Reference Tissue Model Research Articles

Hunger and disinhibition but not cognitive restraint are associated with central norepinephrine transporter availability

The relationship between food-intake related behaviours measured by the Three-Factor Eating Questionnaire (TFEQ) and in vivo norepinephrine transporter (NET) availability has not been explored yet. We investigated ten obese individuals (body mass index (BMI) 42.4 ± 3.7 kg/m2) and ten normal-weight healthy controls (HC, BMI 23.9 ± 2.5 kg/m2) with (S,S)-[11C]-O-methylreboxetine ([11C]MRB) positron emission tomography (PET). All participants completed the TFEQ, which measures cognitive restraint, disinhibition and hunger. Image analysis required magnetic resonance imaging data sets onto which volumes-of-interests were drawn. Tissue time activity curves (TACs) were obtained from the dynamic PET data followed by kinetic modeling of these regional brain TACs applying the multilinear reference tissue model (2 parameters) with the occipital cortex as reference region.Obese individuals scored significantly higher on the hunger subscale of the TFEQ. Correlative data analysis showed that a higher degree of hunger correlated negatively with the NET availability of the insular cortex in both obese individuals and HC; however, this finding was more pronounced in obesity. Further, for obese individuals, a negative correlation between disinhibition and NET BPND of the locus coeruleus was detected.In conclusion, these initial data provide in vivo imaging support for the involvement of the central NE system in maladaptive eating behaviors such as susceptibility to hunger.

Fenfluramine Reduces [11C]Cimbi-36 Binding to the 5-HT2A Receptor in the Nonhuman Primate Brain.

Background[11C]Cimbi-36 is a serotonin 2A receptor agonist positron emission tomography radioligand that has recently been examined in humans. The binding of agonist radioligand is expected to be more sensitive to endogenous neurotransmitter concentrations than antagonist radioligands. In the current study, we compared the effect of serotonin releaser fenfluramine on the binding of [11C]Cimbi-36, [11C]MDL 100907 (a serotonin 2A receptor antagonist radioligand), and [11C]AZ10419369 (a serotonin 1B receptor partial agonist radioligand with established serotonin sensitivity) in the monkey brain.MethodsEighteen positron emission tomography measurements, 6 for each radioligand, were performed in 3 rhesus monkeys before or after administration of 5.0 mg/kg fenfluramine. Binding potential values were determined with the simplified reference tissue model using cerebellum as the reference region.ResultsFenfluramine significantly decreased [11C]Cimbi-36 (26–62%) and [11C]AZ10419369 (35–58%) binding potential values in most regions (P < 0.05). Fenfluramine-induced decreases in [11C]MDL 100907 binding potential were 8% to 30% and statistically significant in 3 regions. Decreases in [11C]Cimbi-36 binding potential were larger than for [11C]AZ10419369 in neocortical and limbic regions (~35%) but smaller in striatum and thalamus (~40%). Decreases in [11C]Cimbi-36 binding potential were 0.9 to 2.8 times larger than for [11C]MDL 100907, and the fraction of serotonin 2A receptor in the high-affinity state was estimated as 54% in the neocortex.ConclusionsThe serotonin sensitivity of serotonin 2A receptor agonist radioligand [11C]Cimbi-36 was higher than for antagonist radioligand [11C]MDL 100907. The serotonin sensitivity of [11C]Cimbi-36 was similar to [11C]AZ10419369, which is one of the most sensitive radioligands. [11C]Cimbi-36 is a promising radioligand to examine serotonin release in the primate brain.

The relationship between excitement symptom severity and extrastriatal dopamine D2/3 receptor availability in patients with schizophrenia: a high-resolution PET study with [18F]fallypride.

The purpose of this study was to investigate the relationship between specific symptom severity and D2/3 receptor availability in extrastriatal regions in outpatients with schizophrenia to shed light on the role of extrastriatal dopaminergic neurotransmission in the pathophysiology of symptoms of schizophrenia. Sixteen schizophrenia patients receiving relatively low-dose maintenance atypical antipsychotics and seventeen healthy controls underwent 3-Tesla magnetic resonance imaging and high-resolution positron emission tomography with [18F]fallypride. For D2/3 receptor availability, the binding potential with respect to non-displaceable compartment (BPND) was derived using the simplified reference tissue model. The BPND values were lower in patients on antipsychotic treatment than in controls across all regions with large effect sizes (1.03-1.42). The regions with the largest effect size were the substantia nigra, amygdala, and insula. Symptoms of schizophrenia were assessed using a five-factor model of the Positive and Negative Syndrome Scale (PANSS). The region of interest-based analysis showed that PANSS excitement factor score had a significant positive correlation with the [18F]fallypride BPND in the insula. The equivalent dose of antipsychotics was not significantly correlated with PANSS factor scores or regional BPND values. The voxel-based analysis also revealed a significant positive association between the PANSS excitement factor and the [18F]fallypride BPND in the insula. The present study revealed a significant association between excitement symptom severity and D2/3 receptor availability in the insula in schizophrenia, suggesting a possible important role of D2/3 receptor-mediated neurotransmission in the insula and related limbic system in the pathophysiology of this specific symptom cluster.

