Comparison of [(18)F]altanserin and [(18)F]deuteroaltanserin for PET imaging of serotonin(2A) receptors in baboon brain: pharmacological studies

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Preclinical evaluation of [18F]D3FSP, deuterated AV-45, for imaging of β-amyloid in the brain

INTRODUCTION

Since the approval of three 18F labeled β-amyloid-targeting PET imaging agents, Amyvid (florbetapir f18, AV-45), Neuraceq (florbetaben f18, AV-1) and Vizamyl (flutemetamol f18, F-PIB), they have increasingly been employed to assist differential diagnosis of Alzheimer's disease in patients with dementia. Also, they are frequently used in selecting patients participating drug trials aiming to reduce β-amyloid (Aβ) plaques in the brain. The first approved tracer in this class was [18F]AV-45, which is metabolized rapidly in blood and some of its N-demethylated metabolites cross the blood brain barrier and resulted in lowering the image contrast. To improve metabolic stability of [18F]AV-45, we hypothesized that substituting N-CH3 with N-CD3 at the metabolically labile position, creating [18F]D3FSP, may reduce in vivo N-demethylation. We report the preclinical evaluation of [18F]D3FSP as an Aβ imaging agent.

METHODS

Preclinical pharmacology of [18F]D3FSP was evaluated using in vitro autoradiography and competitive binding assay. Biodistribution of [18F]D3FSP was evaluated in wild-type CD-1 mice. In vivo metabolism in mice and in vitro microsomal metabolism were analyzed by HPLC. Single dose acute toxicity of D3FSP was also performed in rats.

RESULTS

[18F]D3FSP showed high binding affinity to β-amyloid plaques (Ki = 3.44 ± 1.22 nM, a value similar as AV-45 (Ki = 4.02 ± 0.22 nM)), and displayed excellent β-amyloid binding in AD brain sections consistent with known Aβ regional distribution. After an iv injection it exhibited good initial brain uptake and fast washout in wild-type CD-1 mice. In vitro microsomal metabolism and in vivo metabolism in mice did not result in any significant differences between [18F]D3FSP and [18F]AV-45. No treatment-related mortality or any adverse effects were observed in single dose acute toxicity studies administered at hundred-folds of maximum human dose.

CONCLUSION

A new small molecule, [18F]D3FSP, was prepared and tested as an alternative to [18F]AV-45 to reduce N-demethylation in vivo. This strategy did not lead to better in vivo stability. However, [18F]D3FSP displayed very similar Aβ targeting property comparable to [18F]AV-45. Preclinical studies suggest that [18F]D3FSP is useful as a β-amyloid-targeting PET imaging agent.

Serotonin release measured in the human brain: a PET study with [11C]CIMBI-36 and d-amphetamine challenge

Positron emission tomography (PET) enables non-invasive estimation of neurotransmitter fluctuations in the living human brain. While these methods have been applied to dopamine and some other transmitters, estimation of 5-hydroxytryptamine (5-HT; Serotonin) release has proved to be challenging. Here we demonstrate the utility of the novel 5-HT2A receptor agonist radioligand, [¹¹C]CIMBI-36, and a d-amphetamine challenge to evaluate synaptic 5-HT changes in the living human brain. Seventeen healthy male volunteers received [¹¹C]CIMBI-36 PET scans before and 3 h after an oral dose of d-amphetamine (0.5 mg/kg). Dynamic PET data were acquired over 90 min, and the total volume of distribution (V_T) in the frontal cortex and the cerebellum derived from a kinetic analysis using MA1. The frontal cortex binding potential (BP_ND^frontal) was calculated as (V_T^frontal/V_T^cerebellum) - 1. ∆BP_ND^frontal = 1 - (BP_ND^frontal post-dose/BP_ND^frontal baseline) was used as an index of 5-HT release. Statistical inference was tested by means of a paired Students t-test evaluating a reduction in post-amphetamine [¹¹C]CIMBI-36 BP_ND^frontal. Following d-amphetamine administration, [¹¹C]CIMBI-36 BP_ND^frontal was reduced by 14 ± 13% (p = 0.002). Similar effects were observed in other cortical regions examined in an exploratory analysis. [¹¹C]CIMBI-36 binding is sensitive to synaptic serotonin release in the human brain, and when combined with a d-amphetamine challenge, the evaluation of the human brain serotonin system in neuropsychiatric disorders, such as major depression and Parkinson's disease is enabled.

Kinetic analysis of [18F] altanserin bolus injection in the canine brain using PET imaging

BACKGROUND

Currently, [¹⁸F] altanserin is the most frequently used PET-radioligand for serotonin_2A (5-HT_2A) receptor imaging in the human brain but has never been validated in dogs. In vivo imaging of this receptor in the canine brain could improve diagnosis and therapy of several behavioural disorders in dogs. Furthermore, since dogs are considered as a valuable animal model for human psychiatric disorders, the ability to image this receptor in dogs could help to increase our understanding of the pathophysiology of these diseases. Therefore, five healthy laboratory beagles underwent a 90-min dynamic PET scan with arterial blood sampling after [¹⁸F] altanserin bolus injection. Compartmental modelling using metabolite corrected arterial input functions was compared with reference tissue modelling with the cerebellum as reference region.

