Advancements in Non-Dopaminergic Treatments for Schizophrenia: A Systematic Review of Pipeline Developments

INTRODUCTION

Conventional antipsychotic drugs that attenuate dopaminergic neural transmission are ineffective in approximately one-third of patients with schizophrenia. This necessitates the development of non-dopaminergic agents.

METHODS

A systematic search was conducted for completed phase II and III trials of compounds for schizophrenia treatment using the US Clinical Trials Registry and the EU Clinical Trials Register. Compounds demonstrating significant superiority over placebo in the primary outcome measure in the latest phase II and III trials were identified. Collateral information on the included compounds was gathered through manual searches in PubMed and press releases.

RESULTS

Sixteen compounds were identified; four compounds (ulotaront, xanomeline/trospium chloride, vabicaserin, and roluperidone) were investigated as monotherapy and the remaining 12 (pimavanserin, bitopertin, BI 425809, encenicline, tropisetron, pregnenolone, D-serine, estradiol, tolcapone, valacyclovir, cannabidiol, and rimonabant) were examined as add-on therapy. Compared to the placebo, ulotaront, xanomeline/trospium chloride, vabicaserin, bitopertin, estradiol, cannabidiol, rimonabant, and D-serine showed efficacy for positive symptoms; roluperidone and pimavanserin were effective for negative symptoms; and encenicline, tropisetron, pregnenolone, tolcapone, BI 425809, and valacyclovir improved cognitive function.

DISCUSSION

Compounds that function differently from existing antipsychotics may offer novel symptom-specific therapeutic strategies for patients with schizophrenia.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Targeting the 5-HT2C Receptor in Biological Context and the Current State of 5-HT2C Receptor Ligand Development

Serotonin (5-HT) 5-HT2C receptor (5-HT2CR) is recognized as a critical mediator of diseaserelated pathways and behaviors based upon actions in the central nervous system (CNS). Since 5-HT2CR is a class A G protein-coupled receptor (GPCR), drug discovery efforts have traditionally pursued the activation of the receptor through synthetic ligands with agonists proposed for the treatment of obesity, substance use disorders and impulse control disorders while antagonists may add value for the treatment of anxiety, depression and schizophrenia. The most significant agonist discovery to date is the FDAapproved anti-obesity medication lorcaserin. In recent years, efforts towards developing other mechanisms to enhance receptor function have resulted in the discovery of Positive Allosteric Modulators (PAMs) for the 5-HT2CR, with several molecule series now reported. The biological significance and context for signaling and function of the 5-HT2CR, and the current status of 5-HT2CR agonists and PAMs are discussed in this review.

Synthesis and Biological Evaluation of Disubstituted Pyrimidines as Selective 5-HT2C Agonists

Here, we describe the synthesis of disubstituted pyrimidine derivatives and their biological evaluation as selective 5-HT_2C agonists. To improve selectivity for 5-HT_2C over other subtypes, we synthesized two series of disubstituted pyrimidines with fluorophenylalkoxy groups at either the 5-position or 4-position and varying cyclic amines at the 2-position. The in vitro cell-based assay and binding assay identified compounds 10a and 10f as potent 5-HT_2C agonists. Further studies on selectivity to 5-HT subtypes and drug-like properties indicated that 2,4-disubstituted pyrimidine 10a showed a highly agonistic effect on the 5-HT_2C receptor, with excellent selectivity, as well as exceptional drug-like properties, including high plasma and microsomal stability, along with low CYP inhibition. Thus, pyrimidine 10a could be considered a viable lead compound as a 5-HT_2C selective agonist.

