The potential use of selective 5-HT2C agonists in treating obesity

Effects of psilocybin on body weight, body composition, and metabolites in male and female mice

There is growing interest in the therapeutic potential of psilocybin for the treatment of a wide variety of medical problems, and even for the promotion of wellbeing among healthy individuals. Interestingly, among the many proposed indications, both obesity and anorexia nervosa (AN) have been discussed. However, the effect of psilocybin on appetitive behavior and metabolism is not well known. Here, we report the effects of psilocybin on body weight, intake and output, body composition, and metabolic function among lean male and female wild-type mice. In the days immediately following treatment, both male and female mice receiving a single intraperitoneal dose of psilocybin were consistently heavier than saline controls, with no effect of psilocybin on intake or output. Co-administration of the 5-HT2A/2C receptor antagonist ketanserin had no effect on this outcome. Body composition analysis revealed that psilocybin significantly increased lean and water mass among males, with a similar trend among females. A metabolic panel revealed increased creatine kinase (CK), aspartate aminotransferase (AST), and chloride among male and female psilocybin treated mice. Together, these findings begin to investigate the potential mechanisms of psilocybin's effects on body weight and metabolic measures. Such understanding will be critical for the safe, efficacious, and well-informed use of psilocybin in clinical and non-clinical settings.

Semisynthetic Transformations on (+)-Boldine Reveal a 5-HT2A/2CR Antagonist

Aporphine alkaloids have shown affinity for serotonin receptors (5-HTRs), and there has been a recent upsurge of interest in aporphines as 5-HT_2CR ligands. 1,2,9,10-Tetraoxygenated aporphine alkaloids in particular have demonstrated good affinity for 5-HTRs. In continued efforts to understand the impacts of structural modification of the 1,2,9,10-tetraoxygenated aporphine template on affinity, selectivity, and activity at 5-HT₂R subtypes, we used (+)-boldine (8) as a semisynthetic feedstock in the preparation of C-2-alkoxylated (+)-predicentrine analogues. Compound 10n, which contains a benzyloxy group at C-2, has been identified as a novel 5-HT_2CR ligand with strong affinity (4 nM) and moderate selectivity versus 5-HT_2BR and 5-HT_2AR (12-fold and 6-fold, respectively). Compound 10n functions as an antagonist at 5-HT_2A and 5-HT_2C receptors. Computational experiments indicate that several hydrophobic interactions as well as strong H-bond and salt bridge interactions between the protonated amine moiety in 10n and Asp134 are responsible for the potent 5-HT_2CR affinity of this compound. Furthermore, compound 10n displays favorable predicted drug-like characteristics, which is encouraging toward future optimization.

Discovery of Highly Potent Serotonin 5-HT2 Receptor Agonists Inspired by Heteroyohimbine Natural Products

The serotonin 5-HT2 receptors are important pharmaceutical targets involved in signaling pathways underlying various neurological, psychiatric, and cardiac functions and dysfunctions. As such, numerous ligands for the investigation of these receptors' activity and downstream effects have been developed synthetically or discovered in nature. For example, the heteroyohimbine natural product alstonine exhibits antispychotic activity mediated by 5-HT2A/2C agonism. In this work, we identified a heteroyohimbine metabolite containing a serotonin pharmacophore and truncated the scaffold, leading to the discovery of potent agonist activity of substituted tetrahydro-β-carbolines across the 5-HT2 receptor family. Extensive SAR development resulted in compound 106 with EC50 values of 1.7, 0.58, and 0.50 nM at 5-HT2A, 5-HT2B, and 5-HT2C, respectively. Docking studies suggest a π-stacking interaction between the tetrahydro-β-carboline core and conserved residue Trp6.48 as the structural basis for this activity. This work lays a foundation for future investigation of these compounds in neurological and psychiatric disorders.

Epigenetic Regulation of Adipogenesis in Development of Metabolic Syndrome

Obesity is one of the biggest public health concerns identified by an increase in adipose tissue mass as a result of adipocyte hypertrophy and hyperplasia. Pertaining to the importance of adipose tissue in various biological processes, any alteration in its function results in impaired metabolic health. In this review, we discuss how adipose tissue maintains the metabolic health through secretion of various adipokines and inflammatory mediators and how its dysfunction leads to the development of severe metabolic disorders and influences cancer progression. Impairment in the adipocyte function occurs due to individuals' genetics and/or environmental factor(s) that largely affect the epigenetic profile leading to altered gene expression and onset of obesity in adults. Moreover, several crucial aspects of adipose biology, including the regulation of different transcription factors, are controlled by epigenetic events. Therefore, understanding the intricacies of adipogenesis is crucial for recognizing its relevance in underlying disease conditions and identifying the therapeutic interventions for obesity and metabolic syndrome.

