Pimavanserin reverses multiple measures of anxiety in a rodent model of post-traumatic stress disorder

Pimavanserin is a highly selective 5-HT_2A inverse agonist in current medical use. Prior studies suggest that 5-HT_2A serotonin receptors may play a role in anxiety and emotional memory. Therefore, pimavanserin was tested in a rat model of PTSD to determine whether it might ameliorate PTSD-like symptoms. The lifetime prevalence of PTSD is estimated to be 125% higher in women than men. Consequently, in an effort to create a robust model of PTSD that was more representative of human PTSD prevalence, 20-week old female rats of the emotionally hyperreactive Lewis strain were used for these studies. The rats were single-housed and exposed twice to restraint stress coupled with predator odor or to a sham-stressed condition. Twenty days after the second stress or sham-stress exposure, rats were injected with saline alone or with 0.3 or 1.0 mg/kg pimavanserin, doses that were confirmed to substantially block 5-HT_2A receptor activity in this study without causing any non-specific behavioral or adverse effects. One hour later, rats were tested for anxiety through acoustic startle response, the elevated plus-maze and three parameters of open field behavior. Five days later, blood was sampled for plasma corticosterone. The stressed/saline-injected rats had higher anxiety scores and corticosterone levels than sham-stressed/saline-injected rats. Pimavanserin significantly and generally dose-dependently reversed these persistent stress effects, but had no significant effect on the behavioral measures in normal, non-stressed rats. These results, consistent with a role for the 5-HT_2A receptor, suggest that pimavanserin might have potential to reduce some consequences of traumatic stress.

The impact of pimavanserin on psychotic phenotypes and tau phosphorylation in the P301L/COMT- and rTg(P301L)4510 mouse models of Alzheimer's disease

INTRODUCTION

Psychosis in Alzheimer's disease (AD) is associated with grave clinical consequences including a precipitous cognitive decline and a hastened demise. These outcomes are aggravated by use of existing antipsychotic medications, which are also associated with cognitive decline and increased mortality; preclinical models that would develop new therapeutic approaches are desperately needed. The current report evaluates the ability of the neoteric antipsychotic, pimavanserin, to normalize hyperkinesis and sensorimotor gating in the novel catechol-O-methyltransferase (COMT) deleted P301L/COMT- and rTg(P301L)4510 models of psychotic AD, and the impact of pimavanserin on tau pathology.

METHODS

Female P301L/COMT- mice were behaviorally characterized for abnormalities of locomotion and sensorimotor gating, and biochemically characterized for patterns of tau phosphorylation relative to relevant controls utilizing high-sensitivity tau enzyme-linked immunosorbent assay (ELISA). Female P301L/COMT- and rTg(P301L)4510 mice were randomized to pimavanserin or vehicle treatment to study the ability of pimavanserin to normalize locomotion and rescue sensorimotor gating. Additionally, high-sensitivity tau ELISA was used to investigate the impact of treatment on tau phosphorylation.

RESULTS

P301L/COMT- mice evidenced a hyperlocomotive phenotype and deficits of sensorimotor gating relative to wild-type mice on the same background, and increased tau phosphorylation relative to COMT-competent P301L mice. Pimavanserin normalized the hyperkinetic phenotype in both the P301L/COMT- and rTg(P301L)4510 mice but had no impact on sensorimotor gating in either model. Pimavanserin treatment had little impact on tau phosphorylation patterns.

DISCUSSION

These data suggest that pimavanserin ameliorates tau-driven excessive locomotion. Given the morbidity associated with aberrant motor behaviors such as pacing in AD and lack of effective treatments, future studies of the impact of pimavanserin on actigraphy in patients with this syndrome may be warranted.

Pimavanserin augments the efficacy of atypical antipsychotic drugs in a mouse model of treatment-refractory negative symptoms of schizophrenia

Negative symptoms are a core, pervasive, and often treatment-refractory phenotype of schizophrenia, one which contributes to poor functional outcome, ability to work, pursue educational goals, and quality of life, as well as caretaker burden. Improvement of negative symptoms in some patients with schizophrenia has been reported with some atypical antipsychotic drugs [AAPDs], but improvement is absent in many patients and partial in others. Therefore, more effective treatments are needed, and better preclinical models of negative symptoms are needed to identify them. Sub-chronic [sc] treatment of rodents with phencyclidine [PCP], a noncompetitive N-methyl-d-aspartate [NMDAR] antagonist, produces deficits in social interactions [SI] that have been widely studied as a model of negative symptoms in schizophrenia. Acute restraint stress [ARS] also provides a model of treatment-refractory negative symptoms [TRS] to AAPDs. By themselves, in sc-PCP mice, the AAPDs, risperidone, olanzapine, and aripiprazole, but not the selective 5-HT_2AR inverse agonist, pimavanserin [PIM], rescued the SI deficit in sc-PCP mice, as did the combination of PIM with sub-effective doses of each of these AAPDs. These three AAPDs alone did not rescue SI deficit in sc-PCP+2h-ARS mice, indicating these mice were treatment refractory. However, co-administration of PIM with any of the AAPDs significantly restored SI in these mice. PIM may be an effective adjunctive therapy for treating negative symptoms of schizophrenia in some patients who have failed to respond to AAPDs, but further studies are needed.

Pimavanserin Promotes Trophic Factor Release and Protects Cultured Primary Dopaminergic Neurons Exposed to MPP+ in a GDNF-Dependent Manner

Neurodegeneration and impaired neural development are a common feature of many neuropsychiatric disorders. Second-generation antipsychotics (SGAs) and certain atypical antidepressants display neuroprotective effects. Though these drugs interact with many molecular targets, a common shared attribute is high antagonist potency at 5-HT_2A receptors. Pimavanserin is a selective 5-HT_2A inverse agonist/antagonist that was recently FDA approved for treating hallucinations and delusions associated with Parkinson's disease. Unlike SGAs, pimavanserin lacks activity at other targets like dopamine, histamine, muscarinic, and adrenergic receptors. To investigate whether selective 5-HT_2A inverse agonists have neuroprotective properties, pimavanserin and another selective 5-HT_2A inverse agonist, M100907, were applied to primary cultures of dopaminergic neurons treated with 1-methyl-4-phenylpyridinium (MPP+). Both pimavanserin and M100907 protected dopaminergic neurons against MPP+-induced cell death. The neuroprotective effects of pimavanserin required signaling through the extracellular signal-regulated kinase 1/2 pathway, restored mitochondrial function, and reduced oxidative stress. Further investigation showed that pimavanserin promotes the release of brain-derived neurotrophic factor and glial-derived neurotrophic factor (GDNF) and that the neuroprotective effects of pimavanserin were blocked by antibodies to GDNF but not by anti-tyrosine receptor kinase B receptor antibodies. Thus, pimavanserin induces release of neurotrophic factors and protects dopaminergic neurons against MPP+ toxicity in a GDNF-dependent manner.

