Comparison of the effects of sibutramine and other weight-modifying drugs on extracellular dopamine in the nucleus accumbens of freely moving rats

Unraveling the serotonin saga: from discovery to weight regulation and beyond - a comprehensive scientific review

The prevalence of obesity is rapidly increasing worldwide, while the development of effective obesity therapies lags behind. Although new therapeutic targets to alleviate obesity are identified every day, and drug efficacy is improving, adverse side effects and increased health risks remain serious issues facing the weight-loss industry. Serotonin, also known as 5-HT, has been extensively studied in relation to appetite reduction and weight loss. As a result, dozens of upstream and downstream neural targets of 5-HT have been identified, revealing a multitude of neural circuits involved in mediating the anorexigenic effect of 5-HT. Despite the rise and fall of several 5-HT therapeutics in recent decades, the future of 5-HT as a therapeutic target for weight-loss therapy looks promising. This review focuses on the history of serotonin, the state of current central serotonin research, previous serotonergic therapies, and the future of serotonin for treating individuals with obesity.

Prospects for new drugs to treat binge-eating disorder: Insights from psychopathology and neuropharmacology

BACKGROUND

Binge-eating disorder (BED) is a common psychiatric condition with adverse psychological and metabolic consequences. Lisdexamfetamine (LDX) is the only approved BED drug treatment. New drugs to treat BED are urgently needed.

METHODS

A comprehensive review of published psychopathological, pharmacological and clinical findings.

RESULTS

The evidence supports the hypothesis that BED is an impulse control disorder with similarities to ADHD, including responsiveness to catecholaminergic drugs, for example LDX and dasotraline. The target product profile (TPP) of the ideal BED drug combines treating the psychopathological drivers of the disorder with an independent weight-loss effect. Drugs with proven efficacy in BED have a common pharmacology; they potentiate central noradrenergic and dopaminergic neurotransmission. Because of the overlap between pharmacotherapy in attention deficit hyperactivity disorder (ADHD) and BED, drug-candidates from diverse pharmacological classes, which have already failed in ADHD would also be predicted to fail if tested in BED. The failure in BED trials of drugs with diverse pharmacological mechanisms indicates many possible avenues for drug discovery can probably be discounted.

CONCLUSIONS

(1) The efficacy of drugs for BED is dependent on reducing its core psychopathologies of impulsivity, compulsivity and perseveration and by increasing cognitive control of eating. (2) The analysis revealed a large number of pharmacological mechanisms are unlikely to be productive in the search for effective new BED drugs. (3) The most promising areas for new treatments for BED are drugs, which augment noradrenergic and dopaminergic neurotransmission and/or those which are effective in ADHD.

What pharmacological interventions are effective in binge-eating disorder? Insights from a critical evaluation of the evidence from clinical trials

Binge-eating disorder (BED) is the commonest eating disorder and an important causal factor in obesity. Lisdexamfetamine is the only approved pharmacological treatment. Many drugs have been clinically evaluated and several were described as potentially promising treatments. A comprehensive reassessment of the evidence from these clinical trials has been performed. The questions to be answered were: (1) Does the evidence support claims of efficacy? (2) What pharmacological mechanisms show promise for developing new BED drugs? (3) What are the clinical implications for treating BED? PubMed and internal database searches identified every available published drug trial in BED. The trials and their results were summarised and reviewed to re-evaluate the evidence. Factors taken into consideration included psychiatric diagnosis, primary endpoint, secondary outcome measures, trial size, blinding and controls, drop-out rates, placebo response rates and weight-loss. Drugs were classified according to their pharmacology and therapeutic indication to determine which mechanisms were effective and to provide insights into the psychopathology of BED. For most drugs, robust evidence of efficacy in BED is insubstantial or absent. Some catecholaminergic drugs developed for ADHD are also effective in BED; other pharmacological mechanisms are weakly efficacious at best. Reducing BED severity has little impact on weight. Conversely, weight-loss from anti-obesity therapy is ineffective in ameliorating the psychopathological drivers of BED. (1) BED is a psychiatric not a metabolic disorder. (2) Weight-loss drugs are generally ineffective in BED. (3) Efficacy in BED is restricted to powerful catecholaminergic drugs. (4) Drugs acting via noradrenaline, 5-HT, GABA, carbonic anhydrase inhibition, opioid receptors and various ion channels are generally minimally effective at best. (5) Efficacy in BED is dependent on treating its core psychopathology; reducing impulsivity and compulsivity and increasing cognitive restraint over eating. (6) Obese subjects with BED may benefit from separate treatments for these two disorders.

