Characterization of phentermine and related compounds as monoamine oxidase (MAO) inhibitors

Effects of congeners of amphetamine on the human heart

Central stimulatory and hallucinogenic drugs of abuse like amphetamine and most congeners of amphetamine can have cardiac harmful effects. These cardiac side effects can lead to morbidities and death. In this paper, we review current knowledge on the direct and indirect effects of these amphetamine congeners on the mammalian heart-more specifically, the isolated human heart muscle preparation. In detail, we address the question of whether and how these drugs affect cardiac contractility and their mechanisms of action. Based on this information, further research areas are defined, and further research efforts are proposed.

Combination Therapy: A New Tool for the Management of Obesity

Obesity is a chronic lifestyle issue with devastating results. Behavioral changes are one of the initial lines of management strategies for obesity, but they are not very efficient management strategies. Many people also use surgical intervention to maintain a healthy weight, now considered to be the most common and effective obesity management. Chemically synthesized medicines fill the gap between lifestyle interventions and minimally invasive surgical management of obesity. The most common issue associated with monotherapy without side effects is its moderate effectiveness and higher dose requirement. Combination therapy is already used for many serious and complicated disease treatments and management and has shown efficacy as well. Generally, we use two or more medicines with different mechanisms of action for a better effect. The commonly used combination therapy for obesity management includes low-dose phentermine and prolonged and slow-releasing mechanism topiramate; naltrexone, and bupropion. Phentermine with inhibitors of Na-glucose cotransporter-2 or glucagon-like peptide-1 (GLP-1) agonists with gastric hormone or Na-glucose cotransporter-2 are two more viable combo therapy. This combination strategy aims to achieve success in bariatric surgery and the scientific community is working in this direction.

Serotonin and Pulmonary Hypertension; Sex and Drugs and ROCK and Rho

Serotonin is often referred to as a "happy hormone" as it maintains good mood, well-being, and happiness. It is involved in communication between nerve cells and plays a role in sleeping and digestion. However, too much serotonin can have pathogenic effects and serotonin synthesis is elevated in pulmonary artery endothelial cells from patients with pulmonary arterial hypertension (PAH). PAH is characterized by elevated pulmonary pressures, right ventricular failure, inflammation, and pulmonary vascular remodeling; serotonin has been shown to be associated with these pathologies. The rate-limiting enzyme in the synthesis of serotonin in the periphery of the body is tryptophan hydroxylase 1 (TPH1). TPH1 expression and serotonin synthesis are elevated in pulmonary artery endothelial cells in patients with PAH. The serotonin synthesized in the pulmonary arterial endothelium can act on the adjacent pulmonary arterial smooth muscle cells (PASMCs), adventitial macrophages, and fibroblasts, in a paracrine fashion. In humans, serotonin enters PASMCs cells via the serotonin transporter (SERT) and it can cooperate with the 5-HT1B receptor on the plasma membrane; this activates both contractile and proliferative signaling pathways. The "serotonin hypothesis of pulmonary hypertension" arose when serotonin was associated with PAH induced by diet pills such as fenfluramine, aminorex, and chlorphentermine; these act as indirect serotonergic agonists causing the release of serotonin from platelets and cells through the SERT. Here the role of serotonin in PAH is reviewed. Targeting serotonin synthesis or signaling is a promising novel alternative approach which may lead to novel therapies for PAH. © 2022 American Physiological Society. Compr Physiol 12: 1-16, 2022.

In search of an ideal drug for safer treatment of obesity: The false promise of pseudoephedrine

Obesity is a major public health problem worldwide. Only relatively few treatment options are, at present, available for the management of obese patients. Furthermore, treatment of obesity is affected by the widespread misuse of drugs and food supplements. Ephedra sinica is an old medicinal herb, commonly used in the treatment of respiratory tract diseases. Ephedra species contain several alkaloids, including pseudoephedrine, notably endowed with indirect sympathomimetic pharmacodynamic properties. The anorexigenic effect of pseudoephedrine is attributable primarily to the inhibition of neurons located in the hypothalamic paraventricular nucleus (PVN), mediating satiety stimuli. Pseudoephedrine influences lipolysis and thermogenesis through interaction with β3 adrenergic receptors and reduces fat accumulation through down-regulation of transcription factors related to lipogenesis. However, its use is associated with adverse events that involve to a large extent the cardiovascular and the central nervous system. Adverse events of pseudoephedrine also affect the eye, the intestine, and the skin, and, of relevance, sudden cardiovascular death related to dietary supplements containing Ephedra alkaloids has also been reported. In light of the limited availability of clinical data on pseudoephedrine in obesity, along with its significantly unbalanced risk/benefit profile, as well as of the psychophysical susceptibility of obese patients, it appears reasonable to preclude the prescription of pseudoephedrine in obese patients of any order and degree.

Fenfluramine repurposing from weight loss to epilepsy: What we do and do not know

In 2020, racemic-fenfluramine was approved in the U.S. and Europe for the treatment of seizures associated with Dravet syndrome, through a restricted/controlled access program aimed at minimizing safety risks. Fenfluramine had been used extensively in the past as an appetite suppressant, but it was withdrawn from the market in 1997 when it was found to cause cardiac valvulopathy. Available evidence indicates that appetite suppression and cardiac valvulopathy are mediated by different serotonergic mechanisms. In particular, appetite suppression can be ascribed mainly to the enantiomers d-fenfluramine and d-norfenfluramine, the primary metabolite of d-fenfluramine, whereas cardiac valvulopathy can be ascribed mainly to d-norfenfluramine. Because of early observations of markedly improved seizure control in some forms of epilepsy, fenfluramine remained available in Belgium through a Royal Decree after 1997 for use in a clinical trial in patients with Dravet syndrome at average dosages lower than those generally prescribed for appetite suppression. More recently, double-blind placebo-controlled trials established its efficacy in the treatment of convulsive seizures associated with Dravet syndrome and of drop seizures associated with Lennox-Gastaut syndrome, at doses up to 0.7 mg/kg/day (maximum 26 mg/day). Although no cardiovascular toxicity has been associated with the use of fenfluramine in epilepsy, the number of patients exposed to date has been limited and only few patients had duration of exposure longer than 3 years. This article analyzes available evidence on the mechanisms involved in fenfluramine-induced appetite suppression, antiseizure effects and cardiovascular toxicity. Despite evidence that stimulation of 5-HT_2B receptors (the main mechanism leading to cardiac valvulopathy) is not required for antiseizure activity, there are many critical gaps in understanding fenfluramine's properties which are relevant to its use in epilepsy. Particular emphasis is placed on the remarkable lack of publicly accessible information about the comparative activity of the individual enantiomers of fenfluramine and norfenfluramine in experimental models of seizures and epilepsy, and on receptors systems considered to be involved in antiseizure effects. Preliminary data suggest that l-fenfluramine retains prominent antiseizure effects in a genetic zebrafish model of Dravet syndrome. If these findings are confirmed and extended to other seizure/epilepsy models, there would be an incentive for a chiral switch from racemic-fenfluramine to l-fenfluramine, which could minimize the risk of cardiovascular toxicity and reduce the incidence of adverse effects such as loss of appetite and weight loss.