Pimavanserin, a 5HT2A receptor inverse agonist, rapidly suppresses Aβ production and related pathology in a mouse model of Alzheimer's disease

Amyloid-β (Aβ) peptide aggregation into soluble oligomers and insoluble plaques is a precipitating event in the pathogenesis of Alzheimer's disease (AD). Given that synaptic activity can regulate Aβ generation, we postulated that 5HT_2A -Rs may regulate Aβ as well. We treated APP/PS1 transgenic mice with the selective 5HT_2A inverse agonists M100907 or Pimavanserin systemically and measured brain interstitial fluid (ISF) Aβ levels in real-time using in vivo microdialysis. Both compounds reduced ISF Aβ levels by almost 50% within hours, but had no effect on Aβ levels in 5HT_2A -R knock-out mice. The Aβ-lowering effects of Pimavanserin were blocked by extracellular-regulated kinase (ERK) and NMDA receptor inhibitors. Chronic administration of Pimavanserin by subcutaneous osmotic pump to aged APP/PS1 mice significantly reduced CSF Aβ levels and Aβ pathology and improved cognitive function in these mice. Pimavanserin is FDA-approved to treat Parkinson's disease psychosis, and also has been shown to reduce psychosis in a variety of other dementia subtypes including Alzheimer's disease. These data demonstrate that Pimavanserin may have disease-modifying benefits in addition to its efficacy against neuropsychiatric symptoms of Alzheimer's disease. Read the Editorial Highlight for this article on page 560.

Relevance of 5-HT2A Receptor Modulation of Pyramidal Cell Excitability for Dementia-Related Psychosis: Implications for Pharmacotherapy

Psychosis occurs across a wide variety of dementias with differing etiologies, including Alzheimer's dementia, Parkinson's dementia, Lewy body dementia, frontotemporal dementia, and vascular dementia. Pimavanserin, a selective serotonin 5-HT_2A receptor (5-HT_2AR) inverse agonist, has shown promising results in clinical trials by reducing the frequency and/or severity of hallucinations and delusions and the risk of relapse of these symptoms in patients with dementia-related psychosis. A literature review was conducted to identify mechanisms that explain the role of 5-HT_2ARs in both the etiology and treatment of dementia-related psychosis. This review revealed that most pathological changes commonly associated with neurodegenerative diseases cause one or more of the following events to occur: reduced synaptic contact of gamma aminobutyric acid (GABA)-ergic interneurons with glutamatergic pyramidal cells, reduced cortical innervation from subcortical structures, and altered 5-HT_2AR expression levels. Each of these events promotes increased pyramidal cell hyperexcitability and disruption of excitatory/inhibitory balance, facilitating emergence of psychotic behaviors. The brain regions affected by these pathological changes largely coincide with areas expressing high levels of 5-HT_2ARs. At the cellular level, 5-HT_2ARs are most highly expressed on cortical glutamatergic pyramidal cells, where they regulate pyramidal cell excitability. The common effects of different neurodegenerative diseases on pyramidal cell excitability together with the close anatomical and functional connection of 5-HT_2ARs to pyramidal cell excitability may explain why suppressing 5-HT_2AR activity could be an effective strategy to treat dementia-related psychosis.

Ligand Dependent Switch from RXR Homo- to RXR-NURR1 Heterodimerization

Retinoid X receptors (RXRs) play key roles in many physiological processes in both the periphery and central nervous system. In addition, RXRs form heterodimers with other nuclear receptors to exert their physiological effects. The nuclear receptor related 1 protein (NURR1) is particularly interesting because of its role in promoting differentiation and survival of dopamine neurons. However, only a small number of RXR-heterodimer selective modulators are available, with limited chemical diversity. This work describes the synthesis, biochemical evaluation, and structural elucidation of a novel series of RXR ligands with strongly biased interactions with RXRα–NURR1 heterodimers. Targeted modifications to the small molecule biaryl scaffold caused local RXRα side-chain disturbances and displacement of secondary structural elements upon ligand binding. This resulted in the repositioning of protein helices in the heterodimer interface of RXRα, alterations in homo- versus heterodimer formation, and modulation of activation function 2 (AF2). The data provide a rationale for the design of RXR ligands consisting of a highly conserved hydrophilic region, strongly contributing to the ligand affinity, and a variable hydrophobic region, which efficiently probes the effects of structural changes at the level of the ligand on co-regulator recruitment or the RXRα–NURR1 dimerization interface.

Dronedarone Modulates M1 and M3 Muscarinic Receptors with Subtype Selectivity, Functional Selectivity, and Probe Dependence

We have previously reported that amiodarone interacts with a novel allosteric site on muscarinic receptors. Amiodarone's most striking effect is to enhance the maximal response elicited by muscarinic agonists at the M1, M3, and M5 receptors. Furthermore, the quaternary analog N-ethylamiodarone (NEA) is inhibitory at these receptors and appears to compete with amiodarone at that allosteric site. In the present studies, we show that dronedarone also modulates Gq-mediated responses at M1 and M3, although in a more discriminating manner. For example, dronedarone markedly enhances pilocarpine-stimulated release of arachidonic acid from CHO cells, via the M3 receptor subtype, but does not affect the acetylcholine-stimulated response. Such probe-dependent effects are diagnostic of an allosteric interaction. In comparison to these effects at M3, dronedarone is strongly inhibitory toward both pilocarpine and acetylcholine at the M1 subtype. The effects of dronedarone are consistent with an interaction at the amiodarone site: dronedarone inhibits the enhancement of acetylcholine's response produced by amiodarone at the M3 subtype; also, NEA reverses the enhancement of pilocarpine's response at M3 produced by either dronedarone or amiodarone. In studies with the M1-selective allosteric agonist 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC-260584), amiodarone enhanced the maximal response observed, whereas dronedarone was inhibitory. On the other hand, benzyl quinolone carboxylic acid, the well-known allosteric ligand that dramatically enhances the potency of acetylcholine at the M1 subtype, had no effect on the response profile of AC-260584. In summary, dronedarone acts at M1 and M3 muscarinic receptors in a manner that complements amiodarone and provides an additional tool with which to investigate this novel allosteric site.

