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

Predicting the duration of action of β2-adrenergic receptor agonists: Ligand and structure-based approaches

Agonists of the β2 adrenergic receptor (ADRB2) are an important class of medications used for the treatment of respiratory diseases. They can be classified as short acting (SABA) or long acting (LABA), with each class playing a different role in patient management. In this work we explored both ligand-based and structure-based high-throughput approaches to classify β2-agonists based on their duration of action. A completely in-silico prediction pipeline using an AlphaFold generated structure was used for structure-based modelling. Our analysis identified the ligands' 3D structure and lipophilicity as the most relevant features for the prediction of the duration of action. Interaction-based methods were also able to select ligands with the desired duration of action, incorporating the bias directly in the structure-based drug discovery pipeline without the need for further processing.

Kinetic analysis of endogenous β2 -adrenoceptor-mediated cAMP GloSensor™ responses in HEK293 cells

BACKGROUND AND AIM

Standard pharmacological analysis of agonist activity utilises measurements of receptor-mediated responses at a set time-point, or at the peak response level, to characterise ligands. However, the occurrence of non-equilibrium conditions may dramatically impact the properties of the response being measured. Here we have analysed the initial kinetic phases of cAMP responses to β2 -adrenoceptor agonists in HEK293 cells expressing the endogenous β2 -adrenoceptor at extremely low levels.

EXPERIMENTAL APPROACH

The kinetics of β2 -adrenoceptor agonist-stimulated cAMP responses were monitored in real-time, in the presence and absence of antagonists, in HEK293 cells expressing the cAMP GloSensor™ biosensor. Potency (EC50 ) and efficacy (Emax ) values were determined at the peak of the agonist GloSensor™ response and compared to kinetic parameters L50 and IRmax values derived from initial response rates.

KEY RESULTS

The partial agonists salbutamol and salmeterol displayed reduced relative IRmax values (with respect to isoprenaline) when compared with their Emax values. Except for the fast dissociating bisoprolol, preincubation with β2 -adrenoceptor antagonists produced a large reduction in the isoprenaline peak response due to a state of hemi-equilibrium in this low receptor reserve system. This effect was exacerbated when IRmax parameters were measured. Furthermore, bisoprolol produced a large reduction in isoprenaline IRmax consistent with its short residence time.

CONCLUSIONS AND IMPLICATIONS

Kinetic analysis of real-time signalling data can provide valuable insights into the hemi-equilibria that can occur in low receptor reserve systems with agonist-antagonist interactions, due to incomplete dissociation of antagonist whilst the peak agonist response is developing.

An evolving perspective on novel modified release drug delivery systems for inhalational therapy

INTRODUCTION

Drugs delivered via the lungs are predominantly used to treat various respiratory disorders, including asthma, chronic obstructive pulmonary diseases, respiratory tract infections and lung cancers, and pulmonary vascular diseases such as pulmonary hypertension. To treat respiratory diseases, targeted, modified or controlled release inhalation formulations are desirable for improved patient compliance and superior therapeutic outcome.

AREAS COVERED

This review summarizes the important factors that have an impact on the inhalable modified release formulation approaches with a focus toward various formulation strategies, including dissolution rate-controlled systems, drug complexes, site-specific delivery, drug-polymer conjugates, and drug-polymer matrix systems, lipid matrix particles, nanosystems, and formulations that can bypass clearance via mucociliary system and alveolar macrophages.

EXPERT OPINION

Inhaled modified release formulations can potentially reduce dosing frequency by extending drug's residence time in the lungs. However, inhalable modified or controlled release drug delivery systems remain unexplored and underdeveloped from the commercialization perspective. This review paper addresses the current state-of-the-art of inhaled controlled release formulations, elaborates on the avenues for developing newer technologies for formulating various drugs with tailored release profiles after inhalational delivery and explains the challenges associated with translational feasibility of modified release inhalable formulations.

Interaction With the Lipid Membrane Influences Fentanyl Pharmacology

Overdose deaths from fentanyl have reached epidemic proportions in the USA and are increasing worldwide. Fentanyl is a potent opioid agonist that is less well reversed by naloxone than morphine. Due to fentanyl’s high lipophilicity and elongated structure we hypothesised that its unusual pharmacology may be explained by its interactions with the lipid membrane on route to binding to the μ-opioid receptor (MOPr). Through coarse-grained molecular dynamics simulations, electrophysiological recordings and cell signalling assays, we determined how fentanyl and morphine access the orthosteric pocket of MOPr. Morphine accesses MOPr via the aqueous pathway; first binding to an extracellular vestibule, then diffusing into the orthosteric pocket. In contrast, fentanyl may take a novel route; first partitioning into the membrane, before accessing the orthosteric site by diffusing through a ligand-induced gap between the transmembrane helices. In electrophysiological recordings fentanyl-induced currents returned after washout, suggesting fentanyl deposits in the lipid membrane. However, mutation of residues forming the potential MOPr transmembrane access site did not alter fentanyl’s pharmacological profile in vitro. A high local concentration of fentanyl in the lipid membrane, possibly in combination with a novel lipophilic binding route, may explain the high potency and lower susceptibility of fentanyl to reversal by naloxone.