Altered adenosine 2A and dopamine D2 receptor availability in the 6-hydroxydopamine-treated rats with and without levodopa-induced dyskinesia

Several lines of evidence imply alterations in adenosine signaling in Parkinson's disease (PD). Here, we investigated cerebral changes in adenosine 2A receptor (A2AR) availability in 6-hydroxydopamine (6-OHDA)-lesioned rats with and without levodopa-induced dyskinesia (LID) using positron-emission tomography (PET) with [11C]preladenant. In parallel dopamine type 2 receptor (D2R) imaging with [11C]raclopride PET and behavioral tests for motor and cognitive function were performed. MethodsParametric A2AR and D2R binding potential (BPND) images were reconstructed using reference tissue models with midbrain and cerebellum as reference tissue, respectively. All images were anatomically standardized to Paxinos space and analyzed using volume-of-interest (VOI) and voxel-based approaches. The behavioral alternations were assessed with the open field test, Y-maze, novel object recognition test, cylinder test, and abnormal involuntary movement (AIM) score. In total, 28 female Wistar rats were included. ResultsOn the behavioral level, 6-OHDA-lesioned rats showed asymmetry in forepaw use and deficits in spatial memory and explorative behavior as compared to the sham-operated animals. 15-Days of levodopa (L-DOPA) treatment induced dyskinesia but did not alleviate motor deficits in PD rats. Intranigral 6-OHDA injection significantly increased D2R binding in the lesioned striatum (BPND: 2.69 ± 0.40 6-OHDA vs. 2.31 ± 0.18 sham, + 16.6%; p = 0.03), whereas L-DOPA treatment did not affect the D2R binding in the ipsilateral striatum of the PD rats. In addition, intranigral 6-OHDA injection tended to decrease the A2AR availability in the lesioned striatum. The decrease became significant when data were normalized to the non-affected side (BPND: 4.32 ± 0.41 6-OHDA vs. 4.58 ± 0.89 sham; NS, ratio: 0.94 ± 0.03 6-OHDA vs. 1.00 ± 0.02 sham; − 6.1%; p = 0.01). L-DOPA treatment significantly increased A2AR binding in the affected striatum (BPND: 6.02 ± 0.91 L-DOPA vs. 4.90 ± 0.76 saline; + 23.4%; p = 0.02). In PD rats with LID, positive correlations were found between D2R and A2AR BPND values in the ipsilateral striatum (r = 0.88, ppeak = 8.56.10−4 uncorr), and between AIM score and the D2R BPND in the contralateral striatum (r = 0.98; ppeak = 9.55.10−5 uncorr). ConclusionA2AR availability changed in drug-naïve and in L-DOPA-treated PD rats. The observed correlations of striatal D2R availability with A2AR availability and with AIM score may provide new knowledge on striatal physiology and new possibilities to further unravel the functions of these targets in the pathophysiology of PD.

18 F]Nifene test-retest reproducibility in first-in-human imaging of α4β2* nicotinic acetylcholine receptors.

Eight healthy subjects (4 M,4 F;21-69,44 ± 21 yrs) underwent a [18 F]nifene positron emission tomography scan (200 ± 3.7 MBq), and seven underwent a second scan within 58 ± 31 days. Regional estimates of DVR were measured using the multilinear reference tissue model (MRTM2) with the corpus callosum as reference region. DVR reproducibility was evaluated with test-retest variability (TRV) and intraclass correlation coefficient (ICC). The DVR ranged from 1.3 to 2.5 across brain regions with a TRV of 0-7%, and did not demonstrate a systematic difference between test and retest. The ICCs ranged from 0.2 to 0.9. DVR estimates were stable after 40 min. The binding profile and tracer kinetics of [18 F]nifene make it a promising α4β2* nAChR radiotracer for scientific research in humans, with reliable DVR test-retest reproducibility.