Estrogen Receptor Beta Selective Agonists as Agents to Treat Chemotherapeutic-Induced Neuropathic Pain

Chemotherapy-induced neuropathic pain (CINP) remains a major unmet medical need. Estrogen receptor beta (ERβ)-selective agonists represent a novel strategy for treating CINP because they are neuroprotective and may also have anticancer activity. We confirmed that ERβ-selective agonists have antiallodynic effects in the spinal nerve ligation model of neuropathic pain. We then showed that structurally diverse ERβ-selective agonists also relieved allodynia in CINP caused by taxol, oxaliplatin, and vincristine. These effects were receptor subtype specific and mediated by ERβ receptors as ERα-selective and nonselective estrogen agonists were inactive, a mixture of an ERβ and ERα agonist was inactive, and ERβ-selective antagonists blocked the effects of the ERβ-selective agonists. The efficacy and potency of ERβ-agonists was greater in male rats than females. To address the possibility that AC-186 might stimulate proliferation of cancers, rendering it unsuitable for treating CINP, we evaluated proliferative effects of AC-186 on prostate cancer cells and found it inhibited growth (LNCaP cells) or had no effect (PC3 cells) on these cells. Thus, ERβ-selective agonists exhibit potential for treating CINP.

Preclinical PET Neuroimaging of [11C]Bexarotene

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [¹¹C]CO₂ fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [¹¹C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [¹¹C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system.

Bexarotene prodrugs: targeting through cleavage by NQO1 (DT-diaphorase)

Bexarotene, a retinoid X receptor (RXR) agonist, is being tested as a potential disease modifying treatment for neurodegenerative conditions. To limit the peripheral exposure of bexarotene and release it only in the affected areas of the brain, we designed a prodrug strategy based on the enzyme NAD(P)H/quinone oxidoreductase (NQO1) that is elevated in neurodegenerative diseases. A series of indolequinones (known substrates of NQO1) was synthesized and coupled to bexarotene. Bexarotene-3-(hydroxymethyl)-5-methoxy-1,2-dimethyl-1H-indole-4,7-dione ester 7a was cleaved best by NQO1. The prodrugs are not cleaved by esterase.

Low dose bexarotene treatment rescues dopamine neurons and restores behavioral function in models of Parkinson's disease

Nurr1 is a nuclear hormone receptor (NucHR) strongly implicated in the growth, maintenance, and survival of dopaminergic neurons. Nurr1 may be unable to bind ligands directly, but it forms heterodimers with other NucHRs that do. Using bioluminescence resonance energy transfer (BRET) assays to directly monitor interactions of Nurr1 with other NucHRs, we found the cancer drug bexarotene (Targretin, also LGD1069) displayed biased interactions with Nurr1-RXR heterodimers compared with RXR-RXR homodimers. Remarkably, at doses up to 100-fold lower than those effective in rodent cancer models, bexarotene rescued dopamine neurons and reversed behavioral deficits in 6-hydroxydopamine (6-OHDA) lesioned rats. Compared to the high doses used in cancer therapy, low doses of bexarotene have significantly milder side effects including a reduced increase in plasma triglycerides and less suppression of thyroid function. On the basis of extrapolations from rat to human doses, we hypothesize that low oral doses of bexarotene may provide an effective and tolerated therapy for Parkinson's disease (PD).

Identification of the antiarrhythmic drugs amiodarone and lorcainide as potent H3 histamine receptor inverse agonists

Use of molecular pharmacology to reprofile older drugs discovered before the advent of recombinant technologies is a fruitful method to elucidate mechanisms of drug action, expand understanding of structure-activity relationships between drugs and receptors, and in some cases, repurpose approved drugs. The H3 histamine receptor is a G-protein-coupled receptor (GPCR) primarily expressed in the central nervous system where among many things it modulates cognitive processes, nociception, feeding and drinking behavior, and sleep/wakefulness. In binding assays and functional screens of the H3 histamine receptor, the antiarrhythmic drugs lorcainide and amiodarone were identified as potent, selective antagonists/inverse agonists of human and rat H3 histamine receptors, with relatively little or no activity at over 20 other monoamine GPCRs, including H1, H2, and H4 receptors. Potent antagonism of H3 receptors was unique to amiodarone and lorcainide of 20 antiarrhythmic drugs tested, representing six pharmacological classes. These results expand the pharmacophore of H3 histamine receptor antagonist/inverse agonists and may explain, in part, the effects of lorcainide on sleep in humans.

The protease activated receptor 2 (PAR2) polymorphic variant F240S constitutively activates PAR2 receptors and potentiates responses to small-molecule PAR2 agonists

AC-55541 [N-[[1-(3-bromo-phenyl)-eth-(E)-ylidene-hydrazinocarbonyl]-(4-oxo-3,4-dihydro-phthalazin-1-yl)-methyl]-benzamide] and AC-264613 [2-oxo-4-phenylpyrrolidine-3-carboxylic acid [1-(3-bromo-phenyl)-(E/Z)-ethylidene]-hydrazide] are the first two small-molecule agonists described for the G protein-coupled receptor protease-activated receptor 2 (PAR2), but whether they activate PAR2 through a similar mechanism as its tethered peptide ligand or soluble peptide mimetics of its tethered peptide ligand is unclear. Extracellular loop 2 (ECL2) has been shown to play a critical role in the activation mechanism of PAR2. Therefore, we constructed a series of PAR2 receptors mutated in ECL2, including a previously described polymorphic variant of PAR2 (F240S), and compared AC-55541 and AC-264613 to SLIGRL and a potent analog of SLIGRL called 2-furoyl LIGRLO in a series of functional assays, including cellular proliferation, phosphatidylinositol hydrolysis, and β-arrestin recruitment assays. Surprisingly, receptors with the F240S mutation were constitutively active in all functional assays tested. Furthermore, AC-55541 and AC-264613 were potentiated over 30-fold at the receptors with the F240S mutation, whereas SLIGRL and 2-furoyl LIGRLO were much less affected. In contrast, mutagenesis of charged residues in ECL2 confirmed their important role in the actions of peptide agonists of PAR2, whereas these mutations did not significantly affect activation of PAR2 by AC-55541 or AC-264613. These results suggest that F240S PAR2 receptors may be useful in screens to detect novel small-molecule PAR2 modulators and that further work on the biological importance of the F240S PAR2 variant is warranted.