RESULTS

The distribution of [¹⁸F] altanserin in the canine brain corresponded well to the distribution of 5-HT_2A receptors in human and rodent studies. The kinetics could be best described by a 2-Tissue compartment (2-TC) model. All reference tissue models were highly correlated with the 2-TC model, indicating compartmental modelling can be replaced by reference tissue models to avoid arterial blood sampling.

CONCLUSIONS

This study demonstrates that [¹⁸F] altanserin PET is a reliable tool to visualize and quantify the 5-HT_2A receptor in the canine brain.

Radioligand binding analysis of α 2 adrenoceptors with [11C]yohimbine in brain in vivo: Extended Inhibition Plot correction for plasma protein binding

We describe a novel method of kinetic analysis of radioligand binding to neuroreceptors in brain in vivo, here applied to noradrenaline receptors in rat brain. The method uses positron emission tomography (PET) of [¹¹C]yohimbine binding in brain to quantify the density and affinity of α₂ adrenoceptors under condition of changing radioligand binding to plasma proteins. We obtained dynamic PET recordings from brain of Spraque Dawley rats at baseline, followed by pharmacological challenge with unlabeled yohimbine (0.3 mg/kg). The challenge with unlabeled ligand failed to diminish radioligand accumulation in brain tissue, due to the blocking of radioligand binding to plasma proteins that elevated the free fractions of the radioligand in plasma. We devised a method that graphically resolved the masking of unlabeled ligand binding by the increase of radioligand free fractions in plasma. The Extended Inhibition Plot introduced here yielded an estimate of the volume of distribution of non-displaceable ligand in brain tissue that increased with the increase of the free fraction of the radioligand in plasma. The resulting binding potentials of the radioligand declined by 50–60% in the presence of unlabeled ligand. The kinetic unmasking of inhibited binding reflected in the increase of the reference volume of distribution yielded estimates of receptor saturation consistent with the binding of unlabeled ligand.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Effects of serotonin-2A receptor binding and gender on personality traits and suicidal behavior in borderline personality disorder

Impulsivity and aggressiveness are personality traits associated with a vulnerability to suicidal behavior. Behavioral expression of these traits differs by gender and has been related to central serotonergic function. We assessed the relationships between serotonin-2A receptor function, gender, and personality traits in borderline personality disorder (BPD), a disorder characterized by impulsive-aggression and recurrent suicidal behavior. Participants, who included 33 BPD patients and 27 healthy controls (HC), were assessed for Axis I and II disorders with the Structured Clinical Interview for DSM-IV and the International Personality Disorders Examination, and with the Diagnostic Interview for Borderline Patients-Revised for BPD. Depressed mood, impulsivity, aggression, and temperament were assessed with standardized measures. Positron emission tomography with [(18)F]altanserin as ligand and arterial blood sampling was used to determine the binding potentials (BPND) of serotonin-2A receptors in 11 regions of interest. Data were analyzed using Logan graphical analysis, controlling for age and non-specific binding. Among BPD subjects, aggression, Cluster B co-morbidity, antisocial PD, and childhood abuse were each related to altanserin binding. BPND values predicted impulsivity and aggression in BPD females (but not BPD males), and in HC males (but not HC females.) Altanserin binding was greater in BPD females than males in every contrast, but it did not discriminate suicide attempters from non-attempters. Region-specific differences in serotonin-2A receptor binding related to diagnosis and gender predicted clinical expression of aggression and impulsivity. Vulnerability to suicidal behavior in BPD may be related to serotonin-2A binding through expression of personality risk factors.

In vivo evaluation of [¹²³I]mZIENT as a SPECT radioligand for the serotonin transporter

INTRODUCTION

In vivo imaging of the serotonin transporter continues to be a valuable tool in drug development and in monitoring diseases that alter serotonergic function. The purposes of this study were to: 1) evaluate the test/retest reproducibility of [¹²³I] 2β-Carbomethoxy-3β-(3'-((Z)-2-iodoethenyl)phenyl)nortropane ([¹²³I]mZIENT); and 2) to assess displacement of [¹²³I]mZIENT following administration of SERT specific drugs.

METHODS

Six female baboons (Papio anubis) were scanned following i.v. administration of [¹²³I]mZIENT. The regional binding potential (BP(nd)) was determined using a simplified reference tissue model, with the cerebellum used as a reference region. The test/retest reproducibility of BP(nd) was determined following repeated injection of [¹²³I]mZIENT on a different day. To assess the displacement of [¹²³I]mZIENT from SERT, citalopram (0.01-5mg/kg) or sertraline (0.01-0.5mg/kg) was given as iv bolus at ~4h following administration of [¹²³I]mZIENT.