Treatment of Acquired Hypothalamic Obesity: Now and the Future

The hypothalamus is the centre of neuroendocrine regulation of energy homeostasis and appetite. Maldevelopment of, or damage to, the key hypothalamic nuclei disrupts the coordinated balance between energy intake and expenditure leading, to rapid and excessive weight gain. Hypothalamic obesity is compounded by a disruption of the hypothalamic-pituitary axis, sleep disruption, visual compromise, and neurological and vascular sequalae. Amongst suprasellar tumors, craniopharyngioma is the most common cause of acquired hypothalamic obesity, either directly or following surgical or radiotherapeutic intervention. At present, therapy is limited to strategies to manage obesity but with a modest and variable impact. Current approaches include optimizing pituitary hormone replacement, calorie restriction, increased energy expenditure through physical activity, behavioral interventions, pharmacotherapy and bariatric surgery. Current pharmacotherapeutic approaches include stimulants that increase energy consumption, anti-diabetic agents, hypothalamic-pituitary substitution therapy, octreotide, and methionine aminopeptidase 2 (MetAP2) inhibitors. Some pharmacological studies of hypothalamic obesity report weight loss or stabilization but reported intervention periods are short, and others report no effect. The impact of bariatric surgery on weight loss in hypothalamic obesity again is variable. Novel or combined approaches to manage hypothalamic obesity are thus required to achieve credible and sustained weight loss. Identifying etiological factors contributing hypothalamic obesity may lead to multi-faceted interventions targeting hyperphagia, insulin resistance, decreased energy expenditure, sleep disturbance, hypopituitarism and psychosocial morbidity. Placebo-controlled trials using current single, or combination therapies are required to determine the impact of therapeutic agents. A well-defined approach to defining the location of hypothalamic damage may support the use of future targeted therapies. Intranasal oxytocin is currently being investigated as an anorexogenic agent. Novel agents including those targeting pro-opimelanocortin-C and AgRP/NPY expressing neurons and the MC4 receptor may result in better outcomes. This article discusses the current challenges in the management of hypothalamic obesity in children and young people and future therapeutic approaches to increasing weight loss and quality of life in these patients.

Anorectic interaction and safety of 5-hydroxytryptophan/carbidopa plus phentermine or diethylpropion in rat

Drug combinations are being studied as potential therapies to increase the efficacy or improve the safety profile of weight loss medications. This study was designed to determine the anorectic interaction and safety profile of 5-hydroxytryptophan (5-HTP)/carbidopa + diethylpropion and 5-HTP/carbidopa + phentermine combinations in rats. The anorectic effect of individual drugs or in combination was evaluated by the sweetened milk test. Isobologram and interaction index were employed to determine the anorectic interaction between 5-HTP/carbidopa and diethylpropion or phentermine. Plasma serotonin (5-HT) was measured by ELISA. Safety of repeated doses of both combinations in rats was evaluated using the tail sphygmomanometer, cardiac ultrasound, hematic biometry and blood chemistry. A single oral 5-HTP, diethylpropion or phentermine dose increased the anorectic effect, in a dose-dependent fashion, in 12 h-fasted rats. A dose of carbidopa at 30 mg/kg reduced the 5-HTP-induced plasmatic serotonin concentration and augmented the 5-HTP-induced anorectic effect. Isobologram and interaction index indicated a potentiation interaction between 5-HTP/30 mg/kg carbidopa + diethylpropion and 5-HTP/30 mg/kg carbidopa + phentermine. Chronic administration of experimental ED40 of 5-HTP/30 mg/kg carbidopa + phentermine, but not 5-HTP/30 mg/kg carbidopa + diethylpropion, increased the mitral valve leaflets area. Moreover, there were no other significant changes in cardiovascular, hematic or blood parameters. Both combinations induced around 20% body weight loss after 3 months of oral administration. Results suggest that 5-HTP/30 mg/kg carbidopa potentiates the anorectic effect of diethylpropion and phentermine with an acceptable safety profile, but further clinical studies are necessary to establish their therapeutic potential in the obesity treatment.

