Comparative evaluation in nonhuman primates of five PET radiotracers for imaging the serotonin transporters: [11C]McN 5652, [11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM

Positron Emission Tomography Imaging of Synaptic Dysfunction in Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases with a complex pathogenesis. Aggregations formed by abnormal deposition of alpha-synuclein (αSyn) lead to synapse dysfunction of the dopamine and non-dopamine systems. The loss of dopaminergic neurons and concomitant alterations in non-dopaminergic function in PD constitute its primary pathological manifestation. Positron emission tomography (PET), as a representative molecular imaging technique, enables the non-invasive visualization, characterization, and quantification of biological processes at cellular and molecular levels. Imaging synaptic function with PET would provide insights into the mechanisms underlying PD and facilitate the optimization of clinical management. In this review, we focus on the synaptic dysfunction associated with the αSyn pathology of PD, summarize various related targets and radiopharmaceuticals, and discuss applications and perspectives of PET imaging of synaptic dysfunction in PD.

PET reporter systems for the brain

Positron emission tomography (PET) can be used as a noninvasive method to longitudinally monitor and quantify the expression of proteins in the brain in vivo. It can be used to monitor changes in biomarkers of mental health disorders, and to assess therapeutic interventions such as stem cell and molecular genetic therapies. The utility of PET monitoring depends on the availability of a radiotracer with good central nervous system (CNS) penetration and high selectivity for the target protein. This review evaluates existing methods for the visualization of reporter proteins and/or protein function using PET imaging, focusing on engineered systems, and discusses possible approaches for future success in the development of high-sensitivity and high-specificity PET reporter systems for the brain.

Positron Emission Tomography in the Neuroimaging of Autism Spectrum Disorder: A Review

Autism spectrum disorder (ASD) is a basket term for neurodevelopmental disorders characterized by marked impairments in social interactions, repetitive and stereotypical behaviors, and restricted interests and activities. Subtypes include (A) disorders with known genetic abnormalities including fragile X syndrome, Rett syndrome, and tuberous sclerosis and (B) idiopathic ASD, conditions with unknown etiologies. Positron emission tomography (PET) is a molecular imaging technology that can be utilized in vivo for dynamic and quantitative research, and is a valuable tool for exploring pathophysiological mechanisms, evaluating therapeutic efficacy, and accelerating drug development in ASD. Recently, several imaging studies on ASD have been published and physiological changes during ASD progression was disclosed by PET. This paper reviews the specific radioligands for PET imaging of critical biomarkers in ASD, and summarizes and discusses the similar and different discoveries in outcomes of previous studies. It is of great importance to identify general physiological changes in cerebral glucose metabolism, cerebral blood flow perfusion, abnormalities in neurotransmitter systems, and inflammation in the central nervous system in ASD, which may provide excellent points for further ASD research.

Association Between Lack of Insight and Prefrontal Serotonin Transporter Availability in Antipsychotic-Free Patients with Schizophrenia: A High-Resolution PET Study with [11C]DASB

BACKGROUND

Previous studies suggested a link between serotonergic neurotransmission and impaired insight in schizophrenia. In this study, we examined the relationship between serotonin transporter (SERT) availability in regions of the prefrontal cortex (dorsolateral, ventrolateral, ventromedial, and orbitofrontal cortices) and insight deficits in antipsychotic-free patients with schizophrenia using high-resolution positron emission tomography (PET) with [¹¹C]DASB.

METHODS

Nineteen patients underwent [¹¹C]DASB PET and 7-Tesla magnetic resonance imaging scans. To assess SERT availability, the binding potential with respect to non-displaceable compartment (BP_ND) was derived using the simplified reference tissue model. Patients' level of insight was assessed using the Insight and Treatment Attitude Questionnaire (ITAQ). The relationship between ITAQ scores and [¹¹C]DASB BP_ND values was examined using the region-of-interest (ROI)- and voxel-based analyses with relevant variables as covariates. The prefrontal cortex and its four subregions were selected as a priori ROIs since the prefrontal cortex has been implicated as the critical neuroanatomical substrate of impaired insight in schizophrenia.

RESULTS

The ROI-based analysis revealed that the ITAQ illness insight dimension had significant negative correlations with the [¹¹C]DASB BP_ND in the left dorsolateral, left orbitofrontal, and bilateral ventrolateral prefrontal cortices. The ITAQ treatment insight dimension had significant negative correlations with the [¹¹C]DASB BP_ND in the bilateral dorsolateral, left orbitofrontal, and bilateral ventrolateral prefrontal cortices. The ITAQ total score showed significant negative correlations with the [¹¹C]DASB BP_ND in the bilateral prefrontal cortex and three subregions (dorsolateral, ventrolateral, and orbitofrontal cortices). A supplementary voxel-based analysis corroborated a significant negative association between the ITAQ score and the [¹¹C]DASB BP_ND in the prefrontal cortices.

CONCLUSION

Our study provides in vivo evidence of significant negative correlations between insight deficits and prefrontal SERT availability in patients with schizophrenia, suggesting significant involvement of prefrontal serotonergic signaling in impaired insight, one of the core symptoms of schizophrenia.

Markers for the central serotonin system correlate to verbal ability and paralinguistic social voice processing in autism spectrum disorder

Impairment in verbal communication abilities has been reported in autism spectrum disorder (ASD). Dysfunction of the serotonergic system has also been reported in ASD. However, it is still unknown how the brain serotonergic system relates to impairment in verbal communication abilities in individuals with ASD. In the present study, we investigated the correlation between brain serotonergic condition and brain sensitivity to paralinguistic stimuli (i.e., amplitude in the human voice prosodic change-evoked mismatch field) measured by magnetoencephalography (MEG) or verbal ability in 10 adults with ASD. To estimate the brain serotonergic condition, we measured the serotonin transporter nondisplaceable binding potential cerebrum-wide using positron emission tomography with [11C]N,N-dimethyl-2-(2-amino-4-cyanophenylthio)benzylamine ([11C] DASB). The results demonstrated a significant positive correlation between brain activity to paralinguistic stimuli and brain serotonin transporter binding potential in the left lingual gyrus, left fusiform gyrus and left calcarine cortex. In addition, there were significant positive correlations between verbal ability and serotonergic condition in the right anterior insula, right putamen and right central operculum. These results suggested that the occipital cortex is implicated in recognition of the prosodic change in ASD, whereas the right insula-involved serotonergic system is important in nurturing verbal function in ASD.Trial registration: UMIN000011077.

