Zsolt Szabo

Quantitative PET studies of the serotonin transporter in MDMA users and controls using [11C]McN5652 and [11C]DASB

(±)3,4-Methylenedioxymethamphetamine (MDMA, ‘Ecstasy’) is a widely used illicit drug that produces toxic effects on brain serotonin axons and axon terminals in animals. The results of clinical studies addressing MDMAs serotonin neurotoxic potential in humans have been inconclusive. In the present study, 23 abstinent MDMA users and 19 non-MDMA controls underwent quantitative positron emission tomography (PET) studies using [11C]McN5652 and [11C]DASB, first- and second-generation serotonin transporter (SERT) ligands previously validated in baboons for detecting MDMA-induced brain serotonin neurotoxicity. Global and regional distribution volumes (DVs) and two additional SERT-binding parameters (DVspec and DVR) were compared in the two subject populations using parametric statistical analyses. Data from PET studies revealed excellent correlations between the various binding parameters of [11C] McN5652 and [11C]DASB, both in individual brain regions and individual subjects. Global SERT reductions were found in MDMA users with both PET ligands, using all three of the above-mentioned SERT-binding parameters. Preplanned comparisons in 15 regions of interest demonstrated reductions in selected cortical and subcortical structures. Exploratory correlational analyses suggested that SERT measures recover with time, and that loss of the SERT is directly associated with MDMA use intensity. These quantitative PET data, obtained using validated first- and second-generation SERT PET ligands, provide strong evidence of reduced SERT density in some recreational MDMA users.

Mechanisms of Dopaminergic and Serotonergic Neurotransmission in Tourette Syndrome: Clues from an In Vivo Neurochemistry Study with PET

Tourette syndrome (TS) is a neuropsychiatric disorder with childhood onset characterized by motor and phonic tics. Obsessive-compulsive disorder (OCD) is often concomitant with TS. Dysfunctional tonic and phasic dopamine (DA) and serotonin (5-HT) metabolism may play a role in the pathophysiology of TS. We simultaneously measured the density, affinity, and brain distribution of dopamine D2 receptors (D2-Rs), dopamine transporter binding potential (BP), and amphetamine-induced dopamine release (DArel) in 14 adults with TS and 10 normal adult controls. We also measured the brain distribution and BP of serotonin 5-HT2A receptors (5-HT2AR), and serotonin transporter (SERT) BP, in 11 subjects with TS and 10 normal control subjects. As compared with controls, DArel was significantly increased in the ventral striatum among subjects with TS. Adults with TS+OCD exhibited a significant D2-R increase in left ventral striatum. SERT BP in midbrain and caudate/putamen was significantly increased in adults with TS (TS+OCD and TS-OCD). In three subjects with TS+OCD, in whom D2-R, 5-HT2AR, and SERT were measured within a 12-month period, there was a weakly significant elevation of DArel and 5-HT2A BP, when compared with TS–OCD subjects and normal controls. The current study confirms, with a larger sample size and higher resolution PET scanning, our earlier report that elevated DArel is a primary defect in TS. The finding of decreased SERT BP, and the possible elevation in 5-HT2AR in individuals with TS who had increased DArel, suggest a condition of increased phasic DArel modulated by low 5-HT in concomitant OCD.

In Vivo Detection of Short- and Long-Term MDMA Neurotoxicity—A Positron Emission Tomography Study in the Living Baboon Brain

The present study evaluated short‐ and long‐term effects of MDMA (3,4‐methylenedioxymethamphetamine) in the baboon brain using PET and [11C](+)McN 5652, a potent 5‐HT transporter ligand, as well as [11C]RTI‐55, a cocaine derivative which labels both 5‐HT and dopamine transporters. Following baseline PET scans with [11C](+)McN5652, [11C](−)McN5652 (the inactive enantiomer of the active enantiomer [11C](+)McN5652) and [11C]RTI‐55, a baboon was treated with MDMA (5 mg/kg, s.c., twice daily for four consecutive days). PET studies at 13, 19, and 40 days post‐MDMA revealed decreases in mean radioactivity levels in all brain regions when using [11C](+)McN 5652, but not with [11C](−)McN5652 or [11C]RTI‐55. Reductions in specific [11C](+)McN5652 binding (calculated as the difference in radioactivity concentrations between (+) and (−)[11C]McN5652) ranged from 44% in the pons to 89% in the occipital cortex. PET studies at 9 and 13 months showed regional differences in the apparent recovery of 5‐HT transporters, with increases in some brain regions (e.g., hypothalamus) and persistent decreases in others (e.g., neocortex). Data obtained from PET studies correlated well with regional 5‐HT axonal marker concentrations in the CNS measured after sacrifice of the animal. The results of these studies indicate that PET imaging of the living nonhuman primate brain with [11C](+)McN 5652 can detect changes in regional 5‐HT transporter density secondary to MDMA‐induced neurotoxicity. Using PET, it should also be feasible to use [11C](+)McN5652 to determine whether human MDMA users are also susceptible to MDMAs neurotoxic effects. Synapse 29:183–192, 1998.

Column-switching HPLC for the analysis of plasma in PET imaging studies

A column-switch high performance liquid chromatography method for the analysis of 4 mL of plasma is described with six examples of chromatography of [(11)C]-labeled positron-emission tomography imaging agents. Complete extraction of all but the most polar metabolites by the reverse phase capture column is achieved by disruption of plasma protein binding by 8 M urea.