Quantification of Tau Load Using [18F]AV1451 PET

PurposeThe tau tracer [18F]AV1451, also known as flortaucipir, is a promising ligand for imaging tau accumulation in Alzheimer’s disease (AD). Most of the previous studies have quantified tau load using standardized uptake value ratios (SUVr) derived from a static [18F]AV1451 scan. SUVr may, however, be flow dependent and, especially for longitudinal studies, should be validated against a fully quantitative approach. The objective of this study was to identify the optimal tracer kinetic model for measuring tau load using [18F]AV1451.ProceduresFollowing intravenous injection of 225 ± 16 MBq [18F]AV1451, 130 min dynamic PET scans were performed in five biomarker confirmed AD patients and five controls. Arterial blood sampling was performed to obtain a metabolite-corrected plasma input function. Next, regional time–activity curves were generated using PVElab software. These curves were analysed using several pharmacokinetic models.ResultsThe reversible single tissue compartment model (1T2k_VB) was the preferred model for all but one control. For AD patients, however, model preference shifted towards a reversible two tissue compartmental model (2T4k_VB). The simplified reference tissue model (SRTM) derived binding potential (BPND) showed good correlation (AD: r2 = 0.87, slope = 1.06; controls: r2 = 0.87, slope = 0.86) with indirect plasma input binding (distribution volume ratio-1). Standardized uptake value ratios (80–100 min) correlated well with DVR (r2 = 0.93, slope = 1.07) and SRTM-derived BPND (r2 = 0.84, slope = 0.95). In addition, regional differences in tracer binding between subject groups in different tau-specific regions were observed.ConclusionsModel preference of [18F]AV1451 appears to depend on subject status and, in particular, VT. The relationship between model preference and VT suggests that (higher) tau load may be reflected by a second tissue compartment. Nevertheless, consistent results can be obtained using a 2T4k_VB model. In addition, SRTM can be used to derive BPND.

Patterns of age related changes for phosphodiesterase type-10A in comparison with dopamine D2/3 receptors and sub-cortical volumes in the human basal ganglia: A PET study with 18F-MNI-659 and 11C-raclopride with correction for partial volume effect

Phosphodiesterase 10A enzyme (PDE10A) is an important striatal target that has been shown to be affected in patients with neurodegenerative disorders, particularly Huntington´s disease (HD). PDE10A is expressed on striatal neurones in basal ganglia where other known molecular targets are enriched such as dopamine D2/3 receptors (D2/3 R). The aim of this study was to examine the availability of PDE10A enzyme in relation with age and gender and to compare those changes with those related to D2/3 R and volumes in different regions of the basal ganglia. As a secondary objective we examined the relative distribution of D2/3 R and PDE10A enzyme in the striatum and globus pallidus. Forty control subjects (20F/20M; age: 44±11y, age range 27–69) from an ongoing positron emission tomography (PET) study in HD gene expansion carriers were included. Subjects were examined with PET using the high-resolution research tomograph (HRRT) and with 3T magnetic resonance imaging (MRI). The PDE10A radioligand 18F-MNI-659 and D2/3 R radioligand 11C-raclopride were used. The outcome measure was the binding potential (BPND) estimated with the two-tissue compartment model (18F-MNI-659) and the simplified reference tissue model (11C-raclopride) using the cerebellum as reference region. The PET data were corrected for partial volume effects. In the striatum, PDE10A availability showed a significant age-related decline that was larger compared to the age-related decline of D2/3 R availability and to the age-related decline of volumes measured with MRI. In the globus pallidus, a less pronounced decline of PDE10A availability was observed, whereas D2/3 R availability and volumes seemed to be rather stable with aging. The distribution of the PDE10A enzyme was different from the distribution of D2/3 R, with higher availability in the globus pallidus. These results indicate that aging is associated with a considerable physiological reduction of the availability of PDE10A enzyme in the striatum. Moreover as result of the analysis, in the striatum for both the molecular targets, we observed a gender effect with higher BPND the female group.

Pharmacokinetic modeling of [11C]flumazenil kinetics in the rat brain

BackgroundPreferred models for the pharmacokinetic analysis of [11C]flumazenil human studies have been previously established. However, direct translation of these models and settings to animal studies might be sub-optimal. Therefore, this study evaluates pharmacokinetic models for the quantification of [11C]flumazenil binding in the rat brain.Dynamic (60 min) [11C]flumazenil brain PET scans were performed in two groups of male Wistar rats (tracer dose (TD), n = 10 and pre-saturated (PS), n = 2). Time-activity curves from five regions were analyzed, including the pons (pseudo-reference region). Distribution volume (VT) was calculated using one- and two-tissue compartment models (1TCM and 2TCM) and spectral analysis (SA). Binding potential (BPND) was determined from full and simplified reference tissue models with one or two compartments for the reference tissue (FRTM, SRTM, and SRTM-2C). Model preference was determined by Akaike information criterion (AIC), while parameter agreement was assessed by linear regression, repeated measurements ANOVA and Bland-Altman plots.Results1TCM and 2TCM fits of regions with high specific binding showed similar AIC, a preference for the 1TCM, and good VT agreement (0.1% difference). In contrast, the 2TCM was markedly preferred and necessary for fitting low specific-binding regions, where a worse VT agreement (17.6% difference) and significant VT differences between the models (p < 0.005) were seen. The PS group displayed results similar to those of low specific-binding regions. All reference models (FRTM, SRTM, and SRTM-2C) resulted in at least 13% underestimation of BPND.ConclusionsAlthough the 1TCM was sufficient for the quantification of high specific-binding regions, the 2TCM was found to be the most adequate for the quantification of [11C]flumazenil in the rat brain based on (1) higher fit quality, (2) lower AIC values, and (3) ability to provide reliable fits for all regions. Reference models resulted in negatively biased BPND and were affected by specific binding in the pons of the rat.

