Positron emission tomography quantification of serotonin-1A receptor binding in medication-free bipolar depression

BACKGROUND

Little is known about the serotonin-1A receptor (5-HT1A) in bipolar depression despite altered 5-HT1A binding in major depressive disorder. Utilizing positron emission tomography (PET) and the radioligand N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide ([Carbonyl-C-11]WAY-100635), 5-HT1A binding was compared between depressed bipolar disorder (BD) and controls.

METHODS

Brain 5-HT1A binding potential (BP(F) = B(max)/K(D), where B(max) = total available receptors, and 1/K(D) = ligand affinity) was measured in 32 currently depressed, medication-free BD subjects and 47 controls. Participants were genotyped for the 5-HT1A promoter polymorphism C(-1019)G.

RESULTS

The bipolar depressed group demonstrated higher 5-HT1A BP(F) across all regions of interest (ROIs; p = .022). Post hoc analyses indicated that male BD patients had higher 5-HT1A BP(F) than male controls (p = .025), with higher 5-HT1A BP(F) found in every region (by 102% in raphe nuclei and 29% to 50% in the forebrain ROIs); whereas, female subgroups did not differ in 5-HT1A BP(F) (p = .32). Serotonin-1A BP(F) did not correlate with depression severity. The GG genotype was overrepresented at trend level in the BD group (p = .057). Number of G-allele copies was associated with higher 5-HT1A BP(F) in raphe (p = .0050), amygdala (p = .022), and hippocampus (p = .041).

CONCLUSIONS

Higher 5-HT1A BP(F) in bipolar depressed males suggests higher raphe autoreceptor binding, potentially causing less serotonin release and compensatory upregulation of forebrain postsynaptic 5-HT1A receptors. The raphe effect may be partly genetic. No difference in 5-HT1A BP(F) between BD and control females may reflect greater effect of prior antidepressant exposure in BD females.

Pretreatment regional brain glucose uptake in the midbrain on PET may predict remission from a major depressive episode after three months of treatment

In order to test the hypotheses that pretreatment metabolic activity in the midbrain and the rostral anterior cingulate may predict remission in response to medications enhancing monoaminergic transmission, we compared relative regional cerebral metabolic rate of glucose (rCMRglu) using positron emission tomography (PET) in medication-free patients with major depression who remitted after 3 months of monoaminergic medication, with non-remitters on the same treatment. [(18)F]-FDG PET was conducted in a group of 33 drug-free DSM-IV major depression subjects prior to antidepressant treatment. Patients were prescribed paroxetine initially (61%) unless they had failed paroxetine previously. Treatment was then managed by the subjects' own physician with 91% receiving a selective serotonin reuptake inhibitor and 78% another non-selective monoamine reuptake inhibitor during the 3 months of treatment. Voxel-based parametric brain maps of remitters were compared with maps of non-remitters using SPM2. Remission was defined as a >50% decrease in and a final score of <or=10 on the 24-item Hamilton Depression Rating Scale. We found that treatment remitters have lower activity in a single contiguous brain region (with global maxima in the midbrain, cluster level P=0.013, corrected for multiple comparison (CMC)), prior to treatment, compared with non-remitters to 3 months of community-based monoaminergic antidepressant treatment. Degree of improvement correlated with pretreatment midbrain activity. Pretreatment clinical picture and intensity of treatment did not distinguish remitters. No other area of the brain showed a significant difference between remitters and non-remitters even with CMC completely disabled. Lower relative regional brain activity in the region of monoaminergic nuclei prior to treatment predicts remission in response to 3 months of antidepressant treatment, despite no clinical differences at baseline and no difference in treatment intensity. Brain imaging is a potential objective laboratory technique that may guide treatment selection where clinical methods have not shown promise. Prospective studies are needed to replicate these findings and determine whether outcome prediction is limited to a specific class of antidepressants.

Modeling considerations for in vivo quantification of the dopamine transporter using [(11)C]PE2I and positron emission tomography

The dopamine transporter (DAT) is an important imaging target as changes in DAT have been implicated in a variety of neurologic and psychiatric disorders and can result from certain classes of medications. [(11)C]N-(3-iodoprop-2E-enyl)-2beta-carbomethoxy-3beta-(4-methylphenyl)nortropane ([(11)C]PE2I), a radioligand with high specificity for DAT, has been shown to exhibit favorable kinetics and to produce high contrast positron emission tomography (PET) images. To better characterize this ligand and to assess its measurement reliability, PET images of seven subjects were acquired in a test-retest paradigm. For optimal model performance, each subject was scanned for 120 mins, ensuring that high binding regions could reach equilibrium, a validated coregistration method was performed for accurate anatomic delineations and an exhaustive search for a reference region having one-tissue compartment kinetics was undertaken. Eleven modeling methods were tested and six metrics were used for method evaluation. A noniterative two-tissue compartment method with 100 mins of scanning time was found to be optimal for characterizing [(11)C]PE2I.

Reported childhood abuse is associated with low serotonin transporter binding in vivo in major depressive disorder

BACKGROUND

Physical or psychological adversity in childhood is associated with a higher risk for depression in adulthood, and with persistent serotonergic abnormalities in humans and in animal models. We hypothesized that reported childhood abuse would be associated with lower brain serotonin transporter (5-HTT) binding potential (BP(P), proportional to the number of available transporters) in adults. We examined healthy volunteers and subjects with major depressive disorder, a sample enriched for childhood abuse.

METHODS

Regional brain 5-HTT BP(P) was measured using positron emission tomography (PET) with [(11)C]McN 5652 and a metabolite-corrected arterial input function in 43 healthy volunteers and 23 subjects in a major depressive episode, ten of whom reported a history of sexual and/or physical abuse before age 15, and 13 of whom did not. As only two healthy volunteers reported childhood abuse, primary analyses were restricted to the depressed sample, with healthy controls presented as comparators.

RESULTS

Depressed subjects reporting childhood abuse had lower 5-HTT BP(P) than nonabused depressed subjects across all brain regions examined (P = 0.017). The groups did not differ in relevant demographic or clinical variables. Genotype frequencies of a functional polymorphism in the 5-HTT gene promoter (5-HTTLPR) did not differ between the groups.

CONCLUSIONS

Reported childhood abuse is associated with lower 5-HTT BP(P) in this sample of subjects with major depression, consistent with other reports that childhood adversity can lower serotonergic function permanently. Lower 5-HTT BP(P) may represent a biological pathway through which early life stress predisposes to the development of subsequent psychiatric illness, including major depressive disorder.

Evaluation of 14 nonlinear deformation algorithms applied to human brain MRI registration

All fields of neuroscience that employ brain imaging need to communicate their results with reference to anatomical regions. In particular, comparative morphometry and group analysis of functional and physiological data require coregistration of brains to establish correspondences across brain structures. It is well established that linear registration of one brain to another is inadequate for aligning brain structures, so numerous algorithms have emerged to nonlinearly register brains to one another. This study is the largest evaluation of nonlinear deformation algorithms applied to brain image registration ever conducted. Fourteen algorithms from laboratories around the world are evaluated using 8 different error measures. More than 45,000 registrations between 80 manually labeled brains were performed by algorithms including: AIR, ANIMAL, ART, Diffeomorphic Demons, FNIRT, IRTK, JRD-fluid, ROMEO, SICLE, SyN, and four different SPM5 algorithms ("SPM2-type" and regular Normalization, Unified Segmentation, and the DARTEL Toolbox). All of these registrations were preceded by linear registration between the same image pairs using FLIRT. One of the most significant findings of this study is that the relative performances of the registration methods under comparison appear to be little affected by the choice of subject population, labeling protocol, and type of overlap measure. This is important because it suggests that the findings are generalizable to new subject populations that are labeled or evaluated using different labeling protocols. Furthermore, we ranked the 14 methods according to three completely independent analyses (permutation tests, one-way ANOVA tests, and indifference-zone ranking) and derived three almost identical top rankings of the methods. ART, SyN, IRTK, and SPM's DARTEL Toolbox gave the best results according to overlap and distance measures, with ART and SyN delivering the most consistently high accuracy across subjects and label sets. Updates will be published on the http://www.mindboggle.info/papers/ website.

