Inverse agonists - What do they mean for psychiatry?

Ligand bias and inverse agonism on 5-HT2A receptor-mediated modulation of G protein activity in post-mortem human brain

BACKGROUND AND PURPOSE

Whereas biased agonism on the 5-HT2A receptor has been ascribed to hallucinogenic properties of psychedelics, no information about biased inverse agonism on this receptor is available. In schizophrenia, increased 5-HT2A receptor constitutive activity has been suggested, highlighting the therapeutic relevance of inverse agonism. This study characterized the modulation of G protein activity promoted by different drugs, commonly considered as 5-HT2A receptor antagonists, in post-mortem human brain cortex.

EXPERIMENTAL APPROACH

Modulation of [35S]GTPγS binding to different subtypes of Gα proteins exerted by different 5-HT2A receptor drugs was determined by scintillation proximity assays in brain from human, WT and 5-HT2A receptor KO mice.

KEY RESULTS

MDL-11,939 was the only drug having no effect on the basal activity of 5-HT2A receptor. Altanserin and pimavanserin decreased basal activation of Gi1, but not Gq/11 proteins. This effect was blocked by MDL-11,939 and absent in 5-HT2A receptor KO mice. Volinanserin showed 5-HT2A receptor-mediated inverse agonism both on Gi1 and Gq/11 proteins. Ketanserin exhibited 5-HT2A receptor partial agonism exclusively on Gq/11 proteins. On the other hand, eplivanserin and nelotanserin displayed inverse agonism on Gq/11 and/or Gi1 proteins, which was insensitive to MDL-11,939 and was present in KO mice suggesting a role for another receptor.

CONCLUSION AND IMPLICATIONS

The results reveal the existence of constitutively active 5-HT2A receptors in human pre-frontal cortex and demonstrate different pharmacological profiles of various 5-HT2A receptor drugs previously considered antagonists. These findings indicate that altanserin and pimavanserin possess biased inverse agonist profile towards 5-HT2A receptor activation of Gi1 proteins.

Neurodegenerative Diseases: New Hopes and Perspectives

Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, Huntington's disease, and Friedrich ataxia are all incurable neurodegenerative diseases defined by the continuous progressive loss of distinct neuronal subtypes. Despite their rising prevalence among the world's ageing population, fewer advances have been made in the concurrent massive efforts to develop newer drugs. Recently, there has been a shift in research focus towards the discovery of new therapeutic agents for neurodegenerative diseases. In this review, we have summarized the recently developed therapies and their status in the management of neurodegenerative diseases.

Design, Synthesis, Molecular Docking, and Biological Evaluation of Novel Pimavanserin-Based Analogues as Potential Serotonin 5-HT2A Receptor Inverse Agonists

There is concern for important adverse effects with use of second-generation antipsychotics in Parkinson's disease psychosis (PDP) and dementia-related psychosis. Pimavanserin is the only antipsychotic drug authorized for PDP and represents an inverse agonist of 5-HT_2A receptors (5-HT2AR) lacking affinity for dopamine receptors. Therefore, the development of serotonin 5-HT2AR inverse agonists without dopaminergic activity represents a challenge for different neuropsychiatric disorders. Using ligand-based drug design, we discovered a novel structure of pimavanserin analogues (2, 3, and 4). In vitro competition receptor binding and functional G protein coupling assays demonstrated that compounds 2, 3, and 4 showed higher potency than pimavanserin as 5-HT2AR inverse agonists in the human brain cortex and recombinant cells. To assess the effect of molecular substituents for selectivity and inverse agonism at 5-HT2ARs, molecular docking and in silico predicted physicochemical parameters were performed. Docking studies were in agreement with in vitro screenings and the results resembled pimavanserin.

Aβ peptides stabilize GPCRs in inactive form and trigger inverse agonism in Alzheimer's disease

Several G-protein coupled receptors (GPCR) are upregulated in Alzheimer's Disease (AD), which ought to facilitate neurotransmission, and improve cognition. Yet, despite this upregulation, associated physiological effects are not observed in AD patients. This paradox solicits urgent attention to find a suitable justification for disturbed neurotransmission in AD. This article focuses on the role of Aβ granules and their possible interaction with GPCRs that modulate neurotransmission and AD progression.