Differential Effects of Saikosaponins A, B2, B4, C and D on Alcohol and Chocolate Self-Administration in Rats

AIMS

Treatment with saikosaponin A (SSA)-an ingredient of the medicinal herb, Bupleurum falcatum-has been reported to suppress several addictive-like behaviors, including morphine, cocaine, alcohol and chocolate self-administration in male rats. The aim of this investigation was to investigate whether saikosaponins of B. falcatum other than SSA affect alcohol and chocolate self-administration in rats.

METHODS

Ovariectomized female Sardinian alcohol-preferring (sP) and Wistar rats were trained to self-administer alcohol (15%, v/v) and a chocolate solution [5% (w/v) Nesquik® in water], respectively, under fixed ratio schedules of reinforcement. The following saikosaponins were compared to SSA: saikosaponin D (SSD; epimer of SSA), saikosaponin C (SSC), saikosaponin B2 (SSB2) and saikosaponin B4 (SSB4). All saikosaponins were tested acutely at the doses of 0, 0.25, 0.5 and 1 mg/kg (i.p.).

RESULTS

Treatment with SSA and SSD resulted in highly similar, marked reductions in alcohol self-administration; SSC failed to alter lever-responding for alcohol, while SSB2 and SSB4 produced intermediate reductions. Only SSA and SSD reduced chocolate self-administration, with SSC, SSB2 and SSB4 being ineffective.

CONCLUSIONS

The wide spectrum of efficacy of saikosaponins in reducing alcohol and chocolate self-administration suggests that even relatively small structural differences are sufficient to produce remarkable changes in their in vivo pharmacological profile. Together, these results confirm that roots of B. falcatum may be an interesting source of compounds with anti-addictive potential.

Identification of Novel 1-O-Substituted Aporphine Analogues as Potent 5-HT2C Receptor Agonists

The 5-HT_2C receptor has emerged as a promising target in the treatment of a variety of central nervous system disorders. We have first identified aporphines as a new class of 5-HT_2C receptor agonists. Structure-activity relationship results indicate that the aporphine core may be required for 5-HT_2C receptor activity, and substitutions at its C1 position are important for 5-HT_2C receptor activity. Our efforts to optimize our hit 15781 lead to the identification of the highly potent and selective 5-HT_2C agonist 18b (MQ02-439) with an EC₅₀ value of 104 nM and weak antagonism at the 5-HT_2A and 5-HT_2B receptors. The findings may serve as good starting points for the development of more potent and selective 5-HT_2C agonists as valuable pharmacological tools or potential drug candidates.

Identification of fluorinated (R)-(-)-aporphine derivatives as potent and selective ligands at serotonin 5-HT2C receptor

A series of novel aporphine derivatives were synthesized for initial screening at the 5-HT2 receptor subtypes. Among them, Compounds 11a and 11b were identified as potent 5-HT2C hit ligands with high selectivity over other 5-HT2 receptor subtypes. Molecular docking study revealed that compounds 11a and 11b formed two key interactions with the binding site of 5-HT2C receptor, including a salt-bridge to D3.32 and a H-bond interaction with N6.55.

Weight Loss Strategies for Treatment of Obesity: Lifestyle Management and Pharmacotherapy

Obesity is one of the most serious and prevalent non-communicable diseases of the twenty-first century. It is also a patient-centered condition in which affected individuals seek treatment through a variety of commercial, medical and surgical approaches. Considering obesity as a chronic medical disease state helps to frame the concept of using a three-stepped intensification of care approach to weight management. As a foundation, all patients should be counseled on evidence-based lifestyle approaches that include diet, physical activity and behavior change therapies. At the second tier, four new pharmacological agents, lorcaserin, phentermine/topiramate, naltrexone/bupropion and liraglutide have been approved since 2012 as adjuncts to lifestyle modification. The third step, bariatric surgery, has been demonstrated to be the most effective and long-term treatment for individuals with severe obesity or moderate obesity complicated by co-morbid conditions that is not responsive to non-surgical approaches. By using a medical model, clinicians can provide more proactive and effective treatments in assisting their patients with weight loss.