MK212, a 5-hydroxytryptamine 2C receptor agonist, inhibits conditioned avoidance responses independent of blocking endogenous dopamine release in rats

Although it is widely accepted that 5-hydroxytryptamine (5-HT) 2C receptor agonists produce antipsychotic effects by reducing endogenous dopamine release from presynaptic neurons, no direct evidence supports this. The aim of the present study was to investigate whether the antipsychotic effects induced by 5-HT_2C receptor agonists are dependent on the inhibition of endogenous dopamine release. We developed a novel conditioned avoidance response paradigm to test this hypothesis. In this assay, rats in which dopamine was depleted by reserpine failed to show conditioned avoidance responses, and the acute administration of quinpirole reversed the disruption of avoidance responses induced by reserpine. This suggests that animals successfully showed conditioned avoidance responses independent of endogenous dopamine release under these experimental conditions. Our results revealed that MK212 (0.5 mg/kg) reduced avoidance responses triggered by quinpirole in dopamine-depleted rats. Therefore, 5-HT_2C receptor agonists can inhibit conditioned avoidance responses independent of blocking endogenous dopamine release. Furthermore, the 5-HT_2C receptor agonist, MK212, decreased the extracellular concentration of glutamate in the nucleus accumbens, indicating that this mechanism may be critical for the antipsychotic effects of 5-HT_2C receptor agonists.

Association of Serotonin2c Receptor Polymorphisms With Antipsychotic Drug Response in Schizophrenia

There is conflicting evidence for the association between genetic polymorphisms in the serotonin (5-HT)_2C receptor (HTR2C) and response to antipsychotic drugs (APD) in schizophrenic patients. We tested the association between the HTR2C polymorphisms, Cys23Ser, -759C/T, and -697G/C, and response to APDs (mainly clozapine) in a 6 month prospective study in 171 patients with schizophrenia. Ser23 was significantly associated with treatment response (positive symptoms, X ² = 7.540, p = 0.01; negative symptoms, X ² = 4.796, p = 0.03) in male patients only. A -759C-Ser23 haplotype was similar associated with positive (X ² = 6.648, p = 0.01) and negative (X ² = 6.702, p = 0.01) symptom improvement. Logistic regression, after controlling for covariates, also showed significant haplotypic associations. A meta-analysis of six studies for Ser23 and treatment response showed an overall odds ratio of 2.00 (95%CI, 1.38-2.91, p = 0.0003) or 1.94 (95%CI, 1.27-2.99, p = 0.0024) under fixed or random effect models. These results provide additional evidence that HTR2C polymorphisms are associated with treatment response to APD with HTR2C antagonism or inverse agonism, in male schizophrenic patients.

Identification of dual role of piperazine-linked phenyl cyclopropyl methanone as positive allosteric modulator of 5-HT2C and negative allosteric modulator of 5-HT2B receptors

Allosteric modulators of G-protein-coupled receptors have lately gained significant traction in drug discovery. Recent studies have shown that allosteric modulation of serotonin 2C receptor (5-HT_2C) as a viable strategy for the treatment of various central nervous system (CNS) disorders. Considering the critical role of 5-HT_2C in the modulation of appetite, a selective positive allosteric modulator (PAM) of 5-HT_2C offers a new opportunity for anti-obesity therapeutic development. In this study, phenyl cyclopropyl-linked N-heterocycles were synthesized and evaluated at 5-HT_2C for agonist and PAM activity. Our study shows that imidazole linked phenyl cyclopropyl methanones has PAM activity on both 5-HT_2C and serotonin 2B receptor (5-HT_2B). Interestingly, piperazine linked phenyl cyclopropyl methanones (58) was active as PAM of 5-HT_2C (increased the E_max of 5-HT to 139%), and as negative allosteric modulator (NAM) of 5-HT_2B (decreases EC₅₀ of 5-HT 10 times without affecting E_max). Similar effect of compound 58 was observed with synthetic orthosteric agonist lorcaserin on 5-HT_2B. Molecular docking study revealed that all active compounds were binding to the predicted allosteric site on 5-HT_2C and shared a common interacting residues. Finally, compound 58 suppressed food intake in Sprague Dawley (SD) rats similar to lorcaserin after i.c.v. administration. Therefore, these results suggest that piperazine moiety is essential for dual activity (PAM & NAM) of compounds 58, and supports the hypothesis of 5-HT_2C PAM for the treatment of obesity similar to the full agonist.