Plasma polyunsaturated fatty acids and regional cerebral glucose metabolism in major depression

Deficiencies in polyunsaturated essential fatty acids (PUFA) are implicated in mood disorders, although mechanisms of action and regional specificity in the brain are unknown. We hypothesized that plasma phospholipid PUFA levels are correlated with regionally specific relative cerebral metabolic rates of glucose (rCMRglu). Medication-free depressed subjects (N=29) were studied using [(18)F]-fluoro-2-deoxyglucose positron emission tomography. Docosahexaenoic acid (22:6n-3), arachidonic acid (20:4n-6), and eicosapentaenoic acid (20:5n-3) were assessed as a percentage of total phospholipid PUFA (DHA%, AA%, and EPA%, respectively). DHA% and AA% correlated positively with rCMRglu in temporoparietal cortex. In addition, DHA% correlated negatively with rCMRglu in prefrontal cortex and anterior cingulate. No correlations were seen with EPA%. Thus, under conditions of low plasma DHA, rCMRglu was higher in temporoparietal cortex and lower in anterior cingulate/prefrontal cortex. Opposing effects of DHA on these regions is a hypothesis that could be addressed in future prospective studies with n-3 supplementation. This pilot study is the first to demonstrate fatty acid and regionally specific correlations in the brain between plasma PUFA and rCMRglu in humans.

Serotonin transporter binding as a possible predictor of one-year remission in major depressive disorder

OBJECTIVE

Lower serotonin transporter (5-HTT) binding (BP(P)=f(P)B(avail)/K(D)) is reported during a major depressive episode (MDE) compared to healthy controls. Higher 5-HTT binding in the diencephalon has previously been associated with acute response to antidepressant treatment. We assessed baseline 5-HTT binding as a predictor of one-year remission from a MDE, examining binding in brain regions implicated in the pathophysiology of major depressive disorder (MDD).

METHODS

5-HTT binding was quantified using positron emission tomography (PET) with [(11)C]McN5652 in 19 currently depressed subjects with MDD and 41 healthy controls. Depressed subjects received open, naturalistic antidepressant treatment. Remission status was determined one year after PET scan and treatment initiation.

RESULTS

Significant differences in 5-HTT binding among the three groups (healthy controls, remitters, and non-remitters) were observed in a linear mixed-effects model. Post hoc, non-remitters had lower 5-HTT binding than controls in midbrain, amygdala, and anterior cingulate. Remitters did not differ significantly from controls or non-remitters in 5-HTT binding. Remitters did not differ from non-remitters in clinical characteristics apart from greater family history of depression among non-remitters. A logistic regression model fit to determine the capacity of baseline 5-HTT binding to predict remission status at one year yielded a coefficient that was suggestive but not significant (p=0.057).

LIMITATIONS

The small sample size and heterogeneous treatments received reduced statistical power to detect differences in binding based on clinical outcome.

CONCLUSIONS

Lower pretreatment 5-HTT binding may be predictive of non-remission from major depression following one year of naturalistic antidepressant treatment. Future studies using standardized treatment are warranted.

Effect of BDNF val66met polymorphism on age-related amygdala volume changes in healthy subjects

Brain-derived neurotrophic factor (BDNF) has been implicated in the mechanism of age-related regional brain volumetric changes. Healthy volunteers with the valine to methionine polymorphism at codon 66 of the BDNF gene (val66met) exhibit decreased volume of a number of brain structures, including hippocampus, temporal and occipital lobar gray matter volumes, and a negative correlation between age and the volume of bilateral dorsolateral prefrontal cortices. We sought to characterize the relationship between age, BDNF and amygdala volumes among healthy volunteers. We measured amygdala volumes in 55 healthy, right-handed volunteers who underwent structural magnetic resonance imaging (MRI) and were also characterized demographically and genotyped with respect to BDNF. Using an ANCOVA model, we found that amygdala volumes were inversely correlated with age in BDNF val66met carriers but not in non-carriers. This is the first report of age-related BDNF val66met polymorphism effects on amygdala volume.

Voxel-based analysis of 11C-PIB scans for diagnosing Alzheimer's disease

The positron emission tomography (PET) radioligand N-methyl-11C-2-(4-methylaminophenyl)-6-hydroxybenzothiazole (also known as 11C-6-OH-BTA-1 or 11C-PIB) binds to amyloid-beta (Abeta), which accumulates pathologically in Alzheimer's disease (AD). Although 11C-PIB accumulation is greater in patients with AD than in healthy controls at a group level, the optimal method for discriminating between these 2 groups has, to our knowledge, not been established. We assessed the use of data-determined standardized voxels of interest (VOIs) to improve the classification capability of 11C-PIB scans on patients with AD.

METHODS

A total of 16 controls and 14 AD age-matched patients were recruited. All subjects underwent a 11C-PIB scan and structural MRI. Binding potential (a measure of amyloid burden) was calculated for each voxel using the Logan graphical method with cerebellar gray matter as the reference region. Voxel maps were then partial-volume corrected and spatially normalized by MRI onto a standardized template. The subjects were divided into 2 cohorts. The first cohort (control, 12; AD, 9) was used for statistical parametric mapping analysis and delineation of data-based VOIs. These VOIs were tested in the second cohort (control, 4; AD, 5) of subjects.

RESULTS

Statistical parametric mapping analysis revealed significant differences between control and AD groups. The VOI map determined from the first cohort resulted in complete separation between the control and the AD subjects in the second cohort (P < 0.02). Binding potential values based on this VOI were in the same range as other reported individual and mean cortical VOI results.

CONCLUSION

A standardized VOI template that is optimized for control or AD group discrimination provides excellent separation of control and AD subjects on the basis of 11C-PIB uptake. This VOI template can serve as a potential replacement for manual VOI delineation and can eventually be fully automated, facilitating potential use in a clinical setting. To facilitate independent analysis and validation with more and a broader variety of subjects, this VOI template and the software for processing will be made available through the Internet.

Family history of suicidal behavior and early traumatic experiences: additive effect on suicidality and course of bipolar illness?

BACKGROUND

Bipolar disorder (BD) is associated with a high prevalence of suicide attempt and completion. Family history of suicidal behavior and personal history of childhood abuse are reported risk factors for suicide among BD subjects.

METHODS

BD individuals with family history of suicidal behavior and personal history of childhood abuse (BD-BOTH), BD individuals with family history of suicidal behavior or personal history of childhood abuse (BD-ONE), and BD individuals with neither of these two risk factors (BD-NONE) were compared with regard to demographic variables and clinical measures.