Evaluating pimavanserin as a treatment for psychiatric disorders: A pharmacological property in search of an indication

Introduction: Pimavanserin is FDA-approved to treat hallucinations and delusions associated with Parkinson's disease psychosis. As a potent 5-HT_2A inverse agonist/antagonist, it could be efficacious in other psychiatric disorders. Recently, several studies have investigated this potential.Areas covered: The authors review the efficacy and adverse effects of pimavanserin for hallucinations in dementia, major depression, and schizophrenia.Expert opinion: Two controlled studies suggest pimavanserin has potential as a treatment for hallucinations in dementia. In patients with depression who did not respond to antidepressant treatment, pimavanserin augmentation was efficacious in a phase 2 study. Pimavanserin augmentation also alleviated sexual side effects of SSRI and SSNI. However, Acadia Pharmaceuticals stated in a press release that it does not plan further antidepressant trials based on its phase 3 trial, which showed a nonsignificant trend toward an antidepressant effect. Since almost all existing antipsychotics fail to substantially benefit negative symptoms, better treatments are needed. Pimavanserin augmentation of antipsychotics did benefit negative symptoms (effect size≈0.2) but failed to reduce the total PANSS score significantly in two large, well-controlled double-blind studies. Pimavanserin has a good safety profile.

GPCR oligomerization as a target for antidepressants: Focus on GPR39

At present most of the evidence for the relevance of oligomerization for the pharmacology of depression comes from in vitro studies which identified oligomers, and from neuropsychopharmacological studies of receptors which participate in oligomerization. For example, behavioural and biochemical studies in knockout animals suggest that GPR39 may mediate the antidepressant action of monoaminergic antidepressants. We have recently found long-lasting antidepressant-like effects of GPR39 agonist, thus suggesting GPR39 as a target for the development of novel antidepressant drugs. In vitro studies have shown that GPR39 oligomerizes with other GPCRs. Oligomerization of GPR39 should thus be considered in relation to the development of new antidepressants targeting this receptor as well as antidepressants targeting other receptors that may form complexes with GPR39. Here, we summarize recent data suggestive of the importance of oligomerization for the pharmacology of depression and discuss approaches for validation of this phenomenon.

Serotonin 2A receptors and cannabinoids

Accumulating evidence has proven that both exogenous cannabinoids as well as imbalances in the endocannabinoid system are involved in the onset and development of mental disorders such as anxiety, depression, or schizophrenia. Extensive recent research in this topic has mainly focused on the molecular mechanisms by which cannabinoid agonists may contribute to the pathophysiology of these disorders. Initially, serotonin neurotransmitter garnered most attention due to its relationship to mood disorders and mental diseases, with little attention to specific receptors. To date, the focus has redirected toward the understanding of different serotonin receptors, through a demonstration of its versatile pharmacology and synergy with different modulators. Serotonin 2A receptors are a good example of this phenomenon, and the complex signaling that they trigger appears of high relevance in the context of mental disorders, especially in schizophrenia. This chapter will analyze most relevant attributes of serotonin 2A receptors and the endocannabinoid system, and will highlight the evidence toward the functional bidirectional interaction between these elements in the brain as well as the impact of the endocannabinoid system dysregulation on serotonin 2A receptors functionality.

New antipsychotic drugs for the treatment of agitation and psychosis in Alzheimer's disease: focus on brexpiprazole and pimavanserin

