Effects of Phentermine and Phenformin on Biomarkers of Aging in Rats

BACKGROUND

Caloric restriction (CR) is the only treatment known to substantially prolong both average and maximal life span in experimental animals. Interventions that mimic certain effects of CR could be potential anti-aging treatments in humans. Drugs which reduce appetite (anorexiants) represent one class of candidate treatments. Agents that reduce the glucose utilization by the organism could also represent another class of candidate CR mimetics.

OBJECTIVE

In our study, we addressed the following questions: (1) Does treatment with an anorexiant reduce caloric intake and body weight of experimental animals comparable to that caused by CR? (2) Does treatment with an antidiabetic agent influence caloric intake and body weight? (3) Does treatment with any of these drugs affect metabolic parameters of an organism in the way similar to that observed with CR?

METHODS

One hundred and twenty 6-month-old female Wistar-derived LIO rats were randomly subdivided into four groups and exposed to: (1) ad libitum feeding with placebo (controls); (2) the antidiabetic drug phenformin (2 mg/kg); (3) the anorectic drug phentermine (1 mg/kg), and (4) the same amount of food as the group with the least food intake during the previous week (pair-fed controls). Food and water intake, body weight, and rectal temperature were measured weekly during 16 weeks. At the end of the 16th week of the experiment, serum levels of glucose, total beta-lipoprotein and pre-beta-lipoprotein fractions, cholesterol, triglycerides, insulin, total triiodothyronine, and free thyroxine were estimated. The contents of diene conjugates and Schiff's bases, total antioxidant activity, the activities of Cu/Zn superoxide dismutase, glutathione S-transferase, and glutathione peroxidase, and the generation of reactive oxygen species (ROS) were studied in brain and liver homogenates and in the serum.

RESULTS

The controls exposed to pair feeding had a significantly reduced food consumption (about 20%) as compared with the ad libitum fed controls and thus exhibited a moderate CR. Treatment with phentermine reduced the caloric intake by about 12% as compared with the ad libitum fed controls. Body weight and water intake in this group were only slightly decreased (by about 2 and 5%, respectively) as compared with the controls. The mean rectal temperature in the phentermine group (38 degrees C) was significantly higher than in the ad libitum fed (37.8 degrees C) and pair-fed (37.6 degrees C) controls. Treatment with phentermine also resulted in significantly reduced ROS levels in all tissues studied, while the highest ROS production was found in ad libitum (blood serum) and pair-fed (brain) controls. Treatment with phenformin did not significantly influence food and water consumption, body weight, and temperature when compared with the ad libitum fed controls. Rats treated with this antidiabetic drug showed intermediate values of ROS generation. Differences among the groups in total antioxidant activity were not obvious.

CONCLUSIONS

Treatment with phentermine reduces caloric intake slightly less than is commonly observed in CR studies. CR due to forcibly reduced feeding and CR due to substance-suppressed appetite appear to act through different metabolic mechanisms and thus may affect aging and longevity in different ways.

The effect of the anorectic agent, d-fenfluramine, and its primary metabolite, d-norfenfluramine, on intact human platelet serotonin uptake and efflux

Dexfenfluramine, a drug formerly prescribed for treatment of obesity, caused heart valve damage and pulmonary hypertension in some people. The cause of the toxicity has not been defined, but 5-HT has been implicated. The objective of this study was to evaluate the effect of the anorectic agent, d-fenfluramine, and its major metabolite, d-norfenfluramine, on intact human platelet serotonin (5-HT) transport in vitro. The effects of d-fenfluramine and d-norfenfluramine on platelet uptake and efflux of 3H-5-HT were measured in buffer at pH 6.7, to optimize serotonin transporter (SERT) function, and at pH 7.4. Uptake of 3H-5-HT at pH 6.7 and 7.4 was inhibited by both agents at micro m concentrations (IC50, d-fenfluramine approximately 3 microM; d-norfenfluramine approximately 10 microM). However, no efflux of 3H-5-HT from labeled platelets at either pH 6.7 or 7.4 occurred at similar concentrations of d-fenfluramine or d-norfenfluramine. With inhibition of platelet dense granule 3H-5-HT uptake by reserpine, efflux of 3H-5-HT was observed at pH 6, but not at pH 7.4. Fluoxetine, a SERT inhibitor, was a more potent inhibitor of uptake (IC50 0.05 microM) than d-fenfluramine, but the anorectic agent, phentermine, had no effect. Therefore, d-fenfluramine and d-norfenfluramine inhibit human platelet uptake of 5-HT in vitro at tissue concentrations attainable in vivo, but they do not stimulate 5-HT efflux due to dense granule sequestration. Inhibition of platelet 5-HT uptake may play a role in the cardiopulmonary toxicity of d-fenfluramine, but other factors probably contribute, since similar toxicity has not been observed with fluoxetine.