RESULTS

The test/retest variability of BP(nd) was less than 10% for all SERT-rich brain regions. Estimates of ED50 for displacement of [¹²³I]mZIENT in SERT-rich regions were consistent with previous reports for the [¹¹C] analog of [¹²³I]mZIENT. Both citalopram and sertraline displaced [¹²³I]mZIENT from SERT in a dose-dependent manner, with maximal observed displacements of greater than 80% in the diencephalon and greater than 75% in brainstem for both citalopram and sertraline.

CONCLUSIONS

[¹²³I] mZIENT demonstrates good test-retest reproducibility; and initial displacement studies suggest that this compound is highly selective for SERT. Overall, this radioligand has favorable characteristics for use in drug development studies and/or longitudinal studies interrogating SERT.

Decreased defluorination using the novel beta-cell imaging agent [18F]FE-DTBZ-d4 in pigs examined by PET

Background

Fluorine-18 dihydrotetrabenazine [DTBZ] analogues, which selectively target the vesicular monoamine transporter 2 [VMAT2], have been extensively studied for in vivo quantification of beta cell mass by positron-emission tomography [PET]. This study describes a novel deuterated radioligand [¹⁸F]fluoroethyl [FE]-DTBZ-d4, aimed to increase the stability against in vivo defluorination previously observed for [¹⁸F]FE-DTBZ.

Methods

[¹⁸F]FE-DTBZ-d4 was synthesized by alkylation of 9-O-desmethyl-(+)-DTBZ precursor with deuterated [¹⁸F]FE bromide ([¹⁸F]FCD₂CD₂Br). Radioligand binding potential [BP] was assessed by an in vitro saturation homogenate binding assay using human endocrine and exocrine pancreatic tissues. In vivo pharmacokinetics and pharmacodynamics [PK/PD] was studied in a porcine model by PET/computed tomography, and the rate of defluorination was quantified by compartmental modeling.

Results

[¹⁸F]FE-DTBZ-d4 was produced in reproducible good radiochemical yield in 100 ± 20 min. Radiochemical purity of the formulated product was > 98% for up to 5 h with specific radioactivities that ranged from 192 to 529 GBq/μmol at the end of the synthesis. The in vitro BP for VMAT2 in the islet tissue was 27.0 ± 8.8, and for the exocrine tissue, 1.7 ± 1.0. The rate of in vivo defluorination was decreased significantly (k_{defluorination} = 0.0016 ± 0.0007 min^-1) compared to the non-deuterated analogue (k_{defluorination} = 0.012 ± 0.002 min^-1), resulting in a six fold increase in half-life stability.

Conclusions

[¹⁸F]FE-DTBZ-d4 has similar PK and PD properties for VMAT2 imaging as its non-deuterated analogue [¹⁸F]FE-DTBZ in addition to gaining significantly increased stability against defluorination. [¹⁸F]FE-DTBZ-d4 is a prime candidate for future preclinical and clinical studies on focal clusters of beta cells, such as in intramuscular islet grafts.

Assessment of serotonin release capacity in the human brain using dexfenfluramine challenge and [18F]altanserin positron emission tomography

Although alterations of serotonin (5-HT) system functioning have been proposed for a variety of psychiatric disorders, a direct method quantitatively assessing 5-HT release capacity in the living human brain is still lacking. Therefore, we evaluated a novel method to assess 5-HT release capacity in vivo using dexfenfluramine challenge and [(18)F]altanserin positron emission tomography (PET). Thirteen healthy male subjects received placebo and single oral doses of 40 mg (n = 6) or 60 mg (n = 7) of the potent 5-HT releaser dexfenfluramine separated by an interval of 14 days. Three further subjects received placebo on both days. Two hours after placebo/drug administration, 250 MBq of the 5-HT(2A) receptor selective PET-radiotracer [(18)F]altanserin was administered intravenously as a 30s bolus. Dynamic PET data were subsequently acquired over 90 min. Moreover, arterial blood samples were drawn for measurement of total activity and metabolite correction of the input function. Dexfenfluramine as well as cortisol and prolactin plasma concentration-time profiles was quantitatively determined. Tracer distribution volumes for five volumes-of-interest (prefrontal and occipital cortex, insula, thalamus, caudatum) were calculated by the Logan plot and a 2-tissue compartment model. Dexfenfluramine dose-dependently decreased the total distribution volume of [(18)F]altanserin in cortical regions independent of the PET modeling approach. Cortisol and prolactin plasma concentrations were dose-dependently increased by dexfenfluramine. The decrease in cortical [(18)F]altanserin receptor binding under dexfenfluramine was correlated with the increase of plasma prolactin. These data suggest that the combination of a dexfenfluramine-induced 5-HT release and subsequent assessment of 5-HT(2A) receptor availability with [(18)F]altanserin PET is suitable to measure cortical 5-HT release capacity in the human brain.