Supraventricular tachycardia with the use of phentermine: case report and review of literature

Case: A 34-year-old woman with no significant past medical history presented to the hospital with sudden onset of palpitations with associated dyspnea and chest discomfort. She denied any similar previous episodes. Initial electrocardiogram (EKG) was consistent with a short R-P interval supraventricular tachycardia (SVT). Her transthoracic echocardiogram (TTE) revealed no structural abnormalities, TSH levels were normal, and urine drug screen was negative for any recreational drugs. However, the patient had been taking phentermine for weight loss.Discussion: The exact mechanism is not clear; however, we postulate that the sympathomimetic effects of phentermine likely contribute to SVT induction through enhanced AV nodal conduction or increased atrial ectopy. Conclusions: The only medication she was taking at home was phentermine, and the palpitations did not recur after discontinuation of the drug during follow-up. It is important to collect a thorough medication history when patients present with AV nodal reentrant tachycardia (AVNRT) or other SVT.

Amphetamine Derivatives as Monoamine Oxidase Inhibitors

Amphetamine and its derivatives exhibit a wide range of pharmacological activities, including psychostimulant, hallucinogenic, entactogenic, anorectic, or antidepressant effects. The mechanisms of action underlying these effects are usually related to the ability of the different amphetamines to interact with diverse monoamine transporters or receptors. Moreover, many of these compounds are also potent and selective monoamine oxidase inhibitors. In the present work, we review how structural modifications on the aromatic ring, the amino group and/or the aliphatic side chain of the parent scaffold, modulate the enzyme inhibitory properties of hundreds of amphetamine derivatives. Furthermore, we discuss how monoamine oxidase inhibition might influence the pharmacology of these compounds.

Phentermine: A Systematic Review for Plastic and Reconstructive Surgeons

PURPOSE

Phentermine is the most prescribed antiobesity drug in America, with 2.43 million prescriptions written in 2011. Case reports suggest there are anesthetic risks, such as refractory hypotension, involved with its perioperative use. Despite these risks and the frequency of phentermine use among plastic surgery patients, there are no published guidelines for the perioperative management of phentermine use in the plastic surgery literature. To address this patient safety issue, we performed a systematic review and provide management recommendations.

METHODS

A systematic review of the pharmacology of phentermine and the anesthetic risks involved with its perioperative use was undertaken using the search engines PubMed/MEDLINE, EMBASE, and Scopus.

RESULTS

A total of 251 citations were reviewed, yielding 4 articles that discussed perioperative phentermine use and complications with anesthesia. One was a review article, 2 were case reports, and 1 was a letter. Complications included hypotension, hypertension, hypoglycemia, hyperthermia, bradycardia, cardiac depression, and acute pulmonary edema.

CONCLUSIONS

The relationship between phentermine and anesthesia, if any, is unclear. Hypotension on induction of general anesthesia is the most reported complication of perioperative phentermine use. Specifically, phentermine-induced hypotension may be unresponsive to vasopressors that rely on catecholamine release, such as ephedrine. Therefore, the decision to perform surgery, especially elective surgery, in a patient taking phentermine should be made with caution. Because of the half-life of phentermine, we recommend discontinuing phentermine for at least 4 days prior to surgery. This differs from the classic 2-week discontinuation period recommended for "fen-phen." The patient should be made aware of the increased risk of surgery, and a skilled anesthesiologist should monitor intraoperative blood pressure and body temperature for signs of autonomic derailment.

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BACKGROUND

Methylene blue is used in medical practice for various reasons. Recent findings point to a potential interaction with serotonin reuptake inhibitors (SRIs) that could lead to serotonergic toxicity.

OBJECTIVE

To describe the risk of serotonergic toxicity associated with the interaction between methylene blue and SRIs.

DATA SOURCES

Relevant publications were searched systematically via MEDLINE (1946 to March 21, 2013) and Embase (1974 to 2013, week 11) with the following search terms: "methylene blue", "methylthioninium", "monoamine oxidase inhibitors", "serotonin reup-take inhibitors", and "serotonin syndrome". No restrictions were applied in relation to the indication for methylene blue or the language of publication. The reference lists of identified articles were also searched.

STUDY SELECTION AND DATA EXTRACTION

Eighteen case reports and 2 case series were identified for inclusion. To date, no randomized controlled trials have been published.

DATA SYNTHESIS

The first case report indicating suspicion of an interaction between methylene blue and SRIs was published in 2003. Seventeen other case reports describing the same type of interaction have been published since then. The 2 case series provided data from about 325 parathyroidectomies in which methylene blue was used for staining. The 17 patients who experienced central nervous system toxicity were all taking SRIs in the preoperative period.

CONCLUSION

When administered in combination with SRIs, methylene blue may lead to serotonergic toxicity at doses as low as 0.7 mg/kg. Methylene blue would seem to have monoamine oxidase A inhibitory properties. Precautions should be taken to avoid this interaction. [Publisher's translation].

