolol (10(-7) M), while the beta 2-selective blocker ICI 118551 (10(-8) M) was without effect. Pafenolol (10(-7) M) was also shown to inhibit the atrial frequency effect of alprenolol and oxprenolol. In the uterus, ICI 118551 (3 X 10(-9) M, 3 X 10(-8) M, 3 X 10(-7) M) blocked the pindolol effect with a pKB of 9.28. In addition, ICI 118551 (10(-8) M) competitively inhibited the relaxation of the uterus induced by alprenolol and oxprenolol. For alprenolol (right atrium and uterus), oxprenolol (right atrium), and pindolol (right atrium), the concentrations needed for half-maximal response were significantly greater than those required for occupation of half the receptors. This dissociation was most pronounced for pindolol in the right atrium. In this tissue, 80-85% of the beta 1-adrenoceptors had to be occupied by pindolol to initiate a tissue response corresponding to 50% of the maximal effect generated by the compound. The intrinsic activities of alprenolol, oxprenolol and pindolol (expressed as % of the maximal tissue response to isoprenaline) were significantly higher in the uterus than in the right atrium. The intrinsic activity of the compounds varied between individual preparations and, particularly in the uterus, correlated with the sensitivity of the tissue to beta-adrenoceptor stimulation by isoprenaline. 5 Calculation ofefficacy, relative to isoprenaline, of the partial beta-agonists revealed a beta 2-adrenoceptor selectivity for alprenolol (2.0), oxprenolol (1.4) and pindolol (3.0). 6 It is concluded that weak partial agonists such as alprenolol, oxprenolol and pindolol possess complex beta 1- and beta 2-adrenoceptor stimulatory properties in relation to beta-adrenoceptor occupancy and tissue sensitivity to beta-adrenoceptor stimulation.

The quantification of relative efficacy of agonists

While much pharmacological effort has been expended in the measurement, quantification, and comparison of agonist affinity for the classification of drugs and drug receptors, inordinately less emphasis has been placed on the quantification of the other property of agonists, namely intrinsic efficacy. This is unfortunate as the existing studies of the relative intrinsic efficacy of agonists show this to be a most useful scale for the classification of agonists and the prediction of tissue responses. This paper will review some of the theories that describe efficacy on a molecular level, the methods of measuring relative efficacy, and the factors in these procedures which can lead to artifacts and misleading information for the classification of drug receptors. Lastly, the value of the quantification of efficacy will be discussed in terms of the design of agonists for therapeutic advantage.