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.

Assessing the sensitivity of [¹¹C]p943, a novel 5-HT1B radioligand, to endogenous serotonin release

The main objective of the current study was to determine the sensitivity of the positron emission tomography (PET) radioligand [¹¹C]P943 to fenfluramine-induced changes in endogenous 5-HT in nonhuman primate brain. Fenfluramine-induced changes in 5-HT(1B) occupancy were compared to those obtained by self-block with unlabeled P943. Two baboons and 1 rhesus monkey were given preblocking or displacing doses of fenfluramine (1-5 mg/kg) or preblocking doses of unlabeled P943 (0.2 mg/kg) and imaged with [¹¹C]P943 PET. Receptor occupancy by the low dose of fenfluramine (1 mg/kg) in the baboons was 25 and 29% and by the high dose of fenfluramine (5 mg/kg) in the rhesus macaque was 42%. Receptor occupancy by P943 (0.2 mg/kg) was 68 and 86% in the baboons. PET imaging of 5-HT(1B) receptors with [¹¹C]P943 may be a useful approach for measuring changes in endogenous 5-HT in the living human brain.

Measuring endogenous 5-HT release by emission tomography: promises and pitfalls

Molecular in vivo neuroimaging techniques can be used to measure regional changes in endogenous neurotransmitters, evoked by challenges that alter synaptic neurotransmitter concentration. This technique has most successfully been applied to the study of endogenous dopamine release using positron emission tomography, but has not yet been adequately extended to other neurotransmitter systems. This review focuses on how the technique has been applied to the study of the 5-hydroxytryptamine (5-HT) system. The principles behind visualising fluctuations in neurotransmitters are introduced, with reference to the dopaminergic system. Studies that aim to image acute, endogenous 5-HT release or depletion at 5-HT receptor targets are summarised, with particular attention to studies in humans. Radiotracers targeting the 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors and the serotonin reuptake transporter have been explored for their sensitivity to 5-HT fluctuations, but with mixed outcomes; tracers for these targets cannot reliably image endogenous 5-HT in humans. Shortcomings in our basic knowledge of the mechanisms underlying changes in binding potential are addressed, and suggestions are made as to how the selection of targets, radiotracers, challenge paradigms, and experimental design might be optimised to improve our chances of successfully imaging endogenous neurotransmitters in the future.

Gender, personality, and serotonin-2A receptor binding in healthy subjects

The vulnerability to mood disorders, impulsive-aggression, eating disorders, and suicidal behavior varies greatly with gender, and may reflect gender differences in central serotonergic function. We investigated the relationships of gender, mood, impulsivity, aggression and temperament to 5HT(2A) receptor binding in 21 healthy subjects using [18F]altanserin and PET neuroimaging. Binding potentials in pre-defined regions-of-interest (ROI) were calculated using the Logan graphical method, corrected for partial volume effects, and compared by gender with age co-varied. SPM analysis was used for voxel level comparisons. Altanserin binding (BP(P)) was greater in male than female subjects in the following nine ROIs: hippocampus (HIP) and Lt. HIP, lateral orbital frontal cortex (LOF) and Lt. LOF, left medial frontal cortex (Lt. MFC), left medial temporal cortex (Lt. MTC), left occipital cortex (Lt. OCC), thalamus (THL) and Lt. THL. Differences in Lt. HIP and Lt. MTL remained significant after Bonferroni correction. Gender differences were noted in the co-variation of psychological traits with BP(P) values in specific ROIs. Among males alone, aggression was negatively correlated with BP(P) values in Lt. LOF and Lt. MFC, and Suspiciousness positively correlated in LOF, Lt. LOF and Lt. MFC. Among female subjects alone, Negativism was positively correlated with BP(P) values in HIP, and Verbal Hostility in Lt. HIP. Altanserin binding in Lt. MTC was positively correlated with Persistence, with no significant gender effect. Gender differences in 5HT(2A) receptor function in specific ROIs may mediate expression of psychological characteristics such as aggression, suspiciousness and negativism. Future studies of 5HT(2A) receptor function and its relationship to behavior should control for gender.

Imaging neurotransmitter release by drugs of abuse

Previous studies have shown that imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) radiotracers that are specific for brain dopamine receptors can be used to indirectly image the change in the levels of neurotransmitters within the brain. Most of the studies in addiction have focused on dopamine, since the dopamine neurons that project to the striatum have been shown to play a critical role in mediating addictive behavior. These imaging studies have shown that increased extracellular dopamine produced by psychostimulants can be measured with PET and SPECT. However, there are some technical issues associated with imaging changes in dopamine, and these are reviewed in this chapter. Among these are the loss of sensitivity, the time course of dopamine pulse relative to PET and SPECT imaging, and the question of affinity state of the receptor. In addition, animal studies have shown that most drugs of abuse increase extracellular dopamine in the striatum, yet not all produce a change in neurotransmitter that can be measured. As a result, imaging with a psychostimulant has become the preferred method for imaging presynaptic dopamine transmission, and this method has been used in studies of addiction. The results of these studies suggest that cocaine and alcohol addiction are associated with a loss of dopamine transmission, and a number of studies show that this loss correlates with severity of disease.