Reducing Effect of Saikosaponin A, an Active Ingredient of Bupleurum falcatum, on Intake of Highly Palatable Food in a Rat Model of Overeating

Recent lines of experimental evidence have indicated that saikosaponin A (SSA)-a bioactive ingredient of the medicinal plant, Bupleurum falcatum L.-potently and effectively reduced operant self-administration of chocolate and reinstatement of chocolate-seeking behavior in rats. The present study was designed to assess whether the protective properties of SSA on addictive-like, food-related behaviors generalize to a rat model of overeating of palatable food. To this end, rats were habituated to feed on a standard rat chow for 3 h/day; every 4 days, the 3-h chow-feeding session was followed by a 1-h availability of highly palatable, calorie-rich Danish butter cookies or Oreo chocolate cookies. Even though fed, rats consumed large amounts of cookies; intake of calories from cookies (consumed in 1 h) was even larger than that of calories from chow (consumed in 3 h). SSA (0, 0.25, 0.5, and 1 mg/kg, i.p.) was administered 10 min before cookie presentation. Treatment with SSA resulted in a dose-related decrease in intake of both butter and chocolate cookies. Administration of the cannabinoid CB₁ receptor antagonist/inverse agonist, rimonabant (0, 0.3, 1, and 3 mg/kg, i.p.; tested as reference compound), produced a similar reduction in intake of butter cookies. These results (a) contribute to the set-up and validation of a rat model of overeating, characterized by the intake of large amounts of unnecessary calories and (b) provide an additional piece of evidence to the anorectic profile of SSA in rats.

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.

Serotonin, a possible intermediate between disturbed circadian rhythms and metabolic disease

It is evident that eating in misalignment with the biological clock (such as in shift work, eating late at night and skipping breakfast) is associated with increased risk for obesity and diabetes. The biological clock located in the suprachiasmatic nucleus dictates energy balance including feeding behavior and glucose metabolism. Besides eating and sleeping patterns, glucose metabolism also exhibits clear diurnal variations with higher blood glucose concentrations, glucose tolerance and insulin sensitivity prior to waking up. The daily variation in plasma glucose concentrations in rats, is independent of the rhythm in feeding behavior. On the other hand, feeding itself has profound effects on glucose metabolism, but differential effects occur depending on the time of the day. We here review data showing that a disturbed diurnal eating pattern results in alterations in glucose metabolism induced by a disrupted circadian clock. We first describe the role of central serotonin on feeding behavior and glucose metabolism and subsequently describe the effects of central serotonin on the circadian system. We next explore the interaction between the serotonergic system and the circadian clock in conditions of disrupted diurnal rhythms in feeding and how this might be involved in the metabolic dysregulation that occurs with chronodisruption.

Design and Synthesis of (2-(5-Chloro-2,2-dimethyl-2,3-dihydrobenzofuran-7-yl)cyclopropyl)methanamine as a Selective Serotonin 2C Agonist

A conformationally restricted analog of a selective cyclopropane-bearing serotonin 2C agonist was designed and synthesized. A 2,2-dimethyl-2,3-dihydrobenzofuran scaffold was investigated as a constrained variant of a biologically active isopropyl phenyl ether. Construction of the required dimethyl-2,3-dihydrobenzofuran intermediate began using a procedure that relied on a microwave-assisted alkylation reaction. The synthesis of the designed compound as its HCl salt is reported in a total of 12 steps and 17% overall yield. Biological evaluation revealed the constrained analog to be a selective serotonin 2C agonist with modest potency.

Pharmacological characterization of an antisense knockdown zebrafish model of Dravet syndrome: inhibition of epileptic seizures by the serotonin agonist fenfluramine

Dravet syndrome (DS) is one of the most pharmacoresistant and devastating forms of childhood epilepsy syndromes. Distinct de novo mutations in the SCN1A gene are responsible for over 80% of DS cases. While DS is largely resistant to treatment with existing anti-epileptic drugs, promising results have been obtained in clinical trials with human patients treated with the serotonin agonist fenfluramine as an add-on therapeutic. We developed a zebrafish model of DS using morpholino antisense oligomers (MOs) targeting scn1Lab, the zebrafish ortholog of SCN1A. Zebrafish larvae with an antisense knockdown of scn1Lab (scn1Lab morphants) were characterized by automated behavioral tracking and high-resolution video imaging, in addition to measuring brain activity through local field potential recordings. Our findings reveal that scn1Lab morphants display hyperactivity, convulsive seizure-like behavior, loss of posture, repetitive jerking and a myoclonic seizure-like pattern. The occurrence of spontaneous seizures was confirmed by local field potential recordings of the forebrain, measuring epileptiform discharges. Furthermore, we show that these larvae are remarkably sensitive to hyperthermia, similar to what has been described for mouse models of DS, as well as for human DS patients. Pharmacological evaluation revealed that sodium valproate and fenfluramine significantly reduce epileptiform discharges in scn1Lab morphants. Our findings for this zebrafish model of DS are in accordance with clinical data for human DS patients. To our knowledge, this is the first study demonstrating effective seizure inhibition of fenfluramine in an animal model of Dravet syndrome. Moreover, these results provide a basis for identifying novel analogs with improved activity and significantly milder or no side effects.