Effects of preexposure to dexfenfluramine, phentermine, dexfenfluramine-phentermine, or fluoxetine on sibutramine-induced hypophagia in the adult rat

The antiobesity drug sibutramine suppresses food intake via inhibition of reuptake of both norepinephrine (NE) and serotonin (5-HT) into brain terminals. The present study examined whether preexposure to other antiobesity drugs (fluoxetine [FLUOX], phentermine [PHEN], and dexfenfluramine [DEX]) that alter noradrenergic and/or serotonergic activity in brain induces tolerance or sensitization to the subsequent hypophagic action of sibutramine. Accordingly, adult male rats were treated (administered orally once per day for 21 days) with DEX (0, 1, or 3 mg/kg) and/or PHEN (0, 5, or 10 mg/kg), alone and in combination, or with the selective 5-HT reuptake inhibitor FLUOX (0, 15, or 30 mg/kg). Daily administration of PHEN persistently reduced food intake and body weight whereas tolerance developed to the hypophagic action of DEX or of FLUOX within the first week of daily administration. Moreover, low doses of DEX (1 mg/kg) and PHEN (5 mg/kg) interacted in a supra-additive manner to inhibit food intake and water intake and decrease body weight over the 21-day exposure period. After a recovery period of 9 days, a series of food intake trials were conducted to assess the hypophagic action of sibutramine (0, 1, 3, and 9 mg/kg po). Preexposure to PHEN (5 or 10 mg/kg), DEX (3 mg/kg), or FLUOX (30 mg/kg) resulted in a significant attenuation of the hypophagia induced by sibutramine over an 8-h, but not a 2-h, testing period. The pattern of cross-tolerance noted in this study is consistent with the observation that sibutramine inhibits eating via an interaction with noradrenergic and serotonergic mechanisms. Whether PHEN and DEX preexposure in humans alters subsequent sibutramine effectiveness is unknown.

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.

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.

Fluoxetine increases the anorectic and long-term dopamine-depleting effects of phentermine

The anorectic drug phentermine produces dose-related toxic effects on brain dopamine (DA) neurons in animals. Until recently, phentermine was widely used in combination with fenfluramine for purposes of appetite suppression and weight loss. With the recent withdrawal of fenfluramine from the market, many people have begun combining phentermine with fluoxetine, a serotonin reuptake inhibitor which also produces mild anorectic effects. Fluoxetine, in addition to inhibiting serotonin reuptake, inhibits hepatic mixed function oxidase, which plays an important role in the metabolic degradation of amphetamines. The purpose of the present study was to assess the effects of fluoxetine on the anorectic and DA neurotoxic effects of phentermine in mice. Phentermine, in combination with fluoxetine, produced greater reductions in food intake and body weight than phentermine alone. The phentermine/fluoxetine combination also produced greater long-term reductions in brain DA levels than phentermine alone, likely reflecting greater DA neurotoxicity of the drug combination. Brain concentrations of phentermine were also found to be higher in animals pretreated with fluoxetine. These findings indicate that fluoxetine potentiates both the anorectic and DA neurotoxic effects of phentermine, probably by increasing phentermine brain levels. The clinical significance of these findings remains to be ascertained.

