Mathematical modelling of [¹¹C]-(+)-PHNO human competition studies.

The D(2)/D(3) agonist radioligand [(11)C]-(+)-PHNO is currently the most suitable D(3) imaging agent available, despite its limited selectivity for the D(3) over the D(2). Given the collocation of D(2) and D(3) receptors, and generally higher densities of D(2), the separation of D(2) and D(3) information from [(11)C]-(+)-PHNO PET data are somewhat complex. This complexity is compounded by recent data suggesting that [(11)C]-(+)-PHNO PET scans might be routinely performed in non-tracer conditions (with respect to D(3) receptors), and that the cerebellum (used as a reference region) might manifest some displaceable binding signal. Here we present the modelling and analysis of data from two human studies which employed an adequate dose range of selective D(3) antagonists (GSK598809 and GSK618334) to interrogate the [(11)C]-(+)-PHNO PET signal. Models describing the changes observed in the PET volume of distribution (V(T)) and binding potential (BP(ND)) were used to identify and quantify a [(11)C]-(+)-PHNO mass dose effect at the D(3), and displaceable signal in the cerebellum, as well as providing refined estimates of regional D(3) fractions of [(11)C]-(+)-PHNO BP(ND). The dose of (+)-PHNO required to occupy half of the available D(3) receptors (ED(50)(PHNO,D3)) was estimated as 40ng/kg, and the cerebellum BP(ND) was estimated as 0.40. These findings confirm that [(11)C]-(+)-PHNO human PET studies are in fact routinely performed under non-tracer conditions. This suggests that (+)-PHNO injection masses should be minimised and tightly controlled in order to mitigate the mass dose effect. The specific binding detected in the cerebellum was modest but could have a significant effect, for example on estimates of D(3) potency in drug occupancy studies. A range of methods for the analysis of future [(11)C]-(+)-PHNO data, incorporating models for the effects quantified here, were developed and evaluated. The comparisons and conclusions drawn from these can inform the design and analysis of future PET studies with [(11)C]-(+)-PHNO.

Monoamine transporter occupancy of a novel triple reuptake inhibitor in baboons and humans using positron emission tomography.

The selection of a therapeutically meaningful dose of a novel pharmaceutical is a crucial step in drug development. Positron emission tomography (PET) allows the in vivo estimation of the relationship between the plasma concentration of a drug and its target occupancy, optimizing dose selection and reducing the time and cost of early development. Triple reuptake inhibitors (TRIs), also referred to as serotonin-norepinephrine-dopamine reuptake inhibitors, enhance monoaminergic neurotransmission by blocking the action of the monoamine transporters, raising extracellular concentrations of those neurotransmitters. GSK1360707 [(1R,6S)-1-(3,4-dichlorophenyl)-6-(methoxymethyl)-4-azabicyclo[4.1.0]heptane] is a novel TRI that until recently was under development for the treatment of major depressive disorder; its development was put on hold for strategic reasons. We present the results of an in vivo assessment of the relationship between plasma exposure and transporter blockade (occupancy). Studies were performed in baboons (Papio anubis) to determine the relationship between plasma concentration and occupancy of brain serotonin reuptake transporter (SERT), dopamine reuptake transporter (DAT), and norepinephrine uptake transporter (NET) using the radioligands [(11)C]DASB [(N,N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine], [(11)C]PE2I [N-(3-iodoprop-2E-enyl)-2ß-carbomethoxy-3ß-(4-methylphenyl)nortropane], and [(11)C]2-[(2-methoxyphenoxy)phenylmethyl]morpholine (also known as [(11)C]MRB) and in humans using [(11)C]DASB and [(11)C]PE2I. In P. anubis, plasma concentrations resulting in half-maximal occupancy at SERT, DAT, and NET were 15.16, 15.56, and 0.97 ng/ml, respectively. In humans, the corresponding values for SERT and DAT were 6.80 and 18.00 ng/ml. GSK1360707 dose-dependently blocked the signal of SERT-, DAT-, and NET-selective PET ligands, confirming its penetration across the blood-brain barrier and blockade of all three monoamine transporters in vivo.

The potent M1 receptor allosteric agonist GSK1034702 improves episodic memory in humans in the nicotine abstinence model of cognitive dysfunction.