Modern advances in heterocyclic chemistry in drug discovery

New advances in synthetic methodologies that allow rapid access to a wide variety of functionalized heterocyclic compounds are of critical importance to the medicinal chemist as it provides the ability to expand the available drug-like chemical space and drive more efficient delivery of drug discovery programs. Furthermore, the development of robust synthetic routes that can readily generate bulk quantities of a desired compound help to accelerate the drug development process. While established synthetic methodologies are commonly utilized during the course of a drug discovery program, the development of innovative heterocyclic syntheses that allow for different bond forming strategies are having a significant impact in the pharmaceutical industry. This review will focus on recent applications of new methodologies in C-H activation, photoredox chemistry, borrowing hydrogen catalysis, multicomponent reactions, regio- and stereoselective syntheses, as well as other new, innovative general syntheses for the formation and functionalization of heterocycles that have helped drive project delivery. Additionally, the importance and value of collaborations between industry and academia in shaping the development of innovative synthetic approaches to functionalized heterocycles that are of greatest interest to the pharmaceutical industry will be highlighted.

Effects of the serotonin 2C receptor agonist WAY163909 on the abuse-related effects and mesolimbic dopamine neurochemistry induced by abused stimulants in rhesus monkeys

RATIONALE

Accumulating evidence shows that the serotonergic system plays a major role in psychostimulant abuse through its interactions with the dopaminergic system. Studies indicate that serotonin 5-HT2C receptors are one of the main classes of receptors involved in mediating the influence of serotonin in drug abuse.

OBJECTIVE

The aim of the present study was to evaluate the effects of the selective serotonin 5-HT2C receptor agonist WAY163909 on the behavioral neuropharmacology of cocaine and methamphetamine in adult rhesus macaques.

METHODS

Cocaine or methamphetamine self-administration and reinstatement were evaluated under second-order and fixed-ratio schedules of reinforcement, respectively. Cocaine- and methamphetamine-induced increases in dopamine were assessed through in vivo microdialysis targeting the nucleus accumbens.

RESULTS

Pretreatment with WAY163909 dose-dependently attenuated cocaine and methamphetamine self-administration and drug-induced reinstatement of extinguished behavior previously maintained by cocaine or methamphetamine delivery. In an additional experiment, WAY163909 induced a dose-dependent attenuation of cocaine- or methamphetamine-induced dopamine overflow in the nucleus accumbens.

CONCLUSIONS

Our data indicate that selective 5-HT2C receptor activation decreases drug intake and drug-seeking behavior in nonhuman primate models of psychostimulant abuse through neurochemical mechanisms involved in the modulation of mesolimbic dopamine.

Identification of Optically Active Pyrimidine Derivatives as Selective 5-HT2C Modulators

A series of pyrimidine derivatives 4a-i were synthesized and evaluated for their binding affinities towards 5-HT_2C receptors. With regard to designed molecules 4a-i, the influence of the size of alkyl ether and the absolute configuration of a stereogenic center on the 5-HT_2C binding affinity and selectivity was studied. The most promising diasteromeric mixtures 4d and 4e were selected in the initial radioligand binding assay and they were further synthesized as optically active forms starting from optically active alcohols 5d and 5e, prepared by an enzymatic kinetic resolution. Pyrimidine analogue (R,R)-4e displayed an excellent 5-HT_2C binding affinity with good selectivity values against a broad range of other 5-HT receptor subtypes.

β-Hydroxy-tetrahydroquinolines from Quinolines Using Chloroborane: Synthesis of the Peptidomimetic FISLE-412

A new synthetic protocol provides a simple and direct method to generate functionalized β-hydroxy-tetrahydroquinolines (THQs). Hydroboration of quinolines using chloroboranes followed by oxidation with NaBO3 ⋅H2 O led to the formation of functionalized β-hydroxy THQs. High regio- and diastereoselectivities were observed in α and γ substituted quinolines and the trans diastereomer of the β-hydroxy-THQ was the major isostere. This new protocol was utilized to build the novel antibody-targeted lupus peptidomimetic, FISLE-412.