RESULTS

Almost 70% of the sample had a history of a previous suicide attempt. There were significantly higher rates of previous suicide attempts in the BD-BOTH and BD-ONE relative to the BD-NONE group. BD-BOTH were significantly younger at the time of their first suicide attempt and had higher number of suicide attempts compared with BD-NONE. BD-BOTH were significantly younger at the time of their first episode of mood disorder and first psychiatric hospitalization and had significantly higher rates of substance use and borderline personality disorders compared to BD-NONE.

LIMITATIONS

Retrospective study. Use of semi-structured interview for the assessment of risk factors.

CONCLUSIONS

BD individuals with a familial liability for suicidal behavior and exposed to physical and/or sexual abuse during childhood are at a greater risk to have a more impaired course of bipolar illness and greater suicidality compared to those subjects with either only one or none of these risk factors. Prospective studies are needed to confirm these findings.

Modeling considerations for 11C-CUMI-101, an agonist radiotracer for imaging serotonin 1A receptor in vivo with PET

Several lines of evidence demonstrate involvement of serotonin 1A receptors (5-HT1ARs) in the pathophysiology of neuropsychiatric disorders such as depression, suicidal behavior, schizophrenia, and Alzheimer's disease. We recently published the synthesis and initial evaluation of [O-methyl-11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione (11C-MMP), a 5-HT1AR agonist. Here we determine the optimal modeling parameters for 11C-MMP under its new name, 11C-CUMI-101, in Papio anubis.

METHODS

PET scans were performed on 2 adult male P. anubis; 166.5 MBq +/- 43.0 (4.50 +/- 1.16 mCi) of 11C-CUMI-101 were injected as an intravenous bolus, and emission data were collected for 120 min in 3-dimensional mode. We evaluated 4 different models (1- and 2-tissue compartment iterative and noniterative kinetic models, basis pursuit, and likelihood estimation in graphical analysis [LEGA]), using binding potential (BPF = Bmax/Kd) (Bmax = maximum number of binding sites; Kd = dissociation constant) as the outcome measure. Arterial blood sampling and metabolite-corrected arterial input function were used for full quantification of BPF. To assess the performance of each model, we compared results using 6 different metrics (percentage difference, within-subject mean sum of squares [WSMSS] for reproducibility; variance across subjects, intraclass correlation coefficient [ICC] for reliability; identifiability based on bootstrap resampling of residuals; and time stability analysis to determine minimal required scanning time) at each of 6 different scanning durations. Models were also evaluated on scans acquired after injecting the 5-HT1A antagonist [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide] [WAY100635] 0.5 mg/kg, intravenous) and the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin) [8-OH-DPAT] 2 mg/kg, intravenous).

RESULTS

All metabolites are more polar than 11C-CUMI-101, and no significant change in metabolites was observed in the blocking studies. The free fraction is 59% +/- 3%. We determined that 100 min of scanning time is adequate and that for the region-of-interest (ROI)-level analysis, the LEGA model gives the best results. The median test-retest percentage difference for BPF is 11.15% +/- 4.82% across all regions, WSMSS = 2.66, variance = 6.07, ICC = 0.43, and bootstrap identifiability = 0.59. Preadministration of WAY100635 and 8-OH-DPAT resulted in 87% and 76% average reductions in BPF values, respectively, across ROIs.

CONCLUSION

On the basis of the measurable free fraction, high affinity and selectivity, adequate blood-brain permeability, and favorable plasma and brain kinetics, 11C-CUMI-101 is an excellent candidate for imaging high-affinity 5-HT1ARs in humans.

Quantitative wavelet domain image processing of dynamic PET data

Neuroreceptor PET studies consisting of long dynamic data acquisitions result in data with low signal-to-noise ratio and limited spatial resolution. To address these problems we have developed a 3D wavelet-based image processing tool (wavelet filter, WF), containing both denoising and enhancement functionality. The filter is based on multi-scale thresholding and cross-scale regularization. These operations are data-driven, which may lead to non-linearity effects and hamper quantification of dynamic PET data. The aim of the present study was to investigate these effects using both phantom and human PET data. A phantom study was performed with a cylindrical phantom, filled with 18F, containing a number of spherical inserts filled with 11C. Human studies were performed on 9 healthy volunteers after injection of the serotonine transporter tracer [11C]DASB. Images from both phantom and human studies were reconstructed with filtered backprojection and post-processed by WF with a series of different denoising and enhancement parameter values. The phantom study was analyzed by computing the insert-to-background ratio as a function of time. The human study was analyzed with a 1-tissue compartment model for a series of brain regions. For the phantom study, linear relations were found between unprocessed and WF processed data for positive contrasts. However, for negative contrast, non-linearity effects were observed. For the human data, good correlation was obtained between results from unprocessed and WF processed data. Our results showed that, although non-linear effects may appear in low-contrast areas, it is possible to achieve accurate quantification with wavelet-based image processing.

Does first episode polarity predict risk for suicide attempt in bipolar disorder?

BACKGROUND

Defining bipolar disorder (BD) subtypes with increased risk of suicidal behavior may help clinical management. We tested the hypothesis that the polarity of a patient's first mood episode would be a marker for BD subtypes with differential risk for suicidality.

METHODS

One hundred thirteen subjects with DSM-IV defined BD were classified based on whether their first reported episode was manic/hypomanic (FM) or depressed (FD). They were compared on demographic and clinical variables. Logistic regression adjusting for potential confounds tested the association between first episode polarity and history of suicide attempt.

RESULTS

Multiple logistic regression analysis showed that FD group membership was associated with eightfold odds of a past suicide attempt, adjusting for years ill and total number of lifetime major depressive episodes.

LIMITATIONS

Sample size, retrospective design, recall bias, assessment during a mood episode, and imprecise recall of hypomania.

CONCLUSIONS

Polarity of patients' first reported mood episode suggested a depression-prone subtype with a greater probability of past suicide attempt. The FM group had more alcoholism and psychosis, but less likelihood of past suicide attempt. Validation of these putative subtypes requires prospective study.

Positron emission tomography study of regional brain metabolic responses to a serotonergic challenge in major depressive disorder with and without comorbid lifetime alcohol dependence

This is the first study contrasting regional glucose metabolic rate (rCMRglu) responses to a serotonergic challenge in major depressive disorder (MDD) with and without comorbid alcohol dependence. In a university hospital, patients with MDD without a history of alcohol dependence (MDD only) and patients with MDD and comorbid alcohol dependence (MDD/ALC) were enrolled in this study. Subjects with comorbid borderline personality disorder were excluded. A bolus injection of approximately 5 mCi of (18)fluorodeoxyglucose was administered 3 h after the administration of placebo or fenfluramine. We found an anterior medial prefrontal cortical area where MDD/ALC subjects had more severe hypofrontality than MDD only patients. This area encompassed the left medial frontal and left and right anterior cingulate gyri. This group difference disappeared after fenfluramine administration. The fact that the observed group difference disappeared after the fenfluramine challenge suggests that serotonergic mechanisms play a role in the observed differences between the groups.

Optimal Metabolite Curve Fitting for Kinetic Modeling of 11C-WAY-100635

Many quantitative imaging protocols that make use of a metabolite-corrected arterial input function require the use of a mathematic model to describe the rate of metabolism of the radioligand. Commonly, parametric models are fit to metabolism data and then the fitted model is used to correct the plasma input function. (11)C-WAY 100635 is a rapidly metabolized radioligand used extensively in mapping the 5-hydroxytryptamine receptor 1A system.