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

The acute effects of systemic administration of the anti-obesity agent sibutramine on extracellular dopamine (DA) in the nucleus accumbens of freely moving rats were studied using in vivo microdialysis and compared with the actions of phentermine and d-amphetamine at doses 1x and 3x their respective 2 h ED(50) values to reduce food intake in rats. At the lower dose, sibutramine did not elevate extracellular DA concentrations; however, at the higher dose (6.0 mg kg(-1), i.p.) it caused a modest and prolonged increase in extraneuronal DA. A maximal rise was observed at 60 min post-sibutramine treatment (+231% compared to controls) with DA levels remaining elevated for up to 160 min post treatment. In contrast, phentermine and d-amphetamine significantly enhanced DA efflux at both the lower and higher doses. These elevations of DA levels were significantly greater than that seen with the corresponding dose of sibutramine over 0-80 min post treatment. Maximal rises in DA levels resulting from the higher dose of each drug were +733% (phentermine, 3.9 mg kg(-1), i.p.) and +603% (d-amphetamine, 1.5 mg kg(-1), i.p.) compared to controls 40 min post treatment. The highest doses of phentermine and d-amphetamine increased rat locomotor activity up to 100 min and 160 min post treatment, respectively, whereas the equivalent sibutramine dose had no effect. These findings therefore suggest that dopaminergic reward mechanisms are not involved in the reduction of food intake by sibutramine. Furthermore, they are consistent with the view that sibutramine lacks abuse potential.

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

Phentermine was shown in the 1970s to inhibit the metabolism of serotonin by monoamine oxidase (MAO), but never was labeled as an MAO inhibitor; hence, it was widely used in combination with fenfluramine, and continues to be used, in violation of their labels, with other serotonin uptake blockers. We examined the effects of phentermine and several other unlabeled MAO inhibitors on MAO activities in rat lung, brain, and liver, and also the interactions of such drugs when administered together. Rat tissues were assayed for MAO-A and -B, using serotonin and beta-phenylethylamine as substrates. Phentermine inhibited serotonin-metabolizing (MAO-A) activity in all three tissues with K(i) values of 85-88 microM. These potencies were similar to those of the antidepressant MAO inhibitors iproniazid and moclobemide. When phentermine was mixed with other unlabeled reversible MAO inhibitors (e.g. pseudoephedrine, ephedrine, norephedrine; estradiol benzoate), the degree of MAO inhibition was additive. The cardiac valvular lesions and primary pulmonary hypertension that have been reported to be associated with fenfluramine-phentermine use may have resulted from the intermittent concurrent blockage of both serotonin uptake and metabolism.

Effects of phentermine and fenfluramine on extracellular dopamine and serotonin in rat nucleus accumbens: therapeutic implications

Combined administration of the amphetamine analogs phentermine and fenfluramine (PHEN/FEN) has been used in the treatment of obesity. While these medications are thought to modulate monoamine transmission, the precise neurochemical effects of the PHEN/FEN mixture have not been extensively studied. To assess the mechanism of PHEN/FEN action, in vivo microdialysis studies were performed in the nucleus accumbens of conscious freely moving rats. A series of amphetamine derivatives including phentermine, chlorphentermine, fenfluramine, and PHEN/FEN (1:1 ratio), were infused locally into the accumbens via reverse-dialysis (1, 10, 100 microM) or injected systemically (1 mg/kg, ip). Dialysate samples were assayed for dopamine (DA) and serotonin (5-HT) by high-performance liquid chromatography with electrochemical detection. When infused locally, phentermine preferentially increased extracellular DA, whereas fenfluramine selectively increased extracellular 5-HT. Local administration of chlorphentermine or the PHEN/FEN mixture caused parallel elevations of both transmitters. Analogous results were obtained when the drugs were injected systemically. Phentermine stimulated robust locomotor activity in mice, whereas chlorphentermine and fenfluramine did not. PHEN/FEN caused modest locomotor stimulation after a low dose, but had no effect at the highest dose. Accumulating evidence suggests that chronic drug and alcohol abuse is associated with deficits in both DA and 5-HT neuronal function. Thus, dual activation of DA and 5-HT neurotransmission with monoamine releasing agents may be an effective treatment strategy for substance use disorders, as well as for obesity. Synapse 36:102-113, 2000. Published 2000 Wiley-Liss, Inc.