Episodic memory deficits are a core feature of neurodegenerative disorders. Muscarinic M(1) receptors play a critical role in modulating learning and memory and are highly expressed in the hippocampus. We examined the effect of GSK1034702, a potent M(1) receptor allosteric agonist, on cognitive function, and in particular episodic memory, in healthy smokers using the nicotine abstinence model of cognitive dysfunction. The study utilized a randomized, double-blind, placebo-controlled, cross-over design in which 20 male nicotine abstained smokers were tested following single doses of placebo, 4 and 8 mg GSK1034702. Compared to the baseline (nicotine on-state), nicotine abstinence showed statistical significance in reducing immediate (p=0.019) and delayed (p=0.02) recall. GSK1034702 (8 mg) significantly attenuated (i.e. improved) immediate recall (p=0.014) but not delayed recall. None of the other cognitive domains was modulated by either nicotine abstinence or GSK1034702. These findings suggest that stimulating M(1) receptor mediated neurotransmission in humans with GSK1034702 improves memory encoding potentially by modulating hippocampal function. Hence, selective M(1) receptor allosteric agonists may have therapeutic benefits in disorders of impaired learning including Alzheimer's disease.

Extended characterisation of the serotonin 2A (5-HT2A) receptor-selective PET radiotracer 11C-MDL100907 in humans: quantitative analysis, test-retest reproducibility, and vulnerability to endogenous 5-HT tone.

INTRODUCTION: Scanning properties and analytic methodology of the 5-HT2A receptor-selective positron emission tomography (PET) tracer 11C-MDL100907 have been partially characterised in previous reports. We present an extended characterisation in healthy human subjects. METHODS: 64 11C-MDL100907 PET scans with metabolite-corrected arterial input function were performed in 39 healthy adults (18-55 years). 12 subjects were scanned twice (duration 150 min) to provide data on plasma analysis, model order estimation, and stability and test-retest characteristics of outcome measures. All other scans were 90 min duration. 3 subjects completed scanning at baseline and following 5-HT2A receptor antagonist medication (risperidone or ciproheptadine) to provide definitive data on the suitability of the cerebellum as reference region. 10 subjects were scanned under reduced 5-HT and control conditions using rapid tryptophan depletion to investigate vulnerability to competition with endogenous 5-HT. 13 subjects were scanned as controls in clinical protocols. Pooled data were used to analyse the relationship between tracer injected mass and receptor occupancy, and age-related decline in 5-HT2A receptors. RESULTS: Optimum analytic method was a 2-tissue compartment model with arterial input function. However, basis function implementation of SRTM may be suitable for measuring between-group differences non-invasively and warrants further investigation. Scan duration of 90 min achieved stable outcome measures in all cortical regions except orbitofrontal which required 120 min. Binding potential (BPP and BPND) test-retest variability was very good (7-11%) in neocortical regions other than orbitofrontal, and moderately good (14-20%) in orbitofrontal cortex and medial temporal lobe. Saturation occupancy of 5-HT2A receptors by risperidone validates the use of the cerebellum as a region devoid of specific binding for the purposes of PET. We advocate a mass limit of 4.6 µg to remain below 5% receptor occupancy. 11C-MDL100907 specific binding is not vulnerable to competition with endogenous 5-HT in humans. Paradoxical decreases in BPND were found in right prefrontal cortex following reduced 5-HT, possibly representing receptor internalisation. Mean age-related decline in brain 5-HT2A receptors was 14.0±5.0% per decade, and higher in prefrontal regions. CONCLUSIONS: Our data confirm and extend support for 11C-MDL100907 as a PET tracer with very favourable properties for quantifying 5-HT2A receptors in the human brain.

Synthesis and characterization of two PET radioligands for the metabotropic glutamate 1 (mGlu1) receptor.