Regional distribution and behavioral correlates of 5-HT(2A) receptors in Alzheimer's disease with [(18)F]deuteroaltanserin and PET

Postmortem studies show reductions in brain serotonin 2A (5-HT(2A)) receptors in Alzheimer's disease (AD). Converging evidence also suggests that serotonergic dysregulation may contribute to behavioral symptoms that frequently occur in AD. This study aimed to define regional reductions in 5-HT(2A) binding in AD patients and to examine their behavioral correlates. Nine patients with probable AD and eight elderly controls were studied using a constant infusion paradigm for equilibrium modeling of [(18)F]deuteroaltanserin with positron emission tomography (PET). Region of interest analyses were performed on PET images coregistered to MRI scans. The outcome measures BP(P) (ratio of specific brain uptake to total plasma parent concentration) and BP(ND) (ratio of specific to nondisplaceable uptake) were obtained for pertinent cortical and subcortical regions. AD patients showed a statistically significant decrease in the anterior cingulate in both BP(P) and BP(ND), but in no other region. Within the AD patient sample, no significant correlations were observed between regional 5-HT(2A) binding and behavioral measures, including depressive and psychotic symptoms. These results confirm a reduction in cortical 5-HT(2A) receptors in AD, specifically in the anterior cingulate. However, in a limited AD patient sample, they fail to demonstrate a relationship between regional 5-HT(2A) binding and major behavioral symptoms.

Species differences in blood-brain barrier transport of three positron emission tomography radioligands with emphasis on P-glycoprotein transport

Species differences occur in the brain concentrations of drugs, but the reasons for these differences are not yet apparent. This study was designed to compare brain uptake of three radiolabeled P-glycoprotein (P-gp) substrates across species using positron emission tomography. Brain concentrations and brain-to-plasma ratios were compared; [(11)C]verapamil in rats, guinea pigs, and monkeys; [(11)C](S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethylamino)-2(S)-phenylpiperidine (GR205171) in rats, guinea pigs, monkeys, and humans; and [(18)F]altanserin in rats, minipigs, and humans. The fraction of the unbound radioligand in plasma was studied along with its metabolism. The effect of P-gp inhibition was investigated by administering cyclosporin A (CsA). Pronounced species differences were found in the brain and brain-to-plasma concentrations of [(11)C]verapamil, [(11)C]GR205171, and [(18)F]altanserin with higher brain distribution in humans, monkeys, and minipigs than in rats and guinea pigs. For example, the brain-to-plasma ratio of [(11)C]GR205171 was almost 9-fold higher in humans compared with rats. The species differences were still present after P-gp inhibition, although the increase in brain concentrations after P-gp inhibition was somewhat greater in rats than in the other species. Differences in plasma protein binding and metabolism did not explain the species-related differences. The findings are important for interpretation of brain drug delivery when extrapolating preclinical data to humans. Compounds found to be P-gp substrates in rodents are likely to also be substrates in higher species, but sufficient blood-brain barrier permeability may be retained in humans to allow the compound to act at intracerebral targets.

Imaging of cholinergic and monoaminergic neurochemical changes in neurodegenerative disorders

Positron emission tomography (PET) or single photon emission computer tomography (SPECT) imaging provides the means to study neurochemical processes in vivo. These methods have been applied to examine monoaminergic and cholinergic changes in neurodegenerative disorders. These investigations have provided important insights into disorders, such as Alzheimer's disease (AD) and Parkinson's disease (PD). The most intensely studied monoaminergic transmitter is dopamine. The extent of presynaptic nigrostriatal dopaminergic denervation can be quantified in PD and may serve as a diagnostic biomarker. Dopaminergic receptor imaging may help to distinguish idiopathic PD from atypical parkinsonian disorders. Cholinergic denervation has been identified not only in AD but also in PD and more severely in parkinsonian dementia. PET or SPECT can also provide biomarkers to follow progression of disease or evaluate the effects of therapeutic interventions. Cholinergic receptor imaging is expected to play a major role in new drug development for dementing disorders.

5HT2A receptor binding is increased in borderline personality disorder

BACKGROUND

Postmortem studies in suicide victims demonstrate an increase in the number of post-synaptic 5-HT(2A) receptor binding sites in ventral lateral and orbital frontal cortex. Diminished metabolic responses to serotonergic activation are noted in these areas in impulsive subjects with borderline personality disorder (BPD), a group at high risk for suicidal behaviors. We examined 5HT(2A) receptor binding potential (BP) in impulsive subjects with BPD, with positron emission tomography neuroimaging with [(18)F] altanserin.

