Multicompartmental study of fluorine-18 altanserin binding to brain 5HT2 receptors in humans using positron emission tomography

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.

Sex and the serotonergic underpinnings of depression and migraine

Most psychiatric disorders demonstrate sex differences in their prevalence and symptomatology, and in their response to treatment. These differences are particularly pronounced in mood disorders. Differences in sex hormone levels are among the most overt distinctions between males and females and are thus an intuitive underpinning for these clinical observations. In fact, treatment with estrogen and testosterone was shown to exert antidepressant effects, which underscores this link. Changes to monoaminergic signaling in general, and serotonergic transmission in particular, are understood as central components of depressive pathophysiology. Thus, modulation of the serotonin system may serve as a mechanism via which sex hormones exert their clinical effects in mental health disorders. Over the past 20 years, various experimental approaches have been applied to identify modes of influence of sex and sex hormones on the serotonin system. This chapter provides an overview of different molecular components of the serotonin system, followed by a review of studies performed in animals and in humans with the purpose of elucidating sex hormone effects. Particular emphasis will be placed on studies performed with positron emission tomography, a method that allows for human in vivo molecular imaging and, therefore, assessment of effects in a clinically representative context. The studies addressed in this chapter provide a wealth of information on the interaction between sex, sex hormones, and serotonin in the brain. In general, they offer evidence for the concept that the influence of sex hormones on various components of the serotonin system may serve as an underpinning for the clinical effects these hormones demonstrate.

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

Advances in CNS Imaging Agents: Focus on PET and SPECT Tracers in Experimental and Clinical Use

The physiological functioning of the brain is not well-known in current day medicine and the pathologies of many neuropsychiatric disorders are still not yet fully understood. With our aging population and better life expectancies, it has become imperative to find better biomarkers for disease progression as well as receptor target engagements. In the last decade, these major advances in the field of molecular CNS imaging have been made available with tools such as functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT), and neuroreceptor-targeted positron emission tomography (PET). These tools have given researchers, pharmaceutical companies, and clinical physicians a better method of understanding CNS dysfunctions, and the ability to employ improved therapeutic agents. This review is intended to provide an update on brain imaging agents that are currently used in clinical and translational research toward treatment of CNS disorders. The review begins with amyloid and tau imaging, the former of which has at least three [(18)F] agents that have been recently approved and will soon be available for clinical use for specific indications in the USA and elsewhere. Other prevalent PET and SPECT neurotransmitter system agents, including those newly US FDA-approved imaging agents related to the dopaminergic system, are included. A review of both mature and potentially growing PET imaging agents, including those targeting serotonin and opiate receptor systems, is also provided.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

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.

Validation and quantification of [18F]altanserin binding in the rat brain using blood input and reference tissue modeling

The 5-hydroxytryptamine type 2a (5-HT(2A)) selective radiotracer [(18)F]altanserin has been subjected to a quantitative micro-positron emission tomography study in Lister Hooded rats. Metabolite-corrected plasma input modeling was compared with reference tissue modeling using the cerebellum as reference tissue. [(18)F]altanserin showed sufficient brain uptake in a distribution pattern consistent with the known distribution of 5-HT(2A) receptors. Full binding saturation and displacement was documented, and no significant uptake of radioactive metabolites was detected in the brain. Blood input as well as reference tissue models were equally appropriate to describe the radiotracer kinetics. [(18)F]altanserin is suitable for quantification of 5-HT(2A) receptor availability in rats.

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.

GMP-compliant radiosynthesis of [18F]altanserin and human plasma metabolite studies

[(18)F]altanserin is the preferred radiotracer for in-vivo labeling of serotonin 2A receptors by positron emission tomography (PET). We report a modified synthesis procedure suited for reliable production of multi-GBq amounts of [(18)F]altanserin useful for application in humans. We introduced thermal heating for drying of [(18)F]fluoride as well as for the reaction instead of microwave heating. We furthermore describe solid phase extraction and HPLC procedures for quantitative determination of [(18)F]altanserin and metabolites in plasma. The time course of arterial plasma activity with and without metabolite correction was determined. 90 min after bolus injection, 38.4% of total plasma activity derived from unchanged [(18)F]altanserin. Statistical comparison of kinetic profiles of [(18)F]altanserin metabolism in plasma samples collected in the course of two ongoing studies employing placebo, the serotonin releaser dexfenfluramine and the hallucinogen psilocybin, revealed the same tracer metabolism. We conclude that metabolite analysis for correction of individual plasma input functions used in tracer modeling is not necessary for [(18)F]altanserin studies involving psilocybin or dexfenfluramine treatment.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.

Acute S-ketamine application does not alter cerebral [18F]altanserin binding: a pilot PET study in humans

