A two-state receptor model for the interaction between angiotensin II type 1 receptors and non-peptide antagonists

Induced fit versus conformational selection: From rate constants to fluxes… and back to rate constants

Induced fit- (IF) and conformational selection (CS) binding mechanisms have long been regarded to be mutually exclusive. Yet, they are now increasingly considered to produce the final ligand-target complex alongside within a thermodynamic cycle. This viewpoint benefited from the introduction of binding fluxes as a tool for analyzing the overall behavior of such cycle. This study aims to provide more vivid and applicable insights into this emerging field. In this respect, combining differential equation- based simulations and hitherto little explored alternative modes of calculation provide concordant information about the intricate workings of such cycle. In line with previous reports, we observe that the relative contribution of IF increases with the ligand concentration at equilibrium. Yet the baseline contribution may vary from one case to another and simulations as well as calculations show that this parameter is essentially regulated by the dissociation rate of both pathways. Closer attention should be paid to how the contributions of IF and CS compare at physiologically relevant drug/ligand concentrations. To this end, a simple equation discloses how changing a limited set of "microscopic" rate constants can extend the concentration range at which CS contributes most effectively. Finally, it could also be beneficial to extend the utilization of flux- based approaches to more physiologically relevant time scales and alternative binding models.

Crystal structures of the human neurokinin 1 receptor in complex with clinically used antagonists

Neurokinins (or tachykinins) are peptides that modulate a wide variety of human physiology through the neurokinin G protein-coupled receptor family, implicated in a diverse array of pathological processes. Here we report high-resolution crystal structures of the human NK₁ receptor (NK₁R) bound to two small-molecule antagonist therapeutics – aprepitant and netupitant and the progenitor antagonist CP-99,994. The structures reveal the detailed interactions between clinically approved antagonists and NK₁R, which induce a distinct receptor conformation resulting in an interhelical hydrogen-bond network that cross-links the extracellular ends of helices V and VI. Furthermore, the high-resolution details of NK₁R bound to netupitant establish a structural rationale for the lack of basal activity in NK₁R. Taken together, these co-structures provide a comprehensive structural basis of NK₁R antagonism and will facilitate the design of new therapeutics targeting the neurokinin receptor family.

Neurokinin receptors are G protein-coupled receptors. Here the authors present three crystal structures of the neurokinin 1 receptor (NK₁R) in complex with small-molecule antagonists including aprepitant and netupitant and observe that these clinically approved compounds induce a conformational change in the receptor.

Link between a high k on for drug binding and a fast clinical action: to be or not to be?

Review articles on binding kinetics essentially focus on drugs that dissociate slowly from their target since this is required for the successful treatment of many pathophysiological conditions. Recently, the therapeutic benefit of a high k on (i.e. the second order association rate constant) has also been linked to fast association and to a fast clinical action. Other studies, however, called this assertion into question since additional factors, like the dosing paradigm and the binding mechanism, are important as well. The still ongoing reticence about integrating binding kinetics in lead optimization programs motivated us to critically review the link between the drug's kinetic rate constants and their in vitro and in vivo target occupancy profile, with special focus on k on. The presented simulations tally with a positive link between a drug's effective/observed association rate (which is quite easy to determine in vitro) and the swiftness of its clinical action. On the other hand, the simulations show that the k on-concept should not be confounded with the effective association process since increasing this parameter only enhances the drug's in vitro and in vivo association under certain conditions: the binding mechanism should be suitable, rebinding (and thus the factors within the target's micro-environment that favour this mechanism) should not be too prominent and the dosage should not be kept in par with the drug's affinity. Otherwise, increasing k on could be ineffective or even be counter-productive.

Kinetic operational models of agonism for G-protein-coupled receptors

The application of kinetics to research and therapeutic development of G-protein-coupled receptors has become increasingly valuable. Pharmacological models provide the foundation of pharmacology, providing concepts and measurable parameters such as efficacy and potency that have underlain decades of successful drug discovery. Currently there are few pharmacological models that incorporate kinetic activity in such a way as to yield experimentally-accessible drug parameters. In this study, a kinetic model of pharmacological response was developed that provides a kinetic descriptor of efficacy (the transduction rate constant, k_τ) and allows measurement of receptor-ligand binding kinetics from functional data. The model assumes: (1) receptor interacts with a precursor of the response ("Transduction potential") and converts it to the response. (2) The response can decay. Familiar response vs time plots emerge, depending on whether transduction potential is depleted and/or response decays. These are the straight line, the "association" exponential curve, and the rise-and-fall curve. Convenient, familiar methods are described for measuring the model parameters and files are provided for the curve-fitting program Prism (GraphPad Software) that can be used as a guide. The efficacy parameter k_τ is straightforward to measure and accounts for receptor reserve; all that is required is measurement of response over time at a maximally-stimulating concentration of agonist. The modular nature of the model framework allows it to be extended. Here this is done to incorporate antagonist-receptor binding kinetics and slow agonist-receptor equilibration. In principle, the modular framework can incorporate other cellular processes, such as receptor desensitization. The kinetic response model described here can be applied to measure kinetic pharmacological parameters than can be used to advance the understanding of GPCR pharmacology and optimize new and improved therapeutics.

Distinct in vivo target occupancy by bivalent- and induced-fit-like binding drugs

BACKGROUND AND PURPOSE

Optimal drug therapy often requires long-lasting target occupancy While this attribute was usually linked to the drug's pharmacokinetic properties, the dissociation rate is now increasingly recognized to contribute as well. Nearly all the earlier pharmacokinetic-pharmacodynamic (PK-PD) simulations encompassed single-step binding drugs and focused on koff . However, 'micro'-PK mechanisms and more complex binding mechanisms like bivalent- and induced-fit binding may contribute as well. Corresponding binding models are presently explored.

EXPERIMENTAL APPROACH

We compared the 24 h in vivo occupancy over time profiles of prototype bivalent- and induced-fit-like binding drugs (A and B) after one or repeated daily dosings, both without and with rebinding. Special attention was focused on the effect of each of the microscopic rate constants on the occupancy profiles and on the metrics to represent those profiles.