Discovery of N-Substituted (2-Phenylcyclopropyl)methylamines as Functionally Selective Serotonin 2C Receptor Agonists for Potential Use as Antipsychotic Medications

A series of N-substituted (2-phenylcyclopropyl)methylamines were designed and synthesized, with the aim of finding serotonin 2C (5-HT_2C)-selective agonists with a preference for G_q signaling. A number of these compounds exhibit 5-HT_2C selectivity with a preference for G_q-mediated signaling compared with β-arrestin recruitment. Furthermore, the N-methyl compound (+)-15a, which displayed an EC₅₀ of 23 nM in the calcium flux assay while showing no β-arrestin recruitment activity, is the most functionally selective 5-HT_2C agonist reported to date. The N-benzyl compound (+)-19, which showed an EC₅₀ of 24 nM at the 5-HT_2C receptor, is fully selective over the 5-HT_2B receptor. In an amphetamine-induced hyperactivity model, compound (+)-19 showed significant antipsychotic-drug-like activity. These novel compounds shed light on the role of functional selectivity at the 5-HT_2C receptor with respect to antipsychotic activity.

Utilization of an Active Site Mutant Receptor for the Identification of Potent and Selective Atypical 5-HT2C Receptor Agonists

Agonism of the 5-HT_2C receptor represents one of the most well-studied and clinically proven mechanisms for pharmacological weight reduction. Selectivity over the closely related 5-HT_2A and 5-HT_2B receptors is critical as their activation has been shown to lead to undesirable side effects and major safety concerns. In this communication, we report the development of a new screening paradigm that utilizes an active site mutant D134A (D3.32) 5-HT_2C receptor to identify atypical agonist structures. We additionally report the discovery and optimization of a novel class of nonbasic heterocyclic amide agonists of 5-HT_2C. SAR investigations around the screening hits provided a diverse set of potent agonists at 5-HT_2C with high selectivity over the related 5-HT_2A and 5-HT_2B receptor subtypes. Further optimization through replacement of the amide with a variety of five- and six-membered heterocycles led to the identification of 6-(1-ethyl-3-(quinolin-8-yl)-1H-pyrazol-5-yl)pyridazin-3-amine (69). Oral administration of 69 to rats reduced food intake in an ad libitum feeding model, which could be completely reversed by a selective 5-HT_2C antagonist.

18F-FPP: A PET Ligand for the 5-HT2C Receptor?

In the current issue of ACS Chemical Neuroscience, Kim et al. report on the early characterization of 4-(3-[¹⁸F] fluorophenethoxy)pyrimidine (¹⁸F-FPP) as a new positron emission tomography radiotracer for imaging brain 5-HT_2C receptors ( Kim, J., et al. ( 2017 ) A potential PET radiotracer for the 5-HT2c receptor: Synthesis and in vivo evaluation of 4-(3-[¹⁸F]Fluorophenethoxy)pyrimidine. ACS Chem. Neurosci.

, DOI

10.1021/acschemneuro.6b00445 ). At the present time, the tracer properties of ¹⁸F-FPP have only been reported in rats. If ¹⁸F-FPP is indeed shown to be suitable as a 5-HT_2C receptor PET tracer in humans, it will very likely have an important impact both in the development of any new chemical entities (NCEs) targeted to 5-HT_2C receptors, as well as a tool to advance understanding of 5-HT_2C receptor function both in normal and abnormal brain states.

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT_2C receptor (first known as the 5-HT_1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT_2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT_1CR, the 5-HT_2CR was discovered while studying the pharmacological features and the distribution of [³H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [³H]mesulergine-labelling to the rat choroid plexus. [³H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT₂ binding, the new binding site was called 5-HT_1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT_1CR (later named 5-HT_2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT_2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in the GPCR family: many 5-HT_2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT_2CR splice variants. Intense research led to therapeutically active 5-HT_2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5-HT_2CR antagonists/inverse agonists. Agomelatine, a 5-HT_2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT_2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT_2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT_2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT_2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT_2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT_2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain.

Repeated administration of 5-hydroxytryptamine 2C agonist MK212 produces a sensitization effect of antipsychotic activity