Membrane-facilitated receptor access and binding mechanisms of long-acting β2-adrenergic receptor (β2-AR) agonists

The drugs salmeterol, formoterol, and salbutamol constitute the frontline treatment for asthma and other chronic pulmonary diseases. These drugs activate the β2-adrenergic receptors (β2-AR), a class A G-protein-coupled receptor (GPCR) and differ significantly in their clinical onset and duration of actions. According to the "microkinetic model," the long duration of action of salmeterol and formoterol compared to salbutamol were attributed, at least in part, to their high lipophilicity and increased local concentrations in the membrane near the receptor. However, the structural and molecular bases of how the lipophilic drugs reach the binding site of the receptor from the surrounding membrane remain unknown. Using a variety of classical and enhanced molecular dynamics simulation techniques, we investigated the membrane partitioning characteristics, binding, and unbinding mechanisms of the ligands. The obtained results offer remarkable insight into the functional role of membrane lipids in the ligand association process. Strikingly, salmeterol entered the binding site from the bilayer through transmembrane helices 1 and 7. The entry was preceded by membrane-facilitated rearrangement and presentation of its phenyl-alkoxy-alkyl tail as a passkey to an access route gated by F193, a residue known critical for salmeterol's affinity. Formoterol's access is through the aqueous path shared by other β2-AR agents. We observed a novel secondary path for salbutamol that is distinct from its primary route. Our study offers a mechanistic description for the membrane-facilitated access and binding of ligands to β2-AR and establishes a groundwork for recognizing membrane lipids as an integral component in the molecular recognition process. Significance Statement The cell membrane's functional role behind the duration of action of long-acting β2-adrenergic receptor (β2-AR) agonists such as salmeterol has been a subject of debate for a long time. We investigated the binding and unbinding mechanisms of the three commonly used β2-AR agonists, salmeterol, formoterol, and salbutamol, using advanced simulation techniques. The obtained results offer unprecedented insights into the active role of membrane lipids in facilitating access and binding of the ligands, affecting the molecular recognition process and their pharmacology.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

Does the Lipid Bilayer Orchestrate Access and Binding of Ligands to Transmembrane Orthosteric/Allosteric Sites of G Protein-Coupled Receptors?

The ligand-binding sites of many G protein-coupled receptors (GPCRs) are situated around and deeply embedded within the central pocket formed by their seven transmembrane-spanning α-helical domains. Generally, these binding sites are assumed accessible to endogenous ligands from the aqueous phase. Recent advances in the structural biology of GPCRs, along with biophysical and computational studies, suggest that amphiphilic and lipophilic molecules may gain access to these receptors by first partitioning into the membrane and then reaching the binding site via lateral diffusion through the lipid bilayer. In addition, several crystal structures of class A and class B GPCRs bound to their ligands offer unprecedented details on the existence of lipid-facing allosteric binding sites outside the transmembrane helices that can only be reached via lipid pathways. The highly organized structure of the lipid bilayer may direct lipophilic or amphiphilic drugs to a specific depth within the bilayer, changing local concentration of the drug near the binding site and affecting its binding kinetics. Additionally, the constraints of the lipid bilayer, including its composition and biophysical properties, may play a critical role in "pre-organizing" ligand molecules in an optimal orientation and conformation to facilitate receptor binding. Despite its clear involvement in molecular recognition processes, the critical role of the membrane in binding ligands to lipid-exposed transmembrane binding sites remains poorly understood and warrants comprehensive investigation. Understanding the mechanistic basis of the structure-membrane interaction relationship of drugs will not only provide useful insights about receptor binding kinetics but will also enhance our ability to take advantage of the apparent membrane contributions when designing drugs that target transmembrane proteins with improved efficacy and safety. In this minireview, we summarize recent structural and computational studies on membrane contributions to binding processes, elucidating both lipid pathways of ligand access and binding mechanisms for several orthosteric and allosteric ligands of class A and class B GPCRs.

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.

Comparison of the in vitro pharmacological profiles of long-acting muscarinic antagonists in human bronchus

BACKGROUND AND PURPOSE

Long-acting muscarinic antagonists (LAMAs) have been recommended for the treatment of chronic obstructive pulmonary disease and (more recently) asthma. However, the in vitro pharmacological profiles of the four LAMAs currently marketed (tiotropium, umeclidinium, aclidinium and glycopyrronium) have not yet been compared (relative to ipratropium) by using the same experimental approach.

EXPERIMENTAL APPROACH

With a total of 560 human bronchial rings, we investigated the antagonists' potency, onset and duration of action for inhibition of the contractile response evoked by electrical field stimulation. We also evaluated the antagonists' potency for inhibiting cumulative concentration-contraction curves for acetylcholine and carbachol.

