Effects of fenfluramine and phentermine (fen-phen) on dopamine and serotonin release in rat striatum: in vivo microdialysis study in conscious animals

We measured the effects of acute or chronic administration of fenfluramine and phentermine, alone or in combination, on brain dopamine and serotonin release into striatal dialysates of freely moving rats. Samples collected every 30 min were assayed in a single run by high-pressure liquid chromatography. Acute or chronic administration of fenfluramine (1 mg/kg, i.p.) did not significantly change dopamine concentrations in rat striatal dialysates, but increased those of serotonin by 182% (acute) and 124% (chronic). Phentermine (2 mg/kg, i.p.), on the other hand, significantly increased dopamine concentrations by 52% (acute) and 80% (chronic) without affecting those of serotonin. Administration of the drugs in combination (fenfluramine 1 mg/kg and phentermine 2 mg/kg) amplified the effects of each, increasing striatal dopamine concentrations by 209% (acute) and serotonin concentrations by 330% (acute) and 299% (chronic).

Evaluation of the conditioned reinforcing effects of phentermine and fenfluramine in the rat: concordance with clinical studies

An unbiased place-preference conditioning procedure was used to characterize the conditioned reinforcing effects of phentermine (PHEN), fenfluramine (FEN), and their combination (PHEN/FEN) in previously drug-naive rats. Animals exhibited marked preferences for an environment previously associated with the administration of phentermine. The minimum dose producing a significant effect was 3.0 mg/kg. In contrast, FEN produced dose-related place aversions. In animals which received a subthreshold dose of FEN in combination with a dose of PHEN that produced a conditioned place preference, no preference or aversion for the drug-paired place was seen. Similarly, no significant conditioning in response to administration of PHEN (3.0 mg/kg) and FEN (3.0 mg/kg) was seen. The failure of PHEN/FEN to produce conditioned reinforcing effects is in line with recent clinical studies, and suggests that PHEN/FEN and drug combinations sharing the same neurochemical mechanisms of action will have low potential for abuse.

Appetite suppressant drugs as inhibitors of human cytochromes P450: in vitro inhibition of P450-2D6 by D- and L-fenfluramine, but not phentermine

The activity of D-fenfluramine, L-fenfluramine, and phentermine as inhibitors of five human cytochromes P450 was evaluated using human liver microsomes in vitro. All three compounds produced negligible inhibition of P450-1A2, -2C9, -2E1, and -3A. Phentermine also did not inhibit P450-2D6. However, D- and L-fenfluramine significantly inhibited P450-2D6 activity as measured by dextromethorphan O-demethylation, with mean 50% inhibitory concentrations (15.1 microM) within one order of magnitude of that for fluoxetine (2.7 microM). Findings from the in vitro assay are consistent with clinical studies showing significant inhibition of desipramine clearance by coadministration of fenfluramine.

Efficacy of administration of dexfenfluramine and phentermine, alone and in combination, on ingestive behavior and body weight in rats

Recently, a combination of the anorectics fenfluramine (FEN) and phentermine (PHEN) has been used to treat obesity. While each of these agents has been investigated in animals, little is known concerning the effects of the combination on ingestive behavior and body weight. In the present experiments, we report: (1) the effects of acute administration of dexfenfluramine (DFEN) and PHEN individually and in combination on sweetened milk intake and body weight in non-deprived rats and (2) the effects of chronic administration (7 day minipump) of DFEN, PHEN, and their combination on daily food intake and body weight both during and after the treatment period. Additionally, the effects of the 5-HT2C agonist 1-[3-(trifluoromethyl)-phenyl]piperazine (TFMPP) alone and in combination with PHEN on food intake and body weight were assessed. Both acute and chronic administration of DFEN and PHEN revealed that in combination they are more effective than when given individually. However, the DFEN/ PHEN combination does not appear to exert effects that are selective for food intake because water intake was markedly suppressed in water-deprived rats. PHEN alone or in combination with either DFEN or TFMPP also produced increased activity or alertness during the day when controls normally were asleep. While anorectic combinations such as DFEN/PHEN may be effective at promoting weight loss and reducing food intake, future studies on their specificity, safety and efficacy are warranted.