METHODS

Fourteen female subjects with BPD were assessed for Axis I comorbidity, depressed mood, impulsivity, aggression, suicidality, childhood abuse, and compared with 11 healthy female control subjects. The 5HT(2A) receptor binding was evaluated in prefrontal cortex, anterior cingulate, hippocampus, temporal lobe, occipital cortex, and thalamus. Data were analyzed with Logan graphical analysis and a four-compartment (4C) model.

RESULTS

Hippocampal 5HT(2A) receptor binding was significantly increased in BPD subjects compared with control subjects in both Logan and 4C analyses, covarying for age. Hippocampal BP values were related to comorbid major depressive episode, with highest values found in non-depressed BPD subjects and lowest in healthy control subjects. The BP values were not related to depressed mood, impulsivity, aggression, suicidality, or childhood abuse.

CONCLUSIONS

5HT(2A) receptor binding is increased in the hippocampus of BPD subjects independent of depressed mood, impulsivity, aggression, suicidality, or childhood abuse. Dysregulation of serotonergic function in hippocampus might contribute to affective and behavioral symptoms in BPD.

Chemical fate of the nicotinic acetylcholinergic radiotracer [123I]5-IA-85380 in baboon brain and plasma

The fate of the nicotinic acetylcholinergic receptor radiotracer [123I]5-IA-85380 ([123I]5-IA) was studied in baboon by analyzing the chemical composition of brain tissue and plasma after intravenous administration of the tracer. Acetonitrile denaturation and high-performance liquid chromatography (HPLC) analysis showed predominantly unchanged (91-98%) parent tracer in all brain tissues examined, compared to significant metabolism (23% parent) in the plasma at 90 min postinjection, and control tissue recovery of 95-98%. [123I]5-IA was distributed to the thalamus with a standardized uptake value of 9.2 (0.04% dose/g) or a concentration 5.8 times higher than that of the cerebellum. The HPLC behavior of a synthesized sample of one hypothesized metabolite, 5-iodo-3-pyridinol (5-IP), was consistent with plasma radiometabolite fraction. Since only parent radiotracer compound was found in brain tissue, these results add confidence that information derived from single photon emission computed tomography images of 123I activity in the brain after [123I]5-IA administration can be interpreted as distribution of an intact radiotracer.

Synthesis and in vivo evaluation of [O-methyl-11C](2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine as a 5-HT2A receptor PET ligand

The serotonin2A (5-HT2A) receptor is implicated in the pathophysiology of schizophrenia and mood disorders, and in vivo studies of this receptor would be of value in studying the pathophysiology of these disorders and in measuring the relationship of clinical response to receptor occupancy for 5-HT2A antagonists such as atypical antipsychotics. Therefore, (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)-phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (MPM) (13), a selective and high-affinity (K(i)=0.79 nM) 5HT2A antagonist, has been radiolabeled with carbon-11 by O-methylation of the corresponding desmethyl analogue (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-hydroxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine (12) with [11C]methyltriflate in order to determine the suitability of [11C]MPM to quantify 5-HT2A in living brain using PET. Desmethyl-MPM 12 and standard MPM were prepared, starting from 3-hydroxymethylphenol (2), in excellent yield. The yield obtained for radiolabeling was 40+/-5% (EOB), and the total synthesis time was 30 min at EOS. PET studies with [11C]MPM in baboon showed a distribution in the brain consistent with the known distribution of 5-HT2A receptors. The time-activity curves for the high-binding regions peaked at approximately 45 min after injection. Blocking studies with M100907 demonstrated not only 38-57% blocking of tracer binding in brain regions known to have 5-HT2A receptors but also 38% blocking in cerebellum, which has a low 5-HT2A receptor concentration. Although [11C]MPM exhibits appropriate kinetics in baboon for imaging 5-HT2A receptors, its specific binding in cerebellum and higher proportion of nonspecific binding limit its usefulness for the in vivo quantification of 5-HT2A receptors with PET.