KEY RESULTS

The onset and duration of action were concentration-dependent. At submaximal, equipotent concentrations, the antagonists' onsets of action were within the same order of magnitude. However, the durations of action differed markedly. After washout, ipratropium's inhibitory activity decreased rapidly (within 30-90 min) but those of tiotropium and umeclidinium remained stable (at above 70%) for at least 9 h. Aclidinium and glycopyrronium displayed less stable inhibitory effects, with a progressive loss of inhibition at submaximal concentrations. In contrast to ipratropium, all the LAMAs behaved as insurmountable antagonists by decreasing the maximum responses to both acetylcholine and carbachol.

CONCLUSIONS AND IMPLICATIONS

The observed differences in the LAMAs' in vitro pharmacological profiles in the human bronchus provide a compelling pharmacological rationale for the differences in the drugs' respective recommended daily doses and frequencies of administration.

Abediterol (LAS100977), an inhaled long-acting β2-adrenoceptor agonist, has a fast association rate and long residence time at receptor

This study describes the association rate and residence time of abediterol, a novel long-acting β₂-adrenoceptor agonist (LABA) in Phase II development for treatment of asthma and COPD, in comparison with indacaterol, olodaterol, vilanterol and salmeterol, for both human β₁- and β₂-adrenoceptors. Abediterol association and dissociation rates were monitored directly by using its tritiated form. Moreover, association was determined indirectly using experimental K_i and k_off obtained from assays performed with unlabelled compound. Dissociation was also studied indirectly by measuring the association rate of ³H-CGP12177 to beta adrenoceptors previously occupied by unlabelled compounds. Abediterol shows a fast association for the β₂-adrenoceptor (k_on 1.4 × 10⁷ ± 1.8 × 10⁶M^-1min^-1) while its dissociation rate is between 30 and 64 times slower than that of the reference LABA compounds tested, with a residence time of 91.3 ± 13.3min (measured directly) and 185.5 ± 7.5min (measured indirectly). Abediterol shows kinetic selectivity for the β₂- over the β₁-adrenoceptor, with a dissociation rate from the β₁-adrenoceptor similar to the other LABA compounds tested. In conclusion, abediterol is a potent LABA with a fast association rate and a long residence time at β₂-adrenoceptors. These data are in agreement with the onset and duration of action of abediterol shown in humans.

Interactions between β2-Adrenoceptor Ligands and Membrane: Atomic-Level Insights from Magic-Angle Spinning NMR

To understand the relationship between structural properties of the β2-adrenoceptor ligands and their interactions with membranes, we have investigated the location and distribution of five β2 agonists with distinct clinical durations and onsets of action (indacaterol, two indacaterol analogues, salmeterol and formoterol) in monounsaturated model membranes using magic angle spinning NMR to measure these interactions through both ¹H nuclear Overhauser enhancement (NOE) and paramagnetic relaxation enhancement (PRE) techniques. The hydrophilic aromatic groups of all five β2 agonists show maximum distribution in the lipid/water interface, but distinct location and dynamic behavior were observed for the lipophilic aromatic rings. Our study elucidates at atomic level that the hydrophobicity and substitution geometry of lipophilic groups play important roles in compound-lipid interactions.

Cell membranes… and how long drugs may exert beneficial pharmacological activity in vivo

The time course of the beneficial pharmacological effect of a drug has long been considered to depend merely on the temporal fluctuation of its free concentration. Only in the last decade has it become widely accepted that target-binding kinetics can also affect in vivo pharmacological activity. Although current reviews still essentially focus on genuine dissociation rates, evidence is accumulating that additional micro-pharmacokinetic (PK) and -pharmacodynamic (PD) mechanisms, in which the cell membrane plays a central role, may also increase the residence time of a drug on its target. The present review provides a compilation of otherwise widely dispersed information on this topic. The cell membrane can intervene in drug binding via the following three major mechanisms: (i) by acting as a sink/repository for the drug; (ii) by modulating the conformation of the drug and even by participating in the binding process; and (iii) by facilitating the approach (and rebinding) of the drug to the target. To highlight these mechanisms, we focus on drugs that are currently used in clinical therapy, such as the antihypertensive angiotensin II type 1 receptor antagonist candesartan, the atypical antipsychotic agent clozapine and the bronchodilator salmeterol. Although the role of cell membranes in PK-PD modelling is gaining increasing interest, many issues remain unresolved. It is likely that novel biophysical and computational approaches will provide improved insights in the near future.

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.

Uncoupling the Structure-Activity Relationships of β2 Adrenergic Receptor Ligands from Membrane Binding

Ligand binding to membrane proteins may be significantly influenced by the interaction of ligands with the membrane. In particular, the microscopic ligand concentration within the membrane surface solvation layer may exceed that in bulk solvent, resulting in overestimation of the intrinsic protein-ligand binding contribution to the apparent/measured affinity. Using published binding data for a set of small molecules with the β2 adrenergic receptor, we demonstrate that deconvolution of membrane and protein binding contributions allows for improved structure-activity relationship analysis and structure-based drug design. Molecular dynamics simulations of ligand bound membrane protein complexes were used to validate binding poses, allowing analysis of key interactions and binding site solvation to develop structure-activity relationships of β2 ligand binding. The resulting relationships are consistent with intrinsic binding affinity (corrected for membrane interaction). The successful structure-based design of ligands targeting membrane proteins may require an assessment of membrane affinity to uncouple protein binding from membrane interactions.