Effects of phentermine on striatal dopamine and serotonin release in conscious rats: in vivo microdialysis study

OBJECTIVE

To measure the effects of phentermine, an appetite supressant, on the release of brain dopamine (DA) and serotonin (5-HT) into striatal dialysates of freely moving rats.

DESIGN

Microdialysis and high performance liquid chromatography.

SUBJECTS

Unanesthetized rats.

MEASUREMENTS

Samples collected every 20 min were assayed for both neurotransmitters in a single run, using high performance liquid chromatography with electrochemical detection.

RESULTS

Baseline levels of DA and 5-HT in dialysates were 56+/-16 and 3+/-0.6 fmol/20 microl, respectively. Administration of phentermine (2 or 5 mg/kg) increased dialysate DA concentrations to 147+/-17% (P < 0.01) and 320+/-89% (P < 0.01) of baseline, respectively, without significantly affecting 5-HT concentrations. Pretreatment with tetrodotoxin (TTX, 60 min, 1 microM), which abolished the basal release of DA and 5-HT into striatal dialysates, diminished the increase in DA concentrations induced by phentermine, but did not completely block it. Phentermine (2 or 5 mg/kg, i.p.) still stimulated DA release to 27+/-13% and 85+/-15% of baseline, respectively, in the presence of TTX.

CONCLUSION

Phentermine increases brain DA but not 5-HT release in freely moving rats, and TTX reduces, but does not fully block this effect. This pattern is similar to that known to be produced by d-amphetamine.

Behavioural and neurochemical characteristics of phentermine and fenfluramine administered separately and as a mixture in rats

Clinical case studies suggest that combined administration of the serotonergic agent fenfluramine (FEN) and the weak amphetamine-like anorexic agent phentermine (PHEN) may be useful in the treatment of alcohol and cocaine addictions. The present experiment examined the nature of the interaction between the two agonists using the drug discrimination paradigm. In vivo microdialysis served to examine the neurochemical profile of dopamine and serotonin release in the nucleus accumbens. In conscious rats, acute injections of FEN (1.0-2.0 mg/kg i.p.) or PHEN (1.0-2.0 mg/kg i.p.) selectively elevated levels of serotonin and dopamine in the nucleus accumbens, respectively. A mixture (1 mg/kg of each) increased levels of both amines by similar magnitudes to those observed with each individually. Three groups of Sprague-Dawley rats were trained to discriminate (1) FEN (1.0 mg/kg i.p.) alone, (2) PHEN (1.0 mg/kg i.p.) alone or a mixture (3) PHEN+FEN (1 mg/kg of each, i.p.) from saline under a fixed ratio (FR-10) schedule of food reinforcement. Rats acquired the mixture discrimination rapidly, while for the other groups the training dose had to be increased to 2.0 mg/kg to attain stimulus control. The individual components of the mixture at the training dose generalized partially to the mixture, and complete generalisation was observed following 3.0 mg/kg FEN or PHEN. Rats trained to discriminate the individual components showed respective cross-generalisation profiles. Generalisation to cocaine (0.3-10.0 mg/kg i.p.), amphetamine (0.1-3.0 mg/kg i.p.) and nicotine (0.1-0.8 mg/kg s.c.) was greatest in the MIX-trained rats, while partial or no generalisation was observed in rats trained to discriminate the individual compounds. From the present results, it may be concluded that the two drugs given as a mixture do not produce a novel cue. Rather, these aminergics appear to interact additively. Furthermore, the dual stimulation of the amines by the mixture may be the basis for the cueing effects of the FEN+PHEN drug mixture, and its effectiveness in treating drug addictions.