Positron Emission Tomographic Studies of Brain Dopamine and Serotonin Transporters in Abstinent (±) 3,4-Methylenedioxymethamphetamine (“Ecstasy”) Users: Relationship to Cognitive Performance

Background(±)3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is a recreational drug and brain serotonin (5-HT) neurotoxin. Under certain conditions, MDMA can also damage brain dopamine (DA) neurons, at least in rodents. Human MDMA users have been found to have reduced brain 5-HT transporter (SERT) density and cognitive deficits, although it is not known whether these are related. This study sought to determine whether MDMA users who take closely spaced sequential doses, which engender high plasma MDMA concentrations, develop DA transporter (DAT) deficits, in addition to SERT deficits, and whether there is a relationship between transporter binding and cognitive performance.Materials and methodsSixteen abstinent MDMA users with a history of using sequential MDMA doses (two or more doses over a 3- to 12-h period) and 16 age-, gender-, and education-matched controls participated. Subjects underwent positron emission tomography with the DAT and SERT radioligands, [11C]WIN 35,428 and [11C]DASB, respectively. Subjects also underwent formal neuropsychiatric testing.ResultsMDMA users had reductions in SERT binding in multiple brain regions but no reductions in striatal DAT binding. Memory performance in the aggregate subject population was correlated with SERT binding in the dorsolateral prefrontal cortex, orbitofrontal cortex, and parietal cortex, brain regions implicated in memory function. Prior exposure to MDMA significantly diminished the strength of this relationship.ConclusionsUse of sequential MDMA doses is associated with lasting decreases in brain SERT, but not DAT. Memory performance is associated with SERT binding in brain regions involved in memory function. Prior MDMA exposure appears to disrupt this relationship. These data are the first to directly relate memory performance to brain SERT density.

Quantification and segmentation of brain tissues from MR images: a probabilistic neural network approach

This paper presents a probabilistic neural network based technique for unsupervised quantification and segmentation of brain tissues from magnetic resonance images. It is shown that this problem can be solved by distribution learning and relaxation labeling, resulting in an efficient method that may be particularly useful in quantifying and segmenting abnormal brain tissues where the number of tissue types is unknown and the distributions of tissue types heavily overlap. The new technique uses suitable statistical models for both the pixel and context images and formulates the problem in terms of model-histogram fitting and global consistency labeling. The quantification is achieved by probabilistic self-organizing mixtures and the segmentation by a probabilistic constraint relaxation network. The experimental results show the efficient and robust performance of the new algorithm and that it outperforms the conventional classification based approaches.

Modified Regression Model for the Logan Plot

Logans graphical model is a robust estimation of the total distribution volume (DVt) of reversibly bound radiopharmaceuticals, but the resulting DVt values decrease with increasing noise. The authors hypothesized that the noise dependence can be reduced by a linear regression model that minimizes the sum of squared perpendicular rather than vertical (y) distances between the data points and fitted straight line. To test the new method, 15 levels of simulated noise (repeated 2,000 times) were added to synthetic tissue activity curves, calculated from two different sets of kinetic parameters. Contrary to the traditional method, there was no (P > 0.05) or dramatically decreased noise dependence with the perpendicular model. Real dynamic 11C (+) McN5652 serotonin transporter binding data were processed either by applying Logan analysis to average counts of large areas or by averaging the Logan slopes of individual-voxel data. There were no significant differences between the parameters when the perpendicular regression method was used with both approaches. The presented experiments show that the DVt calculated from the Logan plot is much less noise dependent if the linear regression model accounts for errors in both the x and y variables, allowing fast creation of unbiased parametric images from dynamic positron-emission tomography studies.

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 (fmax), its retention value at 75 minutes (fT), and its normalized retention (frel = fT/fmax) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DVT) and its specific binding component (DVs). Both the noncompartmental (fT, frel) 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 frel 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, frel 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 frel 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 frel 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.

Positron emission tomography of 5-HT transporter sites in the baboon brain with [11C]McN5652

[C]McN5652 is a new radioligand specific for 5-hydroxytryptamine (5-HT; serotonin) transporters. In this study we used [11C]McN5652 to image the 5-HT transporter sites in baboon brain by positron emission tomography (PET). Dynamic PET studies were performed in three Papio anubis baboons. The animals were injected intravenously first with 11C-labeled (+)-McN5652([11C](+)McN5652), then with pharmacologically inactive enantiomer 11C-labeled (–)-McN5652 ([11C](–)McN5652); two animals received a third study with [11C](+)McN5652 after pretreatment with the specific 5-HT uptake site inhibitor fluoxetine (5 mg/kg). Initial uptake into the brain was similar for both [11C](+)McN5652 and [11C](–)McN5652. At later times (45–120 min after injection), only [11C](+)McN5652 showed a distribution characteristic for 5-HT uptake sites. In contrast, in studies with [11C](–)McN5652 and in those with [11C](+)McN5652 after 5-HT uptake site blockade with fluoxetine, 11C radioactivity concentrations were significantly lower and the distribution pattern was relatively even. The differences between [11C](+)-and (–)McN5652 were calculated for the time interval 95–125 min postinjection and used to estimate specific binding. Specific binding correlated well (r = 0.95, p < 0.001) with the known density of 5-HT uptake sites in human brain. These results indicate that [11C](+)McN5652 is suitable for PET imaging of 5-HT uptake sites in primate brain.

Kinetic Analysis of l11CrMcN5652c 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 (fmax), its retention value at 75 minutes (fT), and its normalized retention (frel = fT/fmax) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DVT) and its specific binding component (DVs). Both the noncompartmental (fT, frel) 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 frel 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, frel 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 frel 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 frel 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.