Design of a highly selective and potent class of non-planar estrogen receptor β agonists

Selective activation of the estrogen receptor β (ERβ) could be a safe approach to hormone replacement therapy for both women and men, in contrast to the estrogens currently used for women which activate both ERβ and ERα, occasionally causing severe side effects. The selective ERβ agonist AC-131 has shown efficacy in animal models of Parkinson's disease and neuropathic pain. With the use of AC-131 as template, herein we report the discovery, synthesis, and structure-activity relationship (SAR) study of a new class of dihydrobenzofurans as potent and selective ERβ agonists. The SAR was established by enantioselective synthesis, molecular modeling, and whole-cell-based functional assays. The most potent diastereomer, cis-10-SR, was shown to have an EC50 value of <1 nM, potency 100-fold higher than that of AC-131. Even more interestingly, compound trans-10-SS exhibited 1000-fold ERβ/ERα selectivity while still maintaining good potency (∼10 nM). In addition, trans-10-SS showed only partial agonist activity (30-60 % Eff.) toward ERα at 10 μM. This unprecedented selectivity could be rationalized by molecular modeling. Compound trans-10-SS appears to be the first molecule to take advantage of both conservative amino acid differences found in the α- and β-faces of the binding cavities of ERα and ERβ.

Functional enhancement of AT1R potency in the presence of the TPαR is revealed by a comprehensive 7TM receptor co-expression screen

Background

Functional cross-talk between seven transmembrane (7TM) receptors can dramatically alter their pharmacological properties, both in vitro and in vivo. This represents an opportunity for the development of novel therapeutics that potentially target more specific biological effects while causing fewer adverse events. Although several studies convincingly have established the existence of 7TM receptor cross-talk, little is known about the frequencey and biological significance of this phenomenon.

Methodology/Principal Findings

To evaluate the extent of synergism in 7TM receptor signaling, we took a comprehensive approach and co-expressed 123 different 7TM receptors together with the angiotensin II type 1 receptor (AT1R) and analyzed how each receptor affected the angiotensin II (AngII) response. To monitor the effect we used integrative receptor activation/signaling assay called Receptor Selection and Amplification Technology (R-SAT). In this screen the thromboxane A2α receptor (TPαR) was the only receptor which significantly enhanced the AngII-mediated response. The TPαR-mediated enhancement of AngII signaling was significantly reduced when a signaling deficient receptor mutant (TPαR R130V) was co-expressed instead of the wild-type TPαR, and was completely blocked both by TPαR antagonists and COX inhibitors inhibiting formation of thromboxane A₂ (TXA₂).

Conclusions/Significance

We found a functional enhancement of AT1R only when co-expressed with TPαR, but not with 122 other 7TM receptors. In addition, the TPαR must be functionally active, indicating the AT1R enhancement is mediated by a paracrine mechanism. Since we only found one receptor enhancing AT1R potency, our results suggest that functional augmentation through 7TM receptor cross-talk is a rare event that may require specific conditions to occur.

I. In vivo evidence for partial agonist effects of (-)-OSU6162 and (+)-OSU6162 on 5-HT2A serotonin receptors

The locomotor effects of (-)- and (+)-OSU6162 were evaluated in 'low activity' animals (reserpinized mice and habituated rats) and 'high activity' animals (drug-naive mice and non-habituated rats). Both enantiomers of OSU6162 had dual effects on behavior, stimulating locomotor activity in 'low activity' animals and inhibiting locomotor activity in 'high activity' animals. There were also certain differences between the two enantiomers in their behavioral profiles. The stimulatory effects of both enantiomers in reserpinized mice were blocked by the 5-HT2A selective antagonist M100907, but not by the D2-selective antagonists haloperidol or raclopride, or by the D1-selective antagonists SCH23390 or SCH39166. The stimulatory effect in mice was more pronounced for (+)- than for (-)-OSU6162. In drug-naive mice, both enantiomers of OSU6162 produced head twitches, albeit to a much lesser extent than DOI, and both enantiomers inhibited DOI-induced head twitches, the (-)-form more effectively so than the (+)-form. These results suggest that (-)- and (+)-OSU6162 are partial agonists on 5-HT2A receptors and that the (+)-form has a higher intrinsic activity than the (-)-form. At high doses, both enantiomers inhibited locomotor activity in drug-naive mice, with (-)-OSU6162 being more potent than (+)-OSU6162. Similarly, in high-active rats, both enantiomers inhibited locomotor activity, with the (-)-enantiomer being more potent than the (+)-enantiomer. Conversely, in habituated rats, both enantiomers stimulated locomotor activity, and here, as opposed to the case in low-active mice, (-)-OSU6162 was more effective than (+)-OSU6162. The stimulatory effects in habituated rats of both enantiomers could be antagonized with either haloperidol or M100907. Overall, these results indicate that the dual effects on behavior of (-)- and (+)-OSU6162 are mediated through D2 and 5-HT2A receptors, consistent with their in vitro functional selectivity profiles (see Burstein et al., accompanying paper). Thus, both enantiomers of OSU6162 seem to act as stabilizers not only on dopaminergic, but also on serotonergic brain signaling. These discoveries have important implications for the potential clinical utility of both compounds, as well as for several of their congeners.

II. In vitro evidence that (-)-OSU6162 and (+)-OSU6162 produce their behavioral effects through 5-HT2A serotonin and D2 dopamine receptors

(-)-OSU6162 has promise for treating Parkinson's disease, Huntington's disease and schizophrenia. Behavioral tests evaluating the locomotor effects of (-) and (+)-OSU6162 on 'low activity' animals (reserpinized mice and habituated rats) and 'high activity' animals (drug naive mice and non-habituated rats) revealed that both enantiomers of OSU6162 had dual effects on behavior, stimulating locomotor activity in 'low activity' animals and inhibiting locomotor activity in 'high activity' animals. To elucidate a plausible mechanism of action for their behavioral effects, we evaluated the intrinsic actions of (-)- and (+)-OSU6162, and a collection of other antipsychotic and antiparkinsonian agents at 5-HT2A and D2 receptors in functional assays with various degrees of receptor reserve, including cellular proliferation, phosphatidyl inositol hydrolysis, GTPγS and beta-arrestin recruitment assays. We also tested for possible allosteric actions of (-)-OSU6162 at D2 receptors. Both enantiomers of OSU6162 were medium intrinsic activity partial agonists at 5-HT2A receptors and low intrinsic activity partial agonists at D2 receptors. (+)-OSU6162 had higher efficacy at 5-HT2A receptors, which correlated with its greater stimulatory activity in vivo, but (-)-OSU6162 had higher potency at D2 receptors, which correlated with its greater inhibitory activity in vivo. (-)-OSU6162 did not display any convincing allosteric properties. Both (+)- and (-)-OSU6162 were significantly less active at 27 other monoaminergic receptors and reuptake transporters tested suggesting that D2 and 5-HT2A receptors play crucial roles in mediating their behavioral effects. Compounds with balanced effects on these two receptor systems may offer promise for treating neuropsychiatric diseases.