5-alpha-dihydrotestosterone elevations associated with phentermine use

BACKGROUND

A case of 5-alpha-dihydrotestosterone (DHT) elevation associated with phentermine initiation is reported, and possible mechanisms are discussed. There are no published reports of this association in the literature.

METHODS

Clinical and laboratory information is described.

RESULTS

A 72-year-old male with metastatic prostate cancer taking dutasteride to lower his DHT levels initiated phentermine 15 mg daily for weight loss. His DHT level drawn within 1 week prior to starting phentermine was 9.9 pg/ml. When reporting for follow up 2 weeks later, his DHT level had increased to 114 pg/ml. The DHT level was checked again 2 weeks after that visit, and had increased to 174 pg/ml. At that time, phentermine was discontinued, and 1 week later, the DHT level had decreased to 20.1 pg/ml. Over the next 4 months, the patient's DHT levels were maintained at less than 20 pg/ml. Phentermine 15 mg daily was then reinitiated while his DHT level was 7.5 pg/ml. Two weeks after resuming phentermine, his DHT level had again increased to 196 pg/ml. The patient's phentermine was then discontinued, and around 1 week later, his DHT level had fallen to 5.1 pg/ml.

CONCLUSION

A 72-year-old male with metastatic prostate cancer experienced profound increases in DHT upon initiation of phentermine despite continuation of his baseline dutasteride therapy. The etiology of these increases is still unclear.

Stimulants for the Control of Hedonic Appetite

The focus of this paper is treatment of obesity in relation to the management of hedonic appetite. Obesity is a complex condition which may be potentiated by excessive reward seeking in combination with executive functioning deficits that impair cognitive control of behavior. Stimulant medications address both reward deficiency and enhance motivation, as well as suppressing appetite. They have long been recognized to be effective for treating obesity. However, stimulants can be abused for their euphoric effect. They induce euphoria via the same neural pathway that underlies their therapeutic effect in obesity. For this reason they have generally not been endorsed for use in obesity. Among the stimulants, only phentermine (either alone or in combination with topiramate) and bupropion (which has stimulant-like properties and is used in combination with naltrexone), are approved by the United States Food and Drug Administration (FDA) for obesity, although dexamphetamine and methylpenidate are approved and widely used for treating attention deficit hyperactivity disorder (ADHD) in adults and children. Experience gained over many years in the treatment of ADHD demonstrates that with careful dose titration, stimulants can be used safely. In obesity, improvement in mood and executive functioning could assist with the lifestyle changes necessary for weight control, acting synergistically with appetite suppression. The obesity crisis has reached the stage that strong consideration should be given to adequate utilization of this effective and inexpensive class of drug.

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

Preclinical pharmacokinetics, pharmacology and toxicology of lisdexamfetamine: a novel d-amphetamine pro-drug

Lisdexamfetamine dimesylate (LDX) is a novel pro-drug of d-amphetamine that is currently used for the treatment of attention-deficit/hyperactivity disorder in children aged ≥ 6 years and adults. LDX is enzymatically cleaved to form d-amphetamine following contact with red blood cells, which reduces the rate of appearance and magnitude of d-amphetamine concentration in the blood and hence the brain when compared with immediate-release d-amphetamine at equimolar doses. Thus, the increase of striatal dopamine efflux and subsequent increase of locomotor activity following d-amphetamine is less prominent and slower to attain maximal effect following an equimolar dose of LDX. Furthermore, unlike d-amphetamine, the pharmacodynamic effects of LDX are independent of the route of administration underlining the requirement to be hydrolyzed by contact with red blood cells. It is conceivable that these pharmacokinetic and pharmacodynamic differences may impact the psychostimulant properties of LDX in the clinic. This article reviews the preclinical pharmacokinetics, pharmacology, and toxicology of LDX. This article is part of the Special Issue entitled 'CNS Stimulants'.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Amphetamine, past and present--a pharmacological and clinical perspective