Regioselective alkylation of 1,3,4,5-tetrahydrobenzo[d]azepin-2-one and biological evaluation of the resulting alkylated products as potentially selective 5-HT₂c agonists

The benzazepine ring system has offered interesting CNS-active medicinal agents. Taking this privileged structure as the basic scaffold, [Formula: see text] and/or [Formula: see text]-alkylated benzazepin-2-one derivatives and their reduced analogs have been prepared as potential [Formula: see text] receptor agonists. The selective alkylation at the [Formula: see text] and/or [Formula: see text] positions of this seven-membered lactam ring is here reported for the first time under different reaction conditions. The synthesized compounds were evaluated for their biological profile as potential [Formula: see text] agonists using a classic pharmacological approach. Three derivatives (15, 17, and 20) have shown promising [Formula: see text] agonistic activity which can be further optimized as anti-obesity agents for the treatment of male sexual dysfunction. Further, a homology model for [Formula: see text] receptor was generated using MODELLER, and ligand-receptor interactions for these potential molecules were studied.

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.

3-Substituted 1-methyl-3-benzazepin-2-ones as 5-HT2C receptor agonists

In search of potent and selective 5-HT_2C receptor agonists, a series of novel 3-substituted 1-methyl-3-benzazepin-2-ones and 8-chloro-1-methyl-3-benzazepin-2-ones have been synthesized and evaluated for their 5-HT_2C receptor agonistic potential.

CNS drug development: lessons from the development of ondansetron, aprepitant, ramelteon, varenicline, lorcaserin, and suvorexant. Part I

This column is the first in a two-part series exploring lessons for psychiatric drug development that can be learned from the development of six central nervous system drugs with novel mechanisms of action over the past 25 years. Part 1 presents a brief overview of the neuroscience that supported the development of each drug, including the rationale for selecting a) the target, which in each case was a receptor for a specific neurotransmitter system, and b) the indication, which was based on an understanding of the role that target played in a specific neural circuit in the brain. The neurotransmitter systems on which the development of these agents were based included serotonin for ondansetron and lorcaserin, dopamine for varenicline, substance P (or neurokinin) for aprepitant, melatonin for ramelteon, and orexin for suvorexant. The indications were chemotherapy-induced nausea and vomiting for ondansetron and aprepitant, smoking cessation for varenicline, weight loss for lorcaserin, and insomnia for suvorexant and ramelteon.

Weight loss strategies for treatment of obesity

Obesity is one of the most serious and prevalent non-communicable diseases of the 21st century. It is also a patient-centered condition in which affected individuals seek treatment through a variety of commercial, medical and surgical approaches. Considering obesity as a chronic medical disease state helps to frame the concept of using a three-stepped intensification of care approach to weight management. As a foundation, all patients should be counseled on evidence-based lifestyle approaches that include diet, physical activity and behavior change therapies. At the second tier, two new pharmacological agents, phentermine-topiramate and lorcaserin, were approved in 2012 as adjuncts to lifestyle modification. The third step, bariatric surgery, has been demonstrated to be the most effective and long-term treatment for individuals with severe obesity or moderate obesity complicated by comorbid conditions that is not responsive to non-surgical approaches. By using a medical model, clinicians can provide more proactive and effective treatments in assisting their patients with weight loss.

Pharmacologic management of obesity

Recent discoveries of processes that govern regulation of body weight and energy expenditure have led to development of new anti-obesity pharmacological agents. This article will inform health professionals of new anti-obesity medications that target neuronal systems within the central nervous system (CNS) and peripheral humoral proteins that send signals to the CNS. An emerging theme of new therapies is to use combination medications that are directed toward several targets or leverage existing gastrointestinal satiety hormonal signals. By using combination therapies, it is anticipated that greater weight loss will be achieved compared to monotherapy.

Switching from serotonin reuptake inhibitors to agomelatine in patients with refractory obsessive-compulsive disorder: a 3 month follow-up case series

BACKGROUND

Serotonin reuptake inhibitors (SRIs) currently represent the cornerstone of obsessive-compulsive disorder (OCD) pharmacotherapy. However, OCD is characterized by high rates of partial and/or absent response to standard, recommended treatments, often prompting pharmacological and non-pharmacological augmentation or switching of strategies. Agomelatine, a novel melatonin agonist and selective serotonin antagonist (MASSA) antidepressant approved for major depressive disorder (MDD) has recently been additionally proposed as a treatment for anxiety disorders such as social anxiety disorder (SAD) and panic disorder (PD), but not yet OCD. Nonetheless, agomelatine may have a role in the management of OCD, essentially due to its anxiolytic 5-hydroxytryptamine (HT)2C blockade action, while melatonin (MT)1 and MT2 modulation might contribute to circadian rhythm restoration if impaired.

