Fenfluramine-induced loss of serotonin transporters in baboon brain visualized with PET

Quantitative Positron Emission Tomography Studies of the Serotonin Transporter in Humans Previously Treated with the Appetite Suppressants Fenfluramine or Dexfenfluramine

PURPOSE

The appetite suppressants fenfluramine and dexfenfluramine were widely prescribed before being withdrawn from the market in 1997. Both drugs are known to have the potential to damage brain serotonin (5-HT) axons and axon terminals in animals, including nonhuman primates. This study used quantitative positron emission tomography (PET) with [(11)C] McN5652, a serotonin transporter (SERT) ligand to determine whether humans previously exposed to fenfluramines showed reductions in SERT binding parameters.

PROCEDURES

Subjects previously treated with fenfluramines for weight loss (N = 15) and age-matched controls (N = 17) underwent PET studies with [(11)C] McN5652. Global and regional distribution volumes (DVs) of [(11)C] McN5652 were compared in the two subject groups using parametric statistical analyses.

RESULTS

Compared to controls, subjects previously exposed to fenfluramines had significant reductions in [(11)C]McN5652 binding in 14 of 15 regions of interest, more than four years after drug discontinuation.

CONCLUSIONS

These results are the first to provide direct evidence for fenfluramine-induced 5-HT neurotoxicity in humans.

Positron emission tomography imaging of the serotonin transporter in the pig brain using [11C](+)-McN5652 and S-([18F]fluoromethyl)-(+)-McN5652

S-([(18)F]fluoromethyl)-(+)-McN5652 ([(18)F](+)-FMe-McN5652) has recently been synthesized as a new potential radiotracer for positron emission tomography (PET) imaging of the 5-HT transporter. It is an analog of [(11)C](+)McN5652, which has been used in clinical PET studies for 5-HT transporter imaging. This article describes the comparison of these two radiotracers in pigs with respect to their in vivo binding characteristics. PET images revealed that the highest accumulation of both radiotracers was found in the ventral midbrain, thalamus, olfactory lobe, and pons which is consistent with the known density of 5-HT transporters. The specific binding was determined by subtracting the values of the inactive (-) enantiomers or of the occipital cortex from those obtained with [(11)C](+)McN5652 or [(18)F](+)-FMe-McN5652 in the time period between 75 and 115 min after radiotracer injection. The specific binding of the (18)F-labeled derivative was about 40% higher than that of the (11)C-labeled derivative. A strong inhibition of the specific binding was observed for both radiotracers after pretreatment with the selective 5-HT uptake inhibitor citalopram. [(18)F](+)-FMe-McN5652 showed faster kinetics than [(11)C](+)McN5652. It reached the binding equilibrium during a study length of 120 min, which was not the case for [(11)C](+)McN5652. It is concluded that [(18)F](+)-FMe-McN5652 is suitable for 5-HT transporter imaging with PET.

Altered prolactin response to M-chlorophenylpiperazine in monkeys previously treated with 3,4-methylenedioxymethamphetamine (MDMA) or fenfluramine

3,4-Methylenedioxymethamphetamine ("Ecstasy," MDMA) and fenfluramine, widely used by humans, are potent brain serotonin (5-HT) neurotoxins in animals. Thus, there is concern that humans previously exposed to these amphetamine derivatives may have incurred brain 5-HT neurotoxicity. However, assessing the status of brain 5-HT neurons in the living organism is challenging. To determine whether MDMA- and/or fenfluramine-induced 5-HT neurotoxicity can be detected during life using neuroendocrine methods, groups of monkeys previously treated with neurotoxic regimens of MDMA or fenfluramine, along with saline-treated controls, underwent neuroendocrine challenge with the direct 5-HT agonist and 5-HT-releasing drug, m-chlorophenylpiperazine (m-CPP). Animals treated 2 weeks previously with MDMA exhibited a nonsignificant reduction in the prolactin response to m-CPP. In contrast, monkeys treated 3 1/2 years previously with MDMA or 2 years previously with fenfluramine exhibited significantly increased prolactin responses to m-CPP. No significant differences in cortisol concentrations were noted between groups at any time point. These data indicate that neuroendocrine challenge with m-CPP is capable of detecting substituted amphetamine-induced 5-HT neurotoxicity in living primates, but that the recency of drug exposure is an important consideration. Changes in the neuroendocrine response to m-CPP over time in animals with substituted amphetamine-induced neurotoxicity may be related to aberrant 5-HT reinnervation of the basal forebrain that occurs over time in monkeys previously treated with neurotoxic doses of MDMA or fenfluramine.

Intermittent, chronic fenfluramine administration to rats repeatedly suppresses food intake despite substantial brain serotonin reductions

The mechanisms by which fenfluramine suppresses food intake and body weight have been linked to its ability to enhance transmission across serotonin synapses in brain. This drug initially lowers body weight and suppresses food intake, yet after repeated administration food intake soon returns to normal and body weight no longer decreases. Fenfluramine also causes rapid and prolonged reductions in brain serotonin concentrations, which might account for its loss of appetite suppression. This possibility has been evaluated in rats by assessing if intermittent, chronic fenfluramine administration could suppress food intake during each treatment period, and if so, whether such an effect occurs in the presence of reduced brain serotonin levels. Rats were injected once daily with 10 mg/kg D,L-fenfluramine for 5 days, and then received no injections for the next 5 days. Control rats received only vehicle injections. This 10-day sequence was repeated five more times. During each period of fenfluramine administration, daily food intake dropped markedly the first 1-2 days of treatment, but returned to pretreatment values by day 5. Daily food intake was normal or slightly above normal during non-injection periods. Body weight dropped modestly during each period of fenfluramine administration, and rose during each subsequent period when injections had ceased. Serotonin concentrations and synthesis rates in several brain regions were markedly reduced at early, middle, and late periods of the experiment. Despite the long-term reduction in brain serotonin pools produced by fenfluramine, the drug continues to reduce food intake and body weight. Several possible interpretations of these findings are considered, based on the multiple mechanisms through which this drug has been proposed to modify synaptic serotonin transmission.