Modeling short-term psychotic states with subanaesthetic doses of ketamine provides substantial experimental evidence in support of the glutamate hypothesis of schizophrenia. Ketamine exerts its pharmacological effects both directly via interactions with glutamate receptors and indirectly by stimulating presynaptic release of endogenous serotonin (5-HT). The aim of this feasibility study was to examine whether acute ketamine-induced 5-HT release interferes with the binding of the 5-HT(2A) receptor (5-HT(2A)R) radioligand [(18)F]altanserin and positron emission tomography (PET). Two subjects treated with ketamine and one subject treated with placebo underwent [(18)F]altanserin PET at distribution equilibrium conditions. Robust physiological, psychopathological and cognitive effects were present at ketamine plasma concentrations exceeding 100 microg/l during >70 min. Notwithstanding, we observed stable radioligand binding (changes +/-95% CI of -1.0 +/- 1.6% and +4.1 +/- 1.8% versus -1.2 +/- 2.6%) in large cortical regions presenting high basal uptake of both, [(18)F]altanserin and ketamine. Marginal decreases of 4% of radioligand binding were observed in the frontal lobe, and 8% in a posteriorily specified frontomesial subregion. This finding is not compatible with a specific radioligand displacement from 5-HT(2A)R which should occur proportionally throughout the whole brain. Instead, the spatial pattern of these minor reductions was congruent with ketamine-induced increases in cerebral blood flow observed in a previous study using [(15)O]butanol PET. This may caused by accelerated clearance of unspecifically bound [(18)F]altanserin from cerebral tissue with increased perfusion. In conclusion, this study suggests that [(18)F]altanserin PET is not sensitive to acute neurotransmitter fluctuations under ketamine. Advantageously, the stability of [(18)F]altanserin PET towards acute influences is a prerequisite for its future use to detect sub-acute and chronic effects of ketamine.

A review of the literature on neuroimaging of serotoninergic function in Alzheimer's disease and related disorders

Behavioural and psychological disorders are frequent not only in frontotemporal dementia (FTD), but also in Alzheimer's disease (AD), and many of them are related to serotoninergic dysfunction. In vitro biochemical measurements on brain samples show both pre- and post-synaptic impaired brain serotoninergic function in degenerative dementia, sometimes related to hyperactivity or aggressive behaviour. To date, few studies have explored in vivo 5HT2A and 5HT1A brain receptors in AD and FTD. They suggest that brain cells are lost in the associative cortices (5HT2A) and hippocampus (5HT1A) of AD patients, and in the medial prefrontal and orbitofrontal cortices of FTD subjects (5HT2A). Apart from reflecting a loss of local neurons, the meaning of the decrease in 5HT receptors is not yet clear and larger populations are required to establish relationships with clinical symptoms such as dementia severity and search for possible consequences for patients' behavioural and affective status.

Validation of a tracer kinetic model for the quantification of 5-HT(2A) receptors in human brain with [(11)C]MDL 100,907

The positron emission tomography (PET) ligand [(11)C]MDL 100,907 has previously been introduced to image the serotonin 2A (5-HT(2A)) receptor in human brain. The aim of this work was to contribute to the verification of the tracer kinetic modelling in human studies. Five healthy volunteers were scanned twice after intravenous bolus injection of approximately 370 MBq [(11)C]MDL 100,907 using dynamic PET. One scan was performed under baseline condition, the other scan commenced 90 mins after a single oral dose of 30 mg of the antidepressant mirtazapine, which binds to the 5-HT(2A) receptor. There did not appear to be radiolabelled metabolites of [(11)C]MDL 100,907 in human plasma, which are likely to cross the blood-brain barrier. Total volumes of distribution VD in 11 different brain regions were estimated using a reversible, two tissue, four rate constants compartment model with a variable fractional blood volume term and the metabolite-corrected plasma input function. There were no significant changes of the VD in the cerebellum between the baseline and the blocked scans confirming the cerebellum as a region devoid of displaceable binding. Regional estimates of binding potential were then obtained indirectly using the cerebellar VD and occupancies calculated. The mean occupancy with this clinically effective dose of mirtazapine was 60% without significant regional differences. This study confirmed the use of an arterial input kinetic model for the quantification of 5-HT(2A) receptor binding with [(11)C]MDL 100,907 and the use of the cerebellum as a reference region for the free and nonspecific binding.

Reproducibility of 5-HT2A receptor measurements and sample size estimations with [18F]altanserin PET using a bolus/infusion approach

PURPOSE

To determine the reproducibility of measurements of brain 5-HT2A receptors with an [18F]altanserin PET bolus/infusion approach. Further, to estimate the sample size needed to detect regional differences between two groups and, finally, to evaluate how partial volume correction affects reproducibility and the required sample size.

METHODS

For assessment of the variability, six subjects were investigated with [18F]altanserin PET twice, at an interval of less than 2 weeks. The sample size required to detect a 20% difference was estimated from [18F]altanserin PET studies in 84 healthy subjects. Regions of interest were automatically delineated on co-registered MR and PET images.

RESULTS

In cortical brain regions with a high density of 5-HT2A receptors, the outcome parameter (binding potential, BP1) showed high reproducibility, with a median difference between the two group measurements of 6% (range 5-12%), whereas in regions with a low receptor density, BP1 reproducibility was lower, with a median difference of 17% (range 11-39%). Partial volume correction reduced the variability in the sample considerably. The sample size required to detect a 20% difference in brain regions with high receptor density is approximately 27, whereas for low receptor binding regions the required sample size is substantially higher.

CONCLUSION

This study demonstrates that [18F]altanserin PET with a bolus/infusion design has very low variability, particularly in larger brain regions with high 5-HT2A receptor density. Moreover, partial volume correction considerably reduces the sample size required to detect regional changes between groups.

Vertigo and Motion Sickness. Part II: Pharmacologic Treatment

Vertigo is a sensation of movement when no movement is actually occurring. It is often accompanied by visceral autonomic symptoms including pallor, diaphoresis, nausea, and vomiting. Vertigo is similar to motion sickness in that both may be caused by vestibular stimulation that does not match an internal model of expected environmental stimuli. Indeed, a functioning vestibular system is necessary for the perception of motion sickness. For this reason, many of the same drugs are used to treat both conditions. The investigation of drugs that treat motion sickness helps to discover medications that may treat vertigo caused by disease of the vestibular system. In this article, we discuss the pharmacologic agents that are now available for the treatment of vertigo and those agents that are still under study.