Behavioral and psychological symptoms of dementia are symptoms of disturbed perception, mood, behavior, and thought content that occurred frequently. These symptoms, which include apathy, depression, anxiety, psychosis, agitation, and aggression, can serve as predictors of and early clinical diagnostic markers for Alzheimer's disease (AD) and are common precipitants of institutional care. Agitation and psychosis are associated with accelerated disease progression and increased tau phosphorylation in patients with AD. Current guidelines recommend the use of second-generation antipsychotics for the treatment of agitation and psychosis in AD, but only after first-line non-pharmacological interventions and for no longer than 12 weeks because long-term use of these drugs is associated with an increased risk of mortality and an increased frequency of cerebrovascular events. Therefore, new antipsychotic drugs with improved efficacy and safety are needed as an alternative to current antipsychotic drugs. In this report, we discuss some of the most relevant advances in the field of agitation and psychosis in AD and focus on the recent positive clinical evidence observed with two new antipsychotics drugs: brexpiprazole and pimavanserin. Brexpiprazole is a receptor partial agonist (D2, D3, 5-HT1A), receptor antagonist (5-HT2A/B, α1B/α2C) according to the neuroscience-based nomenclature. Two recent phase III clinical trials have shown that brexpiprazole 2 mg/day is effective for the treatment of agitation in patients with AD and has an improved tolerability and safety profile compared with currently available second-generation antipsychotics. Pimavanserin is a receptor antagonist (5-HT2A, 5-HT2C) that has been given market authorization for psychosis occurring in Parkinson's disease. Recent phase II studies suggest that this drug is effective in AD patients with more severe psychosis, although further long-term studies are needed to better define the efficacy and long-term safety profile of pimavanserin for the treatment of psychosis in AD.

Pimavanserin exhibits serotonin 5-HT2A receptor inverse agonism for Gαi1- and neutral antagonism for Gαq/11-proteins in human brain cortex

Pimavanserin is claimed as the first antipsychotic drug that shows selectivity for serotonin 5-HT₂ receptors (5-HT₂Rs) and lacks of affinity for dopamine D₂ receptors (D₂Rs). Cell-based functional assays suggest that pimavanserin and antipsychotics with D₂R/5-HT₂R affinity could act as inverse agonists of 5-HT_2ARs. However, there is not evidence of such pharmacological profile in native brain tissue. 5-HT_2ARs are able to engage both canonical G_αq/11- and non-canonical G_αi1-proteins. In the present study, the response to pimavanserin of the 5-HT_2AR coupling to G_αq/11- and G_αi1-proteins was measured in membranes of postmortem human prefrontal cortex by antibody-capture [³⁵S]GTPγS binding scintillation proximity assays. Pimavanserin promoted a concentration-dependant inhibition of the 5-HT_2AR coupling to G_αi1-proteins whereas the response of G_αq/11-proteins was unaltered, suggesting inverse agonism and neutral antagonism properties, respectively. The inhibition was abolished in the presence of the selective 5-HT_2AR antagonist MDL-11,939 and was absent in brain cortex of 5-HT_2AR knock-out mice when compared to respective 5-HT_2AR wild-type animals. In conclusion, the results demonstrate the existence of constitutive 5-HT_2AR activity in human brain for the signalling pathway mediated by G_αi1-proteins. Pimavanserin demonstrates 5-HT_2AR functional selectivity and exhibits inverse agonist profile towards G_αi1-proteins, which is considered the effector pathway promoting hallucinogenic responses. In contrast, pimavanserin behaves as neutral antagonist on the 5-HT_2AR coupling to the canonical G_αq/11-protein pathway. The results strengthen the relevance of inverse agonism as potential mechanism of antipsychotic activity. Moreover, the existence of functional selectivity of 5-HT_2ARs for different G_α-proteins could contribute to better design of 5-HT_2AR-related antipsychotic drugs.

Big Data Challenges Targeting Proteins in GPCR Signaling Pathways; Combining PTML-ChEMBL Models and [35S]GTPγS Binding Assays