Test-retest measurements of dopamine D1-type receptors using simultaneous PET/MRI imaging.

The role of dopamine D1-type receptor (D1R)-expressing neurons in the regulation of motivated behavior and reward prediction has not yet been fully established. As a prerequisite for future research assessing D1-mediated neuronal network regulation using simultaneous PET/MRI and D1R-selective [11C]SCH23390, this study investigated the stability of central D1R measurements between two independent PET/MRI sessions under baseline conditions. Thirteen healthy volunteers (7 female, age 33 ± 13yrs) underwent 90-min emission scans, each after 90-s bolus injection of 486 ± 16MBq [11C]SCH23390, on two separate days within 2-4 weeks using a PET/MRI system. Parametric images of D1R distribution volume ratio (DVR) and binding potential (BPND) were generated by a multi-linear reference tissue model with two parameters and the cerebellar cortex as receptor-free reference region. Volume-of-interest (VOI) analysis was performed with manual VOIs drawn on consecutive transverse MRI slices for brain regions with high and low D1R density. The DVR varied from 2.5 ± 0.3 to 2.9 ± 0.5 in regions with high D1R density (e.g. the head of the caudate) and from 1.2 ± 0.1 to 1.6 ± 0.2 in regions with low D1R density (e.g. the prefrontal cortex). The absolute variability of the DVR ranged from 2.4% ± 1.3% to 5.1% ± 5.3%, while Bland-Altman analyses revealed very low differences in mean DVR (e.g. 0.013 ± 0.17 for the nucleus accumbens). Intraclass correlation (one-way, random) indicated very high agreement (0.93 in average) for both DVR and BPND values. Accordingly, the absolute variability of BPND ranged from 7.0% ± 4.7% to 12.5% ± 10.6%; however, there were regions with very low D1R content, such as the occipital cortex, with higher mean variability. The test-retest reliability of D1R measurements in this study was very high. This was the case not only for D1R-rich brain areas, but also for regions with low D1R density. These results will provide a solid base for future joint PET/MRI data analyses in stimulation-dependent mapping of D1R-containing neurons and their effects on projections in neuronal circuits that determine behavior.

Microglial activation in normal-appearing brain regions of patients with cerebral glioma: a cross-sectional study

BackgroundTranslocator protein (TSPO) is overexpressed mainly in activated microglia under disease conditions. [11C]-(R)PK11195 is a radioligand for TSPO widely applied in PET studies. Previously we have found upregulated TSPO in neoplastic cells and its correlation with malignant transformation in human gliomas using [11C]-(R)PK11195 PET and neuropathological assessment. We aimed to investigate TSPO expression and microglial activation in normal-appearing brain regions of patients with glioma. Methods14 patients with low-grade glioma (before tumour biopsy or debulking) and ten healthy controls underwent MRI and PET scans. Epilepsy history was reviewed. Binding potential (BPND) of [11C]-(R)PK11195 was calculated by the simplified reference tissue model. BPND of normal-appearing brain regions was compared with that in controls. Post-mortem brains with treatment-naive low-grade glioma were assessed for TSPO and microglia by immunohistochemistry. FindingsBPND of [11C]-(R)PK11195 in low-grade gliomas was lower than in normal-appearing cerebral regions in the contralateral hemisphere (mean −0·108 [SD 0·148] vs 0·047 [0·101], p=0·0006). Compared with controls, BPND in patients' normal-appearing brain regions was increased (0·065 [0·062] vs 0·002 [0·024], p=0·001), being more prominent in the tumour-bearing hemisphere than in the tumour-free hemisphere (0·081 [0·064] vs 0·057 [0·063], p=0·002). The extratumoral BPND correlated with the length of epilepsy history (Spearman rank correlation coefficient r=0·5, p=0·016). An ipsilateral pattern of increased extratumoral BPND was seen in patients with partial seizures, whereas a bilateral pattern of increase was seen in those with generalised seizures. Post-mortem brain tissue showed a 10-fold increase in microglia and elevation in TSPO-expressing microglia in the gyri adjacent to the tumour and in the contralateral tissue compared with normal brains. Interpretation[11C](R)PK11195 PET and neuropathological assessment revealed widespread microglial activation in normal-appearing brain regions of patients with glioma. That the magnitude of activation was associated with length of epilepsy history and seizure type suggests that modulation of microglial activation could be a novel target for seizure control in these patients. TSPO PET provides an in-vivo demonstration of this inflammatory response, which is undetectable by structural MRI. FundingEuropean Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement INMiND (HEALTH-F2-2011-278850), Engineering and Physical Science Research Council grant MIMIT (EP/G041733/1), Astro Brain Tumour Fund (1133561).