Striatal dopamine D1 receptors in type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

A considerable number of human and animal studies have implied the importance of dopamine system and alterations in dopamine receptors in the context of alcoholism. However, it has remained unclear if the alcohol-abuse related dopaminergic deficit is specifically associated with certain receptor subtype. The aim of this study was to compare putative alterations of dopamine D(1) receptors in caudate and putamen of nine type 1 alcoholics, eight type 2 alcoholics and 10 healthy controls by using [(3)H]SCH 23390 as a radioligand in postmortem human whole hemisphere autoradiography. In addition, we compared the present results to our earlier studies on dopamine transporters and dopamine D(2) receptors in these same subjects and evaluated the putative correlations of dopamine D(1) receptor densities between the nucleus accumbens and the above-mentioned structures. Our results show that alcoholics do not have significantly different striatal dopamine D(1) receptor densities compared to controls. Neither were there any significant correlations between the dopamine D(1) receptors and the two other dopamine binding sites. However, the correlations of the dopamine D(1) receptors between nucleus accumbens and dorsal striatal structures were consistently and mostly statistically significantly positive in alcoholics, but not in controls, which may suggest some pathology related to addiction. In addition, considering the facts that dopamine D(1) receptors were more abundant in the mesolimbic nucleus accumbens than in the caudate or putamen and that there was a strong tendency towards lower binding among type 1 alcoholics may suggest the importance of dopamine D(1) receptors in reward and/or alcoholism.

Binding characteristics of the 5-HT2A receptor antagonists altanserin and MDL 100907

To study the 5-HT(2A) receptors in the living human brain, using positron emission tomography (PET), two selective radiotracers are currently in use: [(18)F]altanserin and [(11)C]MDL 100907. It is, however, currently unknown to what extent data obtained with either tracer are directly comparable. The aim of this study was to compare binding characteristics of these two radiotracers in rat brain with respect to affinity (K(d)), receptor binding density (B(max)), binding potential (BP), and nonspecific binding. Further, binding kinetics, sensitivity towards competition with the endogenous transmitter serotonin, and the competitive/noncompetitive interaction between the two radioligands were evaluated. In addition, the selectivity of [(18)F]altanserin for the 5-HT(2A) receptor was assessed. The K(d) value of [(18)F]altanserin and [(3)H]MDL 100907 was in the order of 0.3 nM. B(max) in frontal cortex was 523 and 527 fmol/mg protein, respectively. The binding of [(18)F]altanserin was not influenced by blocking either the 5-HT(2B/2C) or the alpha(1)-adrenergic receptors. At 37 degrees C the association t(1/2) was 2.8 and 2.7 min and the dissociation t(1/2) was 11 and 13.5 min for [(18)F]altanserin and [(3)H]MDL 100907, respectively. Both radioligands were displaced by 5-HT, only at high concentrations; the K(i) value of 5-HT ranging between 650 and 3,300 nM. This indicates that binding of both radioligands in PET studies is not directly influenced by changes in endogenous 5-HT.Overall, the binding of [(18)F]altanserin and [(3)H]MDL 100907 to the 5-HT(2A) receptor was very comparable, showing selective high affinity binding in the subnanomolar range.

[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.

Neuroreceptor imaging of stress and mood disorders

Techniques such as positron emission tomography and single photon emission computed tomography allow for the imaging of neurotransmitter receptors and transporters in the brain. These tools have been used to investigate serotonergic, dopaminergic, and opioidergic function in healthy subjects as well as in patients with major depressive disorder, bipolar disorder, and other mood disorders. Pharmacologic challenges, such as amphetamine challenge, and physiologic stressors, such as pain challenge, have been used to further examine the function of these neurotransmitter systems. Neuroimaging of patient populations before and after medication treatment may be useful to understand changes in neurotransmission that accompany disease remission. As new radiotracers with higher selectivity for the various receptors and transporters are developed, imaging techniques may provide new insights into the pathophysiology of mood disorders, leading to improved diagnosis and treatment.

A database of [18F]-altanserin binding to 5-HT2A receptors in normal volunteers: normative data and relationship to physiological and demographic variables

This study presents the results of an analysis of 5-hydroxytryptamine (5-HT)(2A) receptors in 52 healthy subjects. Thirty men and twenty-two women aged between 21 and 79 years were investigated with magnetic resonance imaging (MRI) and [(18)F]-altanserin positron emission tomography (PET). The distribution volumes of specific tracer binding (DV(3)') was calculated for 15 brain regions using either cerebellum or pons as reference regions and correlations between DV(3)' and physiological and demographic variables were made. The regional distribution of [(18)F]-altanserin binding in the healthy human brain was in agreement with existing in vitro post-mortem human 5-HT(2A) data. Apart from nonspecific cerebellar binding (DV(2)), there was no gender difference in 5-HT(2A) binding. A positive correlation between cerebellar binding and age was observed and negative correlations between age and DV(3)' were found in all cortical regions, except occipital cortex, corresponding to a decrease in DV(3)' of 6% or 4% per decade with cerebellum or pons as reference regions, respectively. In several temporal and frontal cortical regions, positive correlations were found between body mass index (BMI) and DV(3)'. Our findings provide a resource to aid design of clinical studies of the 5-HT(2A) receptors. [(18)F]-altanserin binding appears to be unaffected by gender, but the effects of ageing must be considered for clinical studies. The correlations between different cortical regions' 5-HT(2A) binding and BMI should be explored in future studies.