Priapism: ecstasy related?

Priapism, an uncommon urological emergency, is commonly drug-induced. We present a previously unreported case of a young man with priapism probably related to Ecstasy.

Kinetic analysis of [11C]McN5652: a serotonin transporter radioligand

The impulse response function of a radioligand is the most fundamental way to describe its pharmacokinetics and to assess its tissue uptake and retention pattern. This study investigates the impulse response function of [11C](+)McN5652, a radioligand used for positron emission tomography (PET) imaging of the serotonin transporter (SERT) in the brain. Dynamic PET studies were performed in eight healthy volunteers injected with [11C](+)McN5652 and subsequently with its pharmacologically inactive enantiomer [11C](-)McN5652. The impulse response function was calculated by deconvolution analysis of regional time-activity curves, and its peak value (f(max)), its retention value at 75 minutes (fT), and its normalized retention (f(rel) = fT/f(max)) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DV(T)) and its specific binding component (DV(S)). Both the noncompartmental (fT,f(rel)) and the compartmental parameters (DV) were investigated with and without correction for nonspecific binding by simple subtraction of the corresponding value obtained with [11C](-)McN5652. The impulse response function obtained by deconvolution analysis demonstrated high tracer extraction followed by a slow decline in the form of a monoexponential function. Statistical analysis revealed that the best compartmental model in terms of analysis of variance F and condition number of the parameter variance-covariance matrix was the one that was based on a single tissue compartment with parameters k1 and k2 and that also included the parameter of regional cerebral blood volume (BV). The parameter f(rel) demonstrated low between-subject variance (coefficient of variation [CV] = 19%), a midbrain to cerebellum ratio of 1.85, and high correlation with the known density of SERT (r = 0.787 where r is the coefficient of linear correlation between the parameter and the known density of SERT). After correction for nonspecific binding, f(rel) demonstrated further improvement in correlation (r = 0.814) and midbrain to cerebellum ratio (3.09). The variance of the distribution volumes was acceptable when the logarithmic transform lnDV was used instead of DV (17% for the three-parameter model), but correlation of this compartmental parameter was slightly less (r = 0.652 for the three-parameter model) than the correlation of the noncompartmental f(rel) with the known density of SERT, and the midbrain to cerebellum ratio was only 1.5 (uncorrected) and 1.8 (corrected). At the expense of increasing variance, the correlation was increased after correction for nonspecific binding using the inactive enantiomer (r = 0.694; CV = 22%). These results indicate that the kinetics of [11C](+)McN5652 can best be described by a one-tissue compartment model with three parameters (k1, k2, and BV), and that both the noncompartmental parameter f(rel) and the compartmental distribution volumes have the potential for quantitative estimation of the density of SERT. Further validation of the radioligand in experimental and clinical situations is warranted.

Reduced in vivo binding to the serotonin transporter in the cerebral cortex of MDMA ('ecstasy') users

BACKGROUND

The use of MDMA ('ecstasy') is common among young people in Western countries. Animal models of MDMA toxicity suggest a loss of serotonergic neurons, and potentially implicate in the development of significant psychiatric morbidity in humans.

AIMS

To test whether long-term use of MDMA can produce abnormalities in cerebral serotonin, but not dopamine, transporter binding measured by single photon emission computed tomography (SPECT).

METHOD

Ten male regular ecstasy users and 10 well-matched controls recruited from the same community sources participated in SPECT with the serotonin transporter (SERT) ligand [123I] beta-CIT. Dopamine transporter binding was determined from scans acquired 23 hours after injection of the tracer.

RESULTS

Ecstasy users showed a cortical reduction of SERT binding, particularly prominent in primary sensory-motor cortex, with normal dopamine transporter binding in lenticular nuclei.

CONCLUSIONS

This cross-sectional association study provides suggestive evidence for specific, at least temporary, serotonergic neurotoxicity of MDMA in humans.

Positron emission tomographic evidence of toxic effect of MDMA ("Ecstasy") on brain serotonin neurons in human beings

BACKGROUND

(+/-)3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") is a popular recreational drug that selectively damages brain serotonin (5-HT) neurons in animals at doses that closely approach those used by humans. We investigated the status of brain 5-HT neurons in MDMA users.

METHODS

We enrolled 14 previous users of MDMA who were currently abstaining from use and 15 controls who had never used MDMA. We used positron emission tomography (PET) with the radioligand carbon-11-labelled McN-5652, which selectively labels the 5-HT transporter. We analysed whether there were differences in 5-HT transporter binding between abstinent MDMA users and participants in the control group. Blood and urine samples were taken and tested to check for abstinence.

FINDINGS

MDMA users showed decreased global and regional brain 5-HT transporter binding compared with controls. Decreases in 5-HT transporter binding positively correlated with the extent of previous MDMA use.

INTERPRETATION

Quantitative PET studies with a ligand selective for 5-HT transporters can be used to assess the status of 5-HT neurons in the living human brain. We show direct evidence of a decrease in a structural component of brain 5-HT neurons in human MDMA users.

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.