Combined phentermine/fenfluramine administration enhances depletion of serotonin from central terminal fields

Administration of phentermine (Phen) together with (+/-) fenfluramine (Fen) enhances the weight reduction that is observed with either drug alone; consequently, these anorectic agents are commonly prescribed together for weight reduction. Repeated administration of Fen is known to cause long-term depletion of axonal serotonin (5-HT) and loss of 5-HT transporters, and is therefore considered neurotoxic. We now report that combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg, and 20 mg/kg/3.125 mg/kg) can enhance the neurotoxic effect of Fen (3.125 mg/kg) and Phen (5 mg/kg and 20 mg/kg) on central 5-HT systems. Rats were repeatedly treated once each hour for a total of four injections with saline, Phen (5 mg/kg and 20 mg/kg), Fen (3.125 mg/kg and 12.5 mg/kg), or combined Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg), and sacrificed either 7 or 28 days after cessation of treatment. Combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg) caused significantly greater reductions of 5-HT levels in the striatum, nucleus accumbens/olfactory tubercle, hypothalamus, amygdala, frontal parietal cortex, and hippocampus than either drug alone. Combined Phen/Fen at the higher drug-dose combination (20 mg/kg/3.125 mg/kg) was observed to reduce the density of 5-HT transporters in rat striatum at both 7 and 28 days after cessation of treatment. In addition, combined administration of Phen/Fen (5 mg/kg/3.125 mg/kg and 20 mg/kg/3.125 mg/kg) caused greater weight loss than that observed with either compound alone. Collectively, the present data demonstrate that combined Phen/Fen administration enhances the neurotoxicity of Phen or Fen on 5-HT neurons.

Phentermine/fenfluramine decreases cocaine self-administration in rhesus monkeys

Dopaminergic agonists can decrease cocaine self-administration at doses that do not decrease food-maintained responding, a pre-clinical effect indicative of a potential treatment for human cocaine abuse. To assess whether similar effects could be obtained with medications currently used to treat substance abuse, phentermine and fenfluramine were given alone and in combination to rhesus monkeys responding under schedules of food and cocaine delivery. Phentermine decreased cocaine-maintained responding with no effect on food-maintained responding. Fenfluramine also selectively decreased cocaine-maintained responding, but only at the highest dose. Combining a lower dose of fenfluramine with phentermine selectively decreased cocaine-maintained responding, but not more than with phentermine alone. These results suggest that phentermine, as well as its combination with fenfluramine, may be useful in the treatment of cocaine abuse.

Phentermine pretreatment antagonizes the cocaine-induced rise in mesolimbic dopamine

Coadministration of phentermine and fenfluramine has been used to treat cocaine dependence. Patients who relapse while receiving this treatment report diminished subjective effects of cocaine. Due to the importance of mesolimbic dopamine (DA) in mediating cocaine reinforcement, we hypothesized that phentermine might attenuate the effects of cocaine on DA transmission. We examined this proposal directly using in vivo microdialysis methods in the nucleus accumbens of awake rats. Rats were pretreated with saline or phentermine (1 mg kg-1, i.v.) and then challenged with cocaine (3 mg kg-1, i.v.). Phentermine alone caused a modest increase in DA, and phentermine pretreatment substantially reduced the cocaine-induced rise in extracellular DA. Phentermine did not alter the stimulatory effect of cocaine on 5-HT. Our findings suggest that phentermine may antagonize the subjective effects of cocaine in humans via a DA mechanism.

Anesthetic considerations for the new antiobesity medications

Combination drug therapy can effectively treat the problem of obesity. The most commonly used combination is a mix of fenfluramine and phentermine. Fenfluramine inhibits the reuptake of serotonin and acts on the hypothalmic appetite control center, while phentermine acts as an appetite suppressant. These drugs along with diet and exercise effectively help people to lose weight with few side effects. However, there are several anesthetic considerations when providing anesthesia services for patients on the fenfluramine and phentermine regimen. Problems of hypotension on induction, hypoglycemia, hyperthermia, and pulmonary hypertension have been reported in the literature. Recently, dexfenfluramine (Redux) was approved by the U.S. Food and Drug Administration. It is the dextrostereoisomer of fenfluramine and is believed to produce the same weight loss with less side effects. Anesthesia providers must understand the potential risks involved when administering a general anesthetic to these patients.