KEY RESULTS

Although both models can be represented by the same mathematical formulation, drugs A and B display quite different occupancy profiles, even though they have the same potency. These differences can be attributed to the different effects of their microscopic rate constants on their composite koff and also on their susceptibility to experience rebinding. This also affects how the occupancy profiles of bivalent- and induced-fit-like binders progress when repeating the dosings and by changing the dosage.

CONCLUSIONS AND IMPLICATIONS

Closer attention should be paid to more complex binding models in PK-PD simulations. This may help pharmacologists and medicinal chemists to improve the translation of in vitro kinetic measurements from preclinical screening programmes into clinical efficiency.

Influence of the cellular environment on ligand binding kinetics at membrane-bound targets

While historically 'in vitro' binding data were obtained by analyzing equilibrium experiments, kinetic data are increasingly appreciated to provide information on the time a particular compound remains bound to its target. This information is of biological importance to understand the molecular mechanism of a drug not only to evaluate the time a particular receptor/enzyme is blocked in the case of antagonists/inhibitors but also to investigate its contribution to the efficacy to mediate signaling in the case of agonists. There is accumulating evidence that many drugs binding to either membrane-bound receptors or enzymes are found to display long duration of action which can be ascribed to slow dissociation from their target proteins. In the present review three such examples are discussed which encompass ligands that bind to membrane-bound proteins and from which it appears that the tight binding kinetics is influenced by the cellular/membrane environment of the target protein.

Distinctions between non-peptide angiotensin II AT1-receptor antagonists

A far-reaching understanding of the molecular action mechanism of AT1-receptor antagonists (AIIAs) was obtained by using CHO cells expressing transfected human AT 1-receptors. In this model, direct [3H]-antagonist binding and inhibition of agonist-induced responses (inositol phosphate accumulation) can be measured under identical experimental conditions. Whereas preincubation with a surmountable AIIA (losartan) causes parallel shifts of the angiotensin II (Ang II) concentration-response curve, insurmountable antagonists also cause partial (i.e., 30% for irbesartan, 50% for valsartan, 70% for EXP3174,) to almost complete (95% for candesartan) reductions of the maximal response. The main conclusions are that all investigated antagonists are competitive with respect to Ang II. They bind to a common or overlapping site on the receptor in a mutually exclusive way. Insurmountable inhibition is related to the slow dissociation rate of the antagonist-receptor complex (t 1/2 of 7 minutes for irbesartan, 17 minutes for valsartan, 30 minutes for EXP3174 and 120 minutes for candesartan). Antagonist-bound AT1-receptors can adopt a fast and a slow reversible state. This is responsible for the partial nature of the insurmountable inhibition. The long-lasting effect of candesartan, the active metabolite of candesartan cilexetil, in vascular smooth muscle contraction studies, as well as in in vivo experiments on rat and in clinical studies, is compatible with its slow dissociation from, and continuous recycling between AT1-receptors. This recycling, or `rebinding' takes place because of the very high affinity of candesartan for the AT1-receptor.

Effects of target binding kinetics on in vivo drug efficacy: koff , kon and rebinding

BACKGROUND AND PURPOSE

Optimal drug therapy often requires continuing high levels of target occupancy. Besides the traditional pharmacokinetic contribution, target binding kinetics is increasingly considered to play an important role as well. While most attention has been focused on the dissociation rate of the complex, recent reports expressed doubt about the unreserved translatability of this pharmacodynamic property into clinical efficacy. 'Micro'-pharmacokinetic mechanisms like drug rebinding and partitioning into the cell membrane may constitute a potential fix.

EXPERIMENTAL APPROACH

Simulations were based on solving differential equations.

KEY RESULTS

Based on a selected range of association and dissociation rate constants, kon and koff , and rebinding potencies of the drugs as variables, their effects on the temporal in vivo occupancy profile of their targets, after one or multiple repetitive dosings, have here been simulated.

CONCLUSIONS AND IMPLICATIONS

Most strikingly, the simulations show that, when rebinding is also taken into account, increasing kon may produce closely the same outcome as decreasing koff when dosing is performed in accordance with the therapeutically most relevant constant [Lmax ]/KD ratio paradigm. Also, under certain conditions, rebinding may produce closely the same outcome as invoking slow diffusion of the drug between the plasma compartment and a target-containing 'effect' compartment. Although the present simulations should only be regarded as a 'proof of principle', these findings may help pharmacologists and medicinal chemists to devise ex vivo and in vitro binding kinetic assays that are more relevant and translatable to in vivo settings.

On the different experimental manifestations of two-state 'induced-fit' binding of drugs to their cellular targets

'Induced-fit' binding of drugs to a target may lead to high affinity, selectivity and a long residence time, and this mechanism has been proposed to apply to many drugs with high clinical efficacy. It is a multistep process that initially involves the binding of a drug to its target to form a loose RL complex and a subsequent isomerization/conformational change to yield a tighter binding R'L state. Equations with the same mathematical form may also describe the binding of bivalent antibodies and related synthetic drugs. Based on a selected range of 'microscopic' rate constants and variables such as the ligand concentration and incubation time, we have simulated the experimental manifestations that may go along with induced-fit binding. Overall, they validate different experimental procedures that have been used over the years to identify such binding mechanisms. However, they also reveal that each of these manifestations only becomes perceptible at particular combinations of rate constants. The simulations also show that the durable nature of R'L and the propensity of R'L to be formed repeatedly before the ligand dissociates will increase the residence time. This review may help pharmacologists and medicinal chemists obtain preliminary indications for identifying an induced-fit mechanism.

Radioligand binding to intact cells as a tool for extended drug screening in a representative physiological context

Radioligand binding assays on intact cells offer distinct advantages to those on membrane suspensions. Major pharmacological properties like drug affinity and binding kinetics are more physiologically relevant. Complex mechanisms can be studied with a wider choice of experimental approaches and so provide insights into induced-fit type binding, receptor internalisation and even into pharmacomicrokinetic phenomena like drug rebinding and partitioning into the membrane. Hence, intact cell binding constitutes a valuable addition to the pharmacologist's toolbox.