An angiotensin II type 1 receptor activation switch patch revealed through evolutionary trace analysis

Seven transmembrane (7TM) or G protein-coupled receptors constitute a large superfamily of cell surface receptors sharing a structural motif of seven transmembrane spanning alpha helices. Their activation mechanism most likely involves concerted movements of the transmembrane helices, but remains to be completely resolved. Evolutionary Trace (ET) analysis is a computational method, which identifies clusters of functionally important residues by integrating information on evolutionary important residue variations with receptor structure. Combined with known mutational data, ET predicted a patch of residues in the cytoplasmic parts of TM2, TM3, and TM6 to form an activation switch that is common to all family A 7TM receptors. We tested this hypothesis in the rat Angiotensin II (Ang II) type 1a (AT1a) receptor. The receptor has important roles in the cardiovascular system, but has also frequently been applied as a model for 7TM receptor activation and signaling. Six mutations: F66A, L67R, L70R, L119R, D125A, and I245F were targeted to the putative switch and assayed for changes in activation state by their ligand binding, signaling, and trafficking properties. All but one receptor mutant (that was not expressed well) displayed phenotypes associated with changed activation state, such as increased agonist affinity or basal activity, promiscuous activation, or constitutive internalization highlighting the importance of testing different signaling pathways. We conclude that this evolutionary important patch mediates interactions important for maintaining the inactive state. More broadly, these observations in the AT1 receptor are consistent with computational predictions of a generic role for this patch in 7TM receptor activation.

Identification and characterization of novel small-molecule protease-activated receptor 2 agonists

We report the first small-molecule protease-activated receptor (PAR) 2 agonists, AC-55541 [N-[[1-(3-bromo-phenyl)-eth-(E)-ylidene-hydrazinocarbonyl]-(4-oxo-3,4-dihydro-phthalazin-1-yl)-methyl]-benzamide] and AC-264613 [2-oxo-4-phenylpyrrolidine-3-carboxylic acid [1-(3-bromo-phenyl)-(E/Z)-ethylidene]-hydrazide], each representing a distinct chemical series. AC-55541 and AC-264613 each activated PAR2 signaling in cellular proliferation assays, phosphatidylinositol hydrolysis assays, and Ca(2+) mobilization assays, with potencies ranging from 200 to 1000 nM for AC-55541 and 30 to 100 nM for AC-264613. In comparison, the PAR2-activating peptide 2-furoyl-LIGRLO-NH(2) had similar potency, whereas SLIGRL-NH(2) was 30 to 300 times less potent. Neither AC-55541 nor AC-264613 had activity at any of the other PAR receptor subtypes, nor did they have any significant affinity for over 30 other molecular targets involved in nociception. Visualization of EYFP-tagged PAR2 receptors showed that each compound stimulated internalization of PAR2 receptors. AC-55541 and AC-264613 were well absorbed when administered intraperitoneally to rats, each reaching micromolar peak plasma concentrations. AC-55541 and AC-264613 were each stable to metabolism by liver microsomes and maintained sustained exposure in rats, with elimination half-lives of 6.1 and 2.5 h, respectively. Intrapaw administration of AC-55541 or AC-264613 elicited robust and persistent thermal hyperalgesia and edema. Coadministration of either a tachykinin 1 (neurokinin 1) receptor antagonist or a transient receptor potential vanilloid (TRPV) 1 antagonist completely blocked these effects. Systemic administration of either AC-55541 or AC-264613 produced a similar degree of hyperalgesia as was observed when the compounds were administered locally. These compounds represent novel small-molecule PAR2 agonists that will be useful in probing the physiological functions of PAR2 receptors.

Discovery of potent and selective small-molecule PAR-2 agonists

Proteinase activated receptor-2 plays a crucial role in a wide variety of conditions with a strong inflammatory component. We present the discovery and characterization of two structurally different, potent, selective, and metabolically stable small-molecule PAR-2 agonists. These ligands may be useful as pharmacological tools for elucidating the complex physiological role of the PAR-2 receptors as well as for the development of PAR-2 antagonists.

Pharmacology of N-desmethylclozapine

Currently available treatments for schizophrenia have limited efficacy and are generally poorly tolerated. However, among these antipsychotic agents, clozapine stands apart in having generally superior motoric tolerability and efficacy. One intriguing possibility, based on clinical correlations, receptor activity profiles and studies with animal models predictive of antipsychotic or cognitive action is that the activity of N-desmethylclozapine (NDMC), a major metabolite of clozapine, may, at least in part, underlie the unique efficacy of clozapine. In this review we compare the pharmacological properties of NDMC to those of clozapine and consider how they may contribute to the overall clinical properties of clozapine. We also consider whether NDMC, in its own right, might be a superior antipsychotic drug.

Identification of the Atypical L-Type Ca2+ Channel Blocker Diltiazem and Its Metabolites As Ghrelin Receptor Agonists

Using a high-throughput functional screen, the atypical L-type Ca2+ channel blocker diltiazem was discovered to be an agonist at the human ghrelin (GHSR1a) receptor. In cellular proliferation, Ca2+ mobilization, and bioluminescence resonance energy transfer (BRET-2) assays, diltiazem was a partial agonist at GHSR1a receptors, with 50 to 80% relative efficacy compared with the GHSR1a peptide agonist GHRP-6, and high nanomolar to low micromolar potency, depending upon the assay. Seven of the known primary metabolites of diltiazem were synthesized, and three of them (MA, M1, and M2) were more efficacious and/or more potent than diltiazem at GHSR1a receptors, with a rank order of agonist activity of M2 > M1 > MA > diltiazem, whereas M4 and M6 metabolites displayed weak agonist activity, and the M8 and M9 metabolites were inactive. Binding affinities of diltiazem and these metabolites to GHSR1a receptors followed a similar rank order. In vivo tests showed that diltiazem and M2 each stimulated growth hormone release in male Sprague-Dawley neonatal rats, although to a lesser degree than GHRP-6. Thus, diltiazem and chemical analogs of diltiazem represent a new class of GHSR1a receptor agonists. The possible contributions of GHSR1a receptor activation to the clinical actions of diltiazem are discussed in the context of the known beneficial cardiovascular effects of ghrelin.

In Vitro Pharmacology of Clinically Used Central Nervous System-Active Drugs as Inverse H1 Receptor Agonists

The human histamine H(1) receptor (H(1)R) is a prototypical G protein-coupled receptor and an important, well characterized target for the development of antagonists to treat allergic conditions. Many neuropsychiatric drugs are also known to potently antagonize this receptor, underlying aspects of their side effect profiles. We have used the cell-based receptor selection and amplification technology assay to further define the clinical pharmacology of the human H(1)R by evaluating >130 therapeutic and reference drugs for functional receptor activity. Based on this screen, we have reported on the identification of 8R-lisuride as a potent stereospecific partial H(1)R agonist (Mol Pharmacol 65:538-549, 2004). In contrast, herein we report on a large number of varied clinical and chemical classes of drugs that are active in the central nervous system that display potent H(1)R inverse agonist activity. Absolute and rank order of functional potency of these clinically relevant brain-penetrating drugs may possibly be used to predict aspects of their clinical profiles, including propensity for sedation.