Targeting the serotonin pathway for the treatment of pulmonary arterial hypertension

As we uncover the complex pathophysiology underlying idiopathic and familial pulmonary arterial hypertension, multiple disease associated pathways, cell types and processes reveal links to elements of the serotonin system. Beyond the original 'serotonin hypothesis' observed with anorexigens, and the latterly demonstrated association with vascular tone and pulmonary artery smooth muscle cell proliferation, recent studies suggest links to BMPR2, PDGF and RhoK pathways, as well as an impact upon more complex lesion formation and pathologic bone marrow progenitor mobilization. Clinical experience with antagonists targeting the various elements of the serotonin pathway has been unsatisfactory, yet perhaps this is less than surprising given our expanding knowledge around serotonin production and signaling biology, which indicate opportunities for novel therapeutic options.

Potential repurposing of known drugs as potent bacterial β-glucuronidase inhibitors

The active metabolite of the chemotherapeutic irinotecan, SN-38, is detoxified through glucuronidation and then excreted into the gastrointestinal tract. Intestinal bacteria convert the glucuronidated metabolite back to the toxic SN-38 using β-glucuronidase (GUS), resulting in debilitating diarrhea. Inhibiting GUS activity may relieve this side effect of irinotecan. In this study, we sought to determine whether any known drugs have GUS inhibitory activity. We screened a library of Food and Drug Administration-approved drugs with a cell-free biochemical enzyme assay using purified bacterial GUS. After triage, five drugs were confirmed to inhibit purified bacterial GUS. Three of these were the monoamine oxidase inhibitors nialamide, isocarboxazid, and phenelzine with average IC(50) values for inhibiting GUS of 71, 128, and 2300 nM, respectively. The tricyclic antidepressant amoxapine (IC(50) = 388 nM) and the antimalarial mefloquine (IC(50) = 1.2 µM) also had activity. Nialamide, isocarboxazid, and amoxapine had no significant activity against purified mammalian GUS but showed potent activity for inhibiting endogenous GUS activity in a cell-based assay using living intact Escherichia coli with average IC(50) values of 17, 336, and 119 nM, respectively. Thus, nialamide, isocarboxazid, and amoxapine have potential to be repurposed as therapeutics to reduce diarrhea associated with irinotecan chemotherapy and warrant further investigation for this use.

Mitochondrial monoamine oxidase-A-mediated hydrogen peroxide generation enhances 5-hydroxytryptamine-induced contraction of rat basilar artery

BACKGROUND AND PURPOSE

We evaluated the role(s) of monoamine oxidase (MAO)-mediated H₂O₂ generation on 5-hydroxytryptamine (5-HT)-induced tension development of isolated basilar artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats.

EXPERIMENTAL APPROACH

Basilar artery (endothelium-denuded) was isolated for tension measurement and Western blots. Enzymically dissociated single myocytes from basilar arteries were used for patch-clamp electrophysiological and confocal microscopic studies.

KEY RESULTS

Under resting tension, 5-HT elicited a concentration-dependent tension development with a greater sensitivity (with unchanged maximum tension development) in SHR compared with WKY (EC(50) : 28.4 ± 4.1 nM vs. 98.2 ± 9.4 nM). The exaggerated component of 5-HT-induced tension development in SHR was eradicated by polyethylene glycol-catalase, clorgyline and citalopram whereas exogenously applied H₂O₂ enhanced the 5-HT-elicited tension development in WKY. A greater protein expression of MAO-A was detected in basilar arteries from SHR than in those from WKY. In single myocytes and the entire basilar artery, 5-HT generated (clorgyline-sensitive) a greater amount of H₂O₂ in SHR compared with WKY. Whole-cell iberiotoxin-sensitive Ca(2+) -activated K(+) (BK(Ca) ) amplitude measured in myocytes of SHR was approximately threefold greater than that in WKY (at +60 mV: 7.61 ± 0.89 pA·pF(-1) vs. 2.61 ± 0.66 pA·pF(-1) ). In SHR myocytes, 5-HT caused a greater inhibition (clorgyline-, polyethylene glycol-catalase- and reduced glutathione-sensitive) of BK(Ca) amplitude than in those from WKY.

CONCLUSIONS AND IMPLICATIONS

5-HT caused an increased generation of mitochondrial H₂O₂ via MAO-A-mediated 5-HT metabolism, which caused a greater inhibition of BK(Ca) gating in basilar artery myocytes, leading to exaggerated basilar artery tension development in SHR.

Risk of severe serotonin toxicity following co-administration of methylene blue and serotonin reuptake inhibitors: an update on a case report of post-operative delirium

In a previous case report, published in this journal, we described a postoperative delirium in a patient during recovery from parathyroidectomy. We noted that the delirium resembled serotonin toxicity and that the patient had been taking paroxetine until 2 days before surgery. We offered several tentative explanations for this event, including an adverse interaction between paroxetine and other agent(s) used in the course of the anaesthesia. Recent developments in characterisation of serotonin toxicity have prompted us to re-examine the clinical details surrounding this life-threatening event. It is now known to be important that the patient was given methylene blue, pre-operatively, to enable visualisation of the parathyroid glands. Methylene blue has been found to be a potent inhibitor of monoamine oxidase (MAO), and several cases of serotonin toxicity have been reported recently following its administration. All these cases are consistent with the well-known risk of serotonin toxicity when drugs that augment serotonergic transmission are given in combination with an MAO inhibitor. Methylene blue is used in a variety of surgical settings as well as for treatment of various types of hypotensive shock and methemoglobinaemia. It is also being studied for treatment of Alzheimer's disease and malaria. In this paper, we outline the pharmacology of methylene blue and the aetiology of serotonin toxicity to help prevent further unintentional co-administration of drugs that risk precipitating this life-threatening drug interaction.