METHODS

To evaluate the adequacy of fit of 4 metabolite models, we examined data from 92 subjects who received an injection of (11)C-WAY 100635, were imaged with PET, and underwent measurement of total plasma concentration and metabolites. The performance of these models was assessed according to residual plots, as well as fit and information criteria.

RESULTS

The study showed that the choice of model has a substantial effect on the resulting estimates of outcome measures.

CONCLUSION

Among the models considered, the Hill model provides the best fit across all criteria.

PET Imaging of CRF1 with [11C]R121920 and [11C]DMP696: is the target of sufficient density?

AIM

Overstimulation of the CRF type 1 receptor (CRF1) is implicated in anxiety and depressive disorders. The aim of this study was to investigate the in vivo binding characteristics of [11C]R121920 and [11C]DMP696 in the nonhuman primate for application in positron emission tomography (PET) studies of CRF1.

METHODS

PET imaging with the two novel CRF1 radioligands was performed in baboon. In vitro binding studies for CRF1 were performed in postmortem brain tissue of baboon and human to assess sufficiency of receptor density for PET.

RESULTS

Both [11C]R121920 and [11C]DMP696 distributed rapidly and uniformly throughout the brain. Washout was comparable across brain regions, without differences in volume of distribution between regions reported to have high and low in vitro CRF1 binding. Membrane-enriched tissue homogenate assay using [(125)I]Tyr(0)-sauvagine and specific CRF1 antagonists CP154,526 and SN003 in human occipital cortex yielded maximal binding (Bmax) of 63.3 and 147.3 fmol/mg protein, respectively, and in human cerebellar cortex yielded Bmax of 103.6 and 64.6 fmol/mg protein, respectively. Dissociation constants (K(D)) were subnanomolar. In baboon, specific binding was not detectable in the same regions; therefore, Bmax and K(D) were not measurable. Autoradiographic results were consistent except there was also detectable CRF1-specific binding in baboon cerebellum.

CONCLUSION

Neither [11C]R121920 nor [11C]DMP696 demonstrated quantifiable regional binding in vivo in baboon. In vitro results suggest CRF1 density in baboon may be insufficient for PET. Studies in man may generate more promising results due to the higher CRF1 density compared with baboon in cerebral cortex and cerebellum.

Synthesis and in vivo evaluation of [18F]-4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide as a PET imaging probe for COX-2 expression

Synthesis of [18F]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide ([18F]celecoxib), a selective COX-2 inhibitor, is achieved via a bromide to [18F]F- exchange reaction. Synthesis of the precursor for radiolabeling was achieved from 4'-methylacetophenone in four steps with 22% overall yield. Under non-radioactive conditions, fluorination was achieved using TBAF in DMSO at 135 degrees C in 80% yield. Synthesis of [18F]celecoxib was achieved using [18F]TBAF in DMSO at 135 degrees C in 10+/-2% yield (EOS) with >99% chemical and radiochemical purities. The specific activity was 120+/-40 mCi/micromol (EOB). [18F]celecoxib was found to be stable in ethanol, however, de[18F]fluorination (6.5%) was observed after 4 h in 10% ethanol-saline solution. Rodent PET studies show bone labeling indicating in vivo de[18F]fluorination of [18F]celecoxib. PET studies in baboon indicated a lower rate of de[18F]fluorination than rat and retention of radioactivity in brain regions consistent with the known distribution of COX-2. A radiolabeling method that can generate consistent high specific activity is needed for routine human use.

Brain serotonin transporter binding in depressed patients with bipolar disorder using positron emission tomography

CONTEXT

Depression in bipolar disorder is clinically indistinguishable from that observed in major depressive disorder. As in major depression, selective serotonin reuptake inhibitors targeting brain serotonin transporters are first-line treatments for bipolar depression. Associations of serotonin transporter promoter polymorphisms and bipolarity have been reported; however, research on alterations in serotonergic neurotransmission in bipolar depression remains scant.

OBJECTIVES

To assess in vivo brain serotonin transporter binding potential (BP(1), proportional to serotonin transporter number) in patients with bipolar depression and controls and to examine the relationship between serotonin transporter binding and genotype.

DESIGN

Case-control study.

SETTING

University hospital.

PARTICIPANTS

A sample of 18 medication-free patients with bipolar depression and 41 controls.

MAIN OUTCOME MEASURES

In vivo brain serotonin transporter binding was measured using positron emission tomography and radiolabeled trans-1,2,3,5,6,10-beta-hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]-isoquinoline ([(11)C](+)-McNeil 5652). Participants were genotyped assessing biallelic and triallelic 5-HTTLPR polymorphisms.

RESULTS

Patients with bipolar disorder had 16% to 26% lower serotonin transporter BP(1) in the midbrain, amygdala, hippocampus, thalamus, putamen, and anterior cingulate cortex. Triallelic 5-HTTLPR genotypes were unrelated to serotonin transporter BP(1).

CONCLUSIONS

Lower serotonin transporter BP(1) in bipolar depression overlaps with that observed in major depression and suggests that serotonergic dysfunction is common to depressive conditions.

Synthesis and in vivo evaluation of a novel 5-HT1A receptor agonist radioligand [O-methyl-11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione in nonhuman primates

PURPOSE

Serotonin1A (5-HT1A) receptors exist in high- and low-affinity states, and agonist ligands bind preferentially to the high-affinity state of the receptor and provide a measure of functional 5-HT1A receptors. Although the antagonist tracers are established PET ligands in clinical studies, a successful 5-HT1A receptor agonist radiotracer in living brain has not been reported. [11C]MPT, our first-generation agonist radiotracer, shows in vivo specificity in baboons; however, its utility is limited owing to slow washout and immeasurable plasma free fraction. Hence we performed structure-activity relationship studies of MPT to optimize a radiotracer that will permit valid quantification of 5-HT1A receptor binding. We now report the synthesis and evaluation of [11C]MMP as an agonist PET tracer for 5-HT1A receptors in baboons.

METHODS

In vitro binding assays were performed in bovine hippocampal membranes and membranes of CHO cells expressing 5-HT1A receptors. [11C] labeling of MMP was performed by reacting desmethyl-MMP with [11C]CH(3)OTf. In vivo studies were performed in baboons, and blocking studies were conducted by pretreatment with 5-HT1A receptor ligands WAY-100635 and (+/-)-8-OH-DPAT.

RESULTS

MMP is a selective 5-HT1A receptor agonist (Ki 0.15 nM). Radiosynthesis of [11C]MMP was achieved in 30 +/- 5% (n = 15) yield at EOS with a specific activity of 2,600 +/- 500 Ci/mmol (n = 12). PET studies in baboons demonstrated specific binding of [11C]MMP to 5-HT1A receptor-enriched brain regions, as confirmed by blockade with WAY-100635 and (+/-)-8-OH-DPAT.

CONCLUSION

We identified [11C]MMP as an optimal agonist PET tracer that shows quantifiable, specific binding in vivo to 5-HT1A receptors in baboons.