The effects of drugs used to treat obesity on the autonomic nervous system

OBJECTIVE

Body fatness is partly under hypothalamic control with effector limbs, which include the endocrine system and the autonomic nervous system (ANS). In previous studies we have shown, in both obese and never-obese subjects, that weight increase leads to increased sympathetic and decreased parasympathetic activity, whereas weight decrease leads to decreased sympathetic and increased parasympathetic activity. We now report on the involvement of such ANS mechanisms in the action of anti-obesity drugs, independent of change in weight.

RESEARCH METHODS AND PROCEDURES

Normal weight males (ages 22 to 38 years) were fed a solid food diet, carefully measured to maintain body weight, for at least 2 weeks, as inpatients at the Rockefeller University General Clinical Research Center. In a single-blind, placebo/drug/placebo design, eight subjects received dexfenfluramine, seven phentermine (PHE), and seven sibutramine (SIB). ANS measures of parasympathetic and sympathetic activity included: determination of amount of parasympathetic control (PC) and sympathetic control (SC) of heart period (interbeat interval), using sequential pharmacological blockade by intravenous administration of atropine and esmolol. These autonomic controls of heart period are used to estimate the overall level of parasympathetic and sympathetic activities. Norepinephrine, dopamine, and epinephrine levels in 24-hour urine collections were measured and also resting metabolic rate (RMR).

RESULTS

Sufficient food intake maintained constant body weight in all groups. PHE and SIB produced significant increases in SC but no change in PC or in RMR. In contrast, dexfenfluramine produced marked decreases in SC, PC, and RMR. For all three drugs, the effects on urine catecholamines directly paralleled changes in cardiac measures of SC.

DISCUSSION

ANS responses to PHE and SIB were anticipated. The large, and unanticipated, response to dexfenfluramine suggests further study to determine whether there could be any relation of these ANS changes to the adverse cardiovascular effects of treatment with dexfenfluramine.

Supraadditive effect of d-fenfluramine plus phentermine on extracellular acetylcholine in the nucleus accumbens: possible mechanism for inhibition of excessive feeding and drug abuse

The combination of d-fenfluramine plus phentermine (d-FEN/PHEN) provides a tool for exploring neural mechanisms that control food intake and drug abuse. Prior research suggests that dopamine (DA) in the nucleus accumbens can reinforce appetitive behavior and acetylcholine (ACh) inhibits it. When rats were given d-fenfluramine (5 mg/kg, IP) DA increased to 169% (p < 0.01), and ACh decreased slightly. Phentermine (5 mg/kg, IP) increased extracellular DA to 469% of baseline and ACh increased slightly to 124% (both p < 0.01). The d-FEN/PHEN combination, however, increased both DA and ACh with a supraadditive effect on ACh to 172%. One interpretation is that dFEN/PHEN increases DA like a meal or drug of abuse, while also increasing ACh to stop further approach behavior. This leaves the animal "satiated," as defined by reduced intake of food or drugs.

Comparison of either norepinephrine-uptake inhibitors or phentermine combined with serotonergic agents on food intake in rats

RATIONALE

We have shown previously that the anorectic effects of the catecholamine-releasing agent phentermine (PHEN) and the serotonin (5-HT)-releasing agent dexfenfluramine (DFEN) are greater than additive in rats. In the present study, we examined whether the norepinephrine-uptake inhibitors desmethylimipramine (DMI) and thionisoxetine (TNIX) have additive effects with either DFEN or with the 5-HT-uptake inhibitor fluoxetine (FLX). We also examined whether PHEN interacts with a postsynaptically acting 5-HT agonist.

METHODS

Undeprived rats were trained to eat a daily sweet-milk dessert and on test days were systemically administered single or combination drugs and the intakes recorded.