Inhibition of monoamine oxidase activity by cannabinoids

Brain monoamines are involved in many of the same processes affected by neuropsychiatric disorders and psychotropic drugs, including cannabinoids. This study investigated in vitro effects of cannabinoids on the activity of monoamine oxidase (MAO), the enzyme responsible for metabolism of monoamine neurotransmitters and affecting brain development and function. The effects of the phytocannabinoid Delta(9)-tetrahydrocannabinol (THC), the endocannabinoid anandamide (N-arachidonoylethanolamide [AEA]), and the synthetic cannabinoid receptor agonist WIN 55,212-2 (WIN) on the activity of MAO were measured in a crude mitochondrial fraction isolated from pig brain cortex. Monoamine oxidase activity was inhibited by the cannabinoids; however, higher half maximal inhibitory concentrations (IC(50)) of cannabinoids were required compared to the known MAO inhibitor iproniazid. The IC(50) was 24.7 micromol/l for THC, 751 micromol/l for AEA, and 17.9 micromol/l for WIN when serotonin was used as substrate (MAO-A), and 22.6 micromol/l for THC, 1,668 micromol/l for AEA, and 21.2 micromol/l for WIN when phenylethylamine was used as substrate (MAO-B). The inhibition of MAOs by THC was noncompetitive. N-Arachidonoylethanolamide was a competitive inhibitor of MAO-A and a noncompetitive inhibitor of MAO-B. WIN was a noncompetitive inhibitor of MAO-A and an uncompetitive inhibitor of MAO-B. Monoamine oxidase activity is affected by cannabinoids at relatively high drug concentrations, and this effect is inhibitory. Decrease of MAO activity may play a role in some effects of cannabinoids on serotonergic, noradrenergic, and dopaminergic neurotransmission.

Serotonin produces monoamine oxidase-dependent oxidative stress in human heart valves

Heart valve disease and pulmonary hypertension, in patients with carcinoid tumors and people who used the fenfluramine-phentermine combination for weight control, have been associated with high levels of serotonin in blood. The mechanism by which serotonin induces valvular changes is not well understood. We recently reported that increased oxidative stress is associated with valvular changes in aortic valve stenosis in humans and mice. In this study, we tested the hypothesis that serotonin induces oxidative stress in human heart valves, and examined mechanisms by which serotonin may increase reactive oxygen species. Superoxide (O2*.-) was measured in heart valves from explanted human hearts that were not used for transplantation. (O2*.-) levels (lucigenin-enhanced chemoluminescence) were increased in homogenates of cardiac valves and blood vessels after incubation with serotonin. A nonspecific inhibitor of flavin-oxidases (diphenyliodonium), or inhibitors of monoamine oxidase [MAO (tranylcypromine and clorgyline)], prevented the serotonin-induced increase in (O2*.-). Dopamine, another MAO substrate that is increased in patients with carcinoid syndrome, also increased (O2*.-) levels in heart valves, and this effect was attenuated by clorgyline. Apocynin [an inhibitor of NAD(P)H oxidase] did not prevent increases in (O2*.-) during serotonin treatment. Addition of serotonin to recombinant human MAO-A generated (O2*.-), and this effect was prevented by an MAO inhibitor. In conclusion, we have identified a novel mechanism whereby MAO-A can contribute to increased oxidative stress in human heart valves and pulmonary artery exposed to serotonin and dopamine.

Role of the serotonergic system in reduced pulmonary function after exposure to methamphetamine

Although use of methamphetamine (MA) by smoking is the fastest growing method of administration, very limited data are available describing the effects of smoked MA. Using a murine inhalation exposure system, we explored the pulmonary effects of low-dose acute inhalation exposure to MA vapor (smoke). Inhalation of MA vapor resulted in transiently reduced pulmonary function, as measured by transpulmonary resistance, dynamic compliance, and whole-body plethysmography compared with unexposed control animals. These changes were associated with an approximately 34% reduction in serotonin (5-hydroxytryptamine [5-HT]) metabolism/inactivation to 5-hydroxyindolacetic acid, and a nearly 40% reduction in monoamine oxidase (MAO)-A activity in the lung. Pretreatment of mice with a selective 5-HT reuptake inhibitor completely ablated the MA-induced changes in pulmonary function, confirming a key role for the 5-HT transporter (serotonin transporter [SERT]) and the serotonergic system in this effect. Immunofluorescent staining of mouse lung tissue confirmed high expression of SERT in airway epithelial cells. Using mouse airway epithelial cell line, LA-4, and purified human MAO-A, it was demonstrated that MA impedes 5-HT metabolism through direct inhibition of MAO-A activity in vitro. Together, these data demonstrate that low-dose exposure to MA results in reduced pulmonary function mediated via SERT and subsequent perturbation of 5-HT metabolism in the lung. This supports a role for the serotonergic system in MA-mediated pulmonary effects.

Involvement of nicotinic receptors in methamphetamine- and MDMA-induced neurotoxicity: pharmacological implications

During the last years, we have focused on the study of the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) on the central nervous system (CNS) and their pharmacological prevention methods. In the process of this research, we have used a semipurified synaptosomal preparation from striatum of mice or rats as a reliable in vitro model to study reactive oxygen species (ROS) production by these amphetamine derivatives, which is well-correlated with their dopaminergic injury in in vivo models. Using this preparation, we have demonstrated that blockade of alpha7 nicotinic receptors with methyllycaconitine (MLA) prevents ROS production induced by MDMA and METH. Consequently, in vivo, MLA significantly prevents MDMA- and METH-induced neurotoxicity at dopaminergic level (mouse striatum), without affecting hyperthermia induced by these amphetamines. Additionally, when neuroprotection was assayed with memantine (MEM), a dual antagonist of NMDA and alpha7 receptors, an effective neuroprotection was obtained also ahead of serotonergic injury induced by MDMA in rats. MEM also prevents MDMA effect on serotonin transporter functionality and METH effect on dopamine transporter (DAT), suggesting that behavioral effects of these psychostimulants can also be modulated by MEM. Finally, we have demonstrated that MEM prevents the impaired memory function induced by MDMA, and also, using binding studies with radioligands, we have characterized the interaction of these substances with nicotinic receptors. Studies at molecular level showed that both MDMA and METH displaced competitively the binding of radioligands with homomeric alpha7 and heteromeric nicotinic acetylcholine receptors (nAChRs), indicating that they can directly interact with them. In all the cases, MDMA displayed higher affinity than METH and it was higher for heteromeric than for alpha7 subtype. Pre-incubation of differentiated PC12 cells with MDMA or METH induces nAChR upregulation in a concentration- and time-dependent manner, as many nicotinic ligands do, supporting their functional interaction with nAChRs. Such interaction expands the pharmacological profile of amphetamines and can account for some of their effects.