On the 'micro'-pharmacodynamic and pharmacokinetic mechanisms that contribute to long-lasting drug action

INTRODUCTION

Optimal drug therapy often requires continuing high levels of target occupancy. Besides the traditional pharmacokinetic (PK) contribution thereto, drug-target interactions that comprise successive 'microscopic' steps as well as the intervention of the cell membrane and other 'micro'-anatomical structures nearby may help attaining this objective.

AREAS COVERED

This article reviews the 'micro'-pharmacodynamic (PD) and PK mechanisms that may increase a drug's residence time. Special focus is on induced-fit- and bivalent ligand binding models as well as on the ability of the plasma membrane surrounding the target to act as a repository for the drug (e.g., microkinetic model), to actively participate in the binding process (e.g., exosite model) and, along with microanatomical elements like synapses and interstitial spaces, to act on the drug's diffusion properties (reduction in dimensionality and drug-rebinding models).

EXPERT OPINION

The PK profile, as well as the target dissociation kinetics of a drug, may fail to account for its long-lasting efficiency in intact tissues and in vivo. This lacuna could potentially be alleviated by incorporating some of the enumerated 'microscopic' mechanisms and, to unveil them, dedicated experiments on sufficiently physiologically relevant biological material like cell monolayers can already be implemented early on in the lead optimization process.

A study of the molecular mechanism of binding kinetics and long residence times of human CCR5 receptor small molecule allosteric ligands

BACKGROUND AND PURPOSE

The human CCR5 receptor is a co-receptor for HIV-1 infection and a target for anti-viral therapy. A greater understanding of the binding kinetics of small molecule allosteric ligand interactions with CCR5 will lead to a better understanding of the binding process and may help discover new molecules that avoid resistance.

EXPERIMENTAL APPROACH

Using [(3) H] maraviroc as a radioligand, a number of different binding protocols were employed in conjunction with simulations to determine rate constants, kinetic mechanism and mutant kinetic fingerprints for wild-type and mutant human CCR5 with maraviroc, aplaviroc and vicriviroc.

KEY RESULTS

Kinetic characterization of maraviroc binding to the wild-type CCR5 was consistent with a two-step kinetic mechanism that involved an initial receptor-ligand complex (RA), which transitioned to a more stable complex, R'A, with at least a 13-fold increase in affinity. The dissociation rate from R'A, k-2 , was 1.2 × 10(-3) min(-1) . The maraviroc time-dependent transition was influenced by F85L, W86A, Y108A, I198A and Y251A mutations of CCR5.

CONCLUSIONS AND IMPLICATIONS

The interaction between maraviroc and CCR5 proceeded according to a multi-step kinetic mechanism, whereby initial mass action binding and later reorganizations of the initial maraviroc-receptor complex lead to a complex with longer residence time. Site-directed mutagenesis identified a kinetic fingerprint of residues that affected the binding kinetics, leading to the conclusion that allosteric ligand binding to CCR5 involved the rearrangement of the binding site in a manner specific to each allosteric ligand.

'Partial' competition of heterobivalent ligand binding may be mistaken for allosteric interactions: a comparison of different target interaction models

BACKGROUND AND PURPOSE

Non-competitive drugs that confer allosteric modulation of orthosteric ligand binding are of increasing interest as therapeutic agents. Sought-after advantages include a ceiling level to drug effect and greater receptor-subtype selectivity. It is thus important to determine the mode of interaction of newly identified receptor ligands early in the drug discovery process and binding studies with labelled orthosteric ligands constitute a traditional approach for this. According to the general allosteric ternary complex model, allosteric ligands that exhibit negative cooperativity may generate distinctive 'competition' curves: they will not reach baseline levels and their nadir will increase in par with the orthosteric ligand concentration. This behaviour is often considered a key hallmark of allosteric interactions.

EXPERIMENTAL APPROACH

The present study is based on differential equation-based simulations.

KEY RESULTS

The differential equation-based simulations revealed that the same 'competition binding' pattern was also obtained when a monovalent ligand binds to one of the target sites of a heterobivalent ligand, even if this process is exempt of allosteric interactions. This pattern was not strictly reciprocal when the binding of each of the ligands was recorded. The prominence of this phenomenon may vary from one heterobivalent ligand to another and we suggest that this phenomenon may take place with ligands that have been proposed to bind according to 'two-domain' and 'charnière' models.

CONCLUSIONS AND IMPLICATIONS

The present findings indicate a familiar experimental situation where bivalency may give rise to observations that could inadvertently be interpreted as allosteric binding. Yet, both mechanisms could be differentiated based on alternative experiments and structural considerations.

[(3) H]UR-DE257: development of a tritium-labeled squaramide-type selective histamine H2 receptor antagonist

A series of new piperidinomethylphenoxypropylamine-type histamine H2 receptor (H2 R) antagonists with different substituted "urea equivalents" was synthesized and characterized in functional in vitro assays. Based on these data as selection criteria, radiosynthesis of N-[6-(3,4-dioxo-2-{3-[3-(piperidin-1-ylmethyl)phenoxy]propylamino}cyclobut-1-enylamino)hexyl]-(2,3-(3) H2 )propionic amide ([(3) H]UR-DE257) was performed. The radioligand (specific activity: 63 Ci mmol(-1) ) had high affinity for human, rat, and guinea pig H2 R (hH2 R, Sf9 cells: Kd , saturation binding: 31 nM, kinetic studies: 20 nM). UR-DE257 revealed high H2 R selectivity on membranes of Sf9 cells, expressing the respective hHx R subtype (Ki values: hH1 R: >10000 nM, hH2 R: 28 nM, hH3 R: 3800 nM, hH4 R: >10000 nM). In spite of insurmountable antagonism, probably due to rebinding of [(3) H]UR-DE257 to the H2 R (extended residence time), the title compound proved to be a valuable pharmacological tool for the determination of H2 R affinities in competition binding assays.

