Fenfluramine-Phentermine is Associated with an Increase in Cellular Proliferation Ex Vivo and In Vitro

BACKGROUND AND AIM OF THE STUDY

Fenfluraminephentermine (FenPhen) has been implicated in accelerated valvular heart disease, characterized by valvular regurgitation and thickening, and resembling the histopathologic lesions found in carcinoid. The study aim was to determine whether cellular proliferation is present in FenPhen-exposed valves, by utilizing an in-vitro model to test whether FenPhen has a direct mitogenic effect on cardiac valvular cells, as compared to serotonin.

METHODS

Ex-vivo valves were tested for proliferation in surgically removed FenPhen-exposed valves (n = 10) and compared to proliferation levels in normal human cardiac valves removed at autopsy (n = 10). Immunostaining for a DNA polymerase, proliferating cell nuclear antigen (PCNA), was performed and quantified using digital imaging analysis. In-vitro assays were performed for direct proliferative effects of serotonin and FenPhen (10-6, 10-7 and 10-8 M) on porcine aortic valve subendothelial cells, using a [3H]-thymidine incorporation assay.

RESULTS

Ex-vivo PCNA levels in human FenPhenexposed valves were elevated compared to controls (22.8 ± 4.54 versus 1.26 ± 0.47; p <0.001). In vivo, serotonin and FenPhen markedly increased (10-fold) cell proliferation (as measured by [3H]-thymidine incorporation) in subendothelial cells in vitro (p <0.001). This proliferative response was demonstrated by PCNA staining in carcinoid heart valves and FenPhen-exposed valves. Mechanistically, plateletderived growth factor increased cell proliferation in a dose-related manner (p <0.001), the response being inhibited by a MAP kinase inhibitor (determined by monitoring p42/44 levels).

CONCLUSIONS

In vitro, FenPhen acts as a powerful mitogen on subendothelial myofibroblast valve cells. Ex vivo, cellular proliferation was significantly elevated in human FenPhen-exposed cells.

Expression of 5-hydroxytryptamine receptor subtype messenger RNA in interstitial cells from human heart valves

BACKGROUND AND AIM OF THE STUDY

Severe heart valve disorder has been reported in patients receiving a combination of the anorectic drugs fenfluramine and phentermine. The exact molecular mechanisms involved remain unknown. Fenfluramine alters the serotonin level in the brain, while phentermine interferes with the pulmonary clearance of serotonin; these data suggest that serotonin levels affect regulation of valve function. The aim of the present study was to characterize the serotonin receptor (5-hydroxytryptamine) subtypes expressed in the interstitial cells of human heart valves.

METHODS

Interstitial cells were isolated and cultured from the aortic, pulmonary, mitral and tricuspid valves of recipient hearts obtained during transplantation. Total RNA was extracted from cultured cells in order to determine gene expression by reverse transcription-polymerase chain reaction (RT-PCR) using 5-hydroxytryptamine (5-HT) subtype-specific primer pairs.

RESULTS

The results show that: (i) 5-HT 1B and 1D receptor subtypes are expressed in all four heart valves. This is significant as the 1B and 1D receptor subfamilies are the target of the anti-migraine drug sumatriptan, and these receptors regulate cardiac function and movement; (ii) 5-HT 1A, 5-HT 1E and 5-HT 1F are not expressed in interstitial cells isolated from the valves.

CONCLUSION

We conclude that preliminary evidence exists for the presence of distinct subsets of 5-HT receptors in human heart valves, indicating that interstitial cells of the valves potentially respond to serotonin levels.

Neurotoxic effects of +/-fenfluramine and phenteramine, alone and in combination, on monoamine neurons in the mouse brain

Until recently, (+/-)fenfluramine (FEN) was widely prescribed as an appetite suppressant. In animals, FEN is a potent and selective brain serotonin neurotoxin. The present studies assessed the effects of phentermine (PHEN), an appetite suppressant frequently used clinically in combination with FEN, on FEN-induced serotonin neurotoxicity. Groups (n = 6/group) of mice were treated with FEN (10 mg/kg), PHEN (20 mg/kg or 40 mg/kg), FEN (10 mg/kg) plus PHEN (20 mg/kg or 40 mg/kg), or vehicle twice daily for four days. Food intake and body weight were measured during and after drug treatment. Brains were evaluated for regional brain serotonin and dopamine axonal markers two weeks after drug treatment. PHEN enhanced the anorectic and weight-reducing effects of FEN. PHEN also significantly enhanced FEN's long-term toxic effects on 5-HT axons. This effect was evident in some (hypothalamus, striatum) but not all (hippocampus, cortex) brain regions examined. PHEN alone produced no long-term effects on 5-HT axonal markers. However, whether given alone or in combination with FEN, PHEN produced significant, dose-related decreases in striatal DA axonal markers. These results, coupled with those from previous studies, suggest that PHEN has the potential to exacerbate FEN-induced serotonin neurotoxicity, if utilized in certain doses. Further, the present results indicate that PHEN possesses dopamine (DA) neurotoxic potential. The relevance of these data to humans previously treated with FEN/PHEN is discussed.