The metabotropic glutamate 1 receptor (mGlu1) is an important protein in the regulation of glutamate transmission in the brain, and believed to be involved in disorders such as ischemia, epilepsy, neuropathic pain, anxiety, and schizophrenia. The goal of this study was to evaluate two selective mGlu1 antagonists [(11) C]3 and [(18) F]4 as potential PET radioligands for the in vivo imaging of the mGlu1 receptor. Biodistribution studies in rats indicated high uptake of [(11) C]3 and [(18) F]4 in the brain. The highest activity level was found in the cerebellum, followed by striatum, hippocampus, frontal cortex, and medulla, in a pattern consistent with the distribution of mGlu1 receptor in rat. At 30 min postinjection, the activity ratio of cerebellum to medulla was 4.5 for [(11) C]3, indicating a high degree of specific binding, while specific binding was lower for [(18) F]4 (cerebellum to medulla activity ratio of 2.0). Moreover, binding of the radioligands [(11) C]3 and [(18) F]4 in mGlu1 receptor-rich region such as cerebellum was blocked by pretreatment of the rats with their respective unlabeled compound or the selective mGlu1 antagonist (compound 5, 2 mg/kg each), but not by the selective mGlu2 antagonist LY341495, or the selective mGlu5 antagonist MPEP (2 mg/kg), thus indicating the binding specificity and selectivity of [(11) C]3 and [(18) F]4 to the mGlu1 receptor. However, in imaging experiments in baboons [(11) C]3 displayed a small specific binding signal only in the cerebellum, while the specific binding of [(18) F]4 was difficult to detect. Species differences in receptor density and affinity of the radioligands in large part account for the differences in the behavior of [(11) C]3 and [(18) F]4 in rats and baboons. Radioligands with higher affinity and/or lower lipophilicity are needed to successfully image the mGlu1 receptor in humans.

Affinity and selectivity of [¹¹C]-(+)-PHNO for the D3 and D2 receptors in the rhesus monkey brain in vivo.

Although [¹¹C]-(+)-PHNO has enabled quantification of the dopamine-D3 receptor (D3R) in the human brain in vivo, its selectivity for the D3R is not sufficiently high to allow us to disregard its binding to the dopamine-D2 receptor (D2R). We quantified the affinity of [¹¹C]-(+)-PHNO for the D2R and D3R in the living primate brain. Two rhesus monkeys were examined on four occasions each, with [¹¹C]-(+)-PHNO administered in a bolus + infusion paradigm. Varying doses of unlabeled (+)-PHNO were coadministered on each occasion (total doses ranging from 0.09 to 5.61 µg kg⁻¹). The regional binding potential (BP(ND) ) and the corresponding doses of injected (+)-PHNO were used as inputs in a model that quantified the affinity of (+)-PHNO for the D2R and D3R, as well as the regional fractions of the [¹¹C]-(+)-PHNO signal attributable to D3R binding. (+)-PHNO in vivo affinity for the D3R (K(d)/f(ND) ~0.23-0.56 nM) was 25- to 48-fold higher than that for the D2R (K(d)/f(ND) ~11-14 nM). The tracer limits for (+)-PHNO (dose associated with D3R occupancy ~10%) were estimated at ~0.02-0.04 µg kg⁻¹ injected mass for anesthetized primate and at 0.01-0.02 µg kg⁻¹ for awake human positron emission tomography (PET) studies. Our data enabled a rational design and interpretation of future PET studies with [¹¹C]-(+)-PHNO.

Imaging dopamine receptors in humans with [11C]-(+)-PHNO: dissection of D3 signal and anatomy.

[(11)C]-(+)-PHNO is a D3 preferring PET radioligand which has recently opened the possibility of imaging D3 receptors in the human brain in vivo. This imaging tool allows characterisation of the distribution of D3 receptors in vivo and further investigation of their functional role. The specific [(11)C]-(+)-PHNO signal is a mixture of D3 and D2 components with the relative magnitude of each component determined by the regional receptor densities. An accurate and reproducible delineation of regions of interest (ROI) is therefore important for optimal analysis of human PET data. We present a set of anatomical guidelines for the delineation of D3 relevant ROIs including substantia nigra, hypothalamus, ventral pallidum/substantia innominata, ventral striatum, globus pallidus and thalamus. Delineation of these structures using this approach allowed for high intra- and inter-operator reproducibility. Subsequently we used a selective D3 antagonist to dissect the total [(11)C]-(+)-PHNO signal in each region into its D3 and D2 components and estimated the regional fraction of the D3 signal (f(PHNO)(D3)). In descending order of magnitude the following results for the f(PHNO)(D3) were obtained: hypothalamus=100%, substantia nigra=100%, ventral pallidum/substantia innominata=75%, globus pallidus=65%, thalamus=43%, ventral striatum=26% and precommissural-ventral putamen=6%. An automated approach for the delineation of these anatomical regions of interest was also developed and investigated in terms of its reproducibility and accuracy.