Characterization of the Mas-related gene family: structural and functional conservation of human and rhesus MrgX receptors

Recently, a large family of G-protein-coupled receptors called Mas-related genes (Mrgs), which is selectively expressed in small-diameter sensory neurons of dorsal root ganglia, was described. A subgroup of human Mrg receptors (MrgX1-X4) is not found in rodents and this has hampered efforts to define the physiological roles of these receptors. MrgX receptors were cloned from rhesus monkey and functionally characterized alongside their human orthologs. Most of the human and rhesus MrgX receptors displayed high constitutive activity in a cellular proliferation assay. Proliferative responses mediated by human or rhesus MrgX1, or rhesus MrgX2 were partially blocked by pertussis toxin (PTX). Proliferative responses mediated by rhesus MrgX3 and both human and rhesus MrgX4 were PTX insensitive. These results indicate that human and rhesus MrgX1 and MrgX2 receptors activate both Gq- and Gi-regulated pathways, while MrgX3 and MrgX4 receptors primarily stimulate Gq-regulated pathways. Peptides known to activate human MrgX1 and MrgX2 receptors activated the corresponding rhesus receptors in cellular proliferation assays, Ca(2+)-mobilization assays, and GTP-gammaS-binding assays. Cortistatin-14 was selective for human and rhesus MrgX2 receptors over human and rhesus MrgX1 receptors. BAM22 and related peptides strongly activated human MrgX1 receptors, but weakly activated rhesus MrgX1, human MrgX2, and rhesus MrgX2 receptors. These data suggest that the rhesus monkey may be a suitable animal model for exploring the physiological roles of the MrgX receptors.

Structural Requirements of Transmembrane Domain 3 for Activation by the M1Muscarinic Receptor Agonists AC-42, AC-260584, Clozapine, andN-Desmethylclozapine: Evidence for Three Distinct Modes of Receptor Activation

Transmembrane domain 3 (TM3) plays a crucial role mediating muscarinic acetylcholine receptor activation by acetylcholine, carbachol, and other muscarinic agonists. We compared the effects of point mutations throughout TM3 on the interactions of carbachol, 4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine hydrogen chloride (AC-42), a potent structural analog of AC-42 called 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC-260584), N-desmethylclozapine, and clozapine with the M(1) muscarinic receptor. The binding and activation profiles of these ligands fell into three distinct patterns; one exemplified by orthosteric compounds like carbachol, another by structural analogs of AC-42, and a third by structural analogs of N-desmethylclozapine. All mutations tested severely reduced carbachol binding and activation of M(1). In contrast, the agonist actions of AC-42 and AC-260584 were greatly potentiated by the W101A mutation, slightly reduced by Y106A, and slightly increased by S109A. Clozapine and N-desmethylclozapine displayed substantially increased maximum responses at the Y106A and W101A mutants, slightly lower activity at S109A, but no substantial changes in potency. At L102A and N110A, agonist responses to AC-42, AC-260584, clozapine, and N-desmethylclozapine were all substantially reduced, but usually less than carbachol. D105A showed no functional responses to all ligands. Displacement and dissociation rate experiments demonstrated clear allosteric properties of AC-42 and AC-260584 but not for N-desmethylclozapine and clozapine, indicating that they may contact different residues than carbachol to activate M(1) but occupy substantially overlapping spaces, in contrast to AC-42 and AC-260584, which occupy separable spaces. These results show that M(1) receptors can be activated in at least three distinct ways and that there is no requirement for potent muscarinic agonists to mimic acetylcholine interactions with TM3.

Constitutive activity of muscarinic acetylcholine receptors

We review the literature describing constitutive activity of the five muscarinic acetylcholine receptors in native and recombinant systems and discuss the effect of constitutive activity on muscarinic pharmacology in the context of modern models of receptor activation. We include a summary of mutations found to cause constitutive activity and discuss the implications of these data for the structure, function, and activation mechanism of muscarinic receptors. Finally, we discuss the possible physiological significance of constitutive activity of muscarinic receptors, incorporating information provided by targeted deletion of each of the muscarinic subtypes.

Integrative Functional Assays, Chemical Genomics and High Throughput Screening: Harnessing Signal Transduction Pathways to a Common HTS Readout

Chemical genomics is a drug discovery strategy that relies heavily on high-throughput screening (HTS) and therefore benefits from functional assay platforms that allow HTS against all relevant genomic targets. Receptor Selection and Amplification Technology (R-SAT) is a cell-based, high-throughput functional assay where the receptor stimulus is translated into a measurable cellular response through an extensive signaling cascade occurring over several days. The large biological and chronological separation of stimulus from response provides numerous opportunities for enabling assays and increasing assay sensitivity. Here we review strategies for building homogeneous assay platforms across large gene families by redirecting and/or amplifying signal transduction pathways.

Pharmacological and Behavioral Profile of N-(4-Fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N′-(4-(2-methylpropyloxy)phenylmethyl) Carbamide (2R,3R)-Dihydroxybutanedioate (2:1) (ACP-103), a Novel 5-Hydroxytryptamine2A Receptor Inverse Agonist

The in vitro and in vivo pharmacological properties of N-(4-fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl)carbamide (2R,3R)-dihydroxybutanedioate (2:1) (ACP-103) are presented. A potent 5-hydroxytryptamine (5-HT)(2A) receptor inverse agonist ACP-103 competitively antagonized the binding of [(3)H]ketanserin to heterologously expressed human 5-HT(2A) receptors with a mean pK(i) of 9.3 in membranes and 9.70 in whole cells. ACP-103 displayed potent inverse agonist activity in the cell-based functional assay receptor selection and amplification technology (R-SAT), with a mean pIC(50) of 8.7. ACP-103 demonstrated lesser affinity (mean pK(i) of 8.80 in membranes and 8.00 in whole cells, as determined by radioligand binding) and potency as an inverse agonist (mean pIC(50) 7.1 in R-SAT) at human 5-HT(2C) receptors, and lacked affinity and functional activity at 5-HT(2B) receptors, dopamine D(2) receptors, and other human monoaminergic receptors. Behaviorally, ACP-103 attenuated head-twitch behavior (3 mg/kg p.o.), and prepulse inhibition deficits (1-10 mg/kg s.c.) induced by the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride in rats and reduced the hyperactivity induced in mice by the N-methyl-d-aspartate receptor noncompetitive antagonist 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate; MK-801) (0.1 and 0.3 mg/kg s.c.; 3 mg/kg p.o.), consistent with a 5-HT(2A) receptor mechanism of action in vivo and antipsychotic-like efficacy. ACP-103 demonstrated >42.6% oral bioavailability in rats. Thus, ACP-103 is a potent, efficacious, orally active 5-HT(2A) receptor inverse agonist with a behavioral pharmacological profile consistent with utility as an antipsychotic agent.