Avidity and positive allosteric modulation/cooperativity act hand in hand to increase the residence time of bivalent receptor ligands

Bivalent ligands bear two target-binding pharmacophores. Their simultaneous binding increases their affinity (avidity) and residence time. They become 'bitopic' when the binding sites at the target permit the pharmacophores the exert allosteric modulation of each other's affinity and/or activity. Present simulations reveal that positive cooperativity exacerbates these phenomena, whereas negative cooperativity curtails them, irrespective of whether the association or dissociation rates of the individual pharmacophores are affected. Positive cooperativity delays the attainment of equilibrium binding, yielding 'hemi-equilibrium' conditions and only apparent affinity constants under usual experimental conditions. Monovalent ligands that bind to one of the target sites decrease the bitopic ligand's residence time concentration-wise; their potency depends on their association rate and thereon acting cooperativity rather than on affinity. This stems from the repetitive, very fast reformation of fully bound bitopic ligand-target complexes by rebinding of freshly dissociated pharmacophores. These studies deal with kinetic binding properties (of increasing interest in pharmacology) of bitopic ligands (a promising avenue in medicinal chemistry).

Exploring avidity: understanding the potential gains in functional affinity and target residence time of bivalent and heterobivalent ligands

Bivalent ligands are increasingly important therapeutic agents. Although the naturally occurring antibodies are predominant, it is becoming more common to combine different antibody fragments or even low molecular weight compounds to generate heterobivalent ligands. Such ligands exhibit markedly increased affinity (i.e. avidity) and target residence time when both pharmacophores can bind simultaneously to their target sites. This is because binding of one pharmacophore forces the second tethered one to stay close to its corresponding site. This 'forced proximity' favours its binding and rebinding (once dissociated) to that site. However, rebinding will also take place when the diffusion of freshly dissociated ligands is merely slowed down. The present differential equation-based simulations explore the way both situations affect ligand binding. Both delay the attainment of binding equilibrium (resulting in steep saturation curves) and also increase the target residence time. Competitive ligands are able to interfere in a concentration-dependent manner, although much higher concentrations are required in the 'forced proximity' situation. Also, it is only in that situation that the ligand shows increased affinity. These simulations shed light on two practical consequences. Depending on the pharmacokinetic half-life of the bivalent ligand in the body, it may not have sufficient time to achieve equilibrium with the target. This will result in lower potency than expected, although it would have significant advantages in terms of residence time. In in vitro experiments, the manifestation of steep saturation curves and of accelerated dissociation in the presence of competitive ligands could mistakenly be interpreted as evidence for non-competitive, allosteric interactions.

A systematic comparison of the properties of clinically used angiotensin II type 1 receptor antagonists

Angiotensin II type 1 receptor antagonists (ARBs) have become an important drug class in the treatment of hypertension and heart failure and the protection from diabetic nephropathy. Eight ARBs are clinically available [azilsartan, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan]. Azilsartan (in some countries), candesartan, and olmesartan are orally administered as prodrugs, whereas the blocking action of some is mediated through active metabolites. On the basis of their chemical structures, ARBs use different binding pockets in the receptor, which are associated with differences in dissociation times and, in most cases, apparently insurmountable antagonism. The physicochemical differences between ARBs also manifest in different tissue penetration, including passage through the blood-brain barrier. Differences in binding mode and tissue penetration are also associated with differences in pharmacokinetic profile, particularly duration of action. Although generally highly specific for angiotensin II type 1 receptors, some ARBs, particularly telmisartan, are partial agonists at peroxisome proliferator-activated receptor-γ. All of these properties are comprehensively reviewed in this article. Although there is general consensus that a continuous receptor blockade over a 24-hour period is desirable, the clinical relevance of other pharmacological differences between individual ARBs remains to be assessed.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Elusive equilibrium: the challenge of interpreting receptor pharmacology using calcium assays

Calcium is a key intracellular signal that controls manifold cellular processes over a wide temporal range. The development of calcium-sensitive fluorescent dyes and proteins revolutionized our ability to visualize this important second messenger and its complex signalling characteristics. The subsequent advent of high throughput plate-based fluorescence readers has resulted in the calcium assay becoming the most widely utilized assay system for the characterization of novel receptor ligands. In this review we discuss common approaches to calcium assays, paying particular attention to the potential issues associated with interpretation of receptor pharmacology using this system. Topics covered include dye saturation and forced-coupling of receptors to the calcium pathway, but special consideration is given to the influence of non-equilibrium conditions in this rapid signalling system. Modelling the calcium transient in a kinetic mode allows the influence of ligand kinetics, receptor reserve and read time to be explored. This demonstrates that observed ligand pharmacology at very early time points can be quite different to that determined after longer incubations, even resulting in reversal of agonist potency orders that may be misinterpreted as agonist biased signalling. It also shows that estimates of antagonist affinity, whether by Schild analysis or inhibition curves, are similarly affected by hemi-equilibrium conditions. Finally we end with a discussion on practical approaches to accurately estimate the affinity of insurmountable antagonists using calcium assays.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

Rebinding: or why drugs may act longer in vivo than expected from their in vitro target residence time

IMPORTANCE OF THE FIELD

It is well established that the in vivo duration of drug action not only depends on macroscopic pharmacokinetic properties like its plasma half-life, but also on the residence time of the drug-target complexes. However, drug 'rebinding' (i.e., the consecutive binding of dissociated drug molecules to the original target and/or targets nearby) can be influential in vivo as well.

AREAS COVERED IN THIS REVIEW

Information about rebinding is available since the 1980s but it is dispersed in the life sciences literature. This review compiles this information. In this respect, neurochemists and biopohysicians advance the same equations to describe drug rebinding.

WHAT THE READER WILL GAIN

The rebinding mechanism is explained according to the prevailing viewpoint in different life science disciplines. There is a general consensus that high target densities, high association rates and local phenomena that hinder the diffusion of free drug molecules away from their target all promote rebinding.

TAKE HOME MESSAGE

Simulations presented here for the first time suggest that rebinding may increase the duration and even the constancy of the drug's clinical action. Intact cell radioligand dissociation and related ex vivo experiments offer useful indications about a drug's aptitude to experience target rebinding.