Characterization of in vivo pharmacological properties and sensitivity to endogenous serotonin of [11C] P943: a positron emission tomography study in Papio anubis.

[11 C] P943 is a recently developed PET radiotracer for serotonin 5-HT1B receptors. We characterized a number of its in vivo pharmacokinetic properties, including the evaluation of its two stereo-isomers, saturability of specific binding, selectivity for 5-HT1B and 5-HT1D receptors, and vulnerability to pharmacologically induced increases in endogenous 5-HT levels. Six isoflurane-anesthetized baboons were scanned with [11 C] P943 at baseline, and following various pharmacological manipulations. The interventions included the administration of pharmacological doses of P943, SB-616234-S (a 5-HT1B selective antagonist), SB-714786 (a 5-HT1D selective antagonist), as well as the administration of 5-HT releasing agents (fenfluramine, amphetamine) and 5-HT reuptake inhibitor (citalopram). [11 C] P943 was observed to bind saturably and specifically to 5-HT1B receptors and to be sensitive to all three challenges known to alter 5-HT levels in the proximity of receptors. [11 C] P943 shows promise as a tracer to image serotonin function in healthy subjects as well as subjects with psychiatric or neurologic conditions.

Evaluation of dopamine D2/3 specific binding in the cerebellum for the positron emission tomography radiotracer [¹¹C]FLB 457: implications for measuring cortical dopamine release.

In a recent positron emission tomography (PET) study, we demonstrated the ability to measure amphetamine-induced dopamine (DA) release in the human cortex with the DA D2/3 radioligand [¹¹C]FLB 457. As previous studies in animals have shown that a relatively high fraction of the [¹¹C]FLB 457 signal in the cerebellum represents specific binding to D2/3 receptors, there was concern that the use of the cerebellum as a measure of nonspecific binding (i.e., reference region) to derive [¹¹C]FLB 457 binding potential (BP) (BP(ND) ) would bias cortical DA release measurements. Thus, we evaluated the fractional contribution of specific binding to D2/3 receptors in the human cerebellum for [¹¹C]FLB 457. Six healthy human subjects (5M/1F) were studied twice with [¹¹C]FLB 457, once at baseline and again following a single oral dose of 15 mg of aripiprazole, a D2/3 partial agonist. [¹¹C]FLB 457 distribution volume (V(T) ) was estimated using kinetic analysis in the cortical regions of interest and potential reference regions. The change in [¹¹C]FLB 457 V(T) following aripiprazole ranged from -33 to -42% in the cortical regions of interest (ROIs). The aripiprazole-induced change in [¹¹C]FLB 457 V(T) in three potential reference regions suggests significant specific binding the cerebellum (CER, -17 ± 12%), but not pons (PON, -10 ± 10%) and centrum semiovale (CESVL, -3 ± 12%). Nevertheless, a reanalysis of the published [¹¹C]FLB 457 test-retest and amphetamine studies suggests that the use of the PON V(T) and CESVL V(T) as an estimate of nonspecific binding to derive [¹¹C]FLB 457 BP(ND) in DA release studies is unlikely to be successful because it leads to less reproducible outcome measures, which in turn diminishes the ability to measure DA release in the cortex. D2/3 blocking studies with aripiprazole and [¹¹C]FLB 457 suggest specific binding to D2/3 receptors in the cerebellum. These data also suggest that the contribution of specific binding to D2/3 receptors in the cerebellum is lower than that in the cortical ROIs and that CER V(T) is mostly representative of nonspecific binding. Nevertheless, caution is advised when using reference tissue methods that rely solely on the cerebellum signal as an input function to quantify [¹¹C]FLB 457 BP(ND).

Translational characterization of [11C]GSK931145, a PET ligand for the glycine transporter type 1.