In vivo quantification of serotonin transporters using [(11)C]DASB and positron emission tomography in humans: modeling considerations

Positron emission tomography (PET) studies of the serotonin transporter (5-HTT) in the human brain are increasingly using the radioligand [(11)C]N, N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine. A variety of models have been applied to such data in several published articles; however to date, these models have not been validated with test-retest data. We recruited 11 healthy subjects and conducted two identical scans on each subject on the same day. We considered four different models (one- and two-tissue compartment kinetic models, likelihood estimation in graphical analysis (LEGA; a bias-free alternative to the graphical method), and basis pursuit) along with fast noniterative approximations to the kinetic models. We considered four different outcome measures (total volume of distribution (V(T)), binding potential with (BP) and without (BP(1)), free-fraction adjustment, and specific-to-nonspecific equilibrium partition coefficient (BP(2))). To assess the performance of each model, we compared results using six different metrics (percent difference (PD) and within-subject mean sum of squares for reproducibility, interclass coefficient for reliability, variance across subjects, identifiability based on bootstrap resampling of residuals for each method, and time stability analysis to determine minimal required scanning time). We considered analysis of both at the voxel level and at the region of interest (ROI) level and compared results from these two approaches to assess agreement. We determined that 100 mins of scanning time is adequate and that for ROI-level analysis, LEGA gives best results. Average PD is 5.51 for V(T), 20.7 for BP, 17.2 for BP(1), and 16.5 for BP(2) across all regions. For voxel-level analysis we determined that the one-tissue compartment noniterative model is best.

In vivo assessment of [11C]MRB as a prospective PET ligand for imaging the norepinephrine transporter

PURPOSE

Antagonism of norepinephrine reuptake is now an important pharmacological strategy in the treatment of anxiety and depressive disorders, and many antidepressants have substantial potential occupancy of the norepinephrine transporter (NET) at recommended dosages. Despite the importance of understanding this transporter's role in psychiatric disease and treatment, a suitable radioligand for studying NET has been slow to emerge. (S,S)-Methylreboxetine (MRB) is among the more promising ligands recently adapted for positron emission tomography (PET), and the present study aimed to evaluate its potential for use in higher primates.

METHODS

Affinities for various brain targets were determined in vitro. PET studies were conducted in baboon under both test-retest and blocking conditions using 1 mg/kg nisoxetine.

RESULTS

MRB has sixfold higher affinity for NET than the serotonin transporter, and negligible affinity for other sites. PET studies in baboons showed little regional heterogeneity in binding and were minimally affected by pretreatment with the NET antagonist nisoxetine.

CONCLUSION

Despite improvement over previous ligands for imaging NET in vivo, the low signal to noise ratio indicates [(11)C]MRB lacks sensitivity and reliability as a PET radiotracer in humans.

In vitro and in vivo evaluation of [11C]MPEPy as a potential PET ligand for mGlu5 receptors

Excessive activation via the metabotropic glutamate receptor subtype 5 (mGluR(5)) has been implicated in depression, neuropathic pain and other psychiatric, neurological and neurodegenerative diseases. A mGluR(5) radioligand for in vivo quantification by positron emission tomography (PET) would facilitate studies of the role of this receptor in disease and treatment. 3-Methoxy-5-pyridin-2-ylethynylpyridine (MPEPy), a selective and high-affinity antagonist at the mGluR(5) receptor was selected as a candidate ligand; a recent publication by Yu et al. [Nucl Med Biol 32 (2005) 631-640] presented initial micro-PET results for [(11)C]MPEPy with enthusiasm. Building on their efforts, we report as unique contributions (1) an improved chemical synthesis method, (2) the first data using human tissue, (3) phosphor images for rat brain preparations, (4) a novel comparison of anesthetic agents and (5) in vivo data in baboon. In vitro phosphor imaging studies of this ligand using human and rat brain tissue demonstrated high specific binding in the hippocampus, striatum and cortex with minimal specific binding in the cerebellum. In contrast, in vivo micro-PET studies in rats using urethane anesthesia, PET studies in baboons using isoflurane anesthesia and ex vivo micro-PET studies in unanesthetized rats each showed little specific binding in the brain. Despite the promising in vitro results, the low signal-to-noise ratio found in vivo does not justify the use of [(11)C]MPEPy as a PET radiotracer in humans.

Metabolite considerations in the in vivo quantification of serotonin transporters using 11C-DASB and PET in humans

PET studies of the serotonin (5-hydroxytryptamine, or 5-HT) transporter are increasingly using (11)C-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile (DASB). We noted that the percentage of unmetabolized (11)C-DASB is often lower at 2 min after injection than at 12 min. We hypothesized that this is due to initial "trapping" of the unmetabolized (11)C-DASB compound in the lung, a major 5-HT transporter site and dose-limiting organ. To determine whether binding to an extracranial pool of 5-HT transporters contributes to the lower initial level of unmetabolized (11)C-DASB, we examined the effects of sertraline.

METHODS

Eleven healthy volunteers had 2 (11)C-DASB PET scans on the same day, and 6 of the 11 had a third scan after sertraline administration. The unmetabolized (11)C-DASB fraction was measured in arterial plasma as a function of time and was fit with 2 exponentials with no damping, power function damping, or no damping with the first point removed.

RESULTS

Power function damping best fit the data as assessed by visual inspection and residuals and resulted in greater distribution volumes than did no damping with the first point removed. Test-retest reproducibility improved when power function damping was used, as compared with no damping with the first point removed. Oral sertraline raised the 2-min unmetabolized (11)C-DASB percentage.

CONCLUSION

Measurement and fitting of early metabolism time points improves curve fitting, significantly affects volume-of-distribution determination, and improves test-retest reproducibility. Saturation of lung 5-HT transporters by sertraline prevents the initial trapping of (11)C-DASB. Initial trapping of high-affinity radioligands may be important in the quantification of the binding of other ligands with a high concentration of binding sites in the lungs.

Higher 5-HT1A receptor binding potential during a major depressive episode predicts poor treatment response: preliminary data from a naturalistic study

Serotonin 1A (5-HT1A) binding potential (BP) as assessed by positron emission tomography (PET) is higher in major depressive disorder (MDD) in association with the higher expressing GG genotype of the 5-HT1A C-1019G polymorphism. We hypothesize that higher 5-HT1A BP and the GG genotype predict remission failure on antidepressant treatment. We determined 5-HT1A BP by PET and 5-HT1A C-1019G genotype in 43 controls and 22 medication-free MDD subjects. MDD was treated naturalistically and remission was defined as >50% reduction and a score of <or=10 on the 24 item Hamilton Scale 1 year after initiation of treatment after scanning. Despite equivalent treatment, nonremitters have higher pretreatment cortical BP and the GG genotype is over-represented compared with remitters. Higher 5-HT1A BP, perhaps due to greater gene expression, may predict antidepressant medication nonremission. The findings should be tested in a controlled prospective treatment study.

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.

Acute occupancy of brain serotonin transporter by sertraline as measured by [11C]DASB and positron emission tomography

BACKGROUND

In vivo determination of serotonin transporter (5-HTT) occupancy by selective serotonin reuptake inhibitors (SSRI) using positron emission tomography (PET) can aid in determination of dosing. Previous studies used chronic SSRI administration that may down-regulate 5-HTT and used the cerebellum as reference region despite measurable 5-HTT. We examine the reference region and occupancy after acute sertraline dosing.

METHODS

We conducted autoradiography of human postmortem cerebellum to determine an optimal reference region. We quantified 5-HTT binding using [(11)C]DASB and arterial input functions in 17 healthy volunteers. Baseline PET scans were followed by a scan 4-6 days after 25 mg to 100mg of daily sertraline. Several modeling methods and outcome measures were assessed.