Non-competitive interaction between raclopride and spiperone on human D-receptors in intact Chinese hamster ovary cells

We recently investigated the binding properties of the antagonists [(3)H]-raclopride and [(3)H]-spiperone to intact Chinese hamster ovary cells expressing recombinant human D(2long)-dopamine receptors (CHO-D(2L) cells). Compared with saturation binding with [(3)H]-raclopride, raclopride reduced [(3)H]-spiperone binding with to low potency in competition binding experiments. The present findings illustrate the ability of spiperone to inhibit [(3)H]-raclopride binding non-competitively. While raclopride only decreases the apparent K(D) of [(3)H]-raclopride in saturation binding experiments, spiperone only decreases the number of sites to which [(3)H]-raclopride binds with high affinity. Also, while the IC(50) of raclopride depends on the concentration of [(3)H]-raclopride in competition experiments, this is not the case for spiperone. Kinetic studies reveal that the binding of raclopride at its high affinity sites does not affect the association of subsequently added [(3)H]-spiperone nor the rebinding of freshly dissociated [(3)H]-spiperone to the same or surrounding receptors. Yet, spiperone does not affect the dissociation rate of [(3)H]-raclopride and raclopride does not affect the (genuine) dissociation rate of [(3)H]-spiperone. The easiest way to interpret the present findings in molecular terms is to assume that D(2L)-receptors or their dimeric complexes possess two distinct binding sites: one with high affinity/accessibility for [(3)H]-raclopride and the other one with high affinity/accessibility for [(3)H]-spiperone. The ability of bound spiperone to inhibit high affinity raclopride binding while the reverse is not the case suggests for the occurrence of non-reciprocal allosteric interactions. These new findings could point at the occurrence of allosteric interactions between different classes of D(2)-receptor antagonists.

Molecular mechanisms for the persistent bronchodilatory effect of the beta 2-adrenoceptor agonist salmeterol

BACKGROUND

Beta(2)-adrenoceptor agonists are effective bronchodilators. In vitro studies demonstrated long-lasting airway smooth muscle relaxation by salmeterol after washout, the quick disappearance of this effect in presence of antagonists and its recovery after antagonist removal. Current explanations invoke salmeterol accumulation in the membrane ('diffusion microkinetic' model) or the existence of salmeterol-binding 'exosites'. An alternative model based on 'rebinding' of a dissociated ligand to the receptor molecules also produces an apparent decrease in the ligand's dissociation rate in the absence of competing ligands. PURPOSE AND APPROACH: Computer-assisted simulations were performed to follow the receptor-occupation by a salmeterol-like ligand and a competing ligand as a function of time. The aptness of the models to describe the above in vitro findings was evaluated.

KEY RESULTS

The 'diffusion microkinetic' model is sufficient to explain a long-lasting beta(2)-adrenoceptor stimulation and reassertion as long as the membrane harbors a high concentration of the agonist. At lower concentration, 'rebinding' and, in second place, 'exosite' binding are likely to become operational.

CONCLUSIONS AND IMPLICATIONS

The 'rebinding' and 'exosite' binding mechanisms take place at a sub-cellular/molecular scale. Pending their demonstration by experiments on appropriate, simple models such as intact cells or membranes thereof, these mechanisms remain hypothetical in the case of salmeterol. Airway smooth muscle contraction could also be governed by additional mechanisms that are particular to this macroscopic approach.

Sartan-AT1 receptor interactions: in vitro evidence for insurmountable antagonism and inverse agonism

Sartans are non-peptide AT(1) receptor antagonists used to treat hypertension and related pathologies. Their effects on the G protein-dependent responses of angiotensin II (Ang II) were the same in vascular tissues and in isolated cell systems. All are competitive but, when pre-incubated, they act surmountably (only rightward shift of the Ang II concentration-response curve) or insurmountably (also decreasing the maximal response). Insurmountable behaviour reflects the formation of tight sartan-receptor complexes; it is often partial due to the co-existence of tight and loose complexes. Their ratio positively correlates with the dissociation half-life of the tight complexes and depends on the sartan: i.e. candesartan>olmesartan>telmisartan approximately equal EXP3174>valsartan>irbesartan>>losartan. When AT(1) receptors display sufficient basal activity (in case of receptor over-expression, mutation and, especially, tissue stretching) sartans may also act as inverse agonists. This rather affects long-term, G protein-independent hypertrophic responses leading to cardiovascular remodelling.

Identification and characterization of pseudoirreversible nonpeptide antagonists of the neuropeptide Y Y5 receptor and development of a novel Y5-selective radioligand

The neuropeptide Y (NPY) Y(5) receptor is believed to be involved in the central regulation of appetite. Thus, antagonists of this receptor have been pursued as potential therapeutic agents for the treatment of obesity. A novel series of potent and selective phenylamide or biaryl urea NPY Y(5) receptor antagonists was identified. Four representative compounds from this series, SCH 208639 (N-[4-[(1,1-dimethylbutyl)thio]phenyl]-2,2-dimethylpropanamide), SCH 430765 (N-[[[3'-fluoro[1,1'-biphenyl]-4-yl]amino]carbonyl]-N-methyl-1-(methylsulfonyl)-4-piperidinamine), SCH 488106 (N-[[[3',5'-difluoro[1,1'-biphenyl]-4-yl]amino]carbonyl]-N-methyl-1-[(5-methyl-3-pyridinyl)carbonyl]-4-piperidinamine) and SCH 500946 (N-[[[5-(3,5-difluorophenyl)-2-pyrazinyl]amino]carbonyl]-N-methyl-1-(methylsulfonyl)-4-piperidinamine), behaved as competitive antagonists in radioligand binding assays, but displayed apparently insurmountable antagonism in a cell-based functional assay. The apparently insurmountable antagonism was due to slow receptor dissociation rates rather than covalent binding, because the antagonists' effects could be reduced by extensive washing of cells after antagonist exposure. A novel radioligand, [(35)S]SCH 500946, was also developed and used to characterize the interaction of these antagonists with the NPY Y(5) receptor. [(35)S]SCH 500946 had high affinity for the NPY Y(5) receptor (K(d)=0.29 nM), and the binding kinetics (k(on) 4.414 x 10(7) M(-)(1) min(-1); k(off) 0.009816 min(-1)) confirmed that the compound slowly dissociates from the receptor. In a competition binding assay, NPY failed to displace [(35)S]SCH 500946 completely, indicating that the binding sites for NPY and [(35)S]SCH 500946 are not identical. These data indicate that the apparent insurmountable antagonism of these NPY Y(5) receptor antagonists is attributable both to slow receptor dissociation rates and to binding at a site distinct from NPY.