Amphetamine was discovered over 100 years ago. Since then, it has transformed from a drug that was freely available without prescription as a panacea for a broad range of disorders into a highly restricted Controlled Drug with therapeutic applications restricted to attention deficit hyperactivity disorder (ADHD) and narcolepsy. This review describes the relationship between chemical structure and pharmacology of amphetamine and its congeners. Amphetamine's diverse pharmacological actions translate not only into therapeutic efficacy, but also into the production of adverse events and liability for recreational abuse. Accordingly, the balance of benefit/risk is the key challenge for its clinical use. The review charts advances in pharmaceutical development from the introduction of once-daily formulations of amphetamine through to lisdexamfetamine, which is the first d-amphetamine prodrug approved for the management of ADHD in children, adolescents and adults. The unusual metabolic route for lisdexamfetamine to deliver d-amphetamine makes an important contribution to its pharmacology. How lisdexamfetamine's distinctive pharmacokinetic/pharmacodynamic profile translates into sustained efficacy as a treatment for ADHD and its reduced potential for recreational abuse is also discussed.

Investigating the roles of different monoamine transmitters and impulse control using the 5-choice serial reaction time task

Previous studies have shown that drugs which block the reuptake of catecholamine neurotransmitters improve impulse control in diseases such as attention deficit hyperactivity disorder (ADHD). Serotonin-specific reuptake inhibitors (SSRI) lack efficacy in ADHD and have been linked to increased suicide risk. The present study investigated drugs with affinity for one or more of the monoamine reuptake transporters using the 5-choice serial reaction time task, a model of attention and impulsivity in rodents. We also tested the effects of the alpha(2)-adreoceptor antagonist, idazoxan and novel antidepressant, agomelatine, which both increase cortical noradrenaline concentrations through non-reuptake mechanisms. Improvements in impulse control were observed with venlafaxine, a serotonin and noradrenaline re-uptake inhibitor (SNRI) but not bupropion (dopamine and noradrenaline re-uptake inhibitor). Sibutramine (SNRI) reduced premature responses by ~50% at the highest dose tested but this was not significant. All three of the SSRIs tested reduced premature responding in a dose-dependent manner, although also slowed response and collection latencies. Neither idazoxan nor agomelatine significantly reduced premature responding, suggesting a lack of efficacy at the doses tested. None of the drugs tested improved attention in this task but sibutramine (SNRI), fluoxetine (SSRI) and paroxetine (SSRI) all increased omissions at the highest dose tested. These data suggest that the SNRIs and SSRIs reduce premature responding but tend to be less specific than noradrenaline specific reuptake inhibitors in this model. SSRIs did not induce any specific impairment in impulse control in this model.

Human abuse potential and cognitive effects of taranabant, a cannabinoid 1 receptor inverse agonist: a randomized, double-blind, placebo- and active-controlled, crossover study in recreational polydrug users

INTRODUCTION

Taranabant is a cannabinoid 1 receptor inverse agonist that was in development for treatment of obesity. Because of central nervous system effects, the study was performed to assess the abuse potential and cognitive effects of taranabant in recreational polydrug users compared with phentermine, dronabinol, and placebo.

METHODS

Stimulant- and cannabis-experienced polydrug users (N = 30) were randomized in a double-blind crossover study to receive taranabant 2, 4, 10, and 20 mg; phentermine 45 and 90 mg; dronabinol 20 mg; and placebo. Subjective and neurocognitive measures were administered for 24 hours, and peak/peak change from baseline effects were analyzed using a linear mixed-effects model.

RESULTS

Phentermine 45 and 90 mg showed abuse-related subjective effects versus placebo, including drug liking, overall drug liking, and other positive/stimulant effects, whereas dronabinol 20 mg showed abuse-related positive, cannabis-like, and sedative effects. Taranabant was not significantly different from placebo on most of the subjective measures other than negative/dysphoric effects at the highest dose, and its effects were significantly less pronounced relative to phentermine and dronabinol on most measures. Phentermine improved cognitive/motor performance and dronabinol impaired motor/cognitive performance on some measures, whereas taranabant 4 and 20 mg had minor impairment effects on manual tracking.

CONCLUSIONS

The phentermine and dronabinol results demonstrate the validity and sensitivity of the study. Taranabant did not consistently show stimulant/cannabis-like effects or abuse potential in recreational polydrug users, indicating that cannabinoid 1 receptor inverse agonists/antagonists are unlikely to be recreationally abused.