The Relationship Between Serotonin-2A Receptor and Cognitive Functions in Nondemented Parkinson's Disease Patients with Visual Hallucinations.

There is growing evidence that the serotonergic system, in particular serotonin 2A receptors, is involved in neuropsychiatric symptoms in Parkinson's disease (PD), including cognitive processing and visual hallucinations. However, the relationship between serotonin 2A receptor availability, visual hallucinations, and cognitive profile is unknown. The objective of this study was to investigate the level of serotonin 2A receptor availability in brain regions affected by visual hallucinations and to test the association with cognitive/behavioral changes in patients who have PD with visual hallucinations. Nondemented patients who had PD with (n = 11) and without (n = 8) visual hallucinations and age-matched controls (n = 10) were recruited. All participants completed neuropsychological testing, which consisted of visuoperceptual, executive, memory, language, and frontal-behavioral function. Positron emission tomography scans using [18F]setoperone, a serotonin 2A antagonist radioligand, were acquired in patients with PD, and a parametric binding potential map of [18F]setoperone was calculated with the simplified reference tissue model using the cerebellum as a reference. Patients who had PD with visual hallucinations exhibited significantly lower scores on measures of executive and visuoperceptual functions compared with age-matched controls. These changes were paralleled by decreased [18F]setoperone binding in the right insula, bilateral dorsolateral prefrontal cortex, right orbitofrontal cortex, right middle temporal gyrus, and right fusiform gyrus. The psychometric correlation analysis revealed significant relationships among tests associated with visuoperceptual function, memory and learning, and serotonin 2A binding in different prefrontal and ventral visual stream regions. There was also reduced serotonin 2A receptor binding in patients who had PD with depression. These findings support a complex interaction between serotonin 2A receptor function and cognitive processing in patients who have PD with visual hallucinations.

Simple and rapid quantification of serotonin transporter binding using [11C]DASB bolus plus constant infusion

IntroductionIn-vivo quantification of serotonin transporters (SERT) in human brain has been a mainstay of molecular imaging in the field of neuropsychiatric disorders and helped to explore the underpinnings of several medical conditions, therapeutic and environmental influences. The emergence of PET/MR hybrid systems and the heterogeneity of SERT binding call for the development of efficient methods making the investigation of larger or vulnerable populations with limited scanner time and simultaneous changes in molecular and functional measures possible. We propose [11C]DASB bolus plus constant infusion for these applications and validate it against standard analyses of dynamic PET data. Methods[11C]DASB bolus/infusion optimization was performed on data acquired after [11C]DASB bolus in 8 healthy subjects. Subsequently, 16 subjects underwent one scan using [11C]DASB bolus plus constant infusion with Kbol 160–179min and one scan after [11C]DASB bolus for inter-method reliability analysis. Arterial blood sampling and metabolite analysis were performed for all scans. Distribution volumes (VT) were obtained using Logan plots for bolus scans and ratios between tissue and plasma parent activity for bolus plus infusion scans for different time spans of the scan (VT-70 for 60–70min after start of tracer infusion, VT-90 for 75–90min, VT-120 for 100–120min) in 9 subjects. Omitting blood data, binding potentials (BPND) obtained using multilinear reference tissue modeling (MRTM2) and cerebellar gray matter as reference region were compared in 11 subjects. ResultsA Kbol of 160min was observed to be optimal for rapid equilibration in thalamus and striatum. VT-70 showed good intraclass correlation coefficients (ICCs) of 0.61–0.70 for thalamus, striatal regions and olfactory cortex with bias ≤5.1% compared to bolus scans. ICCs increased to 0.72–0.78 for VT-90 and 0.77–0.93 for VT-120 in these regions. BPND-90 had negligible bias ≤2.5%, low variability ≤7.9% and ICCs of 0.74–0.87; BPND-120 had ICCs of 0.73–0.90. Low-binding cortical regions and cerebellar gray matter showed a positive bias of ~8% and ICCs 0.57–0.68 at VT-90. Cortical BPND suffered from high variability and bias, best results were obtained for olfactory cortex and anterior cingulate cortex with ICC=0.74–0.75 for BPND-90. High-density regions amygdala and midbrain had a negative bias of −5.5% and −22.5% at VT-90 with ICC 0.70 and 0.63, respectively. ConclusionsWe have optimized the equilibrium method with [11C]DASB bolus plus constant infusion and demonstrated good inter-method reliability with accepted standard methods and for SERT quantification using both VT and BPND in a range of different brain regions. With as little as 10–15min of scanning valid estimates of SERT VT and BPND in thalamus, amygdala, striatal and high-binding cortical regions could be obtained. Blood sampling seems vital for valid quantification of SERT in low-binding cortical regions. These methods allow the investigation of up to three subjects with a single radiosynthesis.