Antagonist-radioligand binding to D2L-receptors in intact cells

D(2)-dopamine receptors mediate most of the physiological actions of dopamine and are important recognition sites for antipsychotic drugs. Earlier binding studies were predominantly done with broken cell preparations with the tritiated D(2)-receptor antagonists [(3)H]-raclopride, a hydrophilic benzamide, and [(3)H]-spiperone, a highly hydrophobic butyrophenone. Here we compared [(3)H]-raclopride and [(3)H]-spiperone binding properties in intact Chinese Hamster Ovary cells stably expressing recombinant human D(2L)-receptors. Specific binding of both radioligands occurred to a comparable number of sites. In contrast to the rapid dissociation of [(3)H]-raclopride in both medium only and in the presence of an excess of unlabelled ligand [(3)H]-spiperone dissociation was only observed in the latter condition, and it was still slower than in broken cell preparations. However, this could not explain the pronounced difference in the potency of some unlabelled ligands to compete with both radioligands. To integrate these new findings, a model is proposed in which raclopride approaches the receptor from the aqueous phase, while spiperone approaches the receptor by lateral diffusion within the membrane.

Molecular characterisation of the interactions between olmesartan and telmisartan and the human angiotensin II AT1 receptor

BACKGROUND AND PURPOSE

Whereas some angiotensin II (Ang II) type 1 receptor blockers (ARBs) produce surmountable antagonism of AT(1) receptors, others such as olmesartan and telmisartan display varying degrees of insurmountability. This study compared the molecular interactions of olmesartan and telmisartan with the human AT(1) receptor, using well characterised in vitro methods and model systems.

EXPERIMENTAL APPROACH

CHO-K1 cells that stably express human AT(1) receptors (CHO-hAT(1) cells) were used in several pharmacological studies of olmesartan and telmisartan, including direct radioligand binding and inhibition of Ang II-induced inositol phosphate (IP) accumulation.

KEY RESULTS

Both ARBs were found to be competitive antagonists that displayed high affinity, slow dissociation, and a high degree of insurmountability for the AT(1) receptor (the latter greater with olmesartan). Their receptor interactions could be described by a two-step process with the initial formation of a loose complex (IR) and subsequent transformation into a tight binding complex (IR*). In washout experiments, [(3)H] telmisartan dissociated from the receptor with a half-life of 29 min and the Ang II-mediated IP accumulation response was 50% maximally restored within 24 min, whereas values for [(3)H] olmesartan were 72 min and 76 min, respectively.

CONCLUSIONS AND IMPLICATIONS

The high degree of insurmountability, slow dissociation, and high affinity of olmesartan for its receptor may relate to its ability to stabilise IR* via the carboxyl group of its imidazole core. In comparison, telmisartan displays a less potent interaction with the receptor.

Neurokinin 1 Receptor Antagonists: Correlation between in Vitro Receptor Interaction and in Vivo Efficacy

We compared the neurokinin 1 receptor (NK(1)R) antagonists aprepitant, CP-99994 [(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine], and ZD6021 [3-cyano-N-((2S)-2-(3,4-dichlorophenyl)-4-[4-[2-(methyl-(S)-sulfinyl)phenyl]piperidino]butyl)-N-methyl]napthamide]] with respect to receptor interactions and duration of efficacy in vivo. In Ca(2+) mobilization assays (fluorometric imaging plate reader), antagonists were applied to human U373MG cells simultaneously with or 2.5 min before substance P (SP). In reversibility studies, antagonists were present for 30 min before washing, and responses to SP were repeatedly measured afterward. The compounds were administered i.p. to gerbils, and the gerbil foot tap (GFT) response was monitored at various time points. The NK(1)R receptor occupancy for aprepitant was determined in striatal regions. Levels of compound in brain and plasma were measured. Antagonists were equipotent at human NK(1)R and acted competitively with SP. After preincubation, aprepitant and ZD6021 attenuated the maximal responses, whereas CP-99994 only shifted the SP concentration-response curve to the right. The inhibitory effect of CP-99994 was over within 30 min, whereas for ZD6021, 50% inhibition still persisted after 60 min. Aprepitant produced maximal inhibition lasting at least 60 min. CP-99994 (3 micromol/kg) inhibited GFT by 100% 15 min after administration, but the effect declined rapidly together with brain levels thereafter. The efficacy of ZD6021 (10 micromol/kg) lasted 4 h and correlated well with brain levels. Aprepitant (3 micromol/kg) inhibited GFT and occupied striatal NK(1)R by 100% for >48 h despite that brain levels of compound were below the limit of detection after 24 h. Slow functional reversibility is associated with long-lasting in vivo efficacy of NK(1)R antagonists, whereas the efficacy of compounds with rapid reversibility is reflected by their pharmacokinetics.

Kinetics of nonpeptide antagonist binding to the human gonadotropin-releasing hormone receptor: Implications for structure–activity relationships and insurmountable antagonism

Numerous nonpeptide ligands have been developed for the human gonadotropin-releasing hormone (GnRH) receptor as potential agents for treatment of disorders of the reproductive-endocrine axis. While the equilibrium binding of these ligands has been studied in detail, little is known of the kinetics of their receptor interaction. In this study we evaluated the kinetic structure-activity relationships (SAR) of uracil-series antagonists by measuring their association and dissociation rate constants. These constants were measured directly using a novel radioligand, [3H] NBI 42902, and indirectly for unlabeled ligands. Receptor association and dissociation of [3H] NBI 42902 was monophasic, with an association rate constant of 93+/-10 microM(-1) min(-1) and a dissociation rate constant of 0.16+/-0.02 h(-1) (t(1/2) of 4.3 h). Four unlabeled compounds were tested with varying substituents at the 2-position of the benzyl group at position 1 of the uracil (-F, -SO(CH3), -SO2(CH3) and -CF3). The nature of the substituent did not appreciably affect the association rate constant but varied the dissociation rate constant >50-fold (t(1/2) ranging from 52 min for -SO(CH3) to >43 h for -CF3). This SAR was poorly resolved in standard competition assays due to lack of equilibration. The functional consequences of the varying dissociation rate were investigated by measuring antagonism of GnRH-stimulated [3H] inositol phosphates accumulation. Slowly dissociating ligands displayed insurmountable antagonism (decrease of the GnRH E(max)) while antagonism by more rapidly dissociating ligands was surmountable (without effect on the GnRH E(max)). Therefore, evaluating the receptor binding kinetics of nonpeptide antagonists revealed SAR, not evident in standard competition assays, that defined at least in part the mode of functional antagonism by the ligands. These findings are of importance for the future definition of nonpeptide ligand SAR and for the identification of potentially useful slowly dissociating antagonists for the GnRH receptor.