A review of late-stage CNS drug candidates for the treatment of obesity

Obesity is an important causative factor in morbidity, disability and premature death. Increasing levels of obesity will impose enormous health, financial and social burdens on worldwide society unless effective interventions are implemented. For many obese individuals, diet and behavioural modification need to be supplemented by pharmacotherapy. Preclinical research has revealed a greater understanding of the complex nature of the hypothalamic regulation of food intake and has generated a wide range of new molecular targets for the development of drug candidates for obesity treatment. As shown by the clinical results that have been obtained with this next generation of therapies, some approaches, for example, fixed-dose drug combinations, have already demonstrated an ability to deliver levels of efficacy that are not achievable with the current antiobesity drug therapies. The regulatory and marketing landscape for development, registration and commercialisation of novel centrally acting drugs for treatment of obesity and related metabolic disorders has changed substantially in recent years. Now a much greater emphasis is placed on tolerability and safety, as well as efficacy. In this review we briefly describe the therapeutic approaches to tackle obesity that are in late-stage clinical development. We then discuss drugs in late-stage development for the treatment of obesity and also future directions.

The use of serotonergic drugs to treat obesity--is there any hope?

Surgical interventional strategies for the treatment of obesity are being implemented at an increasing rate. The safety and feasibility of these procedures are questionable for most overweight or obese individuals. The use of long-term pharmacotherapy options, on the other hand, can target a greater portion of the obese population and provide early intervention to help individuals maintain a healthy lifestyle to promote weight loss. Medications that act on the central serotonergic pathways have been a relative mainstay for the treatment of obesity for the last 35 years. The clinical efficacy of these drugs, however, has been encumbered by the potential for drug-associated complications. Two drugs that act, albeit by different mechanisms, on the central serotonergic system to reduce food intake and decrease body weight are sibutramine and lorcaserin. Sibutramine is a serotonin and norepinephrine reuptake inhibitor, whereas lorcaserin is a selective 5HT_2C receptor agonist. The recent worldwide withdrawal of sibutramine and FDA rejection of lorcaserin has changed the landscape not only for serotonin-based therapeutics specifically, but for obesity pharmacotherapy in general. The purpose of this review is to focus on the importance of the serotonergic system in the control of feeding and its potential as a target for obesity pharmacotherapy. Advances in refining and screening more selective receptor agonists and a better understanding of the potential off-target effects of serotonergic drugs are needed to produce beneficial pharmacotherapy.

MSI-1436 reduces acute food intake without affecting dopamine transporter activity

Many therapies designed to reduce food intake and body weight act, in part, by blocking the dopamine transporter (DAT) - a protein responsible for clearing extracellular dopamine (DA) after release thereby terminating its action. Here, we found that a single injection of the drug trodusquemine (MSI-1436) decreased food intake in rats. To assess the effects of MSI-1436 on DAT function, fast-scan cyclic voltammetry was used to measure DA concentration changes in the ventral striatum. DA release was evoked by electrical stimulation of the ventral tegmental area every 5 min. After 3 baseline measurements, rats were injected with MSI-1436 (10 mg/kg), the known DAT blocker bupropion (80 mg/kg) or saline and evoked DA release and reuptake were monitored for an additional hour. Neither saline nor MSI-1436 caused a significant change in the magnitude of evoked release from baseline values whereas bupropion caused a significant increase. In addition, neither saline nor MSI-1436 significantly increased DA decay rates while such an increase was observed with bupropion. Thus, over a time course when MSI-1436 suppresses food intake it does not affect DAT function. The results support MSI-1436 as an anti-obesity treatment which spares DAT.

The anorexic agents, sibutramine and fenfluramine, depress GABA(B)-induced inhibitory postsynaptic potentials in rat mesencephalic dopaminergic cells

BACKGROUND AND PURPOSE

Nutrition is the result of a complex interaction among environmental, homeostatic and reward-related processes. Accumulating evidence supports key roles for the dopaminergic neurons of the ventral midbrain in regulating feeding behaviour. For this reason, in the present study, we have investigated the electrophysiological effects of two centrally acting anorexic agents, fenfluramine and sibutramine, on these cells.