Examining the underpinnings of loudness dependence of auditory evoked potentials with positron emission tomography

Loudness dependence of auditory evoked potentials (LDAEP) has long been considered to reflect central basal serotonin transmission. However, the relationship between LDAEP and individual serotonin receptors and transporters has not been fully explored in humans and may involve other neurotransmitter systems. To examine LDAEP's relationship with the serotonin system, we performed PET using serotonin-1A (5-HT_1A) imaging via [¹¹C]CUMI-101 and serotonin transporter (5-HTT) imaging via [¹¹C]DASB on a mixed sample of healthy controls (n ​= ​4: 4 females, 0 males), patients with unipolar (MDD, n ​= ​11: 4 females, 7 males) and bipolar depression (BD, n ​= ​8: 4 females, 4 males). On these same participants, we also performed electroencephalography (EEG) within a week of PET scanning, using 1000 ​Hz tones of varying intensity to evoke LDAEP. We then evaluated the relationship between LDAEP and 5-HT_1A or 5-HTT binding in both the raphe (5-HT_1A)/midbrain (5-HTT) areas and in the temporal cortex. We found that LDAEP was significantly correlated with 5-HT_1A positively and with 5-HTT negatively in the temporal cortex (p ​< ​0.05), but not correlated with either in midbrain or raphe. In males only, exploratory analysis showed multiple regions in which LDAEP significantly correlated with 5-HT_1A throughout the brain; we did not find this with 5-HTT. This multimodal study partially validates preclinical models of a serotonergic influence on LDAEP. Replication in larger samples is necessary to further clarify our understanding of the role of serotonin in perception of auditory tones.

Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A₁ adenosine receptor (A₁AR). Using the half-life (t_1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

PET imaging of the mouse brain reveals a dynamic regulation of SERT density in a chronic stress model

The serotonin transporter (SERT, Slc6a4) plays an important role in the regulation of serotonergic neurotransmission and its aberrant expression has been linked to several psychiatric conditions. While SERT density has been proven to be amenable to in vivo quantitative evaluation by positron emission tomography (PET) in humans, this approach is in its infancy for rodents. Here we set out to evaluate the feasibility of using small-animal PET employing [¹¹C]DASB ([¹¹C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) as a radiotracer to measure SERT density in designated areas of the mouse brain. Using Slc6a4^+/+, Slc6a4^+/-, and Slc6a4^-/- mice as a genetic model of different SERT expression levels, we showed the feasibility of SERT imaging in the mouse brain with [¹¹C]DASB-PET. The PET analysis was complemented by an evaluation of SERT protein expression using western blot, which revealed a highly significant correlation between in vivo and ex vivo measurements. [¹¹C]DASB-PET was then applied to the examination of dynamic changes of SERT levels in different brain areas in the chronic corticosterone mouse model of chronic stress. The observed significant reduction in SERT density in corticosterone-treated mice was independently validated by and correlated with western blot analysis. This is the first demonstration of a quantitative in vivo evaluation of SERT density in subregions of the mouse brain using [¹¹C]DASB-PET. The evidenced decrease in SERT density in response to chronic corticosterone treatment adds a new dimension to the complex involvement of SERT in the pathophysiology of stress-induced mental illnesses.

Molecular imaging of autism spectrum disorder

Autism spectrum disorder (ASD) is a condition with onset in early childhood characterized by marked deficits in interpersonal interactions and communication and by a restricted and repetitive range of interests and activities. This review points out key recent findings utilizing molecular imaging including magnetic resonance spectroscopy (MRS) and nuclear neuroimaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). MRS indicates an excitatory/inhibitory imbalance in high-functioning autism. Dysfunction of neurotransmitter and glucose metabolism has been demonstrated by PET and SPECT. Levels of serotonin synthesis in typically developing children are approximately twice those of adults; after the age of 5 years, levels decrease to those of adults. In contrast, levels of serotonin synthesis of children with ASD increase between ages 2 and 15 to 1.5-times adult values. The dopamine transporter is increased in the orbitofrontal cortex of men with ASD. The serotonin transporter is reduced in the brains of children, adolescents, and adults with ASD. Reduced serotonin receptors in the thalamus of adults with ASD are associated with communication difficulties. Glucose metabolism is reduced in the brains of people with ASD. Molecular imaging will provide the preliminary data for promising therapeutic interventions.

Expanding LogP: Present possibilities

INTRODUCTION

Due to the high candidate exclusion rate during a drug development process, an early prediction of the pharmacokinetic behavior would be needed. Accordingly, high performance bioaffinity chromatography (HPBAC) approaches are growing in popularity, however, there is a lack of knowledge and no consensus about the relation between HPBAC measurements, in vivo distribution and blood brain barrier (BBB) penetration behavior. With respect to radiotracers, there is almost no reference data available for plasma protein binding (PPB), permeability (Pm) and the membrane coefficient (K_IAM). Thus, this study was aimed at exploring the relevance of measuring PPB, Pm and K_IAM for the prediction of BBB penetration.

METHODS

Measurements of %PPB, Pm and K_IAM were performed using HPBAC. In total, 113 compounds were tested, 43 with brain uptake, 30 not showing brain uptake and 40 with known interactions with efflux transporters. Additionally, ClogP and HPLC logP_ow^pH7.4 data were collected.

RESULTS

%PPB, K_IAM, Pm and ClogP values were in the same range for each of the three groups. A significant difference was observed for the HPLC logP_ow^pH7.4 between CNS penetrating drug group (CNS_pos) and the non-penetrating drug group (CNS_neg), as well as for the CNS_neg towards the drug group interacting with efflux transporters (DRUG_efflux). However, as the other experimental data, also the HPLC logP_ow^pH7.4 showed a broad overlapping of the single values between the groupings.

CONCLUSION

Experimental reference values (logP, Pm, K_IAM & PPB) of commonly used PET tracers and drugs showing different BBB penetration behavior are provided. The influence of the logP on brain uptake depends strongly on the selected method. However, using a single parameter (experimental or calculated) to predict BBB penetration or for the classification of drug groups is inexpedient.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

Oxytocin effects on emotional response to others' faces via serotonin system in autism: A pilot study

The oxytocin (OT)-related serotonergic system is thought to play an important role in the etiology and social symptoms of autism spectrum disorder (ASD). However, no evidence exists for the relation between the prosocial effect of chronic OT administration and the brain serotonergic system. Ten male subjects with ASD were administered OT for 8-10 weeks in an open-label, single-arm, non-randomized, uncontrolled manner. Before and during the OT treatment, positron emission tomography was used with the (¹¹C)-3-amino-4-(2-[(demethylamino)methyl]phenylthio)benzonitrile(¹¹C-DASB) radiotracer. Then binding of serotonin transporter (¹¹C-DASB BP_ND) was estimated. The main outcome measures were changes in ¹¹C-DASB BP_ND and changes in the emotional response to others' faces. No significant change was found in the emotional response to others' faces after the 8-10 week OT treatment. However, the increased serotonin transporter (SERT) level in the striatum after treatment was correlated significantly with increased negative emotional response to human faces. This study revealed a relation between changes in the serotonergic system and in prosociality after chronic OT administration. Additional studies must be conducted to verify the chronic OT effects on social behavior via the serotonergic system.