Intrinsic efficacy of antipsychotics at human D2, D3, and D4 dopamine receptors: identification of the clozapine metabolite N-desmethylclozapine as a D2/D3 partial agonist

Drugs that antagonize D2-like receptors are effective antipsychotics, but the debilitating movement disorder side effects associated with these drugs cannot be dissociated from dopamine receptor blockade. The "atypical" antipsychotics have a lower propensity to cause extrapyramidal symptoms (EPS), but the molecular basis for this is not fully understood nor is the impact of inverse agonism upon their clinical properties. Using a cell-based functional assay, we demonstrate that overexpression of Galphao induces constitutive activity in the human D2-like receptors (D2, D3, and D4). A large collection of typical and atypical antipsychotics was profiled for activity at these receptors. Virtually all were D2 and D3 inverse agonists, whereas none was D4 inverse agonist, although many were potent D4 antagonists. The inverse agonist activity of haloperidol at D2 and D3 receptors could be reversed by mesoridazine demonstrating that there were significant differences in the degrees of inverse agonism among the compounds tested. Aripiprazole and the principle active metabolite of clozapine NDMC [8-chloro-11-(1-piperazinyl)-5H-dibenzo [b,e] [1,4] diazepine] were identified as partial agonists at D2 and D3 receptors, although clozapine itself was an inverse agonist at these receptors. NDMC-induced functional responses could be reversed by clozapine. It is proposed that the low incidence of EPS associated with clozapine and aripiprazole used may be due, in part, to these partial agonist properties of NDMC and aripiprazole and that bypassing clozapine blockade through direct administration of NDMC to patients may provide superior antipsychotic efficacy.

The role of M1 muscarinic receptor agonism of N-desmethylclozapine in the unique clinical effects of clozapine

RATIONALE

Clozapine is a unique antipsychotic, with efficacy against positive symptoms in treatment-resistant schizophrenic patients, and the ability to improve cognition and treat the negative symptoms characteristic of this disease. Despite its unique clinical actions, no specific molecular mechanism responsible for these actions has yet been described.

OBJECTIVES AND METHODS

To comprehensively profile a large library of neuropsychiatric drugs, including most antipsychotics, at human monoamine receptors using R-SAT, an in vitro functional assay.

RESULTS

Profiling revealed that N-desmethylclozapine (NDMC), the principal metabolite of clozapine, but not clozapine itself, is a potent and efficacious muscarinic receptor agonist, a molecular property not shared by any other antipsychotic. To further explore the role of NDMC muscarinic receptor agonist properties in mediating the physiological actions of clozapine, systemically administered NDMC was found to stimulate the phosphorylation of mitogen-activated protein kinase (MAP kinase) in mouse CA1 hippocampal neurons, an effect that was blocked by scopolamine, confirming central M1 muscarinic receptor agonist activity in vivo. Lastly, an analysis of clozapine and NDMC serum levels in schizophrenic patients indicated that high NDMC/clozapine ratios better predicted improvement in cognitive functioning and quality of life than the levels of either compound alone.

CONCLUSIONS

The muscarinic receptor agonist activities of NDMC are unique among antipsychotics, and provide a possible molecular basis for the superior clinical effects of clozapine pharmacotherapy.

Discovery of novel peptide/receptor interactions: identification of PHM-27 as a potent agonist of the human calcitonin receptor

Many naturally occurring peptides exhibit a high degree of promiscuity across G-protein coupled receptor subtypes. The degree to which this phenomenon occurs, and its physiological significance is not well characterized. In addition, many 'orphan' peptides exist for which there are no known receptors. Therefore, to identify novel interactions between biologically active peptides and G-protein coupled receptors, a library of nearly 200 peptides was screened against the human calcitonin (hCTr), human Parathyroid Hormone (PTH1R), human Corticotropin Releasing Factor (CRF1), and the human Glucagon-like peptide (GLP1) receptors using a cell-based functional assay (Receptor Selection and Amplification Technology). Functional profiling revealed that the 'orphan peptide' PHM-27 selectively activated the hCTr; no activity was observed at the PTH1, CRF1, or GLP1 receptors. PHM-27 was a potent agonist at the hCTr, with similar efficacy as human calcitonin, and a potency of 11 nM. These results were confirmed in cyclic AMP assays. Responses to calcitonin and PHM-27 could be suppressed by the antagonist salmon calcitonin (8-32). In competition binding studies, salmon calcitonin (8-32), calcitonin, and PHM-27 were each able to inhibit (125)I-calcitonin from cell membranes containing transiently expressed hCTr. These results indicate that the orphan peptide PHM-27 is a potent agonist at the hCTr.

Pharmacology of polymorphic variants of the human 5-HT1A receptor

The 5-HT1A receptor is a critical mediator of serotonergic (5-HT) function. We have identified 13 potential single nucleotide polymorphisms resulting in amino acid changes throughout the human 5-HT1A receptor. The pharmacological profiles of these 13 polymorphic variants were then characterized using a high-throughput assay based on ligand-dependent transformation of NIH/3T3 cells. The majority of the polymorphic variants displayed wild-type pharmacological profiles in response to a panel of well-established agonists at the 5-HT1A receptor. However, the A50V polymorphic variant, which had an alanine to valine substitution in transmembrane 1, exhibited a loss of detectable response to 5-HT. Interestingly, all other agonists tested, including buspirone, lisuride, and (+)8-OH-DPAT, exhibited efficacies similar to that of the wild-type receptor. The competitive antagonist, methiothepin, also displayed a 19-fold decrease in potency at the A50V variant receptor. However, both 5-HT and methiothepin were able to compete for [3H]WAY-100635 binding to the A50V variant with affinities similar to the wild-type receptor. Moreover, the Bmax of [3H]WAY-100635 binding was 14-fold lower for the A50V variant than for the wild-type receptor. Thus, the A50V receptor variant exhibited ligand-specific functional alterations in addition to lower expression levels. These data suggest a previously unappreciated role for transmembrane 1 in mediating 5-HT response at the 5-HT1A receptor. Furthermore, individuals that potentially harbor the A50V polymorphism might display aberrant affective behaviors and altered responses to drugs targeting the 5-HT1A receptor.