RESULTS

Cerebellar gray matter is the optimal reference region. Occupation of 5-HTT sites saturates at low plasma sertraline levels (K(D) = 1.9 ng/ml) with maximal occupancies of 106.8 +/- 8.3% across all brain regions. There is a weak correlation between oral sertraline and plasma sertraline. Occupancy measures vary based on the reference region and outcome measure used.

CONCLUSIONS

Occupancy studies and postmortem autoradiography can help define the optimal reference region. Reference tissue modeling using the optimal reference region returns the same occupancy measures as those determined using an arterial input function.

Synthesis and in vivo evaluation of [11C]SN003 as a PET ligand for CRF1 receptors

Synthesis and evaluation of [O-methyl-11C](4-methoxy-2-methylphenyl)[1-(1-methoxymethylpropyl)-6-methyl-1H-[1,2,3]triazolo[4,5-c]pyridin-4-yl]amine or [11C]SN003 ([11C]6), as a PET imaging agent for CRF1 receptors, in baboons is described. 4-[1-(1-Methoxymethylpropyl)-6-methyl-1H-[1,2,3]triazolo[4,5-c]pyridin-4-ylamino]-3-methylphenol (5), the precursor molecule for the radiolabeling, was synthesized from 2,4-dichloro-6-methyl-3-nitropyridine in seven steps with 20% overall yield. The total time required for the synthesis of [11C]SN003 is 30 min from EOB using [11C]methyl triflate in the presence of NaOH in acetone. The yield of the synthesis is 22% (EOS) with >99% chemical and radiochemical purities and a specific activity of >2000 Ci/mmol. PET studies in baboon show that [11C]6 penetrates the BBB and accumulates in brain. No detectable specific binding was observed, likely due to the rapid metabolism or low density of CRF1 receptors in primate brain.

Amyloid plaque imaging agent [C-11]-6-OH-BTA-1: biodistribution and radiation dosimetry in baboon

BACKGROUND

The amyloid neuritic plaque is considered to be a toxic collection of amyloid-ss protein found in brain tissue in Alzheimer's disease. A neutral analogue of the amyloid-binding thioflavin-T (BTA), has been radiolabeled as [C-11]-6-OH-BTA-1. It crosses the blood brain barrier, and is a promising tracer for imaging plaques in vivo using positron emission tomography. We now report the biodistribution and dosimetry of [C-11]-6-OH-BTA-1 in baboons.

METHODS

Four 2-hour whole body studies were acquired in an ECAT ACCEL camera in two baboons after the bolus injection of [C-11]-6-OH-BTA-1. After 3.5 minute transmission scans performed per bed position prior to injection, emission scans were collected in 2-D mode over five bed positions. Regions of interest (ROI) were drawn around the brain, left and right lungs, heart, liver, gall bladder, left and right kidneys, spleen and urinary bladder. Since no fluid was removed from the baboons, total body radioactivity was calculated using the injected dose and a calibration factor determined from a cylinder phantom. The area under the curve (AUC) for each ROI was determined by trapezoidal integration of the first few points with subsequent points fit by a decreasing monoexponential. The AUC was then divided by counts in the total body, and resulting residence times were entered into the MIRDOSE3 program.

RESULTS

The animals tolerated the procedure well. The ligand was eliminated via the hepatobiliary and renal systems. In the adult male and female reference the gallbladder received the highest estimated radiation dose and was the critical organ (3.9E-02 mGy/MBq and 4.3E-02 mGy/MBq respectively).

CONCLUSION

In the United States, the absorbed dose to the gallbladder would limit [C-11]-6-OH-BTA-1 administered with the approval of a Radioactive Drug Research Committee (RDRC) to a single injection of 1295 MBq (35 mCi) in the adult male, and 1314 MBq (35 mCi) in the adult female.

Synthesis, in vitro and in vivo evaluation of [O-methyl-11C] 2-{4-[4-(3-methoxyphenyl)piperazin-1-yl]-butyl}-4-methyl-2H-[1,2,4]-triazine-3,5-dione: a novel agonist 5-HT1A receptor PET ligand

Synthesis and in vivo evaluation of 2-{4-[4-(3-methoxyphenyl)piperazin-1-yl]-butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (5 or MMT), a high affinity and selective serotonin 5-HT1AR agonist PET tracer, are described. GTPgammaS assay shows that MMT is an agonist with an EC50 comparable to 5-HT. Radiolabeling of 5 was achieved in 30% yield (EOS) from desmethyl-MMT (4) with >99% chemical and radiochemical purities and a specific activity >1000 Ci/mmol. PET studies in baboon show that [11C]5 penetrates the blood-brain barrier but, because of low specific binding and fast clearance of radioactivity it is not a suitable PET tracer for the in vivo quantification of 5-HT1AR.

Altered serotonin 1A binding in major depression: a [carbonyl-C-11]WAY100635 positron emission tomography study

BACKGROUND

Serotonin 1A receptors (5-HT(1A)) are implicated in the pathophysiology of major depressive disorder (MDD) and in the action of selective serotonin reuptake inhibitors (SSRI). SSRI desensitize 5-HT(1A) and down-regulate 5-HT transporters (5-HTT) with the latter persisting for weeks after discontinuation of SSRI. MDD subjects are more likely to be homozygous for the functional 5-HT(1A) G(-1019) allele of the promoter polymorphism and are postulated to have higher 5-HT(1A) than healthy volunteers (controls). We measure 5-HT(1A) in MDD, assess the effects of antidepressant exposure (AE), and examine the role of the C(-1019)G polymorphism.

METHODS

Genotyped and determined 5-HT(1A) binding potential (BP) by positron emission tomography (PET) using [carbonyl-C-11]-WAY-100635 in 28 medication-free MDD subjects during a current major depressive episode and 43 controls.

RESULTS

No difference in BP between controls and MDD subjects (p = .235). There was a difference in BP comparing the controls, antidepressant naive (AN) MDD subjects, and subjects with AE across all regions (p = .013). Post hoc testing reveals higher BP in AN compared to controls (p = .008) and to AE (p = .007). The GG genotype is overrepresented in MDD subjects (p = .059), and BP appears higher with the G allele.

CONCLUSIONS

AN have higher 5-HT(1A) than controls and AE suggesting a model of depression characterized by an over expression of autoinhibitory somatodendritic 5-HT(1A) receptors, perhaps due to the higher expressing G allele, that may result in reduced terminal field 5-HT release. AE appears to have long-term effects on 5-HT(1A).

Lower serotonin transporter binding potential in the human brain during major depressive episodes

OBJECTIVE

CSF analysis, neuroendocrine challenges, serotonin depletion studies, and treatment studies implicate the serotonergic system in the pathophysiology of major depressive disorder. On the basis of postmortem and imaging studies, the authors hypothesized that subjects with major depressive disorder in a major depressive episode have fewer serotonin transporter sites, compared with healthy subjects.

METHOD

Serotonin transporter binding potential (f(1)B(max)/K(d)) was determined using positron emission tomography with [(11)C]McN 5652 in six brain regions in 25 medication-free subjects with DSM-IV major depressive disorder during a major depressive episode and in 43 healthy volunteer comparison subjects. All subjects had arterial lines placed to determine metabolite-corrected arterial input functions.