Altered interregional correlations between serotonin transporter availability and cerebral glucose metabolism in schizophrenia: A high-resolution PET study using [11C]DASB and [18F]FDG

The purpose of the present study was to investigate the patterns of interregional correlations of serotonin transporter (SERT) availability with glucose metabolism using 7-Tesla magnetic resonance imaging (MRI) and high-resolution positron emission tomography (PET) with ¹¹C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([¹¹C]DASB) and [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) in antipsychotic-free patients with schizophrenia in order to shed new light on the disrupted functional connectivity in schizophrenia. Nineteen patients with schizophrenia and 18 healthy controls underwent high-resolution PET and MRI. The binding potential (BP_ND) of [¹¹C]DASB and standardized uptake value ratio (SUVR) of [¹⁸F]FDG were obtained. In SERT availability, the region of interest (ROI)-based analyses showed no significant group differences in any region, except for the anterior hippocampus where the SERT availability was lower in patients with schizophrenia than in controls. The ROI- and voxel-based analyses revealed that the [¹⁸F]FDG SUVR values were significantly lower in patients than in controls in the right superior frontal gyrus and medial part of the left superior frontal gyrus. Regarding the interregional correlations of [¹¹C]DASB BP_ND with [¹⁸F]FDG SUVR, more widespread positive correlations across the brain regions were observed in control subjects than in patients with schizophrenia. Notably, the patients and control subjects showed statistically significant differences in correlations between the SERT availability in the parietal and temporal cortices and the glucose metabolism in the posterior cingulate cortex. These results suggest abnormal functional connectivity between the higher-order cortical regions in schizophrenia and a possible important role of the posterior cingulate gyrus and its related circuitry in the pathophysiology of schizophrenia.

LogP, a yesterday's value?

INTRODUCTION

There is an increasing demand for high throughput methods at early stages of preclinical radioligand development, in order to predict pharmacokinetic properties (e.g., biodistribution) and blood brain barrier (BBB) penetration. One of the most important physicochemical properties is the lipophilicity, measured by means of shake-flask (logP) or HPLC methods. Yet, a plethora of experimental methods are described in the literature for the determination of logP values. These varying methods often lead to different results for one identical compound, which complicates any comparison or prediction for subsequent preclinical studies. However, a standardized and internationally applied and accepted database with logP values for a reliable comparison of the lipophilic character of radiotracers is still missing.

METHOD

Lipophilicity measurements were performed with 121 molecules using a high throughput HPLC method and ClogP values were calculated using ChemBioDraw®. Furthermore, logP measurements for six representative radiotracers were performed with the conventional shake-flask method and the results were statistically compared to the ClogP and HPLC logP results. Different logP thresholds, suggesting optimal BBB penetration according to literature, were selected and put into relation with the acquired HPLC logP and ClogP values of cerebral tracers.

RESULTS

The results of the tested compounds ranged from -2.1 to 5.4 with the applied HPLC method. The acquired database comprises ClogP values of the whole set of compounds ranging from -4.11 to 6.12. LogP data from different methods were not comparable. The correlation of the obtained logP data to thresholds suggesting an optimal brain uptake resulted in a high number of false positive classifications.

CONCLUSION

The logP determination for prediction of BBB penetration is obsolete. The extensive database, including clinical relevant radiotracers, can be used as comparative set of values for preclinical studies, and serves as a basis for further critical discussions concerning the eligibility of logP.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Radiosynthesis and first preclinical evaluation of the novel norepinephrine transporter pet-ligand [(11)C]ME@HAPTHI

Background

The norepinephrine transporter (NET) has been demonstrated to be relevant to a multitude of neurological, psychiatric and cardiovascular pathologies. Due to the wide range of possible applications for PET imaging of the NET together with the limitations of currently available radioligands, novel PET tracers for imaging of the cerebral NET with improved pharmacological and pharmacodynamic properties are needed.

Methods

The present study addresses the radiosynthesis and first preclinical evaluation of the novel NET PET tracer [¹¹C]Me@HAPTHI by describing its affinity, selectivity, metabolic stability, plasma free fraction, blood–brain barrier (BBB) penetration and binding behaviour in in vitro autoradiography.

Results

[¹¹C]Me@HAPTHI was prepared and displayed outstanding affinity and selectivity as well as excellent in vitro metabolic stability, and it is likely to penetrate the BBB. Moreover, selective NET binding in in vitro autoradiography was observed in human brain and rat heart tissue samples.

Conclusions

All preclinical results and radiosynthetic key-parameters indicate that the novel benzothiadiazole dioxide-based PET tracer [¹¹C]Me@HAPTHI is a feasible and improved NET radioligand and might prospectively facilitate clinical NET imaging.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0113-3) contains supplementary material, which is available to authorized users.

A graphical method to compare the in vivo binding potential of PET radioligands in the absence of a reference region: application to [¹¹C]PBR28 and [¹⁸F]PBR111 for TSPO imaging

Positron emission tomography (PET) radioligands for a reversible central nervous system (CNS) demand a high specific to nonspecific signal characterized by the binding potential (BPND). The quantification of BPND requires the determination of the nondisplaceable binding usually derived from a reference region devoid of the target of interest. However, for many CNS targets, there is no valid reference region available. In such cases, the total volume of distribution (VT) is often used as the outcome measure, which includes both the specific and nonspecific binding signals. Here we present a graphical method that allows for direct comparison of the binding potential of ligands using the regional VT data alone via linear regression. The method was first validated using literature data for five serotonin transporter ligands, for which a reference region exists, and then applied to two second generation 18 kDa translocator protein radioligands, namely [(11)C]PBR28 and [(18)F]PBR111. The analysis determined that [(11)C]PBR28 had a higher BPND than [(18)F]PBR111.

Tracer kinetic modeling of [(11)C]AFM, a new PET imaging agent for the serotonin transporter

[(11)C]AFM, or [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine, is a new positron emission tomography (PET) radioligand with high affinity and selectivity for the serotonin transporter (SERT). The purpose of this study was to determine the most appropriate kinetic model to quantify [(11)C]AFM binding in the healthy human brain. Positron emission tomography data and arterial input functions were acquired from 10 subjects. Compartmental modeling and the multilinear analysis-1(MA1) method were tested using the arterial input functions. The one-tissue model showed a lack of fit in low-binding regions, and the two-tissue model failed to estimate parameters reliably. Regional time-activity curves were well described by MA1. The rank order of [(11)C]AFM binding potential (BPND) matched well with the known regional SERT densities. For routine use of [(11)C]AFM, several noninvasive methods for quantification of regional binding were evaluated, including simplified reference tissue models (SRTM and SRTM2), and multilinear reference tissue models (MRTM and MRTM2). The best methods for region of interest (ROI) analysis were MA1, MRTM2, and SRTM2, with fixed population kinetic values ( or b') for the reference methods. The MA1 and MRTM2 methods were best for parametric imaging. These results showed that [(11)C]AFM is a suitable PET radioligand to image and quantify SERT in humans.