Phentermine+fenfluramine produce cocaine-like discriminative cues

Drug discrimination studies were conducted in six male Sprague-Dawley rats trained to discriminate the interoceptive cues produced by 10 mg/kg cocaine in an effort to investigate if there is stimulus generalization to phentermine or phentermine + fenfluramine. Once having reached criterion performance, these rats were tested with lower doses of cocaine and generated a typical dose-response curve allowing for calculation of an ED50 value: 2.798 mg/kg. Testing of phentermine in doses of 1.25-5.0 mg/kg indicated generalization with the highest dose producing 80% cocaine-appropriate responding and allowing for an ED50 value of 2.356 mg/kg. When the phentermine doses were tested in combination 2.0 mg/kg fenfluramine, however, there was an increase in the discriminability of the highest phentermine dose and a slight decrease in the ED50 value of the combination. Thus, administration of phentermine + fenfluramine, having both dopamine-releasing and serotonin-releasing properties, respectively, may mimic the neurochemical activity by which cocaine acts in the central nervous system and may possibly allow for cocaine-like effects as these two drugs see increased use in obesity control.

Discriminative stimulus effects of dopaminergic agents in rhesus monkeys

Recent reports have shown that treatment with dopamine reuptake inhibitors can selectively decrease responding maintained by low doses of cocaine in rhesus monkeys. This may occur because response-independent delivery of a reuptake inhibitor and response-dependent cocaine have common effects. One behavioral effect that dopamine reuptake inhibitors and cocaine share is their ability to serve as a discriminative stimulus. To compare discriminative effects of several dopaminergic agents with their ability to attenuate cocaine-maintained responding, three rhesus monkeys were first trained to discriminate intravenous injections of cocaine (0.1 mg/kg) from saline. Following generalization testing with various doses of cocaine (0.001-1.0 mg/kg), the relative potencies of phentermine (0.03-1.0 mg/kg), d-amphetamine (0.01-1.0 mg/kg), GBR 12,909 (0.01-1.0 mg/kg), and buspirone (0.03-0.56 mg/kg) to substitute for cocaine were assessed. Each drug except buspirone resulted in predominantly cocaine-appropriate responding at doses that were generally without rate-decreasing effect. The ED50 for the ability of these drugs to substitute for cocaine exhibited the same rank order as that for their effectiveness in decreasing cocaine-maintained responding. Thus, the current results show that the potencies of dopaminergic drugs to decrease cocaine-maintained responding and substitute for cocaine in a drug discrimination paradigm are related.

Evaluation of phentermine and fenfluramine, alone and in combination, in normal, healthy volunteers

Recent clinical reports indicate that combined administration of phentermine and fenfluramine may have useful effects in the treatment of drug abuse. The present study was designed to evaluate the subjective and mood-altering effects of these drugs, alone and in combination, in normal healthy volunteers. Seven male and five female volunteers participated in an eight-session, double-blind study in which each subject received each of the following drug conditions: d-amphetamine (10 and 20 mg), phentermine (30 mg), fenfluramine (40 and 80 mg), phentermine (30 mg) with fenfluramine (40 mg), phentermine (30 mg) with fenfluramine (80 mg), and placebo. Sessions were conducted in a laboratory setting two or three days a week. Subjects completed standardized self-report questionnaires and psychomotor tests before and at regular intervals after each drug administration. Phentermine produced effects that were similar to those of d-amphetamine, whereas fenfluramine produced different and apparently aversive effects (e.g., it increased measures of anxiety and confusion). Phentermine reduced the apparently aversive effects of fenfluramine when the two drugs were given together. These results suggest that the combination of phentermine and fenfluramine would have a low potential for abuse.

Action of fenfluramine, phenylpropanolamine, phentermine and diethylpropion on acoustic startle in rats

Four commonly used anorectics which are amphetamine analogues were tested for their action on responsiveness in an acoustic startle test when rats were given daily IP injections adequate to produce a change in body weight. Drugs were given for 22 days. None of these drugs increased startle responsiveness as does the amphetamine parent compound. Instead, fenfluramine and phenylpropanolamine decreased startle responsiveness and phentermine and diethylpropion produced no change. There was no relationship between drug action and body weight. Partial tolerance was found for the fenfluramine action on startle and complete tolerance was found for its action on body weight gain. The fenfluramine action is compatible with the extensive literature on humans and animals indicating sedative properties.