Pharmacology of stimulants prohibited by the World Anti-Doping Agency (WADA)

This review examines the pharmacology of stimulants prohibited by the World Anti-Doping Agency (WADA). Stimulants that increase alertness/reduce fatigue or activate the cardiovascular system can include drugs like ephedrine available in many over-the-counter medicines. Others such as amphetamines, cocaine and hallucinogenic drugs, available on prescription or illegally, can modify mood. A total of 62 stimulants (61 chemical entities) are listed in the WADA List, prohibited in competition. Athletes may have stimulants in their body for one of three main reasons: inadvertent consumption in a propriety medicine; deliberate consumption for misuse as a recreational drug and deliberate consumption to enhance performance. The majority of stimulants on the list act on the monoaminergic systems: adrenergic (sympathetic, transmitter noradrenaline), dopaminergic (transmitter dopamine) and serotonergic (transmitter serotonin, 5-HT). Sympathomimetic describes agents, which mimic sympathetic responses, and dopaminomimetic and serotoninomimetic can be used to describe actions on the dopamine and serotonin systems. However, many agents act to mimic more than one of these monoamines, so that a collective term of monoaminomimetic may be useful. Monoaminomimietic actions of stimulants can include blockade of re-uptake of neurotransmitter, indirect release of neurotransmitter, direct activation of monoaminergic receptors. Many of the stimulants are amphetamines or amphetamine derivatives, including agents with abuse potential as recreational drugs. A number of agents are metabolized to amphetamine or metamphetamine. In addition to the monoaminomimetic agents, a small number of agents with different modes of action are on the list. A number of commonly used stimulants are not considered as Prohibited Substances.

Different oxidative profile and nicotinic receptor interaction of amphetamine and 3,4-methylenedioxy-methamphetamine

d-Amphetamine (AMPH) and MDMA increased intracellular production of reactive oxygen species (ROS) in isolated mouse striatal synaptosomes. MDMA showed a maximal oxidative effect at 50-100 microM. However, for AMPH a double maximum was obtained, the first between 0.1 and 1 microM and the second at 1mM. No oxidative effect was present in synaptosomes from reserpinized mice. Cocaine and l-deprenyl inhibited MDMA and AMPH (0.1 microM) ROS production but not that of AMPH at a higher concentration (1mM). When this high concentration was used, its oxidative effect was abolished by a phospholipase A(2) inhibitor. Delta(9)-Tetrahydrocannabinol fully prevented the oxidative effect of AMPH and MDMA, by a CB(1) receptor-independent mechanism, as did it NPC 15437 and genistein. The pro-oxidative effect induced by AMPH and MDMA showed a strong dependence on calcium (extracellular and from internal stores) and also was inhibited by nicotinic receptor (nAChR) antagonists dihydro-beta-erythroidine, methyllycaconitine (MLA) and alpha-bungarotoxin. MDMA displaced [(3)H]epibatidine and [(3)H]MLA binding with higher affinity than AMPH. Both amphetamines competitively displaced [(3)H]epibatidine from heteromeric receptors but results obtained from [(3)H]MLA binding demonstrated a non-competitive profile. Preincubation of PC12 cells with AMPH or MDMA reduced [(3)H]dopamine uptake. For MDMA, this effect was prevented by MLA. To summarize, comparing AMPH and MDMA we have demonstrated that these drugs induce an oxidative effect dependent on drug concentration and also reduce dopamine uptake. Processes that are known to affect dopamine transporter functionality also seem to modulate amphetamine derivatives-induced ROS production. For MDMA, acute effects tested are blocked by nAChR antagonists, which points to the possibility that these antagonists could be used to treat some of the adverse effects described in MDMA abusers. Conversely, no implication of nicotinic receptors has been proved for AMPH-induced effects at concentrations achievable in CNS after its administration.

Triple uptake inhibitors: therapeutic potential in depression and beyond

Drugs that interfere with the uptake and/or metabolism of biogenic amines have been used to treat depression for > 4 decades. Early medications such as tricyclic antidepressants and monoamine oxidase inhibitors are effective but possess many side effects that limit their usefulness. Selective serotonin reuptake inhibitors (SSRIs) or selective noradrenaline reuptake inhibitors (SNRIs) are the results of rational design to find drugs that are as effective as the tricyclic antidepressants, but with more selectivity towards a single monoamine transporter. The SSRI class of drugs, which includes fluoxetine, paroxetine and sertraline, were previously viewed as the agents of choice for treating major depression. Recently, inhibitors of both serotonin and noradrenaline uptake ('dual uptake inhibitors'; SSRI/SNRI such as venlafaxine, duloxetine and milnacipran) have gained acceptance in the market. However, neither the SSRIs nor the SSRI/SNRI are fully satisfactory due to a delayed onset of action, low rate of response and side effect that can affect compliance. An important recent development has been the emergence of the triple uptake inhibitors (SSRI/SNRI/selective dopamine reuptake inhibitor), which inhibit the uptake of all three neurotransmitters that are most closely linked to depression: serotonin, noradrenaline and dopamine. Preclinical studies and clinical trials indicate that a drug inhibiting the uptake of all three of these neurotransmitters could produce a more rapid onset of action and possess greater efficacy than traditional antidepressants. This review discusses the evolution of biogenic amine-based therapies, the emerging strategies involved in the design and synthesis of novel triple uptake inhibitors as antidepressants and the therapeutic potential of triple uptake inhibitors.

Comparison of combinations of drugs for treatment of obesity: body weight and echocardiographic status

BACKGROUND

Obesity treatment with single drugs produces weight losses of about 8-10% of initial body weight. Few studies of combinations of drugs for treating obesity have been published. The combination of phentermine, an adrenergic agent, and fenfluramine, a serotonergic agent, (phen-fen) produced weight losses of about 15% of initial body weight. Fenfluramine is no longer available because it was associated with cardiac valve lesions. Phentermine-fluoxetine (phen-flu) has been proposed as an alternative for phen-fen.