RESULTS

Both DMI and TNIX produced dose-related suppressions of food intake. However, by isobolographic analysis, they did not enhance the anorectic actions of either DFEN or FLX. In contrast, confirming and extending our previous work, PHEN had a greater potentiating effect on the anorectic actions of DFEN and FLX than TNIX. Further, the anorectic action of the 5-HT2c receptor agonist TFMPP was enhanced by PHEN.

CONCLUSIONS

These and other data are consistent with the idea that 5-HT agents may work "upstream" of critical catecholaminergic synapses in the production of anorexia, and explain the diminished efficacy of norepinephrine-uptake inhibitors relative to PHEN. The implications for clinically useful anorectic agents are discussed briefly.

Effects of phentermine and fenfluramine on alcohol consumption and alcohol withdrawal seizures in rats

The drug combination of phentermine plus fenfluramine has been used clinically in both the treatment of obesity and alcoholism. The aim of the current study was to assess the interaction of the two drugs on consumption of both an alcohol-containing and a nonalcoholic diet. Furthermore, the efficacy of the drug combination on suppression of withdrawal seizures was determined. Animals were either maintained on a 6% alcohol-containing diet, free-fed an isocaloric control, or pair-fed the control diet. It was observed that, with regard to body weight growth curves, alcohol provides about 2.5 kcal/g. Both phentermine and fenfluramine caused a decrease in consumption 1 h after administration; however, during the next 23 h, 4 mg/kg phentermine significantly increased consumption of all diets. At doses of 1 and 2 mg/kg, fenfluramine selectively reduced consumption of the alcohol-containing diet as compared to the isocaloric diets. Lower doses of fenfluramine blocked the increases in consumption induced by phentermine. Furthermore, in animals fed the nonalcoholic diet, the drug combination of 2 mg/kg fenfluramine plus 8 mg/kg phentermine produced a 63-82% reduction in consumption, an effect not seen when either drug was administered alone. This greater than additive effect was also seen in the earlier time periods in animals pair-fed the control diet. Neurochemical analysis from these animals revealed that the alcohol-dependent animals displayed a significant reduction of DOPAC and 5-HIAA levels in the striatum, frontal cortex, and hypothalamus after a 9-h withdrawal period, further implicating the serotonergic and dopaminergic systems in mediation of withdrawal symptoms and alcohol craving. Finally, 8 mg/kg phentermine plus 8 mg/kg fenfluramine completely abolished alcohol withdrawal seizures, compared to a 78% rate in saline treated rats. In conclusion, the coadministration of phentermine plus fenfluramine produced a moderate reduction of alcohol consumption and was completely effective at reducing alcohol withdrawal seizures.

Effects of phentermine on responding maintained by progressive-ratio schedules of cocaine and food delivery in rhesus monkeys

Previous reports indicate that intravenous pretreatment with phentermine can decrease cocaine-maintained responding without affecting food-reinforced responding under fixed-ratio schedules. The present experiments were designed to explore the generality of this effect using progressive-ratio schedules of reinforcement and different routes of phentermine administration. Unit doses of cocaine and food-pellet magnitudes were identified that maintained similar breaking points, and the effects of acute exposure to phentermine were assessed. In Experiment 1, a 'conventional' (one-trial) progressive-ratio schedule was used, in which response requirements increased after each reinforcer delivery; in Experiment 2, a 'modified' (five-trial) progressive-ratio schedule was used, in which response requirements increased after every five reinforcer deliveries. In one group of monkeys, responding was maintained by food; in another, cocaine infusions maintained responding. Phentermine (0.1-5.6mg/kg, intramuscularly (i.m.)) dose-dependently decreased breakpoints on both progressive-ratio schedules. There were no differences in phentermine's effects on cocaine- and food-maintained behavior. In Experiment 3, intravenous administration of phentermine had largely similar effects. Taken together with results from previous reports, these data suggest that the effects of phentermine pretreatment are influenced by the behavioral procedure used to maintain responding and/or by the efficacy of the food and cocaine reinforcers.