METHODS

This case series reports the outcome of six patients with or without comorbid mood and/or other anxiety disorders who were treated with SRIs at adequate doses for at least 8 weeks, showing partial or no response. Patients were then switched to agomelatine 50 mg/day, and followed up for 12 weeks.

RESULTS

Three out of six patients, in particular those with relevant circadian rhythm subjective impairment, showed a Yale-Brown Obsessive Compulsive Scale (Y-BOCS) score reduction of ≥35%. No relevant side effects were observed, but initial, transient, self-remitting dizziness in one patient and weight gain in another were seen.

CONCLUSIONS

Although clinical confounding factors (subthreshold bipolarity and eventually the presence of impaired circadian rhythms) and methodological boundaries (lack of control and neurophysiological recording, tiny sample size and short follow-up) limit the validity of this preliminary observation, it does indicate agomelatine may have a role in some SRI-refractory OCD cases, thus prompting the validity of investigation by further controlled studies, even for drug-naïve OCD patients.

Discovery and development of 5-HT2C receptor agonists for obesity: is there light at the end of the tunnel?

Ever since the observation of late-onset obesity during the phenotypic characterization of the 5-HT2C knock-out mouse, the serotonin 5-HT2C receptor has been a drug target for obesity. Small-molecule agonists have repeatedly been shown to reduce food intake and body weight in rodent models of obesity. To date, however, only one compound, lorcaserin, has completed Phase III trials and currently awaits an US FDA decision following a negative advisory committee meeting. Agonist selectivity versus the highly homologous 5-HT2A and 5-HT2B receptors remains a significant hurdle. Ideally, a specific 5-HT2C agonist (completely devoid of 5-HT2A and 5-HT2B activity) would be preferred. The requirement of a basic amine coupled with larger, often aromatic, hydrophobic domains, to gain selectivity, often leads to additional challenges associated with cationic amphiphilic molecules such as hERG-channel inhibition and phospholipidosis. The success of future 5-HT2C agonists will depend on further improvements in selectivity (or attainment of complete specificity) and pharmaceutical properties to permit greater and sustained receptor stimulation, while avoiding side effects associated with the activation of other 5-HT receptors.

Novel 6,7,8,9-tetrahydro-5H-1,4,7,10a-tetraaza-cyclohepta[f]indene analogues as potent and selective 5-HT(2C) agonists for the treatment of metabolic disorders

The discovery of a novel series of 5-HT(2C) agonists based on a tricyclic pyrazolopyrimidine scaffold is described. Compounds with good levels of in vitro potency and moderate to good levels of selectivity with respect to the 5-HT(2A) and 5-HT(2B) receptors were identified. One of the analogues (7 g) was found to be efficacious in a sub-chronic weight loss model. A key limitation of the series of compounds was that they were found to be potent inhibitors of the hERG ion channel. Some compounds, bearing polar side chains were identified which showed a much reduced hERG liability however these compounds were sub-optimal in terms of their in vitro potency or selectivity.

Evidence of complex involvement of serotonergic genes with restrictive and binge purge subtypes of anorexia nervosa

OBJECTIVES

There is mixed evidence of association of serotoninergic genes with anorexia nervosa (AN), but substantial evidence for the involvement of serotonergic mechanisms in appetite control. This study was designed to investigate possible associations between the two subtypes of AN (Restricting-RAN, and Binge-purging-BPAN) and polymorphisms within five genes encoding for proteins involved in the serotoninergic system.

METHODS

In order to carry out this investigation we have conducted a case-control association study on 226 females meeting the criteria for AN, and 678 matched healthy females.

RESULTS

Our data show a significant association between polymorphisms with the gene encoding HTR2A with both AN subtypes, an association between polymorphisms within the genes encoding HTR1D and HTR1B with RAN, and an association between polymorphisms within the gene encoding HTR2C with BPAN. No associations were found for any polymorphisms of the serotonin transporter gene. This outcome indicates a substantial and complex inter-relationship between serotoninergic genes and AN.

CONCLUSIONS

Given these data we hypothesis that the expression or control of expression of several genes of the serotoninergic system, and interactions between these genes, could exert considerable influence over the specific symptomatology of the subtypes of AN.

Serotonergic drugs and valvular heart disease

BACKGROUND

The serotonin (5-HT) releasers (+/-)-fenfluramine and (+)-fenfluramine were withdrawn from clinical use owing to increased risk of valvular heart disease. One prevailing hypothesis (i.e., the '5-HT hypothesis') suggests that fenfluramine-induced increases in plasma 5-HT underlie the disease.

OBJECTIVE

Here, we critically evaluate the possible mechanisms responsible for fenfluramine-associated valve disease.