PET imaging of serotoninergic neurotransmission with [(11)C]DASB and [(18)F]altanserin after focal cerebral ischemia in rats

The use of selective serotonin reuptake inhibitors has shown functional improvement after stroke. Despite this, the role of serotoninergic neurotransmission after cerebral ischemia evolution and its involvement in functional recovery processes are still largely unknown. For this purpose, we performed in parallel in vivo magnetic resonance imaging and positron emission tomography (PET) with [(11)C]DASB and [(18)F]altanserin at 1, 3, 7, 14, 21, and 28 days after middle cerebral artery occlusion (MCAO) in rats. In the ischemic territory, PET with [(11)C]DASB and [(18)F]altanserin showed a dramatic decline in serotonin transporter (SERT) and 5-HT2A binding potential in the cortex and striatum after cerebral ischemia. Interestingly, a slight increase in [(11)C]DASB binding was observed from days 7 to 21 followed by the uppermost binding at day 28 in the ipsilateral midbrain. In contrast, no changes were observed in the contralateral hemisphere by using both radiotracers. Likewise, both functional and behavior testing showed major impaired outcome at day 1 after ischemia onset followed by a recovery of the sensorimotor function and dexterity from day 21 to day 28 after cerebral ischemia. Taken together, these results might evidence that SERT changes in the midbrain could have a key role in the functional recovery process after cerebral ischemia.

The current state-of-the-art of spinal cord imaging: methods

A first-ever spinal cord imaging meeting was sponsored by the International Spinal Research Trust and the Wings for Life Foundation with the aim of identifying the current state-of-the-art of spinal cord imaging, the current greatest challenges, and greatest needs for future development. This meeting was attended by a small group of invited experts spanning all aspects of spinal cord imaging from basic research to clinical practice. The greatest current challenges for spinal cord imaging were identified as arising from the imaging environment itself; difficult imaging environment created by the bone surrounding the spinal canal, physiological motion of the cord and adjacent tissues, and small cross-sectional dimensions of the spinal cord, exacerbated by metallic implants often present in injured patients. Challenges were also identified as a result of a lack of "critical mass" of researchers taking on the development of spinal cord imaging, affecting both the rate of progress in the field, and the demand for equipment and software to manufacturers to produce the necessary tools. Here we define the current state-of-the-art of spinal cord imaging, discuss the underlying theory and challenges, and present the evidence for the current and potential power of these methods. In two review papers (part I and part II), we propose that the challenges can be overcome with advances in methods, improving availability and effectiveness of methods, and linking existing researchers to create the necessary scientific and clinical network to advance the rate of progress and impact of the research.

PET imaging of the brain serotonin transporters (SERT) with N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM) in humans: a preliminary study

PURPOSE

The aim of this study was to assess the feasibility of using 4-[(18)F]-ADAM as a brain SERT imaging agent in humans.

METHODS

Enrolled in the study were 19 healthy Taiwanese subjects (11 men, 8 women; age 33 ± 9 years). The PET data were semiquantitatively analyzed and expressed as specific uptake ratios (SUR) and distribution volume ratios (DVR) using the software package PMOD. The SUR and DVR of 4-[(18)F]-ADAM in the raphe nucleus (RN), midbrain (MB), thalamus (TH), striatum (STR) and prefrontal cortex (PFC) were determined using the cerebellum (CB) as the reference region.

RESULTS

4-[(18)F]-ADAM bound to known SERT-rich regions in human brain. The order of the regional brain uptake was MB (RN) > TH > STR > PFC > CB. The DVR (n = 4, t* = 60 min) in the RN, TH, STR and PFC were 3.00 ± 0.50, 2.25 ± 0.45, 2.05 ± 0.31 and 1.40 ± 0.13, respectively. The optimal time for imaging brain SERT with 4-[(18)F]-ADAM was 120-140 min after injection. At the optimal imaging time, the SURs (n = 15) in the MB, TH, STR, and PFC were 2.25 ± 0.20, 2.28 ± 0.20, 2.12 ± 0.18 and 1.47 ± 0.14, respectively. There were no significant differences in SERT availability between men and women (p < 0.05).

CONCLUSION

The results of this study showed that 4-[(18)F]-ADAM was safe for human studies and its distribution in human brain appeared to correlate well with the known distribution of SERT in the human brain. In addition, it had high specific binding and a reasonable optimal time for imaging brain SERT in humans. Thus, 4-[(18)F]-ADAM may be feasible for assessing the status of brain SERT in humans.

Sustained recreational use of ecstasy is associated with altered pre and postsynaptic markers of serotonin transmission in neocortical areas: a PET study with [¹¹C]DASB and [¹¹C]MDL 100907

3,4-Methylenedioxymethamphetamine (MDMA), the main psychoactive component of the recreational drug ecstasy, is a potent serotonin (5-HT) releaser. In animals, MDMA induces 5-HT depletion and toxicity in 5-HT neurons. The aim of this study was to investigate both presynaptic (5-HT transporter, SERT) and postsynaptic (5-HT(2A) receptor) markers of 5-HT transmission in recently abstinent chronic MDMA users compared with matched healthy controls. We hypothesized that MDMA use is associated with lower SERT density and concomitant upregulation of 5-HT(2A) receptors. Positron emission tomography studies using the SERT ligand [¹¹C]DASB and the 5-HT(2A) receptor ligand [¹¹C]MDL 100907 were evaluated in 13 current and recently detoxified MDMA users and 13 matched healthy controls. MDMA users reported a mean duration of ecstasy use of 8 years, regular exposure, and at least 2 weeks of abstinence before the scans. SERT and 5-HT(2A) receptor availability (binding potential, BP(ND)) were analyzed with a two-tissue compartment model with arterial input function. Current recreational MDMA use was significantly associated with lower SERT BP(ND) and higher 5-HT(2A) receptor BP(ND) in cortical, but not subcortical regions. Decreased SERT BP(ND) was regionally associated with upregulated 5-HT(2A) receptor BP(ND). In light of the animal literature, the most parsimonious interpretation is that repeated exposure to MDMA in humans, even in moderate amounts, leads to damage in 5-HT neuron terminals innervating the cortex. Alterations in mood, cognition, and impulse control associated with these changes might contribute to sustain MDMA use. The reversibility of these changes upon abstinence remains to be firmly established.

Reduced amygdala serotonin transporter binding in posttraumatic stress disorder

BACKGROUND

The amygdala is a key site where alterations in the regulation of the serotonin transporter (5-HTT) may alter stress response. Deficient 5-HTT function and abnormal amygdala activity have been hypothesized to contribute to the pathophysiology of posttraumatic stress disorder (PTSD), but no study has evaluated the 5-HTT in humans with PTSD. On the basis of translational models, we hypothesized that patients diagnosed with PTSD would exhibit reduced amygdala 5-HTT expression as measured with positron emission tomography and the recently developed 5-HTT-selective radiotracer [(11)C]AFM.

METHODS

Fifteen participants with PTSD and 15 healthy control (HC) subjects without trauma history underwent a resting-state positron emission tomography scan.

RESULTS

[(11)C]AFM binding potential (BP(ND)) within the combined bilateral amygdala region of interest was significantly reduced in the PTSD group compared with the HC group (p = .027; 16.3% reduction), which was largely driven by the between-group difference in the left amygdala (p = .008; 20.5% reduction). Furthermore, amygdala [(11)C]AFM BP(ND) was inversely correlated with both Hamilton Rating Scale for Anxiety scores (r = -.55, p = .035) and Montgomery-Åsberg Depression Rating Scale scores (r = -.56, p = .029).