5-Hydroxytryptamine 2C (5-HT_2C ) receptor agonists have been suggested to possess an antipsychotic activity in several acute preclinical tests of antipsychotic drugs with low extra-pyramidal side effect liability. However, little is known about the long-term effect associated with chronic use of 5-HT_2C receptor agonists. The present study examined whether repeated activation of 5-HT_2C receptor with a highly selective 5-HT_2C receptor agonist MK212 would induce a long-term change in its antipsychotic-like activity (either a sensitization or tolerance) in the conditioned avoidance response and MK801-induced hyperlocomotion tests. Sprague-Dawley rats were first tested under the intraperitoneal (i.p.) treatment of MK212 (0.25, 0.5, 1.0 mg/kg) for 5 consecutive days. Three days later, when all rats were injected with a low dose of MK 212 (0.25 mg/kg) and tested for avoidance responding, rats that had been pretreated with 1.0 and 0.5 mg/kg MK212 made significantly fewer avoidance responses than those that had been treated with vehicle (0.9% saline). However, this past drug exposure-induced group difference was not significant in the MK801-induced hyperlocomotion test. Overall, results from this study suggest that repeated treatment of MK212 is capable of inducing a dose-dependent sensitization of antipsychotic activity in conditioned avoidance response. The discrepancy in sensitization of MK212 in CAR and MK801-induce hyperlocomotion may be related to the different mechanism underlying the effect of MK212 in these two tests. © 2016 IUBMB Life, 68(12):985-993, 2016.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Further Advances in Optimizing (2-Phenylcyclopropyl)methylamines as Novel Serotonin 2C Agonists: Effects on Hyperlocomotion, Prepulse Inhibition, and Cognition Models

A series of novel compounds with two halogen substituents have been designed and synthesized to further optimize the 2-phenylcyclopropylmethylamine scaffold in the quest for drug-like 5-HT2C agonists. Compound (+)-22a was identified as a potent 5-HT2C receptor agonist, with good selectivity against the 5-HT2B and the 5-HT2A receptors. ADMET assays showed that compound (+)-22a possessed desirable properties in terms of its microsomal stability, and CYP and hERG inhibition, along with an excellent brain penetration profile. Evaluation of (+)-22a in animal models of schizophrenia-related behaviors revealed that it had a desirable activity profile, as it reduced d-amphetamine-stimulated hyperlocomotion in the open field test, it restored d-amphetamine-disrupted prepulse inhibition, it induced cognitive improvements in the novel object recognition memory test in NR1-KD animals, and it produced very little catalepsy relative to haloperidol. These data support the further development of (+)-22a as a drug candidate for the treatment of schizophrenia.

We Need 2C but Not 2B: Developing Serotonin 2C (5-HT2C) Receptor Agonists for the Treatment of CNS Disorders

The serotonin 2C (5-HT2C ) receptor has been identified as a potential drug target for the treatment of a variety of central nervous system (CNS) disorders, such as obesity, substance abuse, and schizophrenia. In this Viewpoint article, recent progress in developing selective 5-HT2C agonists for use in treating these disorders is summarized, including the work of our group. Challenges in this field and the possible future directions are described. Homology modeling as a method to predict the binding modes of 5-HT2C ligands to the receptor is also discussed. Compared to known ligands, the improved pharmacological profiles of the 2-phenylcyclopropylmethylamine-based 5-HT2C agonists make them preferred candidates for further studies.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Serotonergic signalling suppresses ataxin 3 aggregation and neurotoxicity in animal models of Machado-Joseph disease

Polyglutamine diseases are a class of dominantly inherited neurodegenerative disorders for which there is no effective treatment. Here we provide evidence that activation of serotonergic signalling is beneficial in animal models of Machado-Joseph disease. We identified citalopram, a selective serotonin reuptake inhibitor, in a small molecule screen of FDA-approved drugs that rescued neuronal dysfunction and reduced aggregation using a Caenorhabditis elegans model of mutant ataxin 3-induced neurotoxicity. MOD-5, the C. elegans orthologue of the serotonin transporter and cellular target of citalopram, and the serotonin receptors SER-1 and SER-4 were strong genetic modifiers of ataxin 3 neurotoxicity and necessary for therapeutic efficacy. Moreover, chronic treatment of CMVMJD135 mice with citalopram significantly reduced ataxin 3 neuronal inclusions and astrogliosis, rescued diminished body weight and strikingly ameliorated motor symptoms. These results suggest that small molecule modulation of serotonergic signalling represents a promising therapeutic target for Machado-Joseph disease.