G-protein-coupled receptors (GPCRs), also known as 7-transmembrane receptors, are the single largest class of drug targets. Consequently, a large amount of preclinical assays having GPCRs as molecular targets has been released to public sources like the Chemical European Molecular Biology Laboratory (ChEMBL) database. These data are also very complex covering changes in drug chemical structure and assay conditions like c₀ = activity parameter (K_i, IC₅₀, etc.), c₁ = target protein, c₂ = cell line, c₃ = assay organism, etc., making difficult the analysis of these databases that are placed in the borders of a Big Data challenge. One of the aims of this work is to develop a computational model able to predict new GPCRs targeting drugs taking into consideration multiple conditions of assay. Another objective is to perform new predictive and experimental studies of selective 5-HTA2 receptor agonist, antagonist, or inverse agonist in human comparing the results with those from the literature. In this work, we combined Perturbation Theory (PT) and Machine Learning (ML) to seek a general PTML model for this data set. We analyzed 343 738 unique compounds with 812 072 end points (assay outcomes), with 185 different experimental parameters, 592 protein targets, 51 cell lines, and/or 55 organisms (species). The best PTML linear model found has three input variables only and predicted 56 202/58 653 positive outcomes (sensitivity = 95.8%) and 470 230/550 401 control cases (specificity = 85.4%) in training series. The model also predicted correctly 18 732/19 549 (95.8%) of positive outcomes and 156 739/183 469 (85.4%) of cases in external validation series. To illustrate its practical use, we used the model to predict the outcomes of six different 5-HT2A receptor drugs, namely, TCB-2, DOI, DOB, altanserin, pimavanserin, and nelotanserin, in a very large number of different pharmacological assays. 5-HT2A receptors are altered in schizophrenia and represent drug target for antipsychotic therapeutic activity. The model correctly predicted 93.83% (76 of 86) experimental results for these compounds reported in ChEMBL. Moreover, [³⁵S]GTPγS binding assays were performed experimentally with the same six drugs with the aim of determining their potency and efficacy in the modulation of G-proteins in human brain tissue. The antagonist ketanserin was included as inactive drug with demonstrated affinity for 5-HT2A/C receptors. Our results demonstrate that some of these drugs, previously described as serotonin 5-HT2A receptor agonists, antagonists, or inverse agonists, are not so specific and show different intrinsic activity to that previously reported. Overall, this work opens a new gate for the prediction of GPCRs targeting compounds.

Making Sense of Pharmacology: Inverse Agonism and Functional Selectivity

Constitutive receptor activity/inverse agonism and functional selectivity/biased agonism are 2 concepts in contemporary pharmacology that have major implications for the use of drugs in medicine and research as well as for the processes of new drug development. Traditional receptor theory postulated that receptors in a population are quiescent unless activated by a ligand. Within this framework ligands could act as agonists with various degrees of intrinsic efficacy, or as antagonists with zero intrinsic efficacy. We now know that receptors can be active without an activating ligand and thus display "constitutive" activity. As a result, a new class of ligand was discovered that can reduce the constitutive activity of a receptor. These ligands produce the opposite effect of an agonist and are called inverse agonists. The second topic discussed is functional selectivity, also commonly referred to as biased agonism. Traditional receptor theory also posited that intrinsic efficacy is a single drug property independent of the system in which the drug acts. However, we now know that a drug, acting at a single receptor subtype, can have multiple intrinsic efficacies that differ depending on which of the multiple responses coupled to a receptor is measured. Thus, a drug can be simultaneously an agonist, an antagonist, and an inverse agonist acting at the same receptor. This means that drugs have an additional level of selectivity (signaling selectivity or "functional selectivity") beyond the traditional receptor selectivity. Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools.

Graphic representation of pharmacology: Development of an alternative model

Introduction:

Providing clinicians with an easy to grasp and understandable representation of pharmacology is important to allow optimal clinical decisions to be made. Two of the most clinically relevant dimensions are receptor binding affinity and functional activity. The binding affinity for an agonist is described by the dissociation constant (K_A), and an antagonist by the inhibition constant (K_i). Functionally, medications can act as superagonists, agonists, partial agonists, antagonists, partial inverse agonists, or inverse agonists at several receptor sites, transporters, or ion channels. Comprehending the differences between agents is complicated by the number and types of binding sites.

Methods:

Binding and functional data are obtained from primary literature, product labels, human cloned receptor binding, and other sources. Binding affinities are converted into ratios relative to the putative primary receptor for that category of agent. Antipsychotic binding is referenced to dopamine type 2 long (D2L) receptor binding. Binding affinity ratios (BARs) generate a 6-spoked diagram, with D2L as the hub. The most avidly bound sites are the spokes, and the disk diameter represents the BAR. Where functional data are available, they are shown as a pie chart shading the binding site's disk.

Results:

Binding and function diagrams are shown for the antipsychotics where binding data are available and are compared to previous methods of pharmacologic comparisons of antipsychotics.

Discussion:

Use of graphic models of psychotropic pharmacology improves clinician comprehension and may serve as an aid to improve rational therapeutics and patient outcomes.