METHODS

Findings from in vitro and in vivo experiments performed in our laboratory are reviewed. The data are integrated with existing literature to address the validity of the 5-HT hypothesis and suggest alternative explanations.

CONCLUSIONS

The overwhelming majority of evidence refutes the 5-HT hypothesis. A more likely cause of fenfluramine-induced valvulopathy is activation of 5-HT(2B) receptors on heart valves by the metabolite norfenfluramine. Future serotonergic medications should be designed to lack 5-HT(2B) agonist activity.

Serotonin and the GI tract

Serotonin (5-hydroxytryptamine, 5-HT) participates in several functions of the gastrointestinal tract. Receptors in seven families (5-HT(1)-5-HT(7)) were identified, many of which are present on enterocytes, intrinsic and extrinsic neurons, interstitial cells, and gut myocytes. Most 5-HT is released from enterochromaffin cells in response to physiologic and pathologic stimuli. Roles of 5-HT in health include control of normal gut motor activity, secretion, and sensation, and regulation of food intake and cell growth. Abnormalities of serotonergic function contribute to symptom genesis in functional bowel disorders, inflammatory and infectious diseases of the gut, emetic responses to varied stimuli, obesity, and dysregulation of cell growth. Therapies acting as agonists or antagonists of 5-HT receptors or that modulate 5-HT reuptake play prominent roles in managing these conditions, although use of many agents is hampered by cardiopulmonary complications. Novel agents are in testing, which may exhibit efficacy without significant toxicity.

Appetite suppressants, cardiac valve disease and combination pharmacotherapy

The prevalence of obesity in the United States is a major health problem associated with significant morbidity, mortality, and economic burden. Although obesity and drug addiction are typically considered distinct clinical entities, both diseases involve dysregulation of biogenic amine neuron systems in the brain. Thus, research efforts to develop medications for treating drug addiction can contribute insights into the pharmacotherapy for obesity. Here, we review the neurochemical mechanisms of selected stimulant medications used in the treatment of obesity and issues related to fenfluramine-associated cardiac valvulopathy. In particular, we discuss the evidence that cardiac valve disease involves activation of mitogenic serotonin 2B (5-HT2B) receptors by norfenfluramine, the major metabolite of fenfluramine. Advances in medication discovery suggest that novel molecular entities that target 2 different neurochemical mechanisms, that is, "combination pharmacotherapy," will yield efficacious antiobesity medications with reduced adverse side effects.

Pharmacological targeting of the serotonergic system for the treatment of obesity

The attenuation of food intake as induced by an increase in serotonergic (5-hydroxytryptamine, 5-HT) efficacy has been a target of antiobesity pharmacotherapies. However, the induction of tolerance and/or side-effects limited the clinical utility of the earliest serotonin-related medications. With the global prevalence of obesity rising, there has been renewed interest in the manipulation of the serotonergic system as a point of pharmacological intervention. The serotonin(2C) receptor (5-HT(2C)R), serotonin(1B) (rodent)/serotonin(1Dbeta) (human) receptor (5-HT(1B/1Dbeta)R) and serotonin(6) receptor (5-HT(6)R) represent the most promising serotonin receptor therapeutic targets. Canonical serotonin receptor compounds have given way to a myriad of novel receptor-selective ligands, many of which have observable anorectic effects. Here we review serotonergic compounds reducing ingestive behaviour and discuss their clinical potential for the treatment of obesity.

Serotonin (5-HT) transporter ligands affect plasma 5-HT in rats

Dual dopamine (DA)/serotonin (5-HT)-releasing agents are promising candidate medications for stimulant addiction and other disorders. However, certain 5-HT transporter (SERT) substrates are associated with development of idiopathic pulmonary arterial hypertension (IPAH) and valvular heart disease (VHD). According to the "5-HT hypothesis," SERT substrates increase the risk for developing IPAH and VHD by increasing plasma 5-HT. To test this hypothesis directly, we determined the effects of acute and chronic fenfluramine, and other SERT ligands, on plasma 5-HT in male rats. For acute treatments, rats received i.v. vehicle or test drug (0.3 and 1.0 mg/kg), and serial blood samples were withdrawn. For chronic treatments, vehicle or test drug was infused via osmotic minipump (3 and 10 mg/kg/d) for 2 weeks. On the last day of infusion, rats received i.v. fenfluramine challenge (1 mg/kg), and serial blood samples were withdrawn. Plasma 5-HT was measured using ex vivo microdialysis in whole-blood samples. Baseline plasma 5-HT was <1.0 nM. Acute injection of fenfluramine or other SERT substrates caused large (up to 24-fold) dose-dependent increases in plasma 5-HT. Chronic fenfluramine at 3 and 10 mg/kg/d produced 1.7- and 3.5-fold increases in baseline plasma 5-HT, while chronic fluoxetine had no effect. Chronic infusions of fenfluramine or fluoxetine diminished the ability of acute fenfluramine to elevate dialysate 5-HT, and both drugs markedly reduced whole-blood 5-HT. Acute fenfluramine increases plasma 5-HT to concentrations that are below the micromolar levels necessary to produce adverse cardiovascular effects. Chronic fenfluramine and fluoxetine have minimal effects on plasma 5-HT, suggesting that the increased risk for IPAH associated with fenfluramine does not depend upon elevations in plasma 5-HT.