OBJECTIVE

To compare the efficacy of treatment and prevalence of cardiac valve abnormalities on phen-flu vs phen-fen.

DESIGN

Retrospective chart review of all patients treated for at least 3 months with phen-flu (N=97) to a random sample of patients treated with phen-fen (N=98) in the Clinical Nutrition Clinic at the University of Wisconsin. Comparison of echocardiograms in all patients treated solely with phen-flu (N=21) to a random sample of patients treated with phen-fen (N=47), and to a group of subjects never treated with obesity drugs (N=26).

RESULTS

With last observation carried forward analysis (LOCF), at 6 months of treatment the phen-fen patients lost 12.6+/-0.6% of baseline weight and phen-flu patients lost 9.0+/-0.6% (P<0.001). With completers analysis, there were no significant differences in weight loss as a percent of baseline weight at 6 months (14.4+/-0.6 vs 13.3+/-0.9%). LOCF decreases in body mass index (BMI) at 6 months were -5.3 and -3.6 kg/m(2) for phen-fen and phen-flu, respectively (P<0.001), and 6.2+/-0.3 vs 5.4+/-0.4 kg/m(2), respectively, for the completers analysis (P - NS). Dropout rate at 6 months was higher in phen-flu subjects (44 vs 28%). In subjects without atherosclerosis of valves (presumably pre-existing), cardiac valve lesions occurred in eight of 38 phen-fen subjects and in none of 15 phen-flu subjects or 25 control subjects who had not been treated with drugs.

CONCLUSIONS

The combination of phentermine and fluoxetine was not as effective as phen-fen, but was not associated with cardiac valve lesions. Longer term, larger scale studies of phen-flu are warranted.

Amphetamine Analogs Increase Plasma Serotonin: Implications for Cardiac and Pulmonary Disease

Elevations in plasma serotonin (5-HT) have been implicated in the pathogenesis of cardiac and pulmonary disease. Normally, plasma 5-HT concentrations are kept low by transporter-mediated uptake of 5-HT into platelets and by metabolism to 5-hydroxyindoleacetic acid (5-HIAA). Many abused drugs (e.g., substituted amphetamines) and prescribed medications (e.g., fluoxetine) target 5-HT transporters and could thereby influence circulating 5-HT. We evaluated the effects of amphetamines analogs [(+/-)-fenfluramine, (+/-)-3,4-methylenedioxymethamphetamine, (+)-methamphetamine, (+)-amphetamine, phentermine] on extracellular levels (i.e., plasma levels) of 5-HT and 5-HIAA in blood from catheterized rats. Effects of the 5-HT uptake blocker fluoxetine were examined for comparison. Drugs were tested in vivo and in vitro; plasma indoles were measured using a novel microdialysis method in whole blood. We found that baseline dialysate levels of 5-HT are approximately 0.22 nM, and amphetamine analogs evoke large dose-dependent increases in plasma 5-HT ranging from 4 to 20 nM. The ability of drugs to elevate plasma 5-HT is positively correlated with their potency as 5-HT transporter substrates. Fluoxetine produced small, but significant, increases in plasma 5-HT. Although the drug-evoked 5-HT concentrations are below the micromolar levels required for contraction of pulmonary arteries, they approach concentrations reported to stimulate mitogenesis in pulmonary artery smooth muscle cells. Additional studies are needed to determine the effects of chronic administration of amphetamines on circulating 5-HT.

Translational neuroimaging: positron emission tomography studies of monoamine oxidase

Positron emission tomography (PET) using radiotracers with high molecular specificity is an important scientific tool in studies of monoamine oxidase (MAO), an important enzyme in the regulation of the neurotransmitters dopamine, norepinephrine, and serotonin as well as the dietary amine, tyramine. MAO occurs in two different subtypes, MAO A and MAO B, which have different substrate and inhibitor specificity and which are different gene products. The highly variable subtype distribution with different species makes human studies of special value. MAO A and B can be imaged in the human brain and certain peripheral organs using PET and carbon-11 (half-life 20.4 minutes) labeled mechanism-based irreversible inhibitors, clorgyline and L -deprenyl, respectively. In this article we introduce MAO and describe the development of these radiotracers and their translation from preclinical studies to the investigation of variables affecting MAO in the human brain and peripheral organs.

Alterations in alcohol consumption, withdrawal seizures, and monoamine transmission in rats treated with phentermine and 5-hydroxy-L-tryptophan

We have previously shown that coadministration of the dopamine (DA) agonist phentermine plus the serotonergic agonist fenfluramine suppresses alcohol intake and withdrawal seizures in rats. In the present study, phentermine and the serotonin (5-HT) precursor, 5-hydroxy-L-tryptophan (5-HTP), were administered alone, or in combination, to rats fed on a 6% alcohol-containing diet or an isocaloric control diet. Following a 9-h withdrawal period from the alcohol-containing diet, phentermine enhanced the effects of 5-HTP on both reduction of alcohol withdrawal seizures as well as changes in striatal serotonin. Food intake was monitored for 24 h after drug treatment, and neurochemical measures were examined at various time points. Phentermine alone reduced food intake in all diet conditions, but this anorectic effect was followed by hyperphagia in control rats. Phentermine plus 5-HTP reduced the consumption of the alcohol-containing diet, while its effects on consumption of control diets were mixed. In vivo microdialysis in rat nucleus accumbens revealed that phentermine increased extracellular DA, whereas 5-HTP caused marked elevations in extracellular 5-HT. Coadministration of phentermine and 5-HTP evoked simultaneous elevations in extracellular DA and 5-HT that mirrored the effects of each drug alone. Collectively, these findings show that coadministered phentermine plus 5-HTP is effective in reducing alcohol intake and suppressing alcohol withdrawal seizures. These therapeutic actions may be related to elevations in synaptic DA and 5-HT in critical brain regions.