Synergistic interactions between fenfluramine and phentermine

'Fen-phen' refers to the off-label combination of the appetite suppressants fenfluramine and phentermine. The rationale for the fen-phen combination was that the two drugs exerted independent actions on brain satiety mechanisms so that it was possible to use lower doses of each drug and yet retain a common action on suppressing appetite while minimizing adverse drug effects. The focus of the present review is to consider whether fenfluramine and phentermine exert actions that are additive in nature or whether these two drugs exhibit drug-drug synergism. The fen-phen combination results in synergism for the suppression of appetite and body weight, the reduction of brain serotonin levels, pulmonary vasoconstriction and valve disease. Fen-phen synergism may reflect changes in the pharmacokinetics of drug distribution, common actions on membrane ion currents, or interactions between neuronal release and reuptake mechanisms with MAO-mediated transmitter degradation. The synergism between fenfluramine and phentermine highlights the need to more completely understand the pharmacology and neurochemistry of appetite suppressants prior to use in combination pharmacotherapy for the treatment of obesity.

Anorectic efficacy of the fenfluramine/phentermine combination in rats: additivity or synergy?

Fenfluramine + phentermine was a widely used combination for weight loss. Fenfluramine and phentermine are believed to act via serotonin and catecholamines, respectively. To what extent these drugs interact has not been well-established. We compared the anorectic efficacy of a range of doses of the combination (using dexfenfluramine instead of fenfluramine) relative to a range of doses of the individual drugs in 90 min sweetened milk intake tests in deprived and nondeprived rats. Results were plotted on isobolograms to determine whether the anorectic effects of the combination were either additive or synergistic. Collectively, the isobolographic analysis revealed that: (1) under acute conditions, dexfenfluramine and phentermine interact for the most part in a synergistic manner, and (2) with the exception of phentermine alone, deprivation state at time of testing did not alter the efficacy of the anorectics. These findings suggest that combined drug treatment for obesity is a theoretical approach that merits further investigation.

Is phentermine an inhibitor of monoamine oxidase? A critical appraisal

Phentermine produces a spectrum of concentration-dependent biochemical effects. It interacts with NE transporters at 0.1 microM, DA transporters at about 1 microM, 5-HT transporters at 15 microM and MAO-A at about 100 microM. When administered at typical anorectic doses, phentermine primarily interacts with DA and NE transporters and does not produce biochemical or neurochemical effects which would occur if it were inhibiting MAO-A. Some other explanation other than MAO inhibition must be sought to explain how oral phentermine increases platelet 5-HT, since platelet MAO-B does not metabolize platelet 5-HT, and since amphetamine-type drugs are even weaker inhibitors of MAO-B than MAO-A. Clinical studies in humans have shown that amphetamine, which is a more potent inhibitor of MAO-A than phentermine, does not inhibit MAO-A at therapeutic doses. Neither phentermine alone, fluoxetine alone or their combined use have been associated with cardiac valvulopathy, and clinical experience has shown their combined use to be free of significant adverse effects. Viewed collectively, there appears to be no data to support the hypothesis that phentermine inhibits MAO at typical therapeutic doses.

Effects of fluoxetine, phentermine, and venlafaxine on pulmonary arterial pressure and electrophysiology