CONCLUSIONS

Our findings of abnormally reduced amygdala 5-HTT binding in PTSD and its association with higher anxiety and depression symptoms in PTSD patients support a translational neurobiological model of PTSD directly implicating dysregulated 5-HTT signaling within neural systems underlying threat detection and fear learning.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

fMRI of cocaine self-administration in macaques reveals functional inhibition of basal ganglia

Disparities in cocaine-induced neurochemical and metabolic responses between human beings and rodents motivate the use of non-human primates (NHP) to model consequences of repeated cocaine exposure in human subjects. To characterize the functional response to cocaine infusion in NHP brain, we employed contrast-enhanced fMRI during both non-contingent injection of drug and self-administration of cocaine in the magnet. Cocaine robustly decreased cerebral blood volume (CBV) throughout basal ganglia and motor/pre-motor cortex and produced subtle functional inhibition of prefrontal cortex. No brain regions exhibited significant elevation of CBV in response to cocaine challenge. Theses effects in NHP brain are opposite in sign to the cocaine-induced fMRI response in rats, but consistent with previous measurements in NHP based on glucose metabolism. Because the striatal ratio of D2 to D1 receptors is larger in human beings and NHP than rats, we hypothesize that the inhibitory effects of D2 receptor binding dominate the functional response in primates, whereas excitatory D1 receptor stimulation predominates in the rat. If the NHP accurately models the human response to cocaine, downregulation of D2 receptors in human cocaine-abusing populations can be expected to blunt cocaine-induced functional responses, contributing to the weak and variable fMRI responses reported in human basal ganglia following cocaine infusion.

Genetic variation in HTR2A influences serotonin transporter binding potential as measured using PET and [11C]DASB

In a previous study we showed that genetic variation in HTR2A, which encodes the serotonin 2A receptor, influenced outcome of citalopram treatment in patients with major depressive disorder. Since chronic administration of citalopram, which selectively and potently inhibits the serotonin transporter (5-HTT), putatively enhances serotonergic transmission, it is conceivable that genetic variation within HTR2A also influences pretreatment 5-HTT function or serotonergic transmission. The present study used positron emission tomography (PET) and the selective 5-HTT ligand, [11C]DASB, to investigate whether the HTR2A marker alleles that predict treatment outcome also predict differences in 5-HTT binding. Brain levels of 5-HTT were assessed in vivo using PET measures of the non-displaceable component of the [11C]DASB binding potential (BPND). DNA from 43 patients and healthy volunteers, all unmedicated, was genotyped with 14 single nucleotide polymorphisms located within or around HTR2A. Allelic association with BPND was assessed in eight brain regions, with covariates to control for race and ethnicity. We detected allelic association between [11C]DASB BPND in thalamus and three markers in a region spanning the 3' untranslated region and second intron of HTR2A (rs7333412, p=0.000045; rs7997012, p=0.000086; rs977003, p=0.000069). The association signal at rs7333412 remained significant (p<0.05) after applying corrections for multiple testing via permutation. Genetic variation in HTR2A that was previously associated with citalopram treatment outcome was also associated with thalamic 5-HTT binding. While further work is needed to identify the actual functional genetic variants involved, these results suggest that a relationship exists between genetic variation in HTR2A and either 5-HTT expression or central serotonergic transmission that influences the therapeutic response to 5-HTT inhibition in major depression.

How have developments in molecular imaging techniques furthered schizophrenia research?

Molecular imaging techniques have led to significant advances in understanding the pathophysiology of schizophrenia and contributed to knowledge regarding potential mechanisms of action of the drugs used to treat this illness. The aim of this article is to provide a review of the major findings related to the application of molecular imaging techniques that have furthered schizophrenia research. This article focuses specifically on neuroreceptor imaging studies with PET and SPECT. After providing a brief overview of neuroreceptor imaging methodology, we consider relevant findings from studies of receptor availability, and dopamine synthesis and release. Results are discussed in the context of current hypotheses regarding neurochemical alterations in the illness. We then selectively review pharmacological occupancy studies and the role of neuroreceptor imaging in drug development for schizophrenia.

Positron emission tomography imaging of D(2/3) agonist binding in healthy human subjects with the radiotracer [(11)C]-N-propyl-norapomorphine: preliminary evaluation and reproducibility studies

OBJECTIVE

(-)-N-[(11)C]-propyl-norapomorphine (NPA) is a full dopamine D(2/3) receptor agonist radiotracer suitable for imaging D(2/3) receptors configured in a state of high affinity for agonists using positron emission tomography. The aim of the present study was to define the optimal analytic method to derive accurate and reliable D(2/3) receptor parameters with [(11)C]NPA.

METHODS

Six healthy subjects (four females/two males) underwent two [(11)C]NPA scans in the same day. D(2/3) receptor-binding parameters were estimated using kinetic analysis (using one- and two-tissue compartment models) as well as simplified reference tissue method in the three functional subdivisions of the striatum (associative striatum, limbic striatum, and sensorimotor striatum). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (V(T)), binding potential relative to plasma concentration (BP(P)), and binding potential relative to nondisplaceable uptake (BP(ND)).

RESULTS

A two-tissue compartment kinetic model adequately described the functional subdivisions of the striatum as well as cerebellum time-activity data. The reproducibility of V(T) was excellent (<or=10%) in all regions, for this approach. The reproducibility of both BP(P) (<or=12%) and BP(ND) (<or=10%) was also excellent. The intraclass correlation coefficients of BP(P) and BP(ND) were acceptable as well (>0.75) in the three functional subdivisions of the striatum. Although SRTM led to an underestimation of BP(ND) values relative to that estimated by kinetic analysis by 8-13%, the values derived using both the methods were reasonably well correlated (r(2) = 0.89, n = 84). Both methods were similarly effective in detecting the differences in [(11)C]NPA BP(ND) between subjects.

CONCLUSION

The results of this study indicate that [(11)C]NPA can be used to measure D(2/3) receptors configured in a state of high affinity for the agonists with high reliability and reproducibility in the functional subdivisions of the human striatum.

Reduced serotonin transporter binding in the insular cortex in patients with obsessive-compulsive disorder: a [11C]DASB PET study

The serotonin transporter (5-HTT) and other markers of the serotonergic system have been of interest in the pathophysiology of obsessive-compulsive disorder (OCD). Previous studies using single photon emission computed tomography (SPECT) with [(123)I]beta-CIT or positron emission tomography (PET) with [(11)C]McN5652 have not shown consistent findings about 5-HTT in OCD patients. The aim of the present study was to investigate 5-HTT binding using [(11)C]DASB, which has higher selectivity or specific binding-to-nonspecific binding ratios for 5-HTT compared to the aforementioned radioligands. Four drug-naive and 6 drug-free patients with OCD who were free of comorbid depression and 18 gender and age-matched healthy subjects underwent PET scans with [(11)C]DASB. The severity of OCD was assessed by Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) (mean+/-SD: 22+/-7.6, range: 7-32). The binding potential (BP(ND)) of [(11)C]DASB was calculated using a two-parameter multilinear reference tissue model (MRTM2). The parametric images of BP(ND) were analyzed using a statistical parametric mapping system. Significant reductions of BP(ND) were observed in the right posterior and left anterior insular cortices in patients with OCD compared to controls. Region-of-interest analysis has also confirmed significant reduction of BP(ND) in the insular cortex. Although significantly reduced BP(ND) in the orbitofrontal cortex was also observed in patients with OCD compared to controls, this finding should be considered with caution because of the very low 5-HTT binding in the region. On the other hand, no significant correlation was observed between the Y-BOCS score and BP(ND). The change in [(11)C]DASB binding in the insular cortex suggests that dysfunction of the serotonergic system in the limbic area might be involved in the pathophysiology of OCD.