EXPERIMENTAL APPROACH

Rat midbrain slices were used to make intracellular recordings from dopaminergic neurons of the substantia nigra and the ventral tegmental area. Gamma-aminobutyric acid (GABA)-mediated synaptic transmission was assessed from the inhibitory postsynaptic potentials (IPSPs) mediated by GABA(A) and GABA(B) receptors.

KEY RESULTS

Fenfluramine and sibutramine reduced, concentration-dependently, the GABA(B) IPSPs, without affecting the GABA(A)-mediated potentials. This effect is presynaptic, as postsynaptic membrane responses induced by application of a GABA(B) receptor agonist, baclofen, were not affected by the two drugs. Furthermore, the selective 5-hydroxytriptamine 1B (5-HT(1B)) receptor antagonist, SB216641, blocked the reduction of GABA(B) IPSPs caused by fenfluramine and sibutramine, indicating that the receptor mediating this effect is 5-HT(1B).

CONCLUSIONS AND IMPLICATIONS

Two anorexic agents, fenfluramine and sibutramine, induced the activation of 5-HT(1B) receptors located on presynaptic GABAergic terminals, thus reducing the release of GABA. This action can alter the strength of synaptic afferents that modify the activity of dopaminergic neurons, inducing neuronal excitation. Our results reveal an additional mechanism of action for fenfluramine and sibutramine that might contribute to reducing food intake, by influencing the pleasurable and motor aspects of feeding behaviour.

The development of tolerance to drugs that suppress food intake

Appetite suppressants have been available as weight-reducing aids for over 50 years. The first discovered was amphetamine, which was potent, but possessed undesirable side effects (it is a stimulant and elevates blood pressure). Subsequently, a variety of appetite drugs was developed, all structurally related to amphetamine, but mostly lacking unwanted side effects. Until recently, fenfluramine (FEN) was the most widely used; presently, sibutramine is the most commonly used appetite suppressant. While these appetite suppressants are effective at reducing hunger and food intake when given as a single dose or for short periods of time, their effectiveness diminishes when administered chronically. The biological mechanisms underlying this tolerance have not been carefully studied, but many possibilities have been identified, including the down-regulation in brain of neurotransmitter receptors that might mediate the action of these drugs and adaptive responses of the appetite control circuitry in brain. To date, however, few studies have examined these possibilities in any detail. This article focuses on the question of why appetite suppressants lose efficacy, when given chronically, because this issue is important to the development of the next generation of appetite suppressants. Chronic efficacy should be an issue studied relatively early in the drug development process. This issue is of particular relevance, since obesity treatment is now recognized as a long-term, not a short-term, process. If appetite suppressants are to become a more important tool in obesity treatment, agents that do not lose efficacy when administered for extended periods of time must be identified.

5-HT has contrasting effects in the frontal cortex, but not the hypothalamus, on changes in noradrenaline efflux induced by the monoamine releasing-agent, d-amphetamine, and the reuptake inhibitor, BTS 54 354

There is extensive evidence for functional interactions between central noradrenergic and serotonergic neurones. Here, dual-probe microdialysis was used in freely-moving rats to compare the effects of 5-HT on noradrenergic transmission in the rat frontal cortex and hypothalamus. We studied the effects of the 5-HT synthesis inhibitor, para-chlorophenylalanine (pCPA; which depleted 5-HT stores in both the frontal cortex and the hypothalamus), on spontaneous efflux of noradrenaline and on the noradrenergic responses to d-amphetamine, and the monoamine reuptake inhibitor, BTS 54 354. pCPA pretreatment alone did not affect spontaneous noradrenaline efflux in either brain region, whether or not alpha2-autoreceptors were inactivated by administration of the alpha2-antagonist, atipamezole (1 mg/kg i.p). However, in the frontal cortex, pCPA pretreatment augmented the amplitude of, and prolonged, the noradrenergic response to local infusion of d-amphetamine (10 microM). In contrast, pCPA abolished the increase in cortical noradrenaline efflux induced by local infusion of BTS 54 354 (50 microM). In the hypothalamus, pCPA did not affect the amplitude of the response to either of these agents but did prolong the effects of d-amphetamine on noradrenaline efflux. These findings suggest that serotonergic transmission has complex effects on the noradrenergic response to drugs that increase noradrenergic transmission in the frontal cortex, but has less influence in the hypothalamus.