RESULTS

Serotonin transporter binding potential differed significantly by brain region and group. Post hoc analysis revealed lower binding potential in subjects with major depressive disorder, relative to the comparison subjects, in the amygdala and midbrain. The lower binding potential was more pronounced in the depressed subjects who had never received antidepressants. No correlation was found between binding potential in the midbrain and severity of depression or number of days without medication. Binding potential did not differ between suicide attempters and nonattempters.

CONCLUSIONS

Subjects in a major depressive episode have lower serotonin transporter binding potential in the amygdala and midbrain, compared to healthy subjects.

Effect of a triallelic functional polymorphism of the serotonin-transporter-linked promoter region on expression of serotonin transporter in the human brain

OBJECTIVE

The authors examined effects of a triallelic functional polymorphism of the human serotonin-transporter-linked promoter region (5-HTTLPR) on in vivo expression of serotonin transporter in the brain in healthy volunteers and subjects with major depressive disorder.

METHOD

Twenty-five medication-free subjects with DSM-IV major depressive disorder during a major depressive episode and 42 healthy volunteers were clinically evaluated and genotyped. Serotonin transporter binding potential (f(1)B(max)/K(d)) was determined by using positron emission tomography with the radiotracer [(11)C]McN 5652 and metabolite-corrected arterial input functions.

RESULTS

There was no difference in serotonin transporter binding potential by genotype in healthy volunteers or in subjects with major depressive disorder. Allelic frequencies did not differ between subjects with major depressive disorder and healthy volunteers.

CONCLUSIONS

Associations of the 5-HTTLPR polymorphism to clinical phenotypes appear to be due to developmental effects of 5-HTTLPR on expression and not due to its direct effect on serotonin transporter binding in adulthood.

Brain serotonin1A receptor binding in major depression is related to psychic and somatic anxiety

BACKGROUND

The anxious phenotype of the 5-HT1A receptor knockout mouse and the anxiolytic properties of 5-HT1A agonists suggest that the 5-HT1A receptor modulates anxiety. We investigated the relationship of anxiety expressed in major depressive disorder (MDD) to regional 5-HT1A binding.

METHODS

Positron emission tomography with [carbonyl-11C]WAY-100635 was used to estimate regional 5-HT1A binding potential (BP) in 28 medication-free MDD subjects. Stepwise linear regression assessed the predictive capacity of three anxiety components, derived from a larger MDD sample and termed psychic, somatic, and motoric anxiety, on regional 5-HT1A BP.

RESULTS

Higher psychic (beta >or= .63) and lower somatic (beta <or= -.70) anxiety predicted over 50% of the variance in 5-HT1A BP in multiple cortical regions, but not in amygdala, hippocampus, or autoreceptors of the raphe nuclei. The psychic and somatic anxiety components were not related to depression severity. Comorbid panic disorder was associated with lower cortical and subcortical 5-HT1A BP.

CONCLUSIONS

The 5-HT1A receptor in the same brain regions has different relationships to psychic anxiety versus somatic anxiety. Lower 5-HT1A BP in panic disorder may be accounted for by higher somatic and lower psychic anxiety. Further study of the pathobiology of these anxiety components may identify distinct therapeutic targets or mechanisms.

Synthesis and in Vivo Validation of [O-Methyl-11C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin- 1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione: A Novel 5-HT1A Receptor Agonist Positron Emission Tomography Ligand

Antagonist 5-HT(1A) PET ligands are available, but an agonist ligand would give more information about signal transduction capacity. Synthesis and in vivo evaluation of [O-methyl-(11)C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (10), a potential high affinity (K(i) = 1.36 nM) 5-HT(1A) agonist PET tracer is described. Piperazine 10 is a 5-HT(1A) agonist with an EC(50) comparable to serotonin, based on cAMP formation and GTP(gamma)S binding assays. Radiosynthesis of [(11)C]10 has been achieved by reacting 2-{4-[4-(7-hydroxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (9) and [(11)C]CH(3)OTf in 25 +/- 5% (n = 15) yield at the end of synthesis (EOS). The chemical and radiochemical purities of [(11)C]10 were >99% with a specific activity 1500 +/- 300 Ci/mmol (n =15). PET studies in anesthetized baboon demonstrate [(11)C]10 specific binding in brain regions rich in 5-HT(1A) receptors. Binding of [(11)C]10 was blocked by WAY100635 and 8-OH-DPAT. The regional brain volumes of distribution (V(T)) of [(11)C]10 in baboon correlate with [(11)C]WAY100635 V(T) in baboons. These data provide evidence that [(11)C]10 is the first promising agonist PET tracer for the 5-HT(1A) receptors.

Biodistribution and radiation dosimetry of [11C]DASB in baboons

OBJECTIVE

The serotonin transporter has been implicated in a variety of conditions including mood disorders and suicidal behavior. In vivo human brain studies with positron emission tomography and the serotonin transporter antagonist [(11)C]DASB ([(11)C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) are ongoing in several laboratories with the maximum administered activity based on dosimetry collected in rodents. We report on the biodistribution and dosimetry of [(11)C]DASB in the baboon as this species may be a more reliable surrogate for human dosimetry.

METHODS

Four baboon studies (two studies in each of two baboons) were acquired in an ECAT ACCEL camera after the bolus injection of 183+/-5 MBq/2.3+/-1.0 nmol of [(11)C]DASB. For each study, six whole-body emission scans were collected in 3D mode over 6/7 bed positions for 2 h. Regions of interest were drawn on brain, lungs, liver, gallbladder, spleen, kidneys, small intestine and bladder. Since no fluid was removed from the animal, total body radioactivity was calculated using the injected dose calibrated to the ACCEL image units.

RESULTS

Uptake was greatest in lungs, followed by the urinary bladder, gallbladder, brain and other organs. The ligand was eliminated via the hepato-billiary and renal systems. The largest absorbed dose was found in the lungs (3.6 x 10(-2) mSv/MBq). The absorbed radiation doses in lungs and gallbladder were four and nine times larger than that previously estimated from rat studies.

CONCLUSION

Based on our baboon biodistribution and dose estimates, the lungs are the critical organs for administration of [(11)C]DASB. In the United States, the absorbed dose to the lungs would limit [(11)C]DASB administered with the approval of a Radioactive Drug Research Committee to 1400 MBq (37 mCi) in the adult male and 1100 MBq (30 mCi) in the adult female.

Regional heterogeneity of 5-HT1A receptors in human cerebellum as assessed by positron emission tomography

Two measures used in brain imaging are binding potential (BP) and the specific to nonspecific equilibrium partition coefficient (V(3)''). V(3)'' determined using the 5-HT(1A) ligand [(11)C]WAY-100635 is sensitive to changes in the free and nonspecific binding of the ligand in the reference region (V(2)). Healthy female volunteers have higher 5-HT(1A) BP but not V(3)'' compared with men, because V(2) is higher in women. While there could be several explanations for this observation, we hypothesized that women have more 5-HT(1A) receptors in the cerebellum. We explore the cerebellum to define a subregion that more accurately represents the free and nonspecific binding, potentially allowing the use of V(3)''. A quantitative autoradiogram in human brain using [(3)H]WAY-100635 identified a cerebellar subregion devoid of 5-HT(1A) receptors. In vivo 5-HT(1A) receptors were evaluated using [(11)C]WAY-100635 in 12 healthy women and 13 healthy men. Each subject had a metabolite-corrected arterial input function. The autoradiogram demonstrates the lowest concentration of 5-HT(1A) receptors in the cerebellar white matter (CW) and highest concentration in the cerebellar vermis (CV). The CW volume of distribution (V(T)) is lower than CV. Cerebellar white matter is adequately modeled by a one-tissue compartmental model, while a two-tissue model is necessary to model CV or the total cerebellum (CT). Women have a higher CW V(T) compared with men, suggesting a difference in V(2). Use of CW improves identifiability and time stability of BP in cortical regions. Cerebellar white matter might be a better reference region for use in future 5-HT(1A) studies using [(11)C]WAY-100635. With CW as a reference region, V(3)'' cannot be used to detect differences in 5-HT(1A) receptors between men and women, suggesting the need for arterial input functions to determine BP.