Compartmental modeling of 11C-HOMADAM binding to the serotonin transporter in the healthy human brain

The novel PET radioligand (11)C-N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine ((11)C-HOMADAM) binds with high affinity and selectively to the serotonin transporter (SERT). The purpose of this study was to develop a reliable kinetic model to describe the uptake of (11)C-HOMADAM in the healthy human brain.

METHODS

Eight volunteers participated in the study; 5 of them were fitted with arterial catheters for blood sampling and all were scanned on a high-resolution research tomograph after the injection of (11)C-HOMADAM. Regional distribution volumes and binding potentials were calculated with 2- and 4-parameter arterial-input compartment models, a 3-parameter reference tissue compartment model, and the Logan graphic approach.

RESULTS

The 2-parameter arterial-input compartment model was statistically superior to the 4-parameter model and described all brain regions. Calculated binding potentials agreed well between the arterial-input model and the reference tissue model when the cerebellum was used as the reference tissue. The Logan graphic approach was not able to estimate the higher concentration of SERT in the dorsal raphe than in the midbrain.

CONCLUSION

(11)C-HOMADAM is a highly promising radioligand with high ratios of specific binding to nonspecific binding in known SERT-rich structures, such as the raphe nuclei. The 3-parameter reference tissue model approach permits a simplified quantitatively accurate method for estimating SERT binding potentials.

Synthesis, radiosynthesis, and biological evaluation of fluorine-18-labeled 2beta-carbo(fluoroalkoxy)-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (K(i) values = 0.3-0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [(18)F]1-[(18)F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min, followed by a steady washout. An awake monkey study was performed with [(18)F]1, which demonstrated that the uptake of [(18)F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [(18)F]1-[(18)F]4, but chase studies with the DAT ligand 16 did not displace [(18)F]1-[(18)F]4 thus indicating that the tracers were binding specifically to the SERT.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

2-(2'-((Dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine for positron emission tomography imaging of serotonin transporters

INTRODUCTION

A new (18)F ligand, 2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine ([(18)F]1), for positron emission tomography (PET) imaging of serotonin transporters (SERT) was evaluated.

METHODS

Binding affinity was determined through in vitro binding assays with LLC-PK1 cells overexpressing SERT, NET or DAT (LLC-SERT, LLC-NET and LLC-DAT) and with rat cortical homogenates. Localization and selectivity of [(18)F]1 binding in vivo were evaluated by biodistribution, autoradiography and A-PET imaging studies in rats.

RESULTS

This compound displayed excellent binding affinity for SERT in vitro with K(i)=0.33 and 0.24 nM in LLC-SERT and rat cortical homogenates, respectively. Biodistribution studies with [(18)F]1 showed good brain uptake (1.61% dose/g at 2 min postinjection), high uptake into the hypothalamus (1.22% dose/g at 30 min) and a high target-to-nontarget (hypothalamus to cerebellum) ratio of 9.66 at 180 min postinjection. Pretreatment with a SERT selective inhibitor considerably inhibited [(18)F]1 binding in biodistribution studies. Ex vivo autoradiography reveals [(18)F]1 localization to brain regions with high SERT density, and this binding was blocked by pretreatment with SERT selective inhibitors. Small animal PET (A-PET) imaging in rats provided clear images of tracer localization in the thalamus, midbrain and striatum. In A-PET chasing experiments, injecting a SERT selective inhibitor 75 min post-tracer injection causes a dramatic reduction in regional radioactivity and the target-to-nontarget ratio.

CONCLUSION

The results of the biological studies and the ease of radiosynthesis with moderately good radiochemical yield (RCY=10-35%) make [(18)F]1 an excellent candidate for SERT PET imaging.

Synthesis and in vivo evaluation of halogenated N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine derivatives as PET serotonin transporter ligands

N, N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (38), substituted on ring A, was reported to display high binding affinity and selectivity to the human brain serotonin transporter (SERT). In an attempt to explore the potential of compounds substituted on ring B of the phenylthiophenyl core structure, three derivatives of 38 were synthesized: N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-fluorobenzylamine (35), N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-bromobenzylamine (36), and N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-iodobenzylamine (37). The in vitro binding studies in cells transfected with human SERT, norepinephrine transporter (NET), and dopamine transporter (DAT) showed that 35, 36, and 37 exhibited high SERT affinity with K is (SERT) = 1.26, 0.29, and 0.31 nM (vs [(3)H]citalopram), respectively. [(11)C]-(35), [(11)C]-(36), and [(11)C]-( 37) were prepared by methylation of their monomethyl precursors 16, 17, and 18, with [(11)C]iodomethane in 28, 11, and 14% radiochemical yields, respectively. The microPET images of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) showed high uptake in the monkey brain regions rich in SERT with peak midbrain to cerebellum ratios of 3.41, 3.24, and 3.00 at 85 min post-injection, respectively. In vivo bindings of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) were shown to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT-rich regions to the cerebellum level. These results suggest that [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) could be potential agents for mapping human SERT by PET and radiolabeling 37 with iodine-123, which could afford the first SPECT SERT imaging agent exhibiting fast kinetics.

Neuroimaging of the serotonin reuptake site requires high-affinity ligands

Numerous attempts have been made to develop suitable radiolabeled tracers for positron emission tomography or single photon emission computed tomography imaging of the serotonin transporter (SERT), but most often, negative outcomes are reported. The aim of this study is to define characteristics of a good SERT radioligand and to investigate species differences. We examined seven different selective serotonin reuptake inhibitors (SSRIs) and that except for one all have been previously tested as emission tomography ligands. The outcome of the ligands as emission tomography tracers was compared in relation with receptor density (Bmax) and/or ligand affinity (Kd) in rat and monkey cerebrum and cerebellum (reference region) membranes. [3H]-(S)-Citalopram and [3H]-(+)-McN5652 display statistically significantly lower affinity, whereas [3H]paroxetine displays statistically significantly higher affinity for SERT in monkey cortex when compared with the rat cerebrum. The affinity of [3H]MADAM, [123I]ADAM, and [11C]DASB for SERT obtained with rat cerebrum and monkey cortex are similar. In monkey cortex, Kd and Bmax could not be determined with [3H]fluoxetine. Of the seven SSRIs, [3H]-(S)-citalopram, [3H]MADAM, and [11C]DASB displayed significant specific binding to SERT in monkey cerebellum, with Bmax cortex:cerebellum ratios being 17, 3, and 4, respectively. In rat brain tissue the ratios were 12, 6, and 3, respectively. In conclusion, it can be estimated that imaging of the human SERT in a high-density region requires radioligands with Kd values between 0.03 and a maximum of 0.3 nM (at 37 degrees C). The differential specific cerebellar binding raises the question of the suitability of cerebellum as a reference region for nonspecific binding.