Long-lasting angiotensin type 1 receptor binding and protection by candesartan: comparison with other biphenyl-tetrazole sartans

BACKGROUND

The ability of biphenyl-tetrazole angiotensin type 1 (AT1) receptor antagonists (BTsartans) to block angiotensin II (Ang II)-mediated responses has been extensively investigated in vascular tissues and, more recently, in cell lines expressing the human AT1-receptor. When pre-incubated, BTsartans acted surmountably (shifting the Ang II concentration-response curve to the right) or insurmountably (also decreasing the maximal response). It was shown that their insurmountable behaviour is due to the formation of tight, long-lasting complexes with the receptor. Partial insurmountable antagonism is due to the co-existence of tight and loose complexes. The proportion of insurmountable antagonism, the potency and the dissociation rate of the BTsartans decreases in the order: candesartan > EXP3174 (losartan's active metabolite) > valsartan > irbesartan >> losartan.

OBJECTIVE

It is of interest to explore how tight AT1-receptor binding of BTsartans such as candesartan might contribute to their long-lasting clinical effect.

METHODS

Computer-assisted simulations (COPASI program) were performed to follow the receptor-occupation and protection by different antagonists as a function of time. Free antagonist concentrations were allowed to decrease exponentially with time.

RESULTS

The simulations suggest that slow dissociation does not tangibly prolong receptor occupancy if the free antagonist is eliminated at a slower pace (as is the case for BTsartans). Yet when surmountable and insurmountable antagonists occupy the same amount of receptors, insurmountable antagonists offer appreciably better protection against fluctuations in natural messenger concentration.

CONCLUSION

Slow receptor dissociation and slow antagonist elimination are likely to act in synergy to produce long-lasting receptor protection.

Design and synthesis of new tetrazolyl- and carboxy-biphenylylmethyl-quinazolin-4-one derivatives as angiotensin II AT1 receptor antagonists

A series of novel quinazolin-4-ones was designed and their molecular modeling simulation fitting to a new HipHop 3D pharmacophore model using CATALYST was examined. Several compounds showed significant high simulation fit values. The designed compounds were synthesized and eight of them were biologically evaluated in vitro using an AT1 receptor binding assay, where compound XX competed weakly against radiolabeled Sar1Ile8-angiotensin II (Ang II) binding, compounds XIV and XXII showed moderate competition, and compound XXV showed almost equal ability to displace radiolabeled Sar1Ile8-Ang II binding to AT1 receptors as losartan. In vivo biological evaluation study of compounds XIV, XXII, and XXV on both normotensive and hypertensive rats revealed that compound XXV demonstrated higher hypotensive and antihypertensive activity than the reference compound losartan. To obtain a highly active compound from a candidate set of only eight tested compounds illustrates the power and utility of our pharmacophore model.

A Review of the Structural and Functional Features of Olmesartan Medoxomil, An Angiotensin Receptor Blocker

The angiotensin II (A-II) type 1 (AT1) receptor-mediated effects of A-II play a key role in the pathophysiology of hypertension. Effective inhibition of A-II is provided by the latest class of antihypertensive medications, the AT1 receptor blockers (ARBs). These orally available agents were developed around a common imidazole-based structural core. The most recent member of this drug class to be approved by the Food and Drug Administration, olmesartan medoxomil, contains unique features that may explain its clinical efficacy. Key structural elements of olmesartan medoxomil include a hydroxyalkyl substituent at the imidazole 4-position and a hydrolyzable ester group at the imidazole 5-position. Inter- and intramolecular hydrogen bonding involving these groups may contribute to the potentiation of antagonist activity. After oral administration, olmesartan medoxomil is deesterified in the intestinal tract to produce the active metabolite olmesartan, which undergoes no additional metabolic change. The marked antihypertensive efficacy of olmesartan medoxomil may result from a unique pharmacological interaction of the drug with the AT1 receptor, resulting in a potent, long-lasting, dose-dependent blockade of A-II. This review article characterizes the structural features of olmesartan that may be responsible for its clinical efficacy. Inferential pharmacological studies compare and contrast the effects of olmesartan to those of other ARBs in comparable preclinical animal models.

G protein-coupled receptors: a count of 1001 conformations

G protein-coupled receptors (GPCRs) were initially regarded to adopt an inactive and an active conformation and to activate a single type of G protein. Studies with recombinant cell systems have led to a more complex picture. First, GPCRs can activate distinct G protein species. Second, GPCR multistate models have been invoked to explain their complex behaviour in the presence of agonists, antagonists and other binding partners. The occurrence of intermediate receptor conformational states during GPCR activation and antagonist binding is suggested by fluorescence measurements and studies with constitutively active receptor mutants and insurmountable antagonists. Different agonists may trigger distinct effector pathways through a single receptor by dictating its preference for certain G proteins (i.e. 'agonist trafficking'). Structural modification and exogenous and endogenous (e.g. other cellular proteins, lipids) allosteric modulators also affect ligand-GPCR interaction and receptor activation. These new developments in GPCR research could lead to the development of more selective therapeutic drugs.

Ligand binding and functional properties of human angiotensin AT1 receptors in transiently and stably expressed CHO-K1 cells

Chinese Hamster Ovary Cells (CHO-K1) were transiently and stably transfected to express the human angiotensin AT(1) receptor. Cell surface receptor expression was maximal 2 days after transient transfection. Their pharmacological and signalling properties differed from stably expressed receptors. Receptor reserve was significant in the transient cells but not in stable cells, explaining the higher potency of angiotensin II and the lower degree of insurmountable inhibition by candesartan in the transient cells. [Sar(1)Ile(8)]angiotensin II (sarile) is a potent angiotensin AT(1) receptor antagonist for the stable cells but is a partial agonist, producing 19% of the maximal response by angiotensin II, in transient cells. Internalization of [(3)H]angiotensin II and [(125)I]sarile (i.e., acid-resistant binding) was more pronounced in stable cells. CHO-K1 cells were also transiently transfected with the enhanced green fluorescence-AT(1) receptor gene. Confocal microscopy revealed rapid internalization induced by angiotensin II and sarile but not by candesartan. The above disparities may result from differences in receptor maturation and/or cellular surrounding.