Self-administration of phentermine by naive rats: effects of body weight and a food delivery schedule

Food deprivation has been shown to increase intravenous self-administration of amphetamine and cocaine. In the present experiment, the response rates of four groups of eight rats for intravenous infusions of phentermine under free-feeding (FF) and 80% free-feeding weight (FFW) conditions in the presence and absence of a fixed time 1 min (FT-1) food delivery schedule, were compared with those of saline reinforced animals under identical conditions. The findings showed that: (1) Overall, response rates of phentermine-reinforced animals were significantly greater than were those of saline-reinforced animals; (2) 80% FFW animals self-injected significantly greater amounts of phentermine than did FF animals; and (3) the operation of an FT-1 schedule failed to affect the rate of phentermine-reinforced responding.

Role of age in amphiphilic drug-induced pulmonary morphological and metabolic responses

The morphological and metabolic responsiveness of pulmonary tissue to amphiphilic agents is dependent on age. Daily oral administration of chlorphentermine (60 mg/kg) for 1 wk produced an accumulation of phospholipid-rich hypertrophic macrophages in pulmonary alveoli accompanied by an increase in relative lung weight and elevation in levels of phosphatidylcholine (PC), sphingomyelin (S), phosphatidylserine (PS) plus phosphatidylinositol (PI), phosphatidylethanolamine (PE) plus phosphatidylglycerol (PG), and total phospholipid (TPL) in newborn rats. Chlorcyclizine treatment (60 mg . kg-1 . day-1) for 1 wk also resulted in an accumulation of hypertrophic macrophages and an increase in both TPL and PC levels in newborn lung. However, unlike chlorphentermine, the chlorcyclizine-stimulated increase in pulmonary cells was associated with no marked change in relative lung weight and levels of S, PG plus PE, and PS plus PI. Phenobarbital (30 mg/kg) reduced the chlorphentermine-induced (60 mg/kg) effect on neonatal relative lung weight, accumulation of hypertrophic macrophages, and phospholipid levels. Withdrawal from drug treatment for 2 wk in newborns previously given chlorphentermine for 7 days resulted in a disappearance of pulmonary hypertrophic macrophages and a return of TPL levels to control values. Our data support the view that age plays a role in the observed amphiphilic drug-induced morphological and metabolic responses in rat lung.

Comparison of the acute physical and mental effects of ephedrine, fenfluramine, phentermine and prolintane

Physical and mental effects of a single oral dose of ephedrine (ephedrine HCl 30 or 40 mg), fenfluramine (fenfluramine HCl 15 or 20 mg), phentermine (7.5 or 11.25 mg) and prolintane (prolintane HCl 10 or 20 mg) were compared in a double-blind placebo-controlled study. Each group consisted of 16-43 healthy volunteer medical students. The subjects fasted for at least 3 hr before drug administration and further until the end of the experiment. All the parameters were measured just prior to giving the drug, and 1.5 hr and 2.5 hr afterwards. Ephedrine significantly increased systolic blood pressure (p less than 0.05) and heart rate (p less than 0.01), whereas the other sympathomimetics affected these parameters only slightly or negligibly. None of the drugs markedly changed the tapping rate of the dominant hand. Mental activity was evaluated with a self-rating check list consisting of various mental modalities. None of the sympathomimetics significantly modified the mental activity. Memory, learning and concentration ability were evaluated with sign recording and digit span tests. In the digit span test no changes were obtained. In the sign recording test (for 3 min), phentermine increased significantly the recording score at both 1.5 hr (p less than 0.05) and 2.5 hr (p less than 0.005), and prolintane at 2.5 hr (p less than 0.05) after drug administration. The results suggest that in the doses given, which are commonly used in medical practice, ephedrine has the most pronounced cardiovascular effects, while phentermine and prolintane seem to be most active in the performance of some mental tasks.