Effects of increased endogenous serotonin on the in vivo binding of [11C]DASB to serotonin transporters in conscious monkey brain

Using a combination of positron emission tomography (PET) and the microdialysis technique, the effects of increased endogenous serotonin (5-hydroxytryptamine; 5-HT) on the binding of [(11)C]DASB to 5-HT transporters (5-HTT) were investigated in the conscious monkey brain. Five rhesus monkeys (Macaca mulatta) were scanned with [(11)C]DASB under the control condition and the increased endogenous 5-HT condition, in which 5-hydroxy-L-tryptophan (5-HTP) was administered (20 mg/kg, i.v.) before the PET scan. Compared with the control scan, the 5-HTP administration significantly decreased the binding potential (BP) (BP = B(max)/K(d)) of [(11)C]DASB in several brain regions. The mean % decrease of BP was biggest in the caudate and putamen. Two monkeys were scanned with [(11)C]5-HTP to assess the amino acid decarboxylase (AADC) activity in the brain, resulting in the high activity in the caudate and putamen. Microdialysis measurements showed that although 5-HTP administration (20 mg/kg, i.v.) increased the extracellular 5-HT levels in both the prefrontal cortex and caudate, the increase of the 5-HT level in the caudate was 27 times higher than that in the prefrontal cortex. These results suggest that the caudate and putamen, both of which show high AADC activity, convert 5-HTP to 5-HT at a high rate, and the increased 5-HT competes with [(11)C]DASB for the 5-HTT.

Neuroreceptor imaging studies in schizophrenia

The ability of SPECT and PET to image specific biomolecules in the living brain provides a unique tool for clinical researchers. It is therefore not surprising that the use of neuroreceptor-imaging techniques has become more widespread over the past decade. This article reviews the application of these techniques to the study of schizophrenia. The design of neuroreceptor-imaging studies performed in the field of schizophrenia research can be broadly divided into two categories: (1) studies of pathophysiology and (2) studies of pharmacology. The former examines neuroreceptor and neurotransmitter parameters in individuals with schizophrenia compared to control subjects in order to provide a better understanding of the disease process. Studies of pharmacology seek to elucidate the mechanism of action for the treatments utilized in schizophrenia. This review will consider both studies of pathophysiology and pharmacology, with a discussion of the application of these techniques to drug development.

3-Amino-4-(2-((4-[18F]fluorobenzyl)methylamino)methylphenylsulfanyl)benzonitrile, an F-18 fluorobenzyl analogue of DASB: synthesis, in vitro binding, and in vivo biodistribution studies

3-amino-4-(2[11C]methylaminomethylphenylsulfanyl)benzonitrile (C-11 DASB) exhibits excellent in vitro and in vivo properties toward the serotonin transporter. If labeled with a longer physical half-life radioisotope, this ligand could be more attractive to research groups lacking an on-site cyclotron or lacking C-11 synthesis capabilities. We produce p-[18F]fluorobenzyl iodide on a routine basis to synthesize several neuroimaging agents. Therefore, it was straightforward for us to substitute the DASB precursor with this prosthetic group and assess its biological properties. We designed a different synthesis strategy to obtain the DASB precursor (desmethyl DASB). Herein we report an efficient and facile synthetic route that provides higher chemical yields of 3-amino-4-(2-aminomethylphenylsulfanyl)benzonitrile and related analogues. In addition, we report our results from incorporating p-[18F]fluorobenzyl iodide in DASB precursor and its affect on the in vitro and in vivo biological properties of the DASB.

Effect on [11C]DASB binding after tranylcypromine-induced increase in serotonin concentration: positron emission tomography studies in monkeys and rats

Several research groups have demonstrated that under specific conditions, in vivo neuroreceptor binding techniques can be used to measure acute changes in the concentrations of endogenous transmitters in the vicinity of neuroreceptors. The aim of this study was to investigate whether [(11)C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([(11)C]DASB) binding to the plasma membrane serotonin transporter (SERT) in the rhesus monkey and rat brain decreased after a pharmacologically-induced increase in the interstitial serotonin (5HT) concentration. Three rhesus monkeys were given repeated single boluses of [(11)C]DASB in sequential positron emission tomography (PET) experiments. Rats were given the tracer as a bolus dose plus a constant infusion. In vivo binding in both models was studied before and after presumably having increased interstitial 5HT concentrations using tranylcypromine (TCP), which inhibits the enzyme (monoamine oxidase, MAO), that degrades 5HT. The rat brain tissue was analyzed using high-performance liquid chromatography (HPLC) to determine the proportion of the PET signal comprising unchanged [(11)C]DASB. The binding of [(11)C]DASB in the thalamus decreased in both rhesus monkeys and rats after TCP administration. The possibility of using [(11)C]DASB as a tool for monitoring changes in endogenous serotonin concentrations merits further investigation.

Serotonin transporter binding in bipolar disorder assessed using [11C]DASB and positron emission tomography

BACKGROUND

Evidence from neuroimaging post-mortem, and genetic studies suggests that bipolar disorder (BD) is associated with abnormalities of the serotonin-transporter (5-HTT) system. Because of various limitations of these studies, however, it has remained unclear whether 5-HTT binding is abnormal in unmedicated BD-subjects. This study used PET and [(11)C]DASB, a radioligand that afforded higher sensitivity and specificity for the 5-HTT than previously available radioligands, to compare 5-HTT binding between BD and control subjects.

METHODS

The 5-HTT binding-potential (BP) was assessed in 18 currently-depressed, unmedicated BD-subjects and 37 healthy controls using PET and [(11)C]DASB.

RESULTS

In BD, the mean 5-HTT BP was increased in thalamus, dorsal cingulate cortex (DCC), medial prefrontal cortex and insula and decreased in the brainstem at the level of the pontine raphe-nuclei. Anxiety ratings correlated positively with 5-HTT BP in insula and DCC, and BP in these regions was higher in subjects manifesting pathological obsessions and compulsions relative to BD-subjects lacking such symptoms. Subjects with a history of suicide attempts showed reduced 5-HTT binding in the midbrain and increased binding in anterior cingulate cortex versus controls and to BD-subjects without attempts.

CONCLUSIONS

This is the first study to report abnormalities in 5-HTT binding in unmedicated BD-subjects. The direction of abnormality in the brainstem was opposite to that found in the cortex, thalamus, and striatum. Elevated 5-HTT binding in the cortex may be related to anxiety symptoms and syndromes associated with BD.