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.

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.

Influence of synaptic serotonin level on [18F]altanserin binding to 5HT2 receptors in man

The feasibility of in vivo serotonin 5HT(2) receptor binding measurement using [18F]altanserin as a radioligand has been well established. In this study, the postsynaptic receptor binding potential of this ligand was examined as a possible indicator of synaptic serotonin content after pharmacological challenge. Studies were performed in 11 subjects with a history of recurrent major depression. Six of them received serotonergic antidepressive treatment at the time of the experiment, the other five patients were untreated. Two PET measurements were carried out in each subject within 2 or 3 days. Before one of the measurements, 25 mg of the serotonin re-uptake inhibitor clomipramine were given intravenously, the other measurement was done without pharmacological challenge. The data were analyzed using non-linear least-square regression and Logan's graphical method. In the whole group of subjects, binding potential and distribution volume of altanserin decreased following clomipramine challenge. The decrease was between 14 (P=0.03) and 23% (P=0.004). This effect was mainly seen in subjects not on antidepressive medication. Clomipramine challenge probably increased the synaptic serotonin level, which competed with altanserin leading to the lowered binding potential. The paradigm might, thus, be useful to estimate serotonin release in vivo. Pretreatment with serotonergic antidepressants reduces the effect of clomipramine.

Radiotracers for positron emission tomography imaging

Over the past 30 years, advances in radiotracer chemistry and positron emission tomography instrumentation have merged to make positron emission tomography a powerful scientific tool in the biomedical sciences. However, despite the increasing reliance of the biomedical sciences on imaging and the new needs for functional information created by the sequencing of the human genome, the development of new radiotracers with the specificity and kinetic characteristics for quantitative analysis in vivo remains a slow process. In this article, we focus on advances in the development of the radiotracers involved in neurotransmission, amino acid transport, protein synthesis, and DNA synthesis. We conclude with a brief section on newer radiotracers that image other molecular targets and conclude with a summary of some of the scientific and infrastructure needs that would expedite the development and introduction of new radiotracers into biomedical research and the practice of medicine.

Analyses of [(18)F] altanserin bolus injection PET data. I: consideration of radiolabeled metabolites in baboons

Positron emission tomography (PET) has been used to study serotonin 2A (5-HT(2A)) receptor binding in human brain using the 5-HT(2A) antagonist, [(18)F]altanserin. Previous analyses of bolus injection [(18)F]altanserin data provided 5-HT(2A) specific binding measures that were highly correlated with the in vitro distribution of 5-HT(2A) receptors and reflected decreased binding after ketanserin (5-HT(2A) antagonist) administration. These observations were made in the presence of a nonspecific tissue component that was consistent with blood-brain barrier (BBB) passage of radiolabeled metabolites (radiometabolites). In this work, we evaluated the in vivo kinetics of [(18)F]altanserin and two major radiometabolites of [(18)F]altanserin, focusing on the kinetics of free and nonspecifically-bound radioactivity. PET studies were performed in baboons after the bolus injection of [(18)F]altanserin or one of its major radiometabolites, either [(18)F]altanserinol or [(18)F]4-(4-fluorobenzoyl)piperidine, at baseline and after pharmacologic receptor blockade (blocking data). The cerebellar and blocking data were analyzed using either single (parent radiotracer) or dual (parent radiotracer and radiometabolites) input function methods. After bolus injection of either [(18)F]altanserin metabolite, radioactivity crossed the BBB and localized nonspecifically. The radio- metabolites were found to contribute to nonspecific "background" radioactivity that was similar in receptor-poor and receptor-rich regions. After bolus injection in baboons, two of the major radiometabolites of [(18)F]altanserin crossed the BBB and contributed to a fairly uniform background of nonspecific radioactivity. This uniformity suggests that conventional analyses are appropriate for human bolus injection [(18)F]altanserin PET data, although these methods may overestimate [(18)F]altanserin nonspecific binding.

Reproducibility of in vivo brain measures of 5-HT2A receptors with PET and

The test/retest reproducibility of brain measures of 5-HT2A receptors with positron emission tomography (PET) and [18F]deuteroaltanserin was examined in a group of eight healthy human subjects. PET measures of 5-HT2A receptors were obtained under an equilibrium paradigm, with a 40-min PET acquisition starting approximately at 300 min (308+/-11 min) after bolus plus constant infusion of the radiotracer. Three brain outcome measures were obtained at equilibrium, V(3) (ratio of specific brain uptake to free parent plasma concentration of radiotracer), V(3)' (ratio of specific brain uptake to total parent plasma concentration) and RT (ratio of specific to non-displaceable brain uptakes). V(3)' and RT had high test/retest reproducibility, as measured by mean intra-subject% change for cortical brain areas of 14.1 and 11.0%, respectively. They also had high reliability, as measured by mean intra-class correlation coefficients (ICC) for cortical brain areas of 0.86 and 0.88, respectively. V(3) had low test/retest reproducibility, due to high variability in the measures of free parent tracer in plasma. This study supports the feasibility of equilibrium imaging of 5-HT2A receptors with PET and [18F]deuteroaltanserin. The equilibrium imaging method with [18F]deuteroaltanserin allows a single acquisition and blood measurement to provide an image whose pixel values equal a receptor volume of distribution. Since the single image pixel values are proportional to receptor densities, the images can be used in pixel-by-pixel statistical methods, such as SPM, to assess the distribution and density of 5-HT2A receptors in neuropsychiatric disorders.