Effects of chlorphentermine and phentermine on the pulmonary disposition of 5-hydroxytryptamine in the rat in vivo

This study was designed to examine whether chlorphentermine (CP) affects pulmonary disposition of 5-hydroxytryptamine (5-HT) in rat in vivo. Further, the effects of CP were compared with those of phentermine (P), the nonchlorinated congener. The right jugular vein and left carotid artery of male Sprague-Dawley rats were cannulated to the level of the superior caval vein and ascending aorta, respectively. After 10 min of stabilization, 0.3 ml fresh saline solution containing 150 micrograms indocyanine green and a mixture of labeled and unlabeled 5-HT was injected into the jugular vein, and arterial blood samples were collected for 20 s. Pulmonary extraction of 5-HT during a single pass was inversely proportional to the injected dose, ranging from 67% cf injected dose for a 2.6 nmol dose to 30% for a 130 nmol dose, indicating a saturable pulmonary uptake for 5-HT. In order to compare the effect of CP and P on pulmonary disposition of 5-HT, 2.6 nmol [14C]-5-HT was employed for in vivo single-pass experiments. Each animal was used for 2 in vivo single-pass experiments. After the first experiment, which served as a control, animals received an indicated dose of CP or P, to commence the second "drug-treated" in vivo experiment. Pulmonary clearance of 5-HT was inhibited by prior administration of CP (1 mg/kg) by 42%, whereas at the highest dose (20 mg/kg) P inhibited 5-HT clearance by only 25%. Pulmonary accumulation of CP was greater than P at higher doses, and the inhibition of 5-HT clearance correlated with the pulmonary accumulation of these drugs. In addition to the in vivo demonstration of the CP inhibition of pulmonary clearance of 5-HT in the rat, these studies also demonstrate a higher affinity of the lung tissue for CP than for P and a greater propensity for the impairment of pulmonary 5-HT clearance.

Role of phospholipase-associated penetration mechanism in cell injury by Rickettsia rickettsii

The cholesterol receptor-binding drugs amphotericin B and digitonin reduced rickettsial plaque formation, presumably by blocking rickettsial attachment, even when added to the plaque model on day 4 post-inoculation when infected foci were well established. The phospholipase inhibitor phentermine reduced plaque formation, presumably by inhibiting the phospholipase-associated entry step of penetration.

The opposing effects of N-hydroxyamphetamine and N-hydroxyphentermine on the H2O2 generated by hepatic cytochrome P-450

The effects of N-hydroxyphentermine (NOHP) and N-hydroxyamphetamine (NOHA) on hydrogen peroxide generated by rat liver microsomes and reconstituted preparations in the presence of NADPH were compared. In microsome incubations, NOHP caused an increase in H2O2 levels and NOHA caused a substantial decrease. When the substances were compared for cytochrome P-450-dependent H2O2 generation in reconstituted preparations, NOHA at mM blocked generation and NOHP had no effect. NOHP appears to be an uncoupler of the cytochrome P-450 system in microsomes whereas NOHA is a potent inhibitor, presumably because of its ability to form a metabolic intermediate complex. During the course of their effects on O2 reduction, NOHP and NOHA are themselves undergoing oxidation, NOHP to 2-methyl-2-nitro-1-phenylpropane and NOHA to phenylacetone oxime. The enzymatic natures of these oxidations differ. Thus, two closely related arylalkylhydroxylamines differ substantially in their interaction with cytochrome P-450 systems.

The contribution of increased thermogenesis to the effect of anorectic drugs on body composition in mice

The study investigated whether changes in body composition of normal and genetically obese C57BL/6J (ob/ob) mice caused by the anorectic drugs phentermine, diethylpropion, fenfluramine, and mazindol are entirely due to reduced food intake. Mice were dosed daily (25 mg/kg po) for 28 days after which time carcass composition was determined. Compared to controls fed at libitum, reductions in food intake were for phentermine, 7%; fenfluramine, 17%; diethylpropion, 17%, whereas reduction in body lipid content were for phentermine, 16%; mazindol, 18%; fenfluramine, 8%; diethylpropion, 10%. Since diet restriction by 22% (in the absence of treatment with any drug) resulted in a body lipid content 12% below that of controls fed ad libitum, these results suggest that some of the lipid loss caused by phentermine and possibly mazindol is due to increased energy expenditure. In support of this conclusion, phentermine and mazindol increased energy expenditure in normal mice by 35% compared to untreated controls in the 6 h after dosing but diethylpropion and fenfluramine had little or no effect. Determination of the carcass composition of the normal mice confirmed that phentermine has a metabolic antiobesity effect. Fenfluramine had an unexpected effect on carcass composition in normal mice.