One for the Price of Two…Are Bivalent Ligands Targeting Cannabinoid Receptor Dimers Capable of Simultaneously Binding to both Receptors?

Bivalent ligands bridging two G-protein-coupled receptors (GPCRs) provide valuable pharmacological tools to target oligomers. The success of therapeutically targeting the cannabinoid CB1 receptor has been limited, in part due to its widespread neuronal distribution. Therefore, CB1 ligands targeting oligomers that exhibit restricted distribution or altered pharmacology are highly desirable, and several bivalent ligands containing a CB1 pharmacophore have been reported. Bivalent ligand action presumes that the ligand simultaneously binds to both receptors within the dimeric complex. However, based on the current understanding of CB1 ligand binding, existing bivalent ligands are too short to bind both receptors simultaneously. However, ligands with longer linkers may not be the solution, because evidence suggests that ligands enter CB1 through the lipid bilayer and, thus, linkers are unlikely to exit the receptor through its external face. Thus, the entire premise of designing bivalent ligands targeting CB1 must be revisited.

Budesonide/salmeterol in fixed-dose combination for the treatment of asthma

Fixed dose combinations (FDC) of inhaled corticosteroid (ICS) and long-acting beta agonist (LABA) are well established in asthma treatment. The budesonide/salmeterol (B/S) FDC is now about to reach the market. It is provided as powder in hard capsules of two strengths: 120/20μg and 240/20μg when expressed as delivered doses, equivalent to 150/25μg and 300/25μg when expressed as nominal doses. Its development involved 9 pharmacokinetic (320 subjects), 3 phase II (123 subjects) and 4 phase III (1206 patients with different asthma severity) studies. Delivery is effectuated via low resistance inhaler device, Axahaler®, generating also fine particles targeting the small airways. B/S safety, assessed in 1401 subjects, did not outline novel concerns specific for this FDC. In conclusion, the B/S dry powder FDC can be used for asthma treatment in adults not adequately controlled on ICS alone, or to maintain control of ICS/LABA treated patients, in whom switching to alternative FDC is indicated.

Mechanistic models enable the rational use of in vitro drug-target binding kinetics for better drug effects in patients

INTRODUCTION

Drug-target binding kinetics are major determinants of the time course of drug action for several drugs, as clearly described for the irreversible binders omeprazole and aspirin. This supports the increasing interest to incorporate newly developed high-throughput assays for drug-target binding kinetics in drug discovery. A meaningful application of in vitro drug-target binding kinetics in drug discovery requires insight into the relation between in vivo drug effect and in vitro measured drug-target binding kinetics.

AREAS COVERED

In this review, the authors discuss both the relation between in vitro and in vivo measured binding kinetics and the relation between in vivo binding kinetics, target occupancy and effect profiles.

EXPERT OPINION

More scientific evidence is required for the rational selection and development of drug-candidates on the basis of in vitro estimates of drug-target binding kinetics. To elucidate the value of in vitro binding kinetics measurements, it is necessary to obtain information on system-specific properties which influence the kinetics of target occupancy and drug effect. Mathematical integration of this information enables the identification of drug-specific properties which lead to optimal target occupancy and drug effect in patients.

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.

Pharmacogenomics of long-acting β2-agonists

INTRODUCTION

Long-acting β2-agonists are an effective class of drugs, when combined with inhaled corticosteroids, for reducing symptoms and exacerbations in patients with asthma that is not adequately controlled by inhaled corticosteroids alone. However, because this class of drugs has been associated with severe adverse events, including hospitalization and death in small numbers of patients, efforts to identify a pharmacogenetic profile for patients at risk has been diligently investigated.

AREAS COVERED

The PubMed search engine of the National Library of Medicine was used to identify English-language and non-English language articles published from 1947 to March 2015 pertinent to asthma, pharmacogenomics, and long-acting β2-agonists. Keywords and topics included: asthma, asthma control, long-acting β2-agonists, salmeterol, formoterol, pharmacogenetics, and pharmacogenomics. This strategy was also used for the Cochrane Library Database and CINAHL. Reference types were randomized controlled trials, reviews, and editorials. Additional publications were culled from reference lists. The publications were reviewed by the authors and those most relevant were used to support the topics covered in this review.

EXPERT OPINION

Children, who carry the ADRB2 Arg16Arg genotype, may be at greater risk than adults for severe adverse events. Rare ADRB2 variants appear to provide better clues for identifying the at-risk population of asthmatics.

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.