Effect of sibutramine on macronutrient selection in male and female rats

Sibutramine, a serotonin-noradrenaline reuptake inhibitor (SNRI), has been shown to be a safe and effective weight-loss drug. The purpose of the present study was to examine whether sibutramine has an effect on macronutrient selection in both female and male rats in addition to total food intake. Wistar rats of both sexes were divided into three groups, and each group was offered a different set of three sensorily contrasting macronutrient-specific diets, each set including carbohydrate-, protein-, and fat-rich diets. Sibutramine (10 mg/kg) was shown to consistently decrease carbohydrate and fat intake at all data points regardless of gender and diet. Intake of carbohydrate differed between male and female rats at 2 h post administration with 2.5 and 5 mg/kg of sibutramine. The effect of sibutramine on protein intake was diet- and gender-specific. All doses of sibutramine decreased total food intake regardless of gender and diet group beginning at 6 h post administration. In conclusion, sibutramine affected macronutrient selection and emphasis on dietary recommendations, as well as appropriate dosage according to gender should be considered during therapy.

Acute cardiovascular effects of sibutramine in conscious rats

Sibutramine is a serotonin and norepinephrine reuptake inhibitor, used in the treatment of obesity. In this study, cardiovascular effects of sibutramine (0.9, 3, or 9 mg kg(-1) i.p.) were measured in conscious Sprague-Dawley rats, in the absence and presence of beta- and/or alpha-adrenoceptor antagonism (with propranolol and/or phentolamine, respectively). Sibutramine caused pressor and tachycardic effects, with celiac and mesenteric vasoconstrictions, and hyperemic hindquarters vasodilatation. Pretreatment with propranolol inhibited the tachycardic and hindquarters vasodilator effect of sibutramine, whereas phentolamine inhibited the pressor and vasoconstrictor effects of sibutramine. In the presence of phentolamine, sibutramine caused hyperemic mesenteric vasodilatation. In preconstricted, isolated, mesenteric vessels, sibutramine and its metabolites BTS 54505 (N-desmethylsibutramine) and BTS 54354 (N-didesmethylsibutramine) (10 microM) produced significant vasodilations. Neither sibutramine nor BTS 54505 enhanced vessel sensitivity to norepinephrine, whereas BTS 54 354 produced a significant leftward shift in the concentration-response curve to norepinephrine. Collectively, the results indicate that the overt cardiovascular effects of sibutramine involve alpha-adrenoceptor-mediated celiac and mesenteric vasoconstrictions, and beta-adrenoceptor-mediated hindquarters vasodilatation and tachycardia. The mesenteric vasodilator response to sibutramine, seen in the presence of phentolamine, may be a direct effect of the drug and/or its metabolites, on vessel tone. The cardiovascular effects of sibutramine in vivo may be secondary to inhibition of peripheral and/or central reuptake of monoamines by the metabolites BTS 54354 and/or BTS 54505. It remains to explain why BTS 54354, but not BTS 54505, enhanced norepinephrine sensitivity in vitro, because both metabolites are potent inhibitors of the norepinephrine transporter.

Antiobesity drugs: current and future issues

Overweight and obesity affect more than 60% of the population in the United States. Many social and environmental factors influence this phenomenon. Because of the tremendous difficulties associated with addressing these factors, health care providers have been looking for a magic pill that resolves this tissue. Although there is no panacea, this article is intended to concisely review commonly prescribed medications for obesity, their efficacy, and side effects. In addition, some pharmacologic agents with a promising antiobesity action are discussed.

Comparison of the thermogenic and hypophagic effects of sibutramine's metabolite 2 and other monoamine reuptake inhibitors

The thermogenic and hypophagic effects of sibutramine's metabolite (metabolite 2), a 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor, were investigated in rats and compared with duloxetine and bupropion. Metabolite 2 increased colonic temperature for 2.5-4.5 h. Duloxetine was also thermogenic but was less effective than metabolite 2. Bupropion similarly increased colonic temperature and was as efficacious, but less potent, than metabolite 2. At -8 degrees C, metabolite 2, duloxetine and bupropion all decreased response to heat reinforcement and reduced colonic temperature. Metabolite 2 produced a sustained increase in oxygen consumption (VO(2)) at 29 degrees C from 90 to 240 min, whereas duloxetine was far less effective. Bupropion rapidly enhanced VO(2), but its effect was less prolonged than that of metabolite 2. Metabolite 2 markedly reduced 24-h food intake. Duloxetine decreased feeding although its effect was shorter-lived, but bupropion was without effect. Thus, sibutramine's antiobesity action is probably attributable to effects on energy intake and expenditure. Duloxetine shares these properties, but is generally less efficacious. Any potential weight-reducing effect of bupropion is probably due to thermogenesis.