Cerebellar heterogeneity and its impact on PET data quantification of 5-HT receptor radioligands

In the quantification of positron emission tomography (PET) radiotracer binding, a commonly used method is reference tissue modeling (RTM). RTM necessitates a proper reference and a ubiquitous choice for G-protein coupled receptors is the cerebellum. We investigated regional differences in uptake within the grey matter of the cerebellar hemispheres (CH), the cerebellar white matter (CW), and the cerebellar vermis (CV) for five PET radioligands targeting the serotonin system. Furthermore, we evaluated the impact of choosing different reference regions when quantifying neocortical binding. The PET and MR images are part of the Cimbi database: 5-HT1AR ([11C]CUMI-101, n = 8), 5-HT1BR ([11C]AZ10419369, n = 36), 5-HT2AR ([11C]Cimbi-36, n = 29), 5-HT4R ([11C]SB207145, n = 59), and 5-HTT ([11C]DASB, n = 100). We employed SUIT and FreeSurfer to delineate CV, CW, and CH and quantified mean standardized uptake values (SUV) and nondisplaceable neocortical binding potential (BPND). Statistical difference was assessed with paired nonparametric two-sided Wilcoxon signed-rank tests and multiple comparison corrected via false discovery rate. We demonstrate significant radioligand specific regional differences in cerebellar uptake. These differences persist when using different cerebellar regions for RTM, but the influence on the neocortical BPND is small. Nevertheless, our data highlight the importance of validating each radioligand carefully for defining the optimal reference region.

Chronic exposure to dopamine agonists affects the integrity of striatal D2 receptors in Parkinson's patients.

We aimed to investigate the integrity and clinical relevance of striatal dopamine receptor type-2 (D2R) availability in Parkinson's disease (PD) patients. We studied 68 PD patients, spanning from early to advanced disease stages, and 12 healthy controls. All participants received one [11C]raclopride PET scan in an OFF medication condition for quantification of striatal D2R availability in vivo. Parametric images of [11C]raclopride non-displaceable binding potential were generated from the dynamic [11C]raclopride scans using implementation of the simplified reference tissue model with cerebellum as the reference tissue. PET data were interrogated for correlations with clinical data related to disease burden and dopaminergic treatment. PD patients showed a mean 16.7% decrease in caudate D2R and a mean 3.5% increase in putaminal D2R availability compared to healthy controls. Lower caudate [11C]raclopride BPND correlated with longer PD duration. PD patients on dopamine agonist treatment had 9.2% reduced D2R availability in the caudate and 12.8% in the putamen compared to PD patients who never received treatment with dopamine agonists. Higher amounts of lifetime dopamine agonist therapy correlated with reduced D2Rs availability in both caudate and putamen. No associations between striatal D2R availability and levodopa treatment and dyskinesias were found. In advancing PD the caudate and putamen D2R availability are differentially affected. Chronic exposure to treatment with dopamine agonists, but no levodopa, suppresses striatal D2R availability, which may have relevance to output signaling to frontal lobes and the occurrence of executive deficits, but not dyskinesias.

GABAA receptor occupancy by subtype selective GABAA\u03b12,3 modulators: PET studies in humans

RationaleSedation, dependence, and abuse liability limit the use of non-selective γ-aminobutyric acid (GABAA) receptor positive modulators for the treatment of anxiety. AZD7325 and AZD6280 are novel, subtype-selective GABAAα2,3 receptor positive modulators with limited sedative effects.ObjectivesThe current study aimed to confirm target engagement at GABAA receptors by AZD7325 and AZD6280 in humans and to determine the relationship between exposure, GABAA receptor occupancy, and tolerability.MethodTwo PET studies, using high-resolution research tomography (HRRT) and the radioligand [11C]flumazenil, were performed in 12 subjects at baseline and after administration of single oral doses of AZD7325 (0.2 to 30 mg) and AZD6280 (5 to 40 mg). PET images were analyzed using a simplified reference tissue model, and regional binding potentials (BPND) were obtained. The relationship between plasma concentration of AZD7325 or AZD6280 and GABAA receptor occupancy was described by hyperbolic function, and Ki,plasma (plasma concentration required for 50% receptor occupancy) was estimated. Assessments of safety and tolerability included recording of adverse events, vital signs, electrocardiogram, and laboratory tests.ResultsThe [11C]flumazenil binding was reduced in a dose-dependent, saturable manner by both agents. Maximum receptor occupancy could be reached for both compounds without causing sedation or cognitive impairment. The Ki,plasma estimates for AZD7325 and AZD6280 were 15 and 440 nmol/l, respectively.ConclusionHigh GABAA receptor occupancy by AZD7325 and AZD6280 could be reached without clear sedative effects.