Relevance of animal models to human eating disorders and obesity

BACKGROUND AND RATIONALE

This review addresses the role animal models play in contributing to our knowledge about the eating disorders anorexia nervosa (AN) and bulimia nervosa (BN) and obesity.

OBJECTIVES

Explore the usefulness of animal models in complex biobehavioral familial conditions, such as AN, BN, and obesity, that involve interactions among genetic, physiologic, psychological, and cultural factors.

RESULTS AND CONCLUSIONS

The most promising animal model to mimic AN is the activity-based anorexia rodent model leading to pathological weight loss. The paradigm incorporates reward elements of the drive for activity in the presence of an appetite and allows the use of genetically modified animals. For BN, the sham-feeding preparation in rodents equipped with a gastric fistula appears to be best suited to reproduce the postprandial emesis and the defects in satiety. Animal models that incorporate genes linked to behavior and mood may clarify biobehavioral processes underlying AN and BN. By contrast, a relative abundance of animal models has contributed to our understanding of human obesity. Both environmental and genetic determinants of obesity have been modeled in rodents. Here, we consider single gene mutant obesity models, along with models of obesigenic environmental conditions. The contributions of animal models to obesity research are illustrated by their utility for identifying genes linked to human obesity, for elucidating the pathways that regulate body weight and for the identification of potential therapeutic targets. The utility of these models may be further improved by exploring the impact of experimental manipulations on the behavioral determinants of energy balance.

Anti-obesity drugs

BACKGROUND

Recent discoveries about the metabolic and genetic control systems that govern regulation of body weight and energy expenditure have afforded the opportunity to develop pharmacological agents that can be used as adjunctive treatments for patients with obesity. However, the scarcity of medications available or those that have made it to Phase III trials reflects the difficult challenge of manipulating the biology of appetite regulation.

OBJECTIVE

The aim of this study was to inform health professionals about new anti-obesity medications that target neuronal systems within the central nervous system and peripheral humoral proteins that send signals to the central nervous system.

METHODS

A critical review was conducted of information available from the medical literature.

RESULTS/CONCLUSIONS

An emerging theme is to use combination therapies that are directed towards several targets or leverage existing gastrointestinal satiety hormonal signals. By using combination therapies, it is anticipated that greater weight loss will be achieved compared to monotherapy.

Lorcaserin, a novel selective human 5-hydroxytryptamine2C agonist: in vitro and in vivo pharmacological characterization

5-Hydroxytryptamine (5-HT)(2C) receptor agonists hold promise for the treatment of obesity. In this study, we describe the in vitro and in vivo characteristics of lorcaserin [(1R)-8-chloro-2,3,4,5-tetrahydro-1-methyl-1H-3 benzazepine], a selective, high affinity 5-HT(2C) full agonist. Lorcaserin bound to human and rat 5-HT(2C) receptors with high affinity (K(i) = 15 +/- 1 nM, 29 +/- 7 nM, respectively), and it was a full agonist for the human 5-HT(2C) receptor in a functional inositol phosphate accumulation assay, with 18- and 104-fold selectivity over 5-HT(2A) and 5-HT(2B) receptors, respectively. Lorcaserin was also highly selective for human 5-HT(2C) over other human 5-HT receptors (5-HT(1A), 5-HT(3), 5-HT(4C), 5-HT5(5A), 5-HT(6), and 5-HT(7)), in addition to a panel of 67 other G protein-coupled receptors and ion channels. Lorcaserin did not compete for binding of ligands to serotonin, dopamine, and norepinephrine transporters, and it did not alter their function in vitro. Behavioral observations indicated that unlike the 5-HT(2A) agonist (+/-)-1-(2,5-dimethoxy-4-phenyl)-2-aminopropane, lorcaserin did not induce behavioral changes indicative of functional 5-HT(2A) agonist activity. Acutely, lorcaserin reduced food intake in rats, an effect that was reversed by pretreatment with the 5-HT(2C)-selective antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl-carbamoyl]indoline (SB242,084) but not the 5-HT(2A) antagonist (R)-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol (MDL 100,907), demonstrating mediation by the 5-HT(2C) receptor. Chronic daily treatment with lorcaserin to rats maintained on a high fat diet produced dose-dependent reductions in food intake and body weight gain that were maintained during the 4-week study. Upon discontinuation, body weight returned to control levels. These data demonstrate lorcaserin to be a potent, selective, and efficacious agonist of the 5-HT(2C) receptor, with potential for the treatment of obesity.