Monoamine oxidase inhibition is unlikely to be relevant to the risks associated with phentermine and fenfluramine: a comparison with their abilities to evoke monoamine release

OBJECTIVE AND DESIGN

It has been proposed that the anti-obesity agent, phentermine, may act in part via inhibition of monoamine oxidase (MAO). The ability of phentermine to inhibit both MAO(A) and MAO(B) in vitro has been examined along with that of the fenfluramine isomers, a range of selective serotonin reuptake inhibitors and sibutramine and its active metabolites.

RESULTS

In rat brain, harmaline and lazabemide showed potent and selective inhibition of MAO(A) and MAO(B), their respective target enzymes, with IC(50) values of 2.3 and 18 nM. In contrast, all other drugs examined were only weak inhibitors of MAO(A) and MAO(B) with IC(50) values for each enzyme in the moderate to high micromolar range. For MAO(A), the IC(50) for phentermine was estimated to be 143 microM, that for S(+)-fenfluramine, 265 microM and that for sertraline, 31 microM. For MAO(B), example IC(50)s were as follows: phentermine (285 microM), S(+)-fenfluramine (800 microM) and paroxetine (16 microM). Sibutramine was unable to inhibit either enzyme, even at its limit of solubility.

CONCLUSION

We therefore suggest that MAO inhibition is unlikely to play a role in the pharmacodynamic properties of any of the tested drugs, including phentermine. Instead, the lack of potency of these drugs as MAO inhibitors is contrasted with their powerful ability either to inhibit the uptake of one or more monoamines (fluoxetine, paroxetine, sertraline, sibutramine's active metabolites) or to evoke the release of one or more monoamines (S(+)-fenfluramine, S(+)-norfenfluramine, phentermine). These differences in mode of action may be linked to the adverse cardiovascular events experienced with some of the releasing agents.

Sibutramine metabolites increase glucose transport by cultured rat muscle cells

BACKGROUND

The anti-obesity agent sibutramine, a serotonin and noradrenaline reuptake inhibitor (SNRI), has been shown to reduce insulin resistance and improve glycaemic control in obese-diabetic ob/ob mice and overweight type 2 diabetic patients.

OBJECTIVE

To investigate whether sibutramine or its metabolites act directly on muscle cells to improve glucose uptake and insulin action.

DESIGN

Uptake of the non-metabolized glucose analogue 2-deoxyglucose was measured in cultured L6 rat muscle cells after incubation with sibutramine, its two pharmacologically active metabolites and related agents.

RESULTS

Sibutramine itself (10(-8)-10(-6) M) did not significantly affect 2-deoxyglucose uptake during incubations up to 72 h. The primary amine metabolite M2 (10(-7) and 10(-6) M) increased basal and insulin-stimulated 2-deoxyglucose uptake (by 12% and 34%) after 24 h incubation. These effects of M2 were lost by 72 h incubation. However, the secondary amine metabolite M1 (10(-6) M) increased basal and insulin-stimulated 2-deoxyglucose uptake (by 50%) after 72 h incubation, although M1 was ineffective after 24 h. M2 stimulated 2-deoxyglucose uptake in the presence of LY-294,002 (an inhibitor of phosphatidylinositol 3-kinase) but the effect of M2 was inhibited by cytochalasin B, which acutely blocks glucose transporters. Incubations with serotoninergic, noradrenergic and dopaminergic agents, or agents known to stimulate release or inhibit reuptake of these substances in nervous tissues indicated that the sibutramine metabolites were not affecting 2-deoxyglucose uptake via mechanisms associated with their SNRI properties.

CONCLUSIONS

Sibutramine metabolites can improve insulin-sensitive 2-deoxyglucose uptake by cultured muscle cells independently of SNRI effects.