Differential effects of paroxetine on raphe and cortical 5-HT1A binding: a PET study in monkeys

Positron emission tomography (PET) ligands that are sensitive to transient changes in serotonin (5-HT) concentration are desirable for studies of neuropsychiatric diseases. Few studies, however, have sought to demonstrate that variations in 5-HT concentration can be closely tracked with available serotonergic ligands. Microdialysis studies in rats have shown a maximal increase in 5-HT concentration in raphe nuclei after systemic infusion of selective serotonergic re-uptake inhibitors (SSRIs). We performed PET scans with [(18)F]FPWAY, an intermediate-affinity antagonist of 5-HT(1A) receptors, in 4 anesthetized rhesus monkeys in control studies and after systemic paroxetine administration (5 mg/kg, i.v.). In addition, a paired [(11)C]DASB study revealed that this paroxetine regimen produced an occupancy of 54-83% of the serotonin transporters. According to the conventional receptor competition model, increased 5-HT concentration produces decreased binding of the radioactive ligand. Over a 3-h period following paroxetine infusion, a progressively increasing reduction (ranging from 8 +/- 6% to 27 +/- 10%) of [(18)F]FPWAY-specific binding was found in the raphe nuclei. This result is interpreted as an SSRI-induced increase in 5-HT concentration, potentially combined with reduced binding to internalized 5-HT(1A) receptors. In addition, a transient (1 h) increase in cerebral cortical binding was observed, attributed primarily to a reduction in cortical 5-HT due to the effects of raphe autoreceptor inhibition. This study is the first demonstration of the feasibility of quantifying dynamic changes in 5-HT neurotransmission in the raphe and the cortex with PET. These results lend promise to the use of these serotonergic neuroimaging techniques to study neuropsychiatric disorders.

Effects of reduced endogenous 5-HT on the in vivo binding of the serotonin transporter radioligand11C-DASB in healthy humans

Although abnormal serotonin (5-HT) function is implicated in a range of mental disorders, there is currently no method to directly assess 5-HT synaptic levels in the living human brain. The in vivo binding of some dopamine (DA) radioligands such as (11)C-raclopride is affected by fluctuations in endogenous DA, thus providing an indirect measure of DA presynaptic activity. Attempts to identify a serotonergic radiotracer with similar properties have proved unsuccessful. Here, we investigated in humans the effects of reduced synaptic 5-HT on the in vivo binding of the 5-HT transporter (SERT) radioligand (11)C-DASB, using Positron Emission Tomography (PET) and the rapid tryptophan depletion (RTD) technique. Eight (8) subjects (5M, 3F) were scanned with (11)C-DASB under control and reduced endogenous 5-HT conditions, in a within-subject, double-blind, counterbalanced, crossover design. Regional distribution volumes (V(T)) were calculated using kinetic modeling and metabolite-corrected arterial input function. (11)C-DASB specific binding was estimated as binding potential (BP) and specific to nonspecific equilibrium partition coefficient (V(")(3)), using the cerebellum as reference region. RTD caused small but significant mean reductions in (11)C-DASB V(T) (-6.1%) and BP (-4.5%) across brain regions, probably explained by a concomitant reduction in (11)C-DASB plasma free fraction (f(1)) of similar magnitude. No significant change in (11)C-DASB V(")(3) was observed between control and reduced 5-HT conditions. Nor was there a significant relationship between the magnitude of tryptophan depletion and change in BP and V(")(3) across individual subjects. These results suggest that (11)C-DASB in vivo binding is not affected by reductions in endogenous 5-HT.

[18F]altanserin binding to human 5HT2A receptors is unaltered after citalopram and pindolol challenge

The aim of the present study was to develop an experimental paradigm for the study of serotonergic neurotransmission in humans using positron emission tomography and the 5-HT2A selective radioligand [18F]altanserin. [18F]altanserin studies were conducted in seven subjects using the bolus/infusion approach designed for attaining steady state in blood and brain 2 hours after the initial [18F]altanserin administration. Three hours after commencement of radiotracer administration, 0.25 mg/kg of the selective serotonin reuptake inhibitor, citalopram (Lundbeck, Valby, Denmark), was administered to all subjects as a constant infusion for 20 minutes. To reduce 5-HT1A-mediated autoinhibition of cortical 5-HT release, four of the seven subjects were pretreated with the partial 5-HT1A agonist pindolol for 3 days at an increasing oral dose (25 mg on the day of scanning). In each subject, the baseline condition (120 to 180 minutes) was compared with the stimulated condition (195 to 300 minutes). Despite a pronounced increase in plasma prolactin and two subjects reporting hot flushes compatible with an 5-HT-induced adverse effect, cortical [18F]altanserin binding was insensitive to the citalopram challenge, even after pindolol pretreatment. The biochemical and cellular events possibly affecting the unsuccessful translation of the citalopram/pindolol challenge into a change in 5-HT2A receptor binding of [18F]altanserin are discussed.