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

The regional distribution in brain, distribution volumes, and pharmacological specificity of the PET 5-HT(2A) receptor radiotracer [(18)F]deuteroaltanserin were evaluated and compared to those of its non-deuterated derivative [(18)F]altanserin. Both radiotracers were administered to baboons by bolus plus constant infusion and PET images were acquired up to 8 h. The time-activity curves for both tracers stabilized between 4 and 6 h. The ratio of total and free parent to metabolites was not significantly different between radiotracers; nevertheless, total cortical R(T) (equilibrium ratio of specific to nondisplaceable brain uptake) was significantly higher (34-78%) for [(18)F]deuteroaltanserin than for [(18)F]altanserin. In contrast, the binding potential (Bmax/K(D)) was similar between radiotracers. [(18)F]Deuteroaltanserin cortical activity was displaced by the 5-HT(2A) receptor antagonist SR 46349B but was not altered by changes in endogenous 5-HT induced by fenfluramine. These findings suggest that [(18)F]deuteroaltanserin is essentially equivalent to [(18)F]altanserin for 5-HT(2A) receptor imaging in the baboon.

Analyses of [18F]altanserin bolus injection PET data. II: Consideration of radiolabeled metabolites in humans

Imaging serotonin-2A (5-HT(2A)) neuroreceptors with positron emission tomography (PET) and [(18)F]altanserin has been the focus of a series of PET studies, as [(18)F]altanserin is one of the most selective 5-HT(2A) antagonist radiotracers. Previous animal studies showed that radiolabeled metabolites (radiometabolites) of [(18)F]altanserin crossed the blood-brain barrier (BBB) to localize nonspecifically in brain, consistent with a constant radioactivity "background." In this work, we evaluated human bolus injection [(18)F]altanserin PET data with detailed consideration of the impact of BBB-permeable metabolites on the specific binding parameters. Data were quantified using either single (parent radiotracer), dual (parent radiotracer and radiometabolites), or no arterial input function(s) (cerebellum as reference tissue input function). A step-gradient high-performance liquid chromatography (HPLC) analysis provided distinct separation of [(18)F]altanserin and four radiolabeled components in plasma. After [(18)F]altanserin injection, the step-gradient data showed that the major BBB-permeable radiometabolites approached constant levels in plasma (>50 min), consistent with a constant metabolite "background." The single-input Logan graphical results were highly correlated with the dual-input results and its bias was fairly constant across regions and subjects, as similarly observed for a nongraphical reference tissue method. The most comprehensive and quantitatively valid analysis for bolus [(18)F]altanserin PET data was the dual-input method that specifically accounted for BBB-permeable metabolites, although the Logan analysis was preferred because it provided a good compromise between validity, sensitivity, and reliability of implementation. Further study is needed to better understand how the cerebellar kinetics of [(18)F]altanserin and its radiometabolites impact the reference tissue measures.

Age-related reduction of [11C]MDL100,907 binding to central 5-HT(2A) receptors: PET study in the conscious monkey brain

Age-related alterations of serotonin (5-hydroxytryptamine; 5-HT) type 2A receptors (5-HT(2A)) in the living brains of young (6.0+/-1. 3 years old) and aged (19.2+/-3.0 years old) monkeys (Macaca mulatta) were evaluated with [11C]MDL100,907 in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of 5-HT(2A) binding in vivo, PET scan of [11C]MDL100,907 was performed with arterial blood sampling in each animal, and the metabolic-corrected arterial input function was used for Logan graphical analysis. Higher cerebral binding of [11C]MDL100, 907 was observed in the hippocampus, cingulate gyrus, frontal, temporal and occipital cortices, regions known to contain high densities of 5-HT(2A), by in vitro assay. Binding was intermediate in the striatum and thalamus, and lower in the pons and cerebellum in both young and aged monkeys. The age-related decrease in [11C]MDL100,907 binding to 5-HT(2A) receptors was prominent in the hippocampus, cingulate gyrus, frontal, temporal and occipital cortices, but not in the striatum, thalamus and pons. These observation demonstrated the usefulness of [11C]MDL100,907 as an labeled compound for assessment of the aging process of the cortical 5-HT(2A) measured by PET.

Equilibrium modeling of 5-HT(2A) receptors with [18F]deuteroaltanserin and PET: feasibility of a constant infusion paradigm

[(18)F]Altanserin has emerged as a promising positron emission tomography (PET) ligand for serotonin-2A (5-HT(2A)) receptors. The deuterium substitution of both of the 2'-hydrogens of altanserin ([(18)F]deuteroaltanserin) yields a metabolically more stable radiotracer with higher ratios of parent tracer to radiometabolites and increased specific brain uptake than [(18)F]altanserin. The slower metabolism of the deuterated analog might preclude the possibility of achieving stable plasma and brain activities with a bolus plus constant infusion within a reasonable time frame for an (18)F-labeled tracer (T(1/2) 110 min). Thus, the purpose of this study was to test the feasibility in human subjects of a constant infusion paradigm for equilibrium modeling of [(18)F]deuteroaltanserin with PET. Seven healthy male subjects were injected with [(18)F]deuteroaltanserin as a bolus plus constant infusion lasting 10 h postinjection. PET acquisitions and venous blood sampling were performed throughout the infusion period. Linear regression analysis revealed that time-activity curves for both specific brain uptake and plasma [(18)F]deuteroaltanserin concentration stabilized after about 5 h. This permitted equilibrium modeling and estimation of V(')(3) (ratio of specific uptake to total plasma parent concentration) and the binding potential V(3) (ratio of specific uptake to free plasma parent concentration). Cortical/cerebellar ratios were increased by 26% relative to those we previously observed with [(18)F]altanserin using similar methodology in a somewhat older subject sample. These results demonstrate feasibility of equilibrium imaging with [(18)F]deuteroaltanserin and suggest that it may be superior to [(18)F]altanserin as a PET radioligand.