A single-scan protocol for absolute D2/3 receptor quantification with [123I]IBZM SPECT

PurposeMolecular imaging of the D2/3 receptor is widely used in neuropsychiatric research. Non-displaceable binding potential (BPND) is a very popular quantitative index, defined as the product of the receptor concentration (Bavail) and the radiotracer affinity for the receptor (1/appKd). As the appKd is influenced by parameters such as the endogenous neurotransmitter dynamics, it often constitutes a confounding factor in research studies. A simplified method for absolute quantification of both these parameters would be of great interest in this context. Here, we describe the use of a partial saturation protocol that permits to produce an in vivo Scatchard plot and thus estimate Bavail and appKd separately, through a single dynamic SPECT session. To validate this approach, a multi-injection protocol is used for the full kinetic modeling of [123I]IBZM using a 3-tissue compartment, 7-parameter model (3T-7k). Finally, more “classic” BPND estimation methods are also validated against the results of the 3T-7k. MethodsTwenty-nine male rats were used. Binding parameters were estimated using the 3T-7k in a multi-injection protocol. A partial saturation protocol was applied at the region- and voxel-level and results were compared to those obtained with the 3T-7k model. The partial saturation protocol was applied after an adenovirus-mediated D2 receptor striatal overexpression and in an amphetamine-induced dopamine release paradigm. The Simplified Reference Tissue Model (SRTM), the Logan's non-invasive graphical analysis (LNIGA) and a simple standardized uptake ratio (SUR) method were equally applied. ResultsThe partial saturation experiments gave similar values as the 3T-7k both at the regional and voxel-level. After adenoviral-mediated D2-receptor overexpression, an increase in Bavail by approximately 18% was observed in the striatum. After amphetamine administration, a 16.93% decrease in Bavail (p<0.05) and a 39.12% increase (p<0.01) in appKd was observed. BPND derived from SRTM, LNIGA and SUR correlated well with the Bavail values from the 3T-7k (r=0.84, r=0.84 and r=0.83, respectively, p<0.0001 for all correlations). ConclusionA partial saturation protocol permits the non-invasive and time-efficient estimation of Bavail and appKd separately. Given the different biological phenomena that underlie these parameters, this method may be applied for the in-depth study of the dopaminergic system in translational molecular imaging studies. It can detect the biological variations in these parameters, dissociating the variations in receptor density (Bavail) from affinity (1/appKd), which reflects the interactions of the receptor with its endogenous ligand.

Generalized paired-agent kinetic model for in vivo quantification of cancer cell-surface receptors under receptor saturation conditions

New precision medicine drugs oftentimes act through binding to specific cell-surface cancer receptors, and thus their efficacy is highly dependent on the availability of those receptors and the receptor concentration per cell. Paired-agent molecular imaging can provide quantitative information on receptor status in vivo, especially in tumor tissue; however, to date, published approaches to paired-agent quantitative imaging require that only ‘trace’ levels of imaging agent exist compared to receptor concentration. This strict requirement may limit applicability, particularly in drug binding studies, which seek to report on a biological effect in response to saturating receptors with a drug moiety. To extend the regime over which paired-agent imaging may be used, this work presents a generalized simplified reference tissue model (GSRTM) for paired-agent imaging developed to approximate receptor concentration in both non-receptor-saturated and receptor-saturated conditions. Extensive simulation studies show that tumor receptor concentration estimates recovered using the GSRTM are more accurate in receptor-saturation conditions than the standard simple reference tissue model (SRTM) (% error (mean ± sd): GSRTM 0 ± 1 and SRTM 50 ± 1) and match the SRTM accuracy in non-saturated conditions (% error (mean ± sd): GSRTM 5 ± 5 and SRTM 0 ± 5). To further test the approach, GSRTM-estimated receptor concentration was compared to SRTM-estimated values extracted from tumor xenograft in vivo mouse model data. The GSRTM estimates were observed to deviate from the SRTM in tumors with low receptor saturation (which are likely in a saturated regime). Finally, a general ‘rule-of-thumb’ algorithm is presented to estimate the expected level of receptor saturation that would be achieved in a given tissue provided dose and pharmacokinetic information about the drug or imaging agent being used, and physiological information about the tissue. These studies suggest that the GSRTM is necessary when receptor saturation exceeds 20% and highlight the potential for GSRTM to accurately measure receptor concentrations under saturation conditions, such as might be required during high dose drug studies, or for imaging applications where high concentrations of imaging agent are required to optimize signal-to-noise conditions. This model can also be applied to PET and SPECT imaging studies that tend to suffer from noisier data, but require one less parameter to fit if images are converted to imaging agent concentration (quantitative PET/SPECT).