Therapeutic Potential of 5-HT2C Receptor Agonists for Addictive Disorders

The neurotransmitter 5-hydroxytryptamine (5-HT; serotonin) has long been associated with the control of a variety of motivated behaviors, including feeding. Much of the evidence linking 5-HT and feeding behavior was obtained from studies of the effects of the 5-HT releaser (dex)fenfluramine in laboratory animals and humans. Recently, the selective 5-HT2C receptor agonist lorcaserin received FDA approval for the treatment of obesity. This review examines evidence to support the use of selective 5-HT2C receptor agonists as treatments for conditions beyond obesity, including substance abuse (particularly nicotine, psychostimulant, and alcohol dependence), obsessive compulsive, and excessive gambling disorder. Following a brief survey of the early literature supporting a role for 5-HT in modulating food and drug reinforcement, we propose that intrinsic differences between SSRI and serotonin releasers may have underestimated the value of serotonin-based pharmacotherapeutics to treat clinical forms of addictive behavior beyond obesity. We then highlight the critical involvement of the 5-HT2C receptor in mediating the effect of (dex)fenfluramine on feeding and body weight gain and the evidence that 5-HT2C receptor agonists reduce measures of drug reward and impulsivity. A recent report of lorcaserin efficacy in a smoking cessation trial further strengthens the idea that 5-HT2C receptor agonists may have potential as a treatment for addiction. This review was prepared as a contribution to the proceedings of the 11th International Society for Serotonin Research Meeting held in Hermanus, South Africa, July 9-12, 2014.

Optimization of 2-phenylcyclopropylmethylamines as selective serotonin 2C receptor agonists and their evaluation as potential antipsychotic agents

The discovery of a new series of compounds that are potent, selective 5-HT2C receptor agonists is described herein as we continue our efforts to optimize the 2-phenylcyclopropylmethylamine scaffold. Modifications focused on the alkoxyl substituent present on the aromatic ring led to the identification of improved ligands with better potency at the 5-HT2C receptor and excellent selectivity against the 5-HT2A and 5-HT2B receptors. ADMET studies coupled with a behavioral test using the amphetamine-induced hyperactivity model identified four compounds possessing drug-like profiles and having antipsychotic properties. Compound (+)-16b, which displayed an EC50 of 4.2 nM at 5-HT2C, no activity at 5-HT2B, and an 89-fold selectivity against 5-HT2A, is one of the most potent and selective 5-HT2C agonists reported to date. The likely binding mode of this series of compounds to the 5-HT2C receptor was also investigated in a modeling study, using optimized models incorporating the structures of β2-adrenergic receptor and 5-HT2B receptor.

A 6-week randomized, double-blind, placebo-controlled, comparator referenced trial of vabicaserin in acute schizophrenia

Vabicaserin, a potent 5-HT2C receptor agonist, decreases nucleus accumbens extracellular dopamine levels in rats, without affecting striatal dopamine, indicating mesolimbic selectivity. This is the first study of efficacy, safety and tolerability of vabicaserin in adults with acute schizophrenia. Three hundred fourteen hospitalized subjects were randomized to: Vabicaserin 200 or 400 mg/day, olanzapine 15 mg/day or placebo. Central raters assessed the PANSS and CGI-S. Site raters performed the BPRS and CGI-I. Central rated PANSS Positive (PANSS-PPS) was the primary endpoint. Two hundred eighty-nine subjects were included in the mITT efficacy analysis. Vabicaserin was well tolerated with no major safety concerns. Olanzapine, but not vabicaserin, caused weight gain. Vabicaserin 200 mg/day and olanzapine demonstrated significant improvement at week 6 vs. placebo on PANSS-PSS. A non-significant decrease vs. placebo was observed for 400 mg/day. Both vabicaserin groups demonstrated significant improvement over baseline on PANSS Negative while placebo worsened. Vabicaserin 200 mg/day and olanzapine demonstrated significantly greater improvement over placebo on PANSS Total whereas 400 mg/day showed a trend toward improvement. There was no significant improvement vs. placebo for either vabicaserin group on site-rated BPRS. Vabicaserin 200 mg/day and olanzapine demonstrated significant improvement vs. placebo on CGI-I and CGI-S but not 400 mg/day vabicaserin. Vabicaserin demonstrated efficacy on primary and secondary endpoints at 200 mg/day, but not at 400 mg/day which showed a trend for efficacy. The 200 mg/day vabicaserin group achieved proof of concept using central ratings. Both vabicaserin doses were well tolerated with no significant safety signals and no weight gain.