Sustained wash-resistant receptor activation responses of GPR119 agonists

G protein-coupled receptor 119 (GPR119) is involved in regulating metabolic homoeostasis, with GPR119 agonists targeted for the treatment of type-2 diabetes and obesity. Using the endogenous agonist oleoylethanolamide and a number of small molecule synthetic agonists we have investigated the temporal dynamics of receptor signalling. Using both a dynamic luminescence biosensor-based assay and an endpoint cAMP accumulation assay we show that agonist-driven desensitization is not a major regulatory mechanism for GPR119 despite robust activation responses, regardless of the agonist used. Temporal analysis of the cAMP responses demonstrated sustained signalling resistant to washout for some, but not all of the agonists tested. Further analysis indicated that the sustained effects of one synthetic agonist AR-231,453 were consistent with a role for slow dissociation kinetics. In contrast, the sustained responses to MBX-2982 and AZ1 appeared to involve membrane deposition. We also detect wash-resistant responses to AR-231,453 at the level of physiologically relevant responses in an endogenous expression system (GLP-1 secretion in GLUTag cells). In conclusion, our findings indicate that in a recombinant expression system GPR119 activation is sustained, with little evidence of pronounced receptor desensitization, and for some ligands persistent agonist responses continue despite removal of excess agonist. This provides novel understanding of the temporal responses profiles of potential drug candidates targetting GPR119, and highlights the importance of carefully examining the the mechanisms through which GPCRs generate sustained responses.

In vitro comparison of duration of action of melatonin agonists on melatonin MT1 receptor: possible link between duration of action and dissociation rate from receptor

Melatonin MT1 and MT2 receptors are Gi protein-coupled receptors and promising therapeutic targets for a number of diseases. A proportion of G protein-coupled receptor agonists and antagonists have been classified according to their duration of action, which influences their pharmacological efficacy. However, the duration of action of melatonin agonists remains unclear. In this study, we investigated the duration of action of melatonin agonists (melatonin, 2-iodomelatonin, ramelteon, and the ramelteon metabolite M-II) at the melatonin MT1 receptor, which is more resistant to agonist-induced desensitization than the melatonin MT2 receptor. In Chinese hamster ovary cells stably expressing the human melatonin MT1 receptor, significant differences in the duration of action were observed after 2-h pretreatment with agonists followed by washout. In contrast to melatonin and M-II, the agonist activities of ramelteon and 2-iodomelatonin were persistent (i.e. inhibition of forskolin-stimulated cAMP formation and increase in ERK 1/2 phosphorylation) even after repeated washouts. Similar activities were observed for INS-1 cells endogenously expressing the rat MT1 receptor. Further, we examined potential factors linked to the duration of action. Residual activities of melatonin agonists after washout strongly correlated with their dissociation rates from the human melatonin MT1 receptor, but not their lipophilicity or extent of desensitization. These data suggest that the in vitro duration of action significantly differs between melatonin agonists and might dictate dissociation kinetics. Characterization of these in vitro properties may facilitate further in vivo study of the duration of action.

'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.

Salmeterol Efficacy and Bias in the Activation and Kinase-Mediated Desensitization of β2-Adrenergic Receptors

Salmeterol is a long-acting β2-adrenergic receptor (β2AR) agonist that is widely used as a bronchodilator for the treatment of persistent asthma and chronic obstructive pulmonary disease in conjunction with steroids. Previous studies demonstrated that salmeterol showed weak efficacy for activation of adenylyl cyclase; however, its efficacy in the complex desensitization of the β2AR remains poorly understood. In this work, we provide insights into the roles played by the G protein-coupled receptor kinase/arrestin and protein kinase A in salmeterol-mediated desensitization through bioluminescence resonance energy transfer (BRET) studies of liganded-β2AR binding to arrestin and through kinetic studies of cAMP turnover. First, BRET demonstrated a much reduced efficacy for salmeterol recruitment of arrestin to β2AR relative to isoproterenol. The ratio of BRETISO/BRETSALM after 5-minute stimulation was 20 and decreased to 5 after 35 minutes, reflecting a progressive decline in BRETISO and a stable BRETSALM. Second, to assess salmeterol efficacy for functional desensitization, we examined the kinetics of salmeterol-induced cAMP accumulation (0-30 minutes) in human airway smooth muscle cells in the presence and absence of phosphodiesterase inhibition. Analysis of shaping of cAMP turnover for both agonists demonstrated significant salmeterol desensitization, although it was reduced relative to isoproterenol. Using an isoproterenol rescue protocol after either short-term (10 minutes) or long-term (2 and 14 hours) salmeterol pretreatments, we found that salmeterol progressively depressed isoproterenol stimulation but did not prevent subsequent rescue by isoproterenol and additional isoproterenol-mediated desensitization. Our findings reveal a complex efficacy for functional desensitization, demonstrating that although salmeterol shows weak efficacy for adenylyl cyclase activation and G protein-coupled receptor kinase/arrestin-mediated desensitization, it acts as a strong agonist in highly amplified protein kinase A-mediated events.

Current and novel bronchodilators in respiratory disease

PURPOSE OF REVIEW

β2-Agonists and muscarinic antagonists are widely used to treat asthma and chronic obstructive pulmonary disease (COPD), and a number of novel drug targets are being investigated for potential clinical utility. This review will summarize current developments in the field.