Fenfluramine evokes 5-HT2A receptor-mediated responses but does not displace [11C]MDL 100907: small animal PET and gene expression studies

The in vivo binding of the 5-HT(2A) receptor-selective positron emission tomography (PET) ligand [(11)C]MDL 100907 and its sensitivity to endogenous 5-HT were quantified in rat brain using quad-HIDAC, a novel high-resolution PET camera for small animals. Specific binding of [(11)C]MDL 100907, estimated using volume of interest (VOI) to cerebellum ratios, corresponded well with both the known distribution of 5-HT(2A) receptors and tissue:cerebellum ratios obtained using ex vivo dissection. Specific binding was blocked by predosing with either nonradioactive MDL 100907 (0.2 or 0.4 mg/kg i.v.) or the 5-HT(2A/2C) receptor antagonist ketanserin (2 mg/kg i.v.), but was unaffected in rats pretreated with the 5-HT releasing agent, fenfluramine (10 mg/kg i.p.). In parallel studies, the same dose of fenfluramine was shown to be sufficient to cause an increase in the expression of the immediate early genes (IEG) c-fos and Arc mRNA in cortical regions with high 5-HT(2A) receptor density. This increase was blocked by MDL 100907 (0.2 mg/kg i.v.), confirming a 5-HT(2A) receptor-mediated effect. The results demonstrate that PET with [(11)C]MDL 100907 is insensitive to an increased concentration of synaptic 5-HT, implying that the ligand can be used clinically to monitor 5-HT(2A) receptor function or dysfunction in disease or during therapy, without the need to consider concomitant changes in neurotransmitter concentration.

Radiosynthesis of [18F] N-(3-Fluoropropyl)-2-β-Carbomethoxy-3-β-(4-Bromophenyl) Nortropane and the regional brain uptake in non human primate using PET

A synthetic procedure for the preparation of [18F]FPCBT, an imaging agent for the dopamine transporter (DAT), has been developed. The radiosynthesis was carried out in a two step procedure. Even though the yield was low, we were able to prepare 20 to 30 mCi of the product, which was enough for two or three studies. The radiochemical purity was greater than 96%. The in vivo properties of this radiotracer were evaluated using baboon and it showed highest uptake in the striatum. The studies also revealed that the maximum uptake was reached within 7 to 10 minutes post injection. Plasma metabolite analysis indicated that there is only one metabolite and it is less lipophilic than the parent compound. [18F]FPCBT displayed good brain uptake and its high target to non target ratio indicate that it is a potential candidate for DAT imaging.

Quantification of 5-HT2A receptors in the human brain using [18F]altanserin-PET and the bolus/infusion approach

The aim of the present study is to describe and validate a method for accurate quantification of 5-hydroxytryptamine (5-HT)(2A) receptors using [18F]altanserin-positron emission tomography (PET) and the bolus/infusion approach. A bolus/infusion ratio of 1.75 h aimed at attaining rapid steady state in blood and brain was predicted from previous bolus studies performed in our laboratory. The infusion schedule was tested in normal subjects (n = 10) using dynamic PET and frequent plasma sampling for 6 h. Steady state was attained in brain and plasma within 2 h, and time-activity curves remained constant for another 3 h. To represent free and nonspecifically bound [18F]altanserin and its radiolabeled metabolites only, cerebellum must show no displacement in 5-HT(2A) displacement studies. To validate this, saturating doses of cold ketanserin were administered and it was found that specific binding of [18F]altanserin decreased uniformly to the level of the cerebellum and no change in the cerebellar time-activity curve was found after ketanserin administration. A shorter experimental setup was tested in a second group (n = 20) including patients with neuropsychiatric disorders. Dynamic PET (five frames of 8 minutes each) and venous blood sampling at midscan time started 2 h after [18F]altanserin administration. The mean percentage rate of change per hour in the outcome parameter, DV(3)', was low (mean -0.3% h-1; range -7.3-7.2% h-1) and no correlation of DV(3)' versus time was demonstrated. It is concluded that 5-HT(2A) receptor studies can be conducted within 2 h of [18F]altanserin infusion, yielding reliable results.

Reduced 5-HT2A receptor binding after recovery from anorexia nervosa

BACKGROUND

Several lines of evidence suggest that a disturbance of serotonin neuronal pathways may contribute to the pathogenesis of anorexia nervosa (AN). This study applied positron emission tomography (PET) to investigate the brain serotonin 2A (5HT2A) receptor, which could contribute to disturbances of appetite and behavior in AN.

METHODS

To avoid the confounding effects of malnutrition, we studied 16 women recovered from AN (REC AN, >1 year normal weight, regular menstrual cycles, no bingeing or purging) compared with 23 healthy control women (CW) using [18F]altanserin, a specific 5-HT2A receptor antagonist on PET imaging.

RESULTS

REC AN women had significantly reduced [18F]altanserin binding relative to CW in mesial temporal (amygdala and hippocampus), as well as cingulate cortical regions. In a subset of subjects (11 CW and 16 REC AN), statistical parametric mapping (SPM) confirmed reduced mesial temporal cortex 5HT2A receptor binding and, in addition, showed reduced occipital and parietal cortex binding.

CONCLUSIONS

This study extends research suggesting that altered 5-HT neuronal system activity persists after recovery from AN and may be related to disturbances of mesial temporal lobe function. Altered 5-HT neurotransmission after recovery also supports the possibility that this may be a trait-related disturbance that contributes to the pathophysiology of AN.