A voxel-by-voxel analysis of [18F]setoperone PET data shows no substantial serotonin 5-HT(2A) receptor changes in schizophrenia

Several postmortem studies have reported regionally localized decreases in serotonin(2A) receptors (5-HT(2A)R) in schizophrenia. This was not confirmed by two recent [18F]setoperone positron emission tomography (PET) studies. In these two studies relatively large regions of interest (ROIs) were used; hence, 5-HT(2A)R changes may have been missed in some brain areas. Therefore, data from one study were analyzed on a voxel-by-voxel basis using Statistical Parametric Mapping (SPM). We also used this method to examine the relationship between 5-HT(2A)R binding potential (BP) and five PANSS-derived factors: negative, positive, activation, dysphoric and autistic preoccupation. Thirteen schizophrenic patients (10 antipsychotic-naïve, 3 antipsychotic-free; 11 M, 2 F; age 31+/-7 years) and 35 age-matched control subjects (15 M, 20 F; age 30+/-7 years) were scanned. The 5-HT(2A)R BP was determined for each voxel using the pseudoequilibrium ratio method on PET data obtained between 65 and 90 min after [18F]setoperone bolus injection. The resulting parametric 5-HT(2A)R BP images were spatially normalized using a ligand specific template. Analyses of covariance were done using SPM99 with age as covariate. In tests for the effect of schizophrenia and for partial correlations between 5-HT(2A)R BP and the five factors, corrected P values <0.05 at cluster or voxel level were considered significant. No significant differences were detected between patients and control subjects, and no significant correlations were observed between 5-HT(2A)R BP and any of the five factors. Thus, in agreement with the previous ROI studies, voxel-by-voxel analysis confirmed the lack of substantial 5-HT(2A)R BP differences between schizophrenic patients and control subjects.

Kinetic analysis of the 5-HT2A ligand [11C]MDL 100,907

The goal of this study was to develop a suitable kinetic analysis method for quantification of 5-HT2A receptor parameters with [11C]MDL 100,907. Twelve control studies and four preblocking studies (400 nmol/kg unlabeled MDL 100,907) were performed in isoflurane-anesthetized rhesus monkeys. The plasma input function was determined from arterial blood samples with metabolite measurements by extraction in ethyl acetate. The preblocking studies showed that a two-tissue compartment model was necessary to fit the time activity curves of all brain regions including the cerebellum--in other words, the need for two compartments is not proof of specific binding. Therefore, a three-tissue compartment model was used to analyze the control studies, with three parameters fixed based on the preblocking data. Reliable fits of control data could be obtained only if no more than three parameters were allowed to vary. For routine use of [11C]MDL 100,907, several simplified methods were evaluated. A two-tissue (2T') compartment with one fixed parameter was the most reliable compartmental approach; a one-compartment model failed to fit the data adequately. The Logan graphical approach was also tested and produced comparable results to the 2T' model. However, a simulation study showed that Logan analysis produced a larger bias at higher noise levels. Thus, the 2T' model is the best choice for analysis of [11C]MDL 100,907 studies.

Nuclear imaging methods for non-invasive drug monitoring

Functional imaging techniques provide complimentary information to that provided by structural studies such as MRI and CT. Functional imaging is based upon known parameters such as physiology, metabolism, biochemistry, pharmacology, and any other biological process. As such, this methodology plays a major role in understanding the basic mechanisms of a multitude of disorders, accurate diagnosis of certain diseases, and developing effective treatment for serious illnesses such as cancer and central nervous system maladies. Although this type of imaging can be performed with various modalities, nuclear procedures have played the leading role in this discipline. Advances made in labeling various radionuclides to biologically important compounds, and development of sophisticated instruments have substantially contributed to the growth of the field of functional imaging. The introduction of positron emission topography (PET), which is based on imaging of compounds labeled with elements such as carbon, nitrogen, and fluorine, has added a major dimension to the evolution of the discipline. This review deals with a brief introduction to the methodologies utilized with radiolabeled tracers and then deals with specific applications of this technology. These applications include assessment of blood flow and metabolism, receptor imaging, elucidating the pathophysiologic process, evaluating role of labeled therapeutic agents, and the potential of these techniques in the development of novel biologic therapies. Functional imaging with radiolabeled tracers will play an increasingly important role in modern medicine, and its impact will be substantial in the management of patients with various disorders.