Interaction of phentermine plus fenfluramine: neurochemical and neurotoxic effects

Previous studies have reported the use of combined serotonergic and dopaminergic agonists in the treatment of obesity and alcoholism. Along these lines, phentermine plus fenfluramine has been suggested as a possible clinical treatment for alcohol craving. To determine the neurochemical effects of a combined treatment of phentermine plus fenfluramine, animals were injected subcutaneously with saline, phentermine 12 mg/kg, fenfluramine 16 mg/kg, or a combination of phentermine plus fenfluramine. One hour after injection, animals were sacrificed and neurochemical analysis performed. Furthermore, separate groups of animals were given the same injections 8 times, 12 hours apart, to determine the effects on body weight and to detect a possible exacerbation of fenfluramine induced toxicity. The drug combination produced a significant rise in dopamine in the striatum, greater than that seen with either drug alone. Furthermore, the addition of phentermine reduced the fenfluramine induced rise in striatal 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindolacetic acid (5-HIAA). Phentermine plus fenfluramine combination produced a greater weight loss than either drug alone, however, it did not produce a significantly greater drop in striatal serotonin or 5-HIAA levels above that induced by fenfluramine alone. Thus, while previous studies report the potentiated neurotoxicity of phentermine plus fenfluramine over fenfluramine alone, the present study does not indicate that such an effect occurs following an administration regimen analogous to that of patients treated with the drug combination.

Effects of phentermine and cocaine on fenfluramine-induced depletion of serotonin in mouse brain

The combined administration of phentermine and fenfluramine (PHEN/FEN) has been used as a treatment for obesity. Recent evidence suggests that this drug mixture may also be an effective medication for substance abuse disorders, including cocaine dependence. It is well-established that repeated high-dose fenfluramine causes serotonin (5-HT) terminal degeneration in laboratory animals, and no studies have addressed possible interactions between phentermine and fenfluramine. The purpose of the present work was to examine the effect of phentermine coadministration on fenfluramine-induced depletion of 5-HT in mouse forebrain. In addition, because of the potential for cocaine abuse in drug addicts taking PHEN/FEN as a medication, we examined the effects of PHEN/FEN on forebrain 5-HT levels in the presence or absence of cocaine. Fenfluramine (0, 3, 10, 30 mg/kg, s.c. twice daily for 4 days) caused a dose-dependent reduction in forebrain 5-HT without affecting dopamine or norepinephrine. Phentermine coadministration (7 mg/kg, s.c. twice daily for 4 days) did not significantly alter the 5-HT-depleting effect of fenfluramine. Likewise, cocaine (10 mg/kg, i.p.), administered 60 min prior to or 60 min after PHEN/FEN, had no effect on the PHEN/FEN-induced decrease in central 5-HT. The present results indicate that doses of phentermine far above those typically administered to humans do not potentiate the 5-HT-depleting effect of repeated high-dose fenfluramine. Moreover, exposure to cocaine does not significantly alter the long-term neurochemical actions of the PHEN/FEN mixture.

Neonatal toxicity in rats following in utero exposure to chlorphentermine or phentermine

The administration of chlorphentermine (30 mg/kg) to pregnant rats during the last 5 days of gestation resulted in the development of a phospholipidosis in the lungs of the dams. The disorder developed in utero, as the phospholipidosis was evident in the lungs of the neonates when examined at 12 h postpartum. In contrast, a phospholipidosis was not observed in the lungs of the dams or neonates following phentermine treatment (30 mg/kg). Concurrently, neonates of the chlorphentermine-treated dams displayed a significant decrease in body weight in comparison to controls. Between 16 h and 24 h postpartum, 83% of the neonates of chlorphentermine-treated dams died. Cross-fostering and starvation experiments revealed that the lethality was not due to aberrant maternal behavior by the chlorphentermine-treated dams or malnutrition of the neonates. Histological examinations revealed endothelial and septal alterations in the lungs of neonates from chlorphentermine-treated dams. No signs of toxicity, as evidenced by the maintenance of body weight, or lethality were observed in the neonates of the phentermine-treated dams.

Modification by hyperoxia of chlorphentermine- or phentermine- induced effects on newborn rat lung morphology and metabolism

Treatment of newborns with 20 mg/kg/day chlorphentermine orally for 1 week increased incorporation of thymidine into lung DNA without an associated change in tissue morphology or cyclic AMP levels. An increase in chlorphentermine dose to 60 mg/kg resulted in an accumulation of alveolar hypertrophic macrophages and a rise in incorporation of thymidine into lung DNA; however, cyclic AMP levels were decreased. In contrast, 20 or 60 mg/kg/day for 1 week phentermine-induced depression in the incorporation of thymidine into pulmonary DNA was accompanied by a decrease in cyclic AMP but no apparent alteration in tissue morphology. Hyperoxia did not modify the phentermine-induced changes in cyclic AMP levels and pulmonary ultrastructure. In contrast, hyperoxia altered the responsiveness of newborns to 20 mg/kg chlorphentermine as evidenced by the presence of foam cells. Data suggest that the chlorphentermine-induced increase in DNA synthesis in newborn lung seems independent of changes in cyclic AMP and tha modification of drug-induced alterations by hyperoxia may be related to the chemical structure of a compound.

The protective effects of methysergide, 6-hydroxydopamine and other agents on the toxicity of amphetamine, phentermine, MDA, PMA, and STP in mice

The ability of various drugs to prevent the lethal effects of 4-methoxyamphetamine (PMA) and 3, 4-methylenedioxyamphetamine (MDA) were reduced by pretreatment with phentolamine and 6-hydroxydopamine suggesting that release of norepinephrine from peripheral adrenergic nerves contributed to their toxicity. Pretreatment with methysergide reduced the lethal effects of (+)- and (-)-amphetamine, MDA, PMA and 2, 5-dimethoxy-4-methylamphetamine (STP) suggesting that an action on serotonergic receptors contributed to their toxicity. Pretreatment with 4-chloroamphetamine, practolol and haloperidol did not alter the lethal effects of the agents studied.