TRIAL REGISTRATION

clinicaltrials.gov. Identifier: NCT00265551.

Prediction of Efficacy of Vabicaserin, a 5-HT2C Agonist, for the Treatment of Schizophrenia Using a Quantitative Systems Pharmacology Model

A quantitative systems pharmacology model that combines in vitro/preclinical neurophysiology data, human imaging data, and patient disease information was used to blindly predict steady-state clinical efficacy of vabicaserin, a 5-HT2C full agonist, in monotherapy and, subsequently, to assess adjunctive therapy in schizophrenia. The model predicted a concentration-dependent improvement of positive and negative syndrome scales (PANSS) in schizophrenia monotherapy with vabicaserin. At the exposures of 100 and 200 mg b.i.d., the predicted improvements on PANSS in virtual patient trials were 5.12 (2.20, 8.56) and 6.37 (2.27, 10.40) (mean (95% confidence interval)), respectively, which are comparable to the observed phase IIa results. At the current clinical exposure limit of vabicaserin, the model predicted an ~9-point PANSS improvement in monotherapy, and <4-point PANSS improvement adjunctive with various antipsychotics, suggesting limited clinical benefit of vabicaserin in schizophrenia treatment. In conclusion, the updated quantitative systems pharmacology model of PANSS informed the clinical development decision of vabicaserin in schizophrenia.

From obesity to substance abuse: therapeutic opportunities for 5-HT2C receptor agonists

The recent US Food and Drug Administration (FDA) approval of the serotonin (5-hydroxytryptamine, 5-HT) 5-HT2C receptor agonist lorcaserin for the treatment of obesity represents a new therapeutic drug class available to the clinic. Preclinical evidence supports the potential for this drug class to treat other related conditions such as substance abuse. In the present article we review this evidence and further suggest that overlapping neurobiological systems may contribute to an anti-addictive and anti-obesity profile. The availability of selective 5-HT2C agonists provides an opportunity to evaluate their potential as treatments for nicotine dependence or psychostimulant abuse, conditions for which there is significant medical need but only limited available treatment options.

Discontinued drugs in 2012: endocrine and metabolic

INTRODUCTION

This perspective summarizes 42 drug projects in the general areas of endocrine and metabolic diseases that were reported discontinued during 2012.

AREAS COVERED

These programs include development projects against diabetes, metabolic complications (including kidney and liver disease), as well as projects that can be described as approaches to treatment of obesity/anorexia, lipids and various inflammatory diseases. Candidates were identified from a search by Informa Healthcare including data available on TreasureTrove as provided by EOID. Additional information was sought using Google, PubMed, HighWire and ClinicalTrials.gov.

EXPERT OPINION

The summary of discontinued projects in this area for 2012 provides little in the way of specific guidance especially since details of the fate of most projects are often lacking. Nonetheless, it seems clear that none of these projects has hit upon the key aspects of the pathophysiology of the disorders that they were intended to treat, and the most likely cause of failure is the lack of the ability to produce compelling evidence to support the respective business case in the sense of efficacy/potential therapeutic profile.

Pharmacological and genetic interventions in serotonin (5-HT)(2C) receptors to alter drug abuse and dependence processes

The present review provides an overview on serotonin (5-hydroxytryptamine; 5-HT)(2C) receptors and their relationship to drug dependence. We have focused our discussion on the impact of 5-HT(2C) receptors on the effects of different classes of addictive drugs, illustrated by reference to data using pharmacological and genetic tools. The neurochemical mechanism of the interaction between 5-HT(2C) receptors, with focus on the mesocorticolimbic dopaminergic system, and drugs of abuse (using cocaine as an example) is discussed. Finally, we integrate recent nonclinical and clinical research and information with marketed products possessing 5-HT(2C) receptor binding affinities. Accordingly, available nonclinical data and some clinical observations targeting 5-HT(2C) receptors may offer innovative translational strategies for combating drug dependence.This article is part of a Special Issue entitled: Brain Integration.