Kinetic and equilibrium analyses of [(123)I]epidepride binding to striatal and extrastriatal dopamine D(2) receptors

Quantitative SPECT measures of dopamine D(2) like receptors with [(123)I]epidepride is complicated by its high affinity and lipophilic metabolites. The purpose of this study was to use both parent (P) and lipophilic metabolites (M) as input functions in a kinetic paradigm and in comparison to the results of equilibrium studies. Kinetic studies on eleven healthy human subjects, ages 32+/- 10 were performed following i.v. injection of approximately 370 MBq of [(123)I]epidepride. Images were acquired for 13.5+/-1.0 hours. Equilibrium studies were done on seven of eleven subjects with a bolus injection of approximately 140 MBq, bolus/infusion ratio of 10 hours, and infusion for 30-32 hours. High (striatum) and low (temporal cortex) density regions were studied. Two (P and M) and one (P) input function models were applied in the kinetic studies. In receptor-rich regions, the distribution volumes in nondisplaceable compartments were fixed to those in cerebellum. In addition, in the two input function model, K(1)(P)/K(1)(M) was fixed to the values in the cerebellum. The one input function model provided V'(3) values (=f(1)*B'(max)/K(D)) which were consistent with those obtained in equilibrium studies in both receptor-rich regions, while the two input function model provided consistent values only in striatum. Poor identifiability of the rate constants of metabolites seemed to be the source of errors in the two input function model. These results suggest that correct V'(3) values can be obtained with the one input function model both in high- and low-density regions.

PET imaging of serotonin type 2A receptors in late-life neuropsychiatric disorders

OBJECTIVE

To determine whether there are abnormalities in the in vivo status of the serotonin type 2A (5-HT2A) receptor in late-life depression and Alzheimer's disease, the authors used positron emission tomography (PET) to assess patients with these two conditions and healthy subjects.

METHOD

PET was performed by using [18F]altanserin to evaluate 5-HT2A receptor binding in 11 elderly patients with depression (four men, seven women; mean age = 65.0 years, SD = 5.5); nine Alzheimer's disease patients, including three with concurrent depression (two men, seven women; mean age = 69.7 years, SD = 5.0); and 10 age-matched healthy subjects (four men, six women; mean age = 69.8 years, SD = 5.0). Partial-volume correction of regional specific binding estimates was performed by using a method based on magnetic resonance imaging.

RESULTS

No significant abnormalities in [18F]altanserin binding (binding potential) were observed in the patients with late-life depression, and no effect of depression on binding potential was present within the Alzheimer's disease group. However, the patients with Alzheimer's disease had significantly lower binding than the normal subjects in several brain regions, including the anterior cingulate, prefrontal cortex, and sensorimotor cortex.

CONCLUSIONS

These results suggest that the 5-HT2A receptor is differentially affected in late-life depression and Alzheimer's disease, a finding that has implications for the etiological basis of mood and cognitive features of neuropsychiatric disorders of late life.

Characterization of radioactive metabolites of 5-HT2A receptor PET ligand [18F]altanserin in human and rodent

This study was performed to identify and characterize the radiometabolites of the serotonin 5-HT2A receptor ligand [18F]altanserin in supporting quantification of the target receptors by positron emission tomography. In analogy to its analog ketanserin, we postulated 4-(4-fluorobenzoyl)piperidine (FBP) and altanserinol for the previously observed two polar radiometabolites, corresponding to dealkylation at the piperidine nitrogen and reduction at the ketone, respectively. To test this hypothesis and characterize the in vivo and in vitro behavior of the radiometabolites, we synthesized nonradioactive authentic compounds altanserinol, 1-(4-fluorophenyl)-1-(piperidin-4-yl)methanol (FBPOH), and isolated nonradioactive FBP metabolite from monkey plasma. [18F]Altanserinol was obtained by NaBH4 reduction of [18F]altanserin, followed by acid hydrolysis. Identification of radiometabolites was carried out by high performance liquid chromatography and thin layer chromatography comparison of the radioactive plasma after injection of tracers with five authentic compounds. Human studies revealed that at least four radiometabolites, one identified as [18F]altanserinol, resulted from reduction of the ketone functionality. The N-dealkylation product [18F]FBP was not detectable; however, a radiometabolite of FBP was present in plasma after administration of [18F]altanserin. Monkey studies showed nonradioactive FBP was converted rapidly to a less polar metabolite. In rat, altanserin and altanserinol were converted to each other in vivo, and all the radiometabolites likely penetrated the blood-brain barrier and entered the brain. Displacement binding of altanserin to cloned serotonin 5-HT2A, 5-HT2C, 5-HT6, and 5-HT7 receptors showed Ki values of 0.3, 6.0, 1,756, and 15 nM; the binding of FBP and altanserinol to these four 5-HT subtypes was negligible. We conclude from these studies that the radiometabolites of [18F]altanserin from N-dealkylation and ketone reduction should not interfere with specific receptor quantification in an equilibrium paradigm.

A complete remote-control system for reliable preparation of [18F]altanserin

A complete remote control system was constructed for production of the PET 5-HT2A ligand [18F]altanserin by nitro-for-fluoro exchange. Comparing with published methods, the key features include (1) conducting azeotropic distillation and nucleophilic displacement in an open vessel heated by a commercial microwave oven; (2) purifying the product by a single HPLC procedure and (3) removing HPLC solvent by solid phase extraction. The preparation took 114 min with 23% yield and high quality.