RECENT FINDINGS

The clinical effectiveness of a number of once a day inhaled β2-agonists and muscarinic antagonists is a major advance providing sustained bronchodilation in asthma and COPD. The identification of novel targets (e.g. bitter taste receptor TASR2), the demonstration of clinical effectiveness of others [e.g. phosphodiesterase (PDE)3/4] and exploring the potential of inverse agonists/biased agonists are evidence of continuing interest in the development of novel bronchodilators.

SUMMARY

Novel long-acting β2-agonists (e.g. indacaterol, vilanterol, olodaterol and carmoterol) and muscarinic antagonists (e.g. tiotropium, aclidinium, glycopyrronium and umeclidinium bromide) document sustained bronchodilation and their combination provides additional benefits over monotherapy. Not surprisingly, inhaled long-acting β2-agonist and long-acting muscarinic antagonists remain the drugs of choice for maintenance bronchodilation. However, there is a continued interest in developing novel bronchodilators illustrated by the clinical effectiveness of long acting mixed PDE3/4 inhibitors, vasointestinal peptide adenylyl cyclase agonists and inverse agonists/biased agonists for the β2-adrenoceptor, and the identification of intracellular (e.g. Rho kinase, exchange proteins activated by cyclic AMP) and cell surface (e.g. TAS2R, natriuretic peptide receptor) targets.

Recent structural advances of β1 and β2 adrenoceptors yield keys for ligand recognition and drug design

Because they represent attractive drug targets, adrenoceptors have been widely studied. Recent progress in structural data of β-adrenoceptors allows us to understand and predict key interactions in ligand recognition and receptor activation. Nevertheless, an important aspect of this process has only begun to be explored: the stabilization of a conformational state of these receptors upon contact with a ligand and the capacity of a ligand to influence receptor conformation through allosteric modulation, biased signaling, and selectivity. The aim of the present Perspective is to identify the well-defined orthosteric binding site and possible allosteric sites and to analyze the importance of the ligand-receptor interaction in the stabilization of certain receptor conformations. For this purpose, we have reviewed recent advances made through the use of X-ray data from ligand-β-adrenoceptor (including ADRB1 and ADRB2) crystal structures. Most importantly, implications in the medicinal chemistry field are explored in relation to drug design.

The dopamine D1 receptor is expressed and facilitates relaxation in airway smooth muscle

Background

Dopamine signaling is mediated by G_s protein-coupled “D₁-like” receptors (D₁ and D₅) and G_i-coupled “D₂-like” receptors (D_2-4). In asthmatic patients, inhaled dopamine induces bronchodilation. Although the G_i-coupled dopamine D₂ receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle (ASM) cells, the G_s-coupled dopamine D₁-like receptor subtypes have never been identified on these cells. Activation of G_s-coupled receptors stimulates cyclic AMP (cAMP) production through the stimulation of adenylyl cyclase, which promotes ASM relaxation. We questioned whether the dopamine D₁-like receptor is expressed on ASM, and modulates its function through G_s-coupling.

Methods

The mRNA and protein expression of dopamine D₁-like receptor subtypes in both native human and guinea pig ASM tissue and cultured human ASM (HASM) cells was measured. To characterize the stimulation of cAMP through the dopamine D₁ receptor, HASM cells were treated with dopamine or the dopamine D₁-like receptor agonists (A68930 or SKF38393) before cAMP measurements. To evaluate whether the activation of dopamine D₁ receptor induces ASM relaxation, guinea pig tracheal rings suspended under isometric tension in organ baths were treated with cumulatively increasing concentrations of dopamine or A68930, following an acetylcholine-induced contraction with or without the cAMP-dependent protein kinase (PKA) inhibitor Rp-cAMPS, the large-conductance calcium-activated potassium (BK_Ca) channel blocker iberiotoxin, or the exchange proteins directly activated by cAMP (Epac) antagonist NSC45576.

Results

Messenger RNA encoding the dopamine D₁ and D₅ receptors were detected in native human ASM tissue and cultured HASM cells. Immunoblots confirmed the protein expression of the dopamine D₁ receptor in both native human and guinea pig ASM tissue and cultured HASM cells. The dopamine D₁ receptor was also immunohistochemically localized to both human and guinea pig ASM. The dopamine D₁-like receptor agonists stimulated cAMP production in HASM cells, which was reversed by the selective dopamine D₁-like receptor antagonists SCH23390 or SCH39166. A68930 relaxed acetylcholine-contracted guinea pig tracheal rings, which was attenuated by Rp-cAMPS but not by iberiotoxin or NSC45576.

Conclusions

These results demonstrate that the dopamine D₁ receptors are expressed on ASM and regulate smooth muscle force via cAMP activation of PKA, and offer a novel target for therapeutic relaxation of ASM.

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.