Noradrenergic and dopaminergic effects of (+)-amphetamine-like stimulants in the baboonPapio anubis

(+)-Amphetamine, (+/-)-ephedrine, and phentermine are commonly used appetite suppressants that release monoamines from nerve cells by acting as substrates for biogenic amine transporters. One key difference among the three drugs is their selectivity for norepinephrine (NE) release vs. dopamine (DA) release. The NE/DA selectivity ratios for these drugs as determined in vitro [(EC50 NE(-1))/(EC50 DA(-1))] are (+/-)-ephedrine (18.6) > phentermine (6.7) > (+)-amphetamine (3.5). The in vitro data suggest that when administered in vivo, these stimulants might differ in their ability to release DA from nerve terminals in the brain. To test this hypothesis, noradrenergic effects (i.e., plasma NE) and dopaminergic effects (i.e., central DA release) were assessed when each drug was administered intravenously (1.5 mg/kg) to anesthetized baboons. Central DA release was determined via positron emission tomography using the method of [11C]raclopride displacement. In the present investigation, high doses of these stimulants increased plasma NE and DA in parallel, but only (+)-amphetamine released central DA from neurons and decreased plasma prolactin. None of the drugs altered plasma amine metabolite levels, indicating no inhibition of monoamine oxidase activity at the administered doses. Plasma drug levels measured in baboons were higher than those measured in human patients taking prescribed doses of the drugs. Viewed collectively, the present data indicate that typical clinical doses of phentermine and (+/-)-ephedrine may not release central DA in humans, a hypothesis that should ultimately be tested in controlled clinical studies.

5-Hydroxy-L-tryptophan suppresses food intake in food-deprived and stressed rats

Giving L-tryptophan, serotonin's circulating precursor, or a serotonin-releasing drug can decrease food intake and body weight. Giving 5-hydroxy-L-tryptophan (5-HTP), serotonin's immediate intracellular precursor, has been thought to be ineffective in enhancing brain serotonin synthesis unless it is coadministered with a dopa decarboxylase inhibitor to protect 5-HTP from destruction outside the brain. We have examined the effect of 5-HTP on food consumption and tissue 5-HTP levels among rats subjected to two different hyperphagic stimuli, food deprivation and a standardized stress (tail pinch), and on plasma 5-HTP levels in humans. In rats, 5-HTP (3-200 mg/kg ip) suppressed food intake in a dose-dependent manner in both models, but was at least eight times more effective in our stress-hyperphagia model. (Differences in the two procedures might have contributed to the observed differences in potencies.) This suppression was blocked by coadministration of another large neutral amino acid (LNAA), L-valine. Brain 5-HTP levels correlated significantly with peak plasma 5-HTP (r(2)=.69) or 5-HTP/LNAA (r(2)=.81) levels. Additionally, among humans, oral 5-HTP (1.2-2.0 mg/kg) produced, after 1 and 2 h, a significant increase in plasma 5-HTP (1.5- to 2.3-fold). These observations suggest that 5-HTP may be useful in controlling the excessive food intake sometimes generated by stress, even if given without decarboxylase inhibitors or other drugs.

Immobilized enzyme reactors based upon the flavoenzymes monoamine oxidase A and B

Monoamine oxidase (MAO) catalyzes the oxidative deamination of amines. The enzyme exists in two forms, MAO-A and MAO-B, which differ in substrate specificity and sensitivity to various inhibitors. Membrane fractions containing either expressed MAO-A or MAO-B have been non-covalently immobilized in the hydrophobic interface of an immobilized artificial membrane (IAM) liquid chromatographic stationary phase. The MAO-containing stationary phases were packed into glass columns to create on-line immobilized enzyme reactors (IMERs) that retained the enzymatic activity of the MAO. The resulting MAO-IMERs were coupled through a switching valve to analytical high performance liquid chromatographic columns. The multi-dimensional chromatographic system was used to characterize the MAO-A (MAO-A-IMER) and MAO-B (MAO-B-IMER) forms of the enzyme including the enzyme kinetic constants associated with enzyme/substrate and enzyme/inhibitor interactions as well as the determination of IC(50) values. The results of the study demonstrate that the MAO-A-IMER and the MAO-B-IMER can be used for the on-line screening of substances for MAO-A and MAO-B substrate/inhibitor properties.

Monoamine oxidase A imaging in peripheral organs in healthy human subjects

Monoamine oxidase (MAO) catalyzes the oxidative deamination of many biogenic and dietary amines. Though studies of MAO have focused mainly on its regulatory role in the brain, MAO in peripheral organs also represents a vast mechanism for detoxifying vasoactive compounds as well as for terminating the action of physiologically active amines, which can cross the blood brain barrier. Indeed, robust central and peripheral MAO activity is a major requirement in the safe use of many CNS drugs, particularly antidepressants, and thus an awareness of the MAO inhibitory potential of drugs is essential in therapeutics. In this study, we examined the feasibility of quantifying MAO A in peripheral organs in healthy human subjects using comparative positron emission tomography (PET) imaging with carbon-11 (t(1/2): 20.4 min) labeled clorgyline ([(11)C]clorgyline) a suicide inactivator of MAO A and its deuterium labeled counterpart ([(11)C]clorgyline-D2). Heart, lungs, kidneys, thyroid, and spleen showed a robust deuterium isotope effect characteristic of MAO and the magnitude of the effect was similar to that of trancylcypromine, an irreversible MAO inhibitor used in the treatment of depression. Liver time-activity curves were not affected by deuterium substitution precluding the estimation of liver MAO in vivo. In organs showing an isotope effect, MAO A is greatest in the lungs and kidneys followed by the thyroid and heart. This method, which has been previously applied in the human brain, opens the possibility to also directly assess the effects of different variables including smoking, dietary substances, drugs, disease, and genetics on peripheral MAO A in humans.