Effect of saponin and filipin on antagonist binding to AT 1 receptors in intact cells

In the present study, [ 3H ]-candesartan binding experiments were performed on intact Chinese Hamster Ovary cells transfected with the human AT1 receptor (CHO-AT1 cells). Cells were pre-treated with 0.01mg/ml saponin or filipin. Both pre-treatments resulted in an increased dissociation rate and decreased affinity of the insurmountable non-peptide antagonist [3H ]-candesartan. A similar decrease in affinity was observed for the peptide antagonist Sar1-Ile8 angiotensin II and for other non-peptide antagonists, irrespectively of their degree of insurmountability. A similar discrepancy in [ 3H ]-candesartan binding was earlier observed when comparing intact CHO-AT1 cells and membrane preparations thereof. This similarity is further highlighted by the observations that saponin or filipin no longer affect [ 3H ]-candesartan binding to CHO-AT1 cell membranes and that both agents permeabilise the CHO-AT1 cells. This suggests that the intracellular composition and/or organisation of living cells play an active role with regard to antagonist-AT1 receptor interactions.

The two-state model of antagonist-AT1 receptor interaction: an hypothesis defended but not tested

A correlation between insurmountable antagonism and slow dissociation has been observed for the non-peptidic AT1 receptor antagonists. This commentary examines the validity of conclusions regarding a two stage binding mechanism that has been proposed in order to account for both the slow dissociation and insurmountable antagonism. Support for that hypothetical mechanism is in the form of the goodness of fit between experimental data and modelled data in a number of papers from the same laboratory. We challenge the idea that a simple match of model and data is an adequate test of an hypothesis by showing that a simpler model matches the data equally well. We conclude that two stage binding is not necessary to explain the behaviour of AT1 receptor antagonists.

Peptide and nonpeptide antagonist interaction with constitutively active human AT1 receptors

Wild type human AT(1) receptors (WT-AT(1)) and mutant receptors, in which Asn(111) was replaced by glycine (N111G), alanine (N111A) and serine (N111S), or in which Asp(281) was replaced by alanine (D281A) or in which N111G and D281A replacements were combined, were transiently expressed in CHO-K1 cells. While the biphenyltetrazole compound candesartan dissociated slowly and behaved as an insurmountable antagonist for WT-AT(1), it dissociated swiftly and only produced a rightward shift of the angiotensin Ang II- and -IV dose-response curves for inositol phosphate (IP) accumulation in cells expressing N111G. [3H]candesartan competition binding yielded the same potency order of the related biphenyltetrazoles for WT-AT(1) and mutated receptors, i.e. candesartan>EXP3174>irbesartan>losartan. Affinities were equal for WT-AT(1) and D281A and 40- to 400-fold lower for all Asn(111) mutants. Mutations did not affect the affinity of the peptide antagonist [Sar(1)Ile(8)]Ang II (SARILE). Basal IP accumulation in cells with WT-AT(1) was not affected by any biphenyltetrazole antagonists and was increased by SARILE to 19% of the maximal Ang II stimulation. Basal IP accumulation was higher for cells expressing the Asn(111)-mutated receptors. For N111G, this accumulation was partially inhibited by all the biphenyltetrazoles upon long-term (18hr) exposure. In these cells SARILE produced the same maximal stimulation as Ang II. Asn(111)-mutated AT(1) receptors are thought to mimic the pre-activated state of the wild type receptor and comparing the efficacy and affinity of ligands for such mutated receptors facilitate the distinction of partial (SARILE) and inverse (biphenyltetrazoles) agonists from true antagonists.

A two-state model of antagonist-AT1 receptor interaction: further support by binding studies at low temperature

The molecular mechanism of insurmountable antagonism was investigated to a large extent in Chinese hamster ovary cells transfected with the human angiotensin II receptor type 1 (AT(1)) receptor. It was proposed that AT(1) receptor antagonists interact with their receptor according to a two-state receptor model. Briefly, this theoretical model reveals that antagonist bound AT(1) receptor can adopt a fast and a slow reversible state. The first, fast reversible state is similar for all antagonists, while the slow reversible state displays the characteristics of each antagonist. In the present study, we performed competition experiments with the AT(1) receptor antagonists candesartan, EXP3174, irbesartan, losartan and ligand [3H]-angiotensin II at 0-4 degrees. This gave the opportunity to verify the two-state model for the first time with experimental data.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.

Distinct binding properties of the AT(1) receptor antagonist [(3)H]candesartan to intact cells and membrane preparations

[(3)H]-2-Ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]-1H-benzimidazoline-7-carboxylic acid ([(3)H]candesartan), a non-peptide angiotensin II type 1 receptor (AT(1) receptor) antagonist bound with high affinity and specificity to intact adherent human AT(1) receptor transfected Chinese hamster ovary cells. The binding characteristics were preserved when cells were suspended, but the dissociation was 3-4-fold faster and the affinity 2-fold lower, while examining [(3)H]candesartan binding to cell membranes. These data suggested the role of the intracellular organisation of living CHO-hAT(1) cells in antagonist-AT(1) receptor interactions. Yet, a specific role of microtubule or actin filaments of the cytoskeleton, receptor phosphorylation by Protein Kinase C, membrane polarity, cytoplasmic components like ATP and the need of an intact cell membrane could be excluded. The potential effect of protease degradation or receptor oxidation during the membrane preparation was also unlikely. The dissociation rate and the equilibrium dissociation constant of [(3)H]candesartan increased with the temperature for both intact cells and membranes. Thermodynamic studies suggested that the bonds between candesartan and the hAT(1) receptor may be of different nature in intact CHO-hAT(1) cells and membranes thereof. Whereas the binding was almost completely enthalpy-driven on intact cells, there was a mixed contribution of both enthalpy and entropy on membranes.