In vitro pharmacological characterization of vilanterol, a novel long-acting β2-adrenoceptor agonist with 24-hour duration of action

Vilanterol trifenatate (vilanterol) is a novel, long-acting β(2)-adrenoceptor (β(2)-AR) agonist with 24 h activity. In this study, we describe the preclinical pharmacological profile of vilanterol using radioligand binding and cAMP studies in recombinant assays as well as human and guinea pig tissue systems to characterize β(2)-AR binding and functional properties. Vilanterol displayed a subnanomolar affinity for the β(2)-AR that was comparable with that of salmeterol but higher than olodaterol, formoterol, and indacaterol. In cAMP functional activity studies, vilanterol demonstrated similar selectivity as salmeterol for β(2)- over β(1)-AR and β(3)-AR, but a significantly improved selectivity profile than formoterol and indacaterol. Vilanterol also showed a level of intrinsic efficacy that was comparable to indacaterol but significantly greater than that of salmeterol. In cellular cAMP production and tissue-based studies measuring persistence and reassertion, vilanterol had a persistence of action comparable with indacaterol and longer than formoterol. In addition, vilanterol demonstrated reassertion activity in both cell and tissue systems that was comparable with salmeterol and indacaterol but longer than formoterol. In human airways, vilanterol was shown to have a faster onset and longer duration of action than salmeterol, exhibiting a significant level of bronchodilation 22 h after treatment. From these investigations, the data for vilanterol are consistent, showing that it is a novel, potent, and selective β(2)-AR receptor agonist with a long duration of action. This pharmacological profile combined with clinical data is consistent with once a day dosing of vilanterol in the treatment of both asthma and chronic obstructive pulmonary disease (COPD).

The dynamics of drug-target interactions: drug-target residence time and its impact on efficacy and safety

The extent and duration of pharmacological action is determined by the lifetime of drug occupancy on a molecular target. This lifetime is defined by dynamic processes that control the rates of drug association and dissociation from the target. Recently, the term residence time has been coined to describe experimental measurements that can be related to the lifetime of the binary drug-target complex, and this in turn to durable, pharmacodynamic activity. The residence time concept and its impact on drug optimization are reviewed here. Examples are provided that demonstrate how a long residence time can improve drug efficacy in vivo. Additionally, optimization of drug-target residence time can help to mitigate off-target mediated toxicity, hence, improving drug safety and tolerability. Recent applications of the residence time concept to both drug discovery and development are also presented.

Design and application of locally delivered agonists of the adenosine A(2A) receptor

The broad spectrum anti-inflammatory actions of adenosine A(2A) receptor agonists are well described. The wide distribution of this receptor, however, suggests that the therapeutic potential of these agents is likely to reside in topical treatments to avoid systemic side effects associated with oral administration. Adenosine A(2A) receptor agonists have been assessed as topical agents: GW328267X (GSK; allergic rhinitis and asthma), UK-432097 (Pfizer; chronic obstructive pulmonary disease [COPD]) and Sonedenoson (MRE0094, King Pharmaceuticals; wound healing). All trials failed to achieve effects against the desired clinical end points. This broad-based review will discuss general principles of chemical design of topically applied agents and potential therapeutic topical applications of current adenosine A(2A) receptor agonists. Potential factors contributing to the lack of efficacy in the above clinical trials will be discussed together with design principles, which may influence efficacy in disease states. Our analysis suggests that adenosine A(2A) receptor agonists have a wide therapeutic potential as topical agents in a wide variety of diseases, such as neutrophil-dependent lung diseases (acute lung injury, exacerbations in asthma and COPD), allergic rhinitis, glaucoma and wound repair. Factors that will influence topical activity include formulation, tissue retention, compound potency, receptor kinetics and pharmacokinetics.

Slow receptor dissociation is not a key factor in the duration of action of inhaled long-acting β2-adrenoceptor agonists

BACKGROUND AND PURPOSE

β(2) -Adrenoceptor agonists are important bronchodilators used for the treatment of chronic obstructive pulmonary disease and asthma. Clinical data on β(2) -adrenoceptor agonists show a range of onset and duration of action. We have investigated whether the receptor binding kinetics of β(2) -adrenoceptor agonists can explain their observed onset of action and duration of effect in the clinic.

EXPERIMENTAL APPROACH

[(3) H]-DHA was used to label β(2) -adrenoceptors expressed in CHO-cell membranes (K(d) of 0.084 nM). Competition kinetic experiments were performed in the presence of unlabelled β(2) agonists at 37°C in HBSS containing GTP. To determine the kinetic parameters, three concentrations (10, 3 and 1 ×K(i) ) of the unlabelled compound were employed against a fixed concentration of [(3) H]-DHA (0.6 nM).

KEY RESULTS

The clinically used β(2) -adrenoceptor agonists exhibited a range of association and dissociation rates. The kinetic K(d) and the competition K(i) values of the eight β(2) -adrenoceptor agonists examined were strongly correlated, suggesting that the method had produced accurate k(off) and k(on) rates. The kinetic on-rate was highly correlated with equilibrium binding affinity.

CONCLUSIONS AND IMPLICATIONS

Although the β(2) -adrenoceptor agonists displayed a range of kinetic rate parameters, simulations at relevant drug concentrations suggest that receptor kinetics do not play an important role in determining onset of action in the clinic. In addition, it is unlikely that receptor kinetics exert an important influence on the duration of action of these agonists, as indacaterol (once daily dosing) had a shorter residency time at the receptor than salmeterol (twice daily dosing).