G-protein-coupled receptor-mediated activation of rap GTPases: characterization of a novel Galphai regulated pathway

Ras proteins mediate the proliferative effects of G-protein-coupled receptors (GPCRs), but the role of Rap proteins in GPCR signaling is unclear. We have developed a novel cellular proliferation assay for examining signal transduction to Rap utilizing Ras-rap chimeras that respond selectively to Rap-specific exchange factors, but which stimulate cellular proliferation through Ras effectors. Both the D1 dopamine receptor (Gs-coupled) and the 5HT1E serotonin receptor (Gi-coupled) mediated cellular proliferation in a Ras/rap chimera-dependent manner. Responses to both receptors were PKA-independent. Both receptors activated Ras/rap and full-length Rap as measured by activation-specific probes. Pertussis toxin blocked Ras/rap-dependent responses to 5HT1E but not D1. Ras/rap-dependent responses to both receptors were insensitive to beta-gamma scavengers. Responses to 5HT1E, but not D1, were sensitive to inhibition by a dominant-negative C3G fragment, by the Src-like kinase inhibitors PP1 and PP2, and by a dominant-negative mutant of Src. Very similar data were obtained for two other Gi-coupled receptors, the D2 dopamine receptor and the alpha2C adrenergic receptor. A constitutively active mutant of Galphai2 also mediated Ras/rap-dependent responses. These data indicate that GPCRs coupled to pertussis-toxin-sensitive G-proteins activate Rap through a Galpha subunit, C3G, and Src-dependent pathway.

Discovery of the first nonpeptide agonist of the GPR14/urotensin-II receptor: 3-(4-chlorophenyl)-3-(2- (dimethylamino)ethyl)isochroman-1-one (AC-7954)

A functional cell-based screen identified 3-(4-chlorophenyl)-3-(2-(dimethylamino)ethyl)isochroman-1-one hydrochloride (AC-7954, 1) as a nonpeptidic agonist of the urotensin-II receptor. Racemic 1 had an EC50 of 300 nM at the human UII receptor and was highly selective. Testing of the enantiopure (+)- and (-)- 1 revealed that the UII receptor activity of racemic 1 resides primarily in (+)-1. Being a selective nonpeptidic druglike UII receptor agonist, (+)-1 will be useful as a pharmacological research tool and a potential drug lead.

Discovery of an ectopic activation site on the M(1) muscarinic receptor

Receptors have well-conserved regions that are recognized and activated by hormones and neurotransmitters. Most drugs bind to these sites and mimic or block the action of the native ligands. Using a high-throughput functional screen, we identified a potent and selective M(1) muscarinic receptor agonist from a novel structural class. Using a series of chimeric receptors, we demonstrated that this ligand activates the receptor through a region that is not conserved among receptor subtypes, explaining its unprecedented selectivity. This region of the receptor is distinct from the conserved region that is recognized by traditional ligands. The finding that receptors for small-molecule transmitters can have multiple, structurally distinct activation sites has broad implications for the study of receptor structure/function and the potential for the discovery of novel ligands with high selectivity.

Molecular cloning and pharmacology of functionally distinct isoforms of the human histamine H(3) receptor

The pharmacology of histamine H(3) receptors suggests the presence of distinct receptor isoforms or subtypes. We herein describe multiple, functionally distinct, alternatively spliced isoforms of the human H(3) receptor. Combinatorial splicing at three different sites creates at least six distinct receptor isoforms, of which isoforms 1, 2, and 4, encode functional proteins. Detailed pharmacology on isoforms 1 (unspliced receptor), and 2 (which has an 80 amino acid deletion within the third intracellular loop of the protein) revealed that both isoforms displayed robust responses to a series of known H(3) agonists, while all agonists tested displayed increased potency at isoform 2 relative to isoform 1. Histamine, N(alpha)-methylhistamine, and R(-) and S(+)-alpha-methylhistamine are 16-23-fold more potent, while immepip and imetit are three to fivefold more potent. Antagonist experiments revealed a rank order of potency at both isoforms of clobenpropit>iodophenpropit>thioperamide, and these drugs are fivefold less potent at isoform 2 than isoform 1. To further explore the pharmacology of H(3) receptor function, we screened 150 clinically relevant neuropsychiatric drugs for H(3) receptor activity, and identified a small number of antipsychotics that possess significant antagonist activity.

Homologous mutations near the junction of the sixth transmembrane domain and the third extracellular loop lead to constitutive activity and enhanced agonist affinity at all muscarinic receptor subtypes

Previous studies have found that a mutation near the junction of the sixth transmembrane domain (TM6) and the third extracellular loop of the M5 muscarinic receptor leads to constitutive activation and enhanced agonist affinity for the mutated receptor. These results were consistent with the extended ternary complex model, which predicts a correlation between agonist affinity and constitutive activity. We have introduced the homologous mutation into all five subtypes of the highly conserved muscarinic receptor family; SerThr-->TyrPro was introduced into M1 and M5, and AsnThr-->TyrPro was introduced into M2, M3, and M4. In binding assays, these mutations produced increases in affinities toward acetylcholine and carbachol that ranged from 5-fold at the M2 receptor to 15- to 20-fold at M1, M3, and M4, to 40-fold at M5. In functional assays, all five mutant receptors exhibited constitutive activity, at levels ranging between 30 and 80% of the maximal response elicited by carbachol. In every case, the muscarinic antagonist atropine inhibited this constitutive activity with high affinity. Thus, despite differences in effector coupling and in wild-type sequence at the mutation site, all five subtypes were activated by this mutation at the top of TM6. Previous studies of the M5 subtype have indicated that TM6 is a ligand-dependent switch that sets the activation state of the receptor. Based on the results of the present study, it is possible that TM6 represents a general switch for the activation of the muscarinic receptor family.

Chemical genomics: massively parallel technologies for rapid lead identification and target validation

Chemical genomics is a new research paradigm with importantapplications in drug discovery. It links genomic targets withsmall-molecule chemistries thereby allowing for efficient targetvalidation and lead compound identification. ACADIA'schemical-genomics platform consists of a large and diverse small-moleculelibrary (800,000), a reference drug library (2,000), druggablegenomic targets (>300) and a cell-based functional assaytechnology (R-SAT(TM); Receptor Selection and AmplificationTechnology) that allows for ultra-high throughput screening(>500,000 data points/week) as well as high throughputpharmacology and profiling over a wide range of targets. Twoexamples are presented that illustrate the success of ourchemical-genomics approach: (i) The validation of inverse agonismat serotonin 5-HT(2A) receptors as an antipsychotic mechanismand the subsequent discovery of potent and selectively acting 5-HT(2A) inverse agonists, currently in preclinical development,and (ii) the discovery of the first ectopically binding subtype-selective muscarinic m1 agonist.