The anorexic agents dexfenfluramine and fenfluramine plus phentermine have been associated with outbreaks of pulmonary hypertension. The fenfluramines release serotonin and reduce serotonin reuptake in neurons. They also inhibit potassium current (IK), causing membrane potential depolarization in pulmonary arterial smooth muscle cells. The recent withdrawal of the fenfluramines has led to the use of fluoxetine and phentermine as an alternative anorexic combination. Because fluoxetine and venlafaxine reduce serotonin reuptake, we compared the effects of these agents with those of phentermine and dexfenfluramine on pulmonary arterial pressure, IK, and membrane potential. Fluoxetine, venlafaxine, and phentermine caused minimal increases in pulmonary arterial pressure at concentrations < 100 microM but did cause a dose-dependent inhibition of IK. The order of potency for inhibition of IK at +50 mV was fluoxetine > dexfenfluramine = venlafaxine > phentermine. Despite the inhibitory effect on IK at more positive membrane potentials, fluoxetine, venlafaxine, and phentermine, in contrast to dexfenfluramine, had minimal effects on the cell resting membrane potential (all at a concentration of 100 microM). However, application of 100 microM fluoxetine to cells that had been depolarized to -30 mV by current injection elicited a further depolarization of >18 mV. These results suggest that fluoxetine, venlafaxine, and phentermine do not inhibit IK at the resting membrane potential. Consequently, they may present less risk of inducing pulmonary hypertension than the fenfluramines, at least by mechanisms involving membrane depolarization.

Effects of phentermine on responding maintained under multiple fixed-ratio schedules of food and cocaine presentation in the rhesus monkey

Drugs that decrease drug-maintained responding at doses that do not decrease other behaviors in animals may be suitable candidates for development as medications to treat drug abuse in humans. The present study examined whether this effect could be obtained with phentermine, a drug that has been reported to decrease cocaine intake in humans. Rhesus monkeys were trained under multiple fixed-ratio 30-response schedules of food and i.v. cocaine delivery. Phentermine was always given as a slow, i.v. infusion. Acute treatment with phentermine (0.3-10 mg/kg) decreased cocaine-maintained responding at doses that did not decrease, or decreased less, food-maintained responding for each of three unit doses of cocaine (10-100 microg/kg/injection). Subacute treatment with phentermine (3 or 5.6 mg/kg, daily) also decreased cocaine-maintained responding more than food-maintained responding. After subacute treatment was terminated, rates of cocaine-maintained responding generally recovered to levels comparable to those seen during untreated control sessions. Phentermine (0.3-3 mg/kg) did not generally increase responding associated with a very low (1 microg/kg/injection) unit dose of cocaine, suggesting that the decrease in cocaine-maintained responding at higher unit doses was not the result of a leftward shift in the cocaine unit dose-effect function. Phentermine (0.1-3 mg/kg) decreased responding maintained by 1-[2-[bis(4-fluorophenyl) methoxy]ethyl]-4-[3-phenylpropyl] piperazine (GBR 12909) (30 microg/kg/injection) at doses similar to those that decreased food-maintained responding. These results show that phentermine is effective in decreasing cocaine self-administration and suggest that it may be an effective medication for cocaine abuse.

Effects of dopamine and serotonin-releasing agents on methamphetamine discrimination and self-administration in rats

To analyze the involvement of dopamine (DA) and serotonin (5-HT) release in the stimulus properties of methamphetamine, two amphetamine analogs that selectively release either brain DA (phentermine) or 5-HT (fenfluramine) were tested for their ability to substitute for methamphetamine in rats discriminating methamphetamine (1.0 mg/kg) from saline. They were subsequently tested for their ability to alter IV methamphetamine (0.06 mg/kg per injection) self-administration in the same species when given as a pretreatment. The DA releaser phentermine, like methamphetamine itself, decreased methamphetamine self-administration (to 70% of baseline responding), but only at a dose of 3.0 mg/kg that fully generalized to the methamphetamine stimulus in the discrimination study. The 5-HT releaser fenfluramine attenuated methamphetamine self-administration to a much larger extent than phentermine (to 37% of baseline responding) at a dose of 1.8 mg/kg that did not generalize to methamphetamine and did not decrease rate of responding in the discrimination study. Tolerance developed to the inhibitory effect of 1.8 mg/kg fenfluramine on methamphetamine self-administration when it was given repeatedly over four consecutive daily sessions. The fenfluramine-induced decrease in methamphetamine self-administration was also attenuated when it was given together with the small 1.0 mg/kg dose of phentermine. These results suggest that DA release plays a dominant role in the discriminative stimulus effects of methamphetamine. However, stimulation of 5-HT release can strongly modify methamphetamine self-administration.