Liquid chromatography studies on the pharmacokinetics of phentermine and fenfluramine in brain and blood microdialysates after intraperitoneal administration to rats

A highly sensitive and simple HPLC method with fluorescence detection for the determination of phentermine (Phen), fenfluramine (Fen) and norfenfluramine (Norf, the active metabolite of Fen) in rat brain and blood microdialysates has been developed. The brain and blood microdialysates were directly subjected to derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride (DIB-Cl) in the presence of carbonate buffer (0.1 M, pH 9.0) at room temperature. The chromatographic conditions consisted of an ODS column and mobile phase composition of acetonitrile and water (65:35, v/v) with flow rate set at 1.0 ml/min. The detection was performed at excitation and emission wavelengths of 325 and 430 nm, respectively. Under these conditions, the DIB-derivatives of Phen, Fen and Norf were well separated and showed good linearities in the studied ranges (5-2000 nM for Phen and 10-2000 nM for Norf and Fen) with correlation coefficients greater than 0.999. The obtained detection limits were less than 23 fmol on column (for the three compounds) in both brain and blood microdialysates at a signal-to-noise ratio of 3 (S/N=3). The intra- and the inter-assay precisions were lower than 10%. The method coupled with microdialysis was applied for a pharmacokinetic drug-drug interaction study of Phen and Fen following individual and combined intraperitoneal administration to rats. In addition, since the role of protein binding in drug interactions can be quite involved, the method was applied for the determination of total and free Phen and Fen in rat plasma and ultrafiltrate, respectively. The results showed that Fen and/or Norf significantly altered the pharmacokinetic parameters of Phen in both blood and brain but did not alter its protein binding. On the other hand, there was no significant difference in the pharmacokinetics of Fen when administered with Phen.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Effects on serotonin in rat hypothalamus of D-fenfluramine, aminorex, phentermine and fluoxetine

Hypothalamic 5-HT (serotonin) regulates food intake, energy expenditure and bodyweight. Using in vivo microdialysis, we determined the effects of various anorectic drugs on hypothalamic extracellular 5-HT levels during the dark phase when rats predominantly feed. Phentermine and aminorex, which were originally considered to be catecholaminergic drugs, markedly increased 5-HT efflux in rat hypothalamus. Their actions were less profound than D-fenfluramine, but considerably greater than that of the selective 5-HT reuptake inhibitor, fluoxetine. This suggests that enhanced hypothalamic 5-HT function could be involved in their anorectic actions. Pharmacological characterization revealed that D-fenfluramine, aminorex and probably also phentermine potentiate synaptic 5-HT function predominantly by release, whereas fluoxetine acts exclusively by reuptake inhibition. The results also revealed that the combined actions of phentermine and D-fenfluramine on hypothalamic extracellular 5-HT levels were additive, but not synergistic. In contrast, there was a significant negative cooperative effect on extraneuronal 5-HT of combining phentermine with fluoxetine.

Phentermine inhibition of recombinant human liver monoamine oxidases A and B

Recent studies with rat tissue preparations have suggested that the anorectic drug phentermine inhibits serotonin degradation by inhibition of monoamine oxidase (MAO) A with a K(I) value of 85-88 microM, a potency suggested to be similar to that of other reversible MAO inhibitors (Ulus et al., Biochem Pharmacol 2000;59:1611-21). Since there are known differences between rats and humans in substrate and inhibitor specificities of MAOs, the interactions of phentermine with recombinant human purified preparations of MAO A and MAO B were determined. Human MAO A was competitively inhibited by phentermine with a K(I) value of 498+/-60 microM, a value approximately 6-fold weaker than that observed for the rat enzyme. Phentermine was also observed to be a competitive inhibitor of recombinant human liver MAO B with a K(I) value of 375+/-42 microM, a value similar to that observed with the rat enzyme (310-416 microM). In contrast to the behavior with rat tissue preparations, no slow time-dependent behavior was observed for phentermine inhibition of purified soluble human MAO preparations. Difference absorption spectral studies showed similar perturbations of the covalent FAD moieties of both human MAO A and MAO B, which suggests a similar mode of binding in both enzymes. These data suggest that phentermine inhibition of human MAO A (or of MAO B) is too weak to be of pharmacological relevance.

Serotonin syndrome presenting as hypotonic coma and apnea: potentially fatal complications of selective serotonin receptor inhibitor therapy

OBJECTIVE

To describe a patient who developed serotonin syndrome on four separate occasions as a result of monotherapy with two different selective serotonin receptor inhibitors (fluoxetine and cetalopram).

DESIGN

Case report.

SETTING

Community hospital.

PATIENTS

Single patient with four episodes of serotonin syndrome.

MEASUREMENTS AND MAIN RESULTS

The syndrome was characterized by coma/unresponsiveness (four episodes), dilated pupils (four episodes), salivation (two episodes), dryness of mouth (two episodes), myoclonus like activity of eyelids (four episodes), oculogyric crisis (four episodes), flaccid paralysis of all extremities (four episodes), tremors (two episodes), apnea (two episodes), restlessness (one episode). Recovery occurred within 24 hrs, although muscle pain and weakness persisted for 2 months after stopping fluoxetine. Apnea occurred in both episodes associated with fluoxetine therapy.

CONCLUSION

Apnea and coma may occur in serotonin syndrome.

Additive effects on rat brain 5HT release of combining phentermine with dexfenfluramine

OBJECTIVE AND DESIGN

This study examined the effects of the anti-obesity agents, phentermine and dexfenfluramine given alone or in combination, on in vitro and in vivo 5HT release from rat brain tissue.

RESULTS

In vitro, phentermine was without effect on basal [3H]5HT efflux from hypothalamic slices whereas dexfenfluramine (10 microM) evoked a 131% increase in [3H]5HT release. In combination, the two drugs did not alter [3H]5HT release beyond that caused by dexfenfluramine alone. At pharmacologically equivalent doses, phentermine (5.7 mg/kg, i.p.) caused a rapid, modest elevation, and dexfenfluramine (3 mg/kg, i.p.) a larger but equally rapid elevation of extracellular 5HT in the microdialysates from the rat anterior hypothalamus. In combination, the increase in extracellular 5HT evoked by these drugs was not significantly greater than the sum of their individual effects.

CONCLUSIONS

This study provides evidence that phentermine's actions are not restricted to catecholamine systems and indicates that combining phentermine with dexfenfluramine results in an additive increase in neuronal 5HT release.

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.