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.

Third-generation long-acting β₂-adrenoceptor agonists: medicinal chemistry strategies employed in the identification of once-daily inhaled β₂-adrenoceptor agonists

Inhaled long-acting β(2)-adrenoceptor agonists (LABAs) are highly effective bronchodilators in the treatment of asthma and chronic obstructive pulmonary disease. There is significant interest in the development of third-generation compounds that improve upon the marketed twice-daily LABAs salmeterol and formoterol. A principal advantage sought from the next generation is duration of action that supports once-daily dosing, although improved efficacy, faster onset, and increased therapeutic index are also frequently cited as objectives. Recent publications detailing medicinal chemistry programs directed at the discovery of third-generation LABAs illustrate a wide variety of strategies that have been successfully employed towards these goals. Some recent scientific advances in the understanding of inhaled bronchodilators are discussed and the reported medicinal chemistry strategies are reviewed in the context of these advances.

A multivalent approach to the discovery of long-acting β(2)-adrenoceptor agonists for the treatment of asthma and COPD

A multivalent approach was applied to the design of long-acting inhaled β(2)-adrenoceptor agonists. A series of dimeric arylethanolamines based on the short acting β(2)-adrenoceptor agonist albuterol were prepared, varying the nature and length of the linker between the basic nitrogens. None of the C(2)-symmetric dimers demonstrated increased potency, however dimer 5j, derived from 4-phenethylamine, was found to have increased binding potency in vitro relative to the parent monomer. Optimization of this structure led to the identification of 22 (milveterol) which demonstrates high potency in vitro and long duration of action in a guinea pig model of bronchoprotection.

Estimation of the dissociation rate of unlabelled ligand-receptor complexes by a 'two-step' competition binding approach

BACKGROUND AND PURPOSE

Because the in vivo effectiveness of ligands may also be determined by the rate by which they dissociate from their target receptors, drug candidates are being increasingly screened for this kinetic property. The dissociation rate of unlabelled ligand-receptor complexes can be estimated indirectly from their ability to slow the association of subsequently added radioligand molecules.

EXPERIMENTAL APPROACH

We used the 'two-step competition' binding approach consisting of pre-incubating the receptor preparation with a wide range of ligand concentrations, washing off free ligand molecules, adding radioligand and monitoring its receptor binding after a fixed time. Based on the rationale that binding of both ligands is mutually exclusive and that they bind according to the law of mass action to a single class of sites, the unlabelled ligand's dissociation rate can be estimated from the upward shift that the competition curve experiences after washing.

KEY RESULTS

The relevance of the 'two-step competition' approach was explored by computer simulations and by comparing the dissociation behaviour of unlabelled D(2) dopamine and CB(1) cannabinoid receptor antagonists in this and alternative approaches. Besides providing satisfactory estimations of dissociation rates, the method also detects the ability of the unlabelled ligand molecules to be released from 'sinks' such as the cell membrane.

CONCLUSIONS AND IMPLICATIONS

As the 'two-step competition' requires rapid intermediate washing steps and needs radioligand binding to be measured at only one time point, this approach is particularly suited for binding studies on intact plated cells.

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.

In vitro characterisation of the duration of action of the histamine-1 receptor antagonist azelastine

Azelastine is a selective antagonist at the human histamine-1 receptor and is used clinically in the treatment of allergic rhinitis. In this study we have investigated its duration of action in vitro in an effort to characterise the receptor and tissue components involved. Chinese hamster ovary cell membrane fragments were used to determine the kinetics of azelastine at the H₁ receptor in a radioligand binding assay. Further duration of action studies were completed in tissue preparations using guinea-pig trachea and human bronchus. In radioligand binding studies, azelastine reached steady state at the H₁ receptor after approximately 41 min and exhibited a significantly slower dissociation rate constant from the receptor than the first generation antihistamine, diphenhydramine. In washout studies completed in guinea-pig and human airway in vitro tissue preparations, azelastine continued to antagonise the effects of histamine at the H₁ receptor for at least 18 h post-washout of the antagonist. This outcome was reversed following removal of the epithelium from guinea-pig isolated tracheal strips. These studies indicate there is a tissue component contributing to azelastine's duration of action, in addition to its direct H₁ receptor binding, with evidence suggesting a role for the epithelial layer.

The effect of plasma protein binding on in vivo efficacy: misconceptions in drug discovery

Data from in vitro plasma protein binding experiments that determine the fraction of protein-bound drug are frequently used in drug discovery to guide structure design and to prioritize compounds for in vivo studies. However, we consider that these practices are usually misleading, because in vivo efficacy is determined by the free (unbound) drug concentration surrounding the therapeutic target, not by the free drug fraction. These practices yield no enhancement of the in vivo free drug concentration. So, decisions based on free drug fraction could result in the wrong compounds being advanced through drug discovery programmes. This Perspective provides guidance on the application of plasma protein binding information in drug discovery.