Chronic fenfluramine administration increases plasma serotonin (5-hydroxytryptamine) to nontoxic levels

Large elevations in blood serotonin (5-hydroxytryptamine; 5-HT) can produce valvular heart disease in humans and laboratory animals. In accordance, one prevailing hypothesis (i.e., the "5-HT hypothesis") suggests that 5-HT transporter substrates like fenfluramine increase the risk for valvular heart disease by elevating plasma 5-HT, secondary to the release of 5-HT from platelets. The main purpose of this study was to determine whether chronic administration of fenfluramine increases plasma 5-HT to concentrations that are associated with the development of valvular heart disease. To the best of our knowledge, this is the first study to address this issue using an in vivo microdialysis method that measures plasma 5-HT in nonhypoxic rats. We examined the effects of chronic (+/-)-fenfluramine and fluoxetine on plasma levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in blood samples from conscious catheterized rats. Plasma indoles were measured by high-performance liquid chromatography with electrochemical detection in the dialysates of whole blood. The baseline plasma 5-HT level was <1.0 nM. Chronic fenfluramine (14-day minipump infusion) produced small increases in baseline plasma 5-HT ( approximately 2-4-fold), whereas chronic fluoxetine had no effect. Chronic fenfluramine and fluoxetine markedly decreased whole-blood 5-HT and reduced the ability of acute fenfluramine to evoke 5-HT release. Elevations in baseline plasma 5-HT produced by chronic fenfluramine are far below the micromolar levels necessary to produce valvular heart disease. Furthermore, chronic fenfluramine reduces the ability of acute fenfluramine to increase plasma 5-HT, suggesting that the 5-HT hypothesis cannot explain the increased risk of valvular heart disease in patients treated with fenfluramine.

Long-term serotonin effects in the rat are prevented by terguride

Long-term hyperserotoninemia induces heart valve disease in rats, and cases of cardiac valvulopathies have been reported in patients using ergolines, possibly through activation of the 5-hydroxytryptamine(2B) (5HT(2B)) receptor. The ergoline terguride (transdihydrolisuride) is a 5HT(2B/2C) receptor antagonist. Using a rat model, we have investigated whether terguride could prevent serotonin-induced changes in general and heart disease specifically. During 4 months, twelve Sprague-Dawley rats were given daily subcutaneous serotonin injections; twelve rats received a combination of serotonin injections and terguride by gavage, whereas ten rats were untreated controls. Using echocardiography, rats with aortic insufficiency were found in all 3 groups, while pulmonary insufficiency was only found in two rats injected with serotonin alone. Animals given serotonin alone had significantly higher heart weights compared to the controls (p=0.029) and rats given terguride (p=0.034). Rats injected with serotonin alone developed macroscopic skin changes at the injection sites, histologically identified as orthokeratosis and acanthosis. Terguride completely prevented these changes (p=0.0001, p=0.0003). Liver weights were higher in the animals given serotonin alone compared to controls (p=0.014) and terguride treated animals (p=0.009). Stomach weights were higher in animals given serotonin alone compared to rats given terguride (p=0.012). In the mesenchymal cell-line MC3T3-E1, terguride almost completely inhibited serotonin-induced proliferation (p<0.01). Serotonin increases heart, liver and stomach weights, possibly through enhanced proliferation. Terguride inhibits these effects. We propose that terguride may have beneficial effects in the treatment of diseases such as carcinoid syndrome, where serotonin plays an important pathogenic role.

Canine obesity: an overview

Canine patients are generally regarded as being clinically obese when their body weight is at least 15% above ideal. The incidence of obesity in dogs is thought to be in the range of 20-40% of the general population and, since obesity is known to predispose or exacerbate a range of serious medical conditions, its importance cannot be overstated. Management of obesity through dietary restriction and increased exercise is often difficult to achieve and dependent upon owner compliance. Until recently there has been no authorized therapeutic medication available for weight reduction in dogs, and drugs used in people have proved unsuitable. However, with the development of microsomal triglyceride transfer protein inhibitors for canine use, such as dirlotapide, the veterinarian has a novel method with which to augment traditional weight control programmes. This approach has the additional advantage that weight loss is achieved without dietary restriction or change in exercise regimen, providing encouragement for the owner to comply with subsequent dietary and exercise recommendations, thereby increasing the likelihood for long-term success.