Dual time point method for the quantification of irreversible tracer kinetics: A reference tissue approach applied to [18F]-FDOPA brain PET

The Patlak graphical analysis (PGAREF) for quantification of irreversible tracer binding with a reference tissue model was approximated by a dual time point imaging approach (DTPREF). The DTPREF was applied to 18 [18F]-FDOPA brain scans using the occipital cortex as reference region (DTPOCC) and compared to both PGAOCC and striatal-to-occipital ratios (SOR). Pearson correlation analysis and Bland–Altman plots showed an excellent correlation and good agreement between DTPOCC and PGAOCC, while correlations between SOR and PGAOCC were consistently lower. Linear discriminant analysis (LDA) demonstrated a similar performance for all methods in differentiating patients with Parkinson’s disease (PD) from healthy controls (HC). Specifically for [18F]-FDOPA brain imaging, these findings validate DTPOCC as an approximation for PGAOCC, providing the same quantitative information while reducing the acquisition time to two short static scans. For PD patients, this approach can greatly improve patient comfort while reducing motion artifacts and increasing image quality. In general, DTPREF can improve the clinical applicability of tracers with irreversible binding characteristics when a reference tissue is available.

PET Quantification of the Norepinephrine Transporter in Human Brain with (S,S)-18F-FMeNER-D2.

Norepinephrine transporter (NET) in the brain plays important roles in human cognition and the pathophysiology of psychiatric disorders. Two radioligands, (S,S)-11C-MRB and (S,S)-18F-FMeNER-D2, have been used for imaging NETs in the thalamus and midbrain (including locus coeruleus) using PET in humans. However, NET density in the equally important cerebral cortex has not been well quantified because of unfavorable kinetics with (S,S)-11C-MRB and defluorination with (S,S)-18F-FMeNER-D2, which can complicate NET quantification in the cerebral cortex adjacent to the skull containing defluorinated 18F radioactivity. In this study, we have established analysis methods of quantification of NET density in the brain including the cerebral cortex using (S,S)-18F-FMeNER-D2 PET. Methods: We analyzed our previous (S,S)-18F-FMeNER-D2 PET data of 10 healthy volunteers dynamically acquired for 240 min with arterial blood sampling. The effects of defluorination on the NET quantification in the superficial cerebral cortex was evaluated by establishing a time stability of NET density estimations with an arterial input 2-tissue-compartment model, which guided the less-invasive reference tissue model and area under the time-activity curve methods to accurately quantify NET density in all brain regions including the cerebral cortex. Results: Defluorination of (S,S)-18F-FMeNER-D2 became prominent toward the latter half of the 240-min scan. Total distribution volumes in the superficial cerebral cortex increased with the scan duration beyond 120 min. We verified that 90-min dynamic scans provided a sufficient amount of data for quantification of NET density unaffected by defluorination. Reference tissue model binding potential values from the 90-min scan data and area under the time-activity curve ratios of 70- to 90-min data allowed for the accurate quantification of NET density in the cerebral cortex. Conclusion: We have established methods of quantification of NET densities in the brain including the cerebral cortex unaffected by defluorination using (S,S)-18F-FMeNER-D2 These results suggest that we can accurately quantify NET density with a 90-min (S,S)-18F-FMeNER-D2 scan in broad brain areas.

Characterization of [11C]Lu AE92686 as a PET radioligand for phosphodiesterase 10A in the nonhuman primate brain

Purpose[11C]Lu AE92686 is a positron emission tomography (PET) radioligand that has recently been validated for examining phosphodiesterase 10A (PDE10A) in the human striatum. [11C]Lu AE92686 has high affinity for PDE10A (IC50 = 0.39 nM) and may also be suitable for examination of the substantia nigra, a region with low density of PDE10A. Here, we report characterization of regional [11C]Lu AE92686 binding to PDE10A in the nonhuman primate (NHP) brain.MethodsA total of 11 PET measurements, seven baseline and four following pretreatment with unlabeled Lu AE92686 or the structurally unrelated PDE10A inhibitor MP-10, were performed in five NHPs using a high resolution research tomograph (HRRT). [11C]Lu AE92686 binding was quantified using a radiometabolite-corrected arterial input function and compartmental and graphical modeling approaches.ResultsRegional time-activity curves were best described with the two-tissue compartment model (2TCM). However, the distribution volume (VT) values for all regions were obtained by the Logan plot analysis, as reliable cerebellar VT values could not be derived by the 2TCM. For cerebellum, a proposed reference region, VT values increased by ∼30 % with increasing PET measurement duration from 63 to 123 min, while VT values in target regions remained stable. Both pretreatment drugs significantly decreased [11C]Lu AE92686 binding in target regions, while no significant effect on cerebellum was observed. Binding potential (BPND) values, derived with the simplified reference tissue model (SRTM), were 13–17 in putamen and 3–5 in substantia nigra and correlated well to values from the Logan plot analysis.ConclusionsThe method proposed for quantification of [11C]Lu AE92686 binding in applied studies in NHP is based on 63 min PET data and SRTM with cerebellum as a reference region. The study supports that [11C]Lu AE92686 can be used for PET examinations of PDE10A binding also in substantia nigra.