Positron emission tomography of regional brain metabolic responses to a serotonergic challenge in major depressive disorder with and without borderline personality disorder

Previous neuroimaging studies of major depression have not controlled for the presence of personality disorders characterized by impulsive aggressive behavior, such as borderline personality disorder (BPD). Using positron emission tomography (PET), we studied regional glucose uptake in response to fenfluramine (FEN) in depressed subjects with BPD (n=11) and depressed patients without Cluster B Axis II disorders (n=8). Subjects were scanned while medication-free after a single blind placebo administration and after FEN on a second day. Brain responses were measured by PET imaging of [18F]fluorodeoxyglucose (FDG) and serial prolactin levels. Scans were compared at a voxel level using statistical parametric mapping. Correlations of changes in relative regional cerebral uptake (rCMRglu) with clinical measures were assessed. Depressed borderline patients had greater relative activity in parietotemporal cortical regions (BA 40, BA 22, and BA 42) before and after FEN activation compared to those without BPD. They also had less relative uptake in the anterior cingulate cortex (BA 32) at baseline compared to depressed patients without BPD and FEN abolished this difference. Impulsivity was positively correlated with rCMRglu in superior and middle frontal cortex (BA 6 and 44). Hostility was positively correlated with rCMRglu in temporal cortical regions (BA 21 and 22). In conclusions, borderline pathology in the context of a Major Depressive Disorder is associated with altered activity in parietotemporal and anterior cingulate cortical regions. Controlling for the presence of BPD in future imaging studies of mood disorders may elucidate similarities and differences in regional serotonergic function in these two often comorbid disorders.

Neuroanatomic correlates of psychopathologic components of major depressive disorder

BACKGROUND

The Hamilton Depression Rating Scale (HDRS) is widely used to measure the severity of depression in mood disorders. Total HDRS score correlates with brain metabolism as measured by fludeoxyglucose F 18 ([(18)F]-FDG) positron emission tomography. The HDRS comprises distinct symptom clusters that may be associated with different patterns of regional brain glucose metabolism.

OBJECTIVE

To examine associations between HDRS component psychopathologic clusters and resting glucose cerebral metabolism assessed by [(18)F]-FDG positron emission tomography. Patients We evaluated 298 drug-free patients who met the DSM-III-R criteria for major depressive disorder.

MAIN OUTCOME MEASURES

Five principal components were extracted from the 24-item HDRS for all subjects and ProMax rotated: psychic depression, loss of motivated behavior, psychosis, anxiety, and sleep disturbance. The [(18)F]-FDG scans were acquired in a subgroup of 43 drug-free patients in twelve 5-minute frames. Voxel-level correlation maps were generated with HDRS total and factor scores.

RESULTS

Total HDRS score correlated positively with activity in a large bilateral ventral cortical and subcortical region that included limbic, thalamic, and basal ganglia structures. Distinct correlation patterns were found with the 3 individual HDRS factors. Psychic depression correlated positively with metabolism in the cingulate gyrus, thalamus, and basal ganglia. Sleep disturbance correlated positively with metabolism in limbic structures and basal ganglia. Loss of motivated behavior was negatively associated with parietal and superior frontal cortical areas.

CONCLUSIONS

Different brain regions correlate with discrete symptom components that compose the overall syndrome of major depression. Future studies should extend knowledge about specific regional networks by identifying responsible neurotransmitters related to specific psychopathologic components of mood disorders.

Biodistribution and radiation dosimetry of 11C-WAY100,635 in humans

Serotonin 1A receptors have been implicated in a variety of conditions including depression, suicidal behavior, and aggression. Dose estimates for current human studies are based on data from rat dosimetry studies. We report the biodistribution and dosimetry of the PET serotonin 1A antagonist 11C-WAY100,635 in humans.

METHODS

PET studies of 6 healthy human volunteers (3 male, 3 female) were acquired after a bolus injection of 11C-WAY100,635. Transmission scans of 3.5 min were obtained at each bed position before injection, and emission scans then were collected in 2-dimensional mode over 8 bed positions. Regions of interest were drawn around the brain, left and right lungs, heart, liver, stomach wall, gallbladder, left and right kidneys, spleen, and urinary bladder. Because no fluid was removed from the subjects, whole-body radioactivity was calculated using the injected dose and a calibration factor determined from a cylinder phantom. The area under the curve for each region of interest was determined by trapezoidal integration of the first 3 points, with subsequent points fit by a decreasing monoexponential. The area under the curve was then divided by counts in the whole body, and the resulting residence times were entered into the MIRDOSE3 program.

RESULTS

Primary elimination was via kidneys to the urinary bladder. There were no sex differences in organ residence times. The urinary bladder wall was the organ with the highest estimated radiation dose (1.94 x 10(-1) +/- 3.57 x 10(-2) mGy/MBq). Except for the kidney and bladder wall, correlation was good between human dosimetry estimates and estimates reported previously from rats. The human dosimetry was 6.6 and 60.6 times higher in the kidneys and urinary bladder wall, respectively, than estimates from rats.

CONCLUSION

The urinary bladder wall is the critical organ for 11C-WAY100,635 in humans. In the United States, according to Radioactive Drug Research Committee guidelines a single dose cannot exceed 300 MBq in a man and 227 MBq in a woman, with up to 3 such injections permitted per annum.

Effects of tryptophan depletion on the binding of [11C]-DASB to the serotonin transporter in baboons: response to acute serotonin deficiency

BACKGROUND

The objective of this study was to evaluate the sensitivity of [(11)C]-N,N-dimethyl-2-(2-amino-4-cyanophenylthio)benzylamine (DASB) binding to the brain serotonin transporter (SERT) to changes in endogenous serotonin (5-hydroxytryptamine [5-HT]) levels. A ligand sensitive to endogenous competition (EC) would enable the measurement of fluctuations of intrasynaptic 5-HT. A ligand insensitive to EC can provide a measure of SERT unaffected by levels of 5-HT. Alternatively, serotonin depletion could accelerate internalization of SERT and reduce binding.

METHODS

Eighteen (14 baseline and 9 tryptophan-depleted) positron emission tomography (PET) scans were carried out in two baboons (Papio anubis). A metabolite-corrected arterial input function was used to estimate the binding potential (BP = B(max)/K(D)).

RESULTS

Depletion of plasma tryptophan by a mean of 65% from the baseline (p = .03) reduces [(11)C]-DASB BP in the six brain regions of interest (ROI). Lower DASB binding correlated with lower plasma tryptophan levels in the ROIs with higher SERT density.

CONCLUSIONS

[(11)C]-DASB binding to SERT in vivo rapidly declines in response to acute reduction in serotonin availability, contrary to what is predicted by a simple competition model. This rapid reduction in SERT availability may be due to accelerated transporter internalization.