Delivery of radioligands for positron emission tomography (PET) in the central nervous system

Positron emission tomography (PET) is an imaging technique to monitor the delivery of tracers labeled with positron emitters ((11)C, (13)N, (15)O and (18)F). A wide variety of probes have been labeled to measure biochemical and physiological parameters in the central nervous system (CNS), such as glucose and oxygen metabolism, protein synthesis, blood flow, and neurotransmitter receptor functions. The delivery of labeled compounds to the target tissue, which directly reflect the distribution and kinetics patterns, especially to the neurotransmitter receptors is modulated by several factors, such as regional cerebral blood flow (rCBF), peripheral metabolism, and neurotransmitter concentration in the synaptic cleft. These factors provide misunderstanding of the apparent results, which do not reflect the true state of the CNS. The present paper will summarize several factors that affect the delivery of labeled compounds related to the neurotransmitter receptors in the CNS.

Test-retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test-retest variability of the binding of [18F]altanserin were measured as important steps in the further development of [18F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5-HT2A receptor. Two high specific activity [18F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2-16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [18F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16-20%). For all regions, the absolute value of the test-retest differences in the Logan DV values averaged 12% or less. The test-retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [18F]altanserin DV values using both of these methods were significantly correlated with literature-based values of the regional concentrations of 5-HT2A receptors determined by postmortem autoradiographic studies (r2 = 0.95, P < 0.001 for the 4C model and r2 = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [18F]altanserin (present at levels of 3-25% of the total radioactivity in human plasma 10-120 min postinjection) were able to penetrate the blood-brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5-HT2A receptor and did not interfere with the interpretation of regional [18F]altanserin-specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test-retest variability of [18F]altanserin-specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [18F]altanserin in human brain was correlated with the known regional distribution of 5-HT2A receptors. These findings support the usefulness of [18F]altanserin as a radioligand for PET studies of 5-HT2A receptors.

Imaging of the serotonergic system: interactions of neuroanatomical and functional abnormalities of depression

For nearly three decades, evidence supporting a role for aberrant serotonergic function in the pathogenesis of depression has accumulated; however, only recently have methodologies and radiotracers suitable for in vivo clinical assessment of depression become available. To date, only a few neurochemical imaging studies have been performed in actively depressed subjects. A preliminary study using single photon emission computed tomography (SPECT) has demonstrated decreased levels of serotonin (5-HT) transporters in the midbrain regions of subjects with major depression. Analysis of the 5-HT2 receptor using positron emission tomography (PET) has suggested that this receptor may not be altered significantly in the depressed brain but may increase in response to antidepressant treatment. These findings are supported by studies in secondary "poststroke" depression that have shown that elevations in 5-HT2 receptor density correlated with the alleviation of symptoms of depressed mood. With the rapid development of novel PET and SPECT radiotracers, future studies of the serotonergic system that evaluate presynaptic (5-HT transporter) and postsynaptic (5-HT1A and 5-HT2A receptors) markers and the interaction of synaptic levels of 5-HT with these sites will make profound contributions to the understanding of the role of the serotonergic synapse in the pathophysiology of depression.

Comparison of the transient equilibrium and continuous infusion method for quantitative PET analysis of [11C]raclopride binding

Several approaches have been applied for quantification of D2 dopamine receptors in positron emission tomography studies using [11C]raclopride. Initial approaches were based on analyses of data obtained after rapid bolus injection of [11C]raclopride. A continuous infusion paradigm has more recently been applied. The current study compares these approaches in healthy men. Two positron emission tomography measurements were performed in each of six healthy men, the first with rapid bolus injection and the second with continuous infusion of [11C]raclopride. In rapid bolus injection, the binding potential was calculated by the following methods. One approach is the kinetic analysis using the standard three-compartment model. Another is to define a transient equilibrium at the moment when the specific binding reaches its maximum. In continuous infusion, binding potential was calculated by using time-activity data at equilibrium condition. All methods gave almost identical binding potential, representing cross-validation of these methods. The continuous infusion method can provide "true" equilibrium condition. The kinetic analysis is a sophisticated approach but requires determination of an arterial input function. The transient equilibrium method thus is suitable for routine clinical research, since it does not require determination of an arterial input function.

Serotonin 5-HT2 receptor imaging in major depression: focal changes in orbito-insular cortex

BACKGROUND

Serotonin receptors may play an important role in the pathophysiology of affective disorders. We studied type-2 serotonin (5-HT2) receptors in the brain of patients with major depression.

METHOD

Using positron emission tomography (PET) and the selective radioligand [18F]altanserin, we investigated 5-HT2 receptor distribution in eight drug-free unipolar depressed patients and 22 healthy subjects. Data were analysed using Statistical Parametric Mapping 95.

RESULTS

In depressed patients, [18F]altanserin uptake was significantly reduced in a region of the right hemisphere including the posterolateral orbitofrontal cortex and the anterior insular cortex. A trend to similar changes was found in the left hemisphere. No correlation was found between the uptake and the Hamilton rating scale score.

CONCLUSIONS

Pathophysiology of depression may involve changes in 5-HT2 receptor in brain regions selectively implicated in mood regulation.

In vivo binding of [18F]altanserin to rat brain 5HT2 receptors: a film and electronic autoradiographic study

To further validate its use in positron emission tomography (PET), we studied the binding of [18F]altanserin, a specific 5HT2 radioligand, in the rat brain using in vivo autoradiography. Distribution of [18F]altanserin binding was comparable to the in vitro mapping of 5HT2 receptors reported in the literature. Selective displacers were used to test the reversibility and the selectivity of this radioligand. Specific binding of [18F]altanserin in the rat frontal cortex was quantified by direct counting with an electronic imaging system and by quantification on digitalized autoradiograms. Close results of about 30 pmol/g were obtained with both methods. Our data confirmed that [18F]altanserin is a valid tracer for 5HT2 receptors binding studies.