Neuroimaging findings with MDMA/ecstasy: technical aspects, conceptual issues and future prospects

Users of ecstasy (3,4-methylenedioxymethamphetamine; MDMA) may be at risk of developing MDMA-induced injury to the serotonin (5-HT) system. Previously, there were no methods available for directly evaluating the neurotoxic effects of MDMA in the living human brain. However, development of in vivoneuroimaging tools have begun to provide insights into the effects of ecstasy on the human brain. Single photon emission computed tomography (SPECT), positron emission computed tomography (PET) and proton magnetic resonance spectroscopy (1H-MRS) studies which have evaluated ecstasy's neurotoxic potential will be reviewed and discussed in terms of technical aspects, conceptual issues and future prospects. Although PET and SPECT may be limited by several factors such as the low cortical uptake and the use of a non-optimal reference region (cerebellum) the few studies conducted so far provide suggestive evidence that people who heavily use ecstasy are at risk of developing subcortical, and probably also cortical reductions in serotonin transporter (SERT) densities, a marker of 5-HT neurotoxicity. There seem to be dose-dependent and transient reductions in SERT for which females may be more vulnerable than males. 1H-MRS appears to be a less sensitive technique for studying ecstasy's neurotoxic potential. Whether individuals with a relatively low ecstasy exposure also demonstrate loss of SERT needs to be determined. Because most studies have had a retrospective design, in which evidence is indirect and differs in the degree to which any causal links can be implied, longitudinal studies in human ecstasy users are needed to draw definite conclusions.

[(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.

Imaging of serotonin transporters and its blockade by citalopram in patients with major depression using a novel SPECT ligand [123I]-ADAM

We studied the midbrain SERT availability in patients with major depression and assessed the relation of SERT occupancy by citalopram to the treatment response. 21 non-medicated patients with major depression and 13 healthy controls were examined by [(123)I]-ADAM SPECT. The midbrain SERT availability (SERT V(3)'') was calculated using individual MRI scans. In 13/21 patients SPECT was repeated 7 days after oral medication with citalopram (10 mg/day). We found no significant difference in the mean midbrain SERT availability between the studied patients with major depression and healthy controls (0.86 +/- 0.27 vs. 0.71 +/- 0.44, p = 0.069). The mean SERT occupancy accounted to 61%. The degree of SERT blockade by citalopram did not correlate with the reduction in HAMD total score. Treatment with low-dosed citalopram caused individually variable occupancy of the midbrain-SERT and a rapid clinical improvement in 54% of the investigated patients.

Comparative evaluation of positron emission tomography radiotracers for imaging the norepinephrine transporter: (S,S) and (R,R) enantiomers of reboxetine analogs ([11C]methylreboxetine, 3-Cl-[11C]methylreboxetine and [18F]fluororeboxetine), (R)-[11C]nisoxetine, [11C]oxaprotiline and [11C]lortalamine

We have synthesized and evaluated several new ligands for imaging the norepinephrine transporter (NET) system in baboons with positron emission tomography (PET). Ligands possessing high brain penetration, high affinity and selectivity, appropriate lipophilicity (log P = 1.0-3.5), high plasma free fraction and reasonable stability in plasma were selected for further studies. Based on our characterization studies in baboons, including 11C-labeled (R)-nisoxetine (Nis), oxaprotiline (Oxap), lortalamine (Lort) and new analogs of methylreboxetine (MRB), in conjunction with our earlier evaluation of 11C and 18F derivatives of reboxetine, MRB and their individual (R,R) and (S,S) enantiomers, we have identified the superiority of (S,S)-[11C]MRB and the suitability of MRB analogs [(S,S)-[11C]MRB > (S,S)-[11C]3-Cl-MRB > (S,S)-[18F]fluororeboxetine] as potential NET ligands for PET. In contrast, Nis, Oxap and Lort displayed high uptake in striatum (higher than in thalamus). The use of these ligands is further limited by high non-specific binding and relatively low specific signal, as is characteristic of many earlier NET ligands. Thus, to our knowledge (S,S)-[11C]MRB remains by far the most promising NET ligand for PET studies.

Serotonin transporter availability in patients with schizophrenia: a positron emission tomography imaging study with [11C]DASB

BACKGROUND

Postmortem studies have reported several alterations in serotonin transporter (SERT) binding parameters in patients with schizophrenia. The aim of this study was to compare SERT availability in vivo in patients with schizophrenia and matched control subjects.

METHODS

Ten medication-free patients with schizophrenia and 10 healthy subjects underwent positron emission tomography (PET) scans for 90 min after 11C-3-amino-4-(2-dimethylaminomethylphenylthio)benzonitrile ([11C]DASB) injection. Metabolite-corrected arterial input function was measured. Regional distribution volumes (mL/g) were derived with a two tissue compartment kinetic model. Outcome measures for SERT availability included binding potential (BP) and the specific-to-nonspecific equilibrium partition coefficient (V3''). Ten brain regions with high density of SERT and where SERT availability can be reliably quantified with [11C]DASB were included in the analysis.

RESULTS

No significant differences were observed in regional BP or V3'' between patients and control subjects. No significant relationships were observed between regional SERT availability and severity of positive, negative, and depressive symptoms.

CONCLUSIONS

This study failed to detect alterations of SERT availability in patients with schizophrenia; however, this study does not rule out the possibility that schizophrenia might be associated with alterations of SERT density in the cortical regions, where the [11C]DASB-specific binding signal is too low for reliable quantification of SERT.

Synthesis, in Vitro Characterization, and Radiolabeling of N,N-Dimethyl-2-(2‘-amino-4‘-substituted-phenylthio)benzylamines: Potential Candidates as Selective Serotonin Transporter Radioligands

A series of N,N-dimethylated and N-monomethylated analogues of N,N-dimethyl-2-(2'-amino-4'-iodophenylthio)benzylamine substituted at the 4'-phenyl position have been prepared and evaluated in vitro for serotonin transporter (SERT) selectivity. Several derivatives were prepared where the 4'-position was either unsubstituted 13 and 33a or substituted with methyl 14a and 33b, ethenyl 14b and 34, ethyl 16 and 35, hydroxymethyl 20 and 41, hydroxyethyl 22, fluoroethyl 23, hydroxypropyl 27, and fluoropropyl 28. Competition binding in cells stably expressing the transfected human SERT, dopamine transporter (DAT), and norepinephrine transporter (NET) using [(3)H]citalopram, [(3)H]WIN 35,428 or [(125)I]RTI-55, and [(3)H]nisoxetine, respectively, demonstrated the following order of SERT affinity (K(i) (nM)): 14a (0.25) > 16 (0.49) > 20 (0.57) > 14b (1.12) > 13 (1.59) > 33b (1.94) = 35 (2.04) >> 23 (8.50) = 28 (8.55) > 41 (15.11) >> 22 (51) > 33a (83.43) > 27 (92). The K(i) values revealed that most of these derivatives displayed a high affinity for the SERT and a high selectivity over the DAT and NET. Moreover, substitution at the 4'-position of the dimethylated and monomethylated benzylamines differently influenced SERT binding: (i) the dimethylated benzylamines exhibited higher SERT affinity than the monomethylated ones, (ii) alkyl, alkenyl, or hydroxymethyl functions at the 4'-position afford compounds with high SERT affinity, and (iii) omega-hydroxy and fluoro-substituted ethyl and propyl groups at the 4'-position decrease the SERT affinity. From this series, the dimethylated derivatives 13, 14a, 14b, 16, and 20 were radiolabeled with carbon-11 and their log P(7.4) was calculated as a measure of their potential brain penetrance as positron emission tomography SERT imaging agents.