The current interest in developing Glycine transporter Type 1 (GlyT-1) inhibitors, for diseases such as schizophrenia, has led to the demand for a GlyT-1 PET molecular imaging tool to aid drug development and dose selection. We report on [(11) C]GSK931145 as a novel GlyT-1 imaging probe in primate and man. Primate PET studies were performed to determine the level of specific binding following homologous competition with GSK931145 and the plasma-occupancy relationship of the GlyT-1 inhibitor GSK1018921. Human PET studies were performed to determine the test-retest reproducibility of [(11) C]GSK931145 and the plasma-occupancy relationship of GSK1018921. [(11) C]GSK931145 entered primate and human brain and yielded a heterogeneous pattern of uptake which was similar in both species with highest uptake in midbrain, thalamus, and cerebellum. Homologous competition in primates indicated no viable reference region and gave binding potential estimates between 1.5 and 3 for midbrain, thalamus and cerebellum, While the distribution and binding potential values were similar across species, both the plasma free fraction (f(P) : 0.8 vs. 8%) and delivery (K(1) : 0.025 vs. 0.126 ml cm(-3) min(-1) ) were significantly lower in humans. Test-retest reproducibility in humans calculated using a two tissue compartmental model was poor (VAR(V(T) ): 29-38%), but was improved using a pseudo reference tissue model (VAR(BP(ND) ): 16-23%). GSK1018921 EC(50) estimates were 22.5 and 45.7 ng/ml in primates and humans, respectively.

Positron emission tomography imaging of dopamine D2/3 receptors in the human cortex with [¹¹C]FLB 457: reproducibility studies.

In a recent PET study, we demonstrated the ability to measure amphetamine-induced DA release in the human cortex with the relatively high affinity dopamine D2/3 radioligand [¹¹C]FLB 457 (Narendran et al., [2009] Synapse 63:447-461). The aim of this study was to evaluate the reproducibility and reliability of [¹¹C]FLB 457 in the same imaging paradigm we used to measure amphetamine-induced DA transmission. Six healthy human subjects (three males/three females)were studied twice with [¹¹C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D2/3 receptor binding parameters were estimated using a two-tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BP(P)), and binding potential relative to non-displaceable uptake (BP(ND)) of [¹¹C]FLB 457. The test-retest variability of [¹¹C]FLB 457 VT, BPP, and BP(ND) were ≤15%, consistent with the published test-retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215-1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666-1673). In addition, no significant decrease in [¹¹C]FLB457 BP(ND) was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [¹¹C]FLB 457 to the measured reduction in[¹¹C]FLB 457 BP(ND) following amphetamine was relatively low. These data support the further validation of [¹¹C]FLB 457 as a tool to measure amphetamine-induced dopamine release in the human cortex.

The 5-HT(2A) receptor and serotonin transporter in Asperger's disorder: A PET study with [¹¹C]MDL 100907 and [¹¹C]DASB.

Evidence from biochemical, imaging, and treatment studies suggest abnormalities of the serotonin system in autism spectrum disorders, in particular in frontolimbic areas of the brain. We used the radiotracers [(11)C]MDL 100907 and [(11)C]DASB to characterize the 5-HT(2A) receptor and serotonin transporter in Asperger's Disorder. Seventeen individuals with Asperger's Disorder (age=34.3 ± 11.1 years) and 17 healthy controls (age=33.0 ± 9.6 years) were scanned with [(11)C]MDL 100907. Of the 17 patients, eight (age=29.7 ± 7.0 years) were also scanned with [¹¹C]DASB, as were eight healthy controls (age=28.7 ± 7.0 years). Patients with Asperger's Disorder and healthy control subjects were matched for age, gender, and ethnicity, and all had normal intelligence. Metabolite-corrected arterial plasma inputs were collected and data analyzed by two-tissue compartment modeling. The primary outcome measure was regional binding potential BP(ND). Neither regional [¹¹C]MDL 100907 BP(ND) nor [¹¹C]DASB BP(ND) was statistically different between the Asperger's and healthy subjects. This study failed to find significant alterations in binding parameters of 5-HT(2A) receptors and serotonin transporters in adult subjects with Asperger's disorder.

Striatal and extrastriatal dopamine release measured with PET and [(18)F] fallypride.

The amphetamine challenge, in which positron emission tomography (PET) or single photon emission computed tomography radioligand binding following administration of amphetamine is compared to baseline values, has been successfully used in a number of brain imaging studies as an indicator of dopaminergic function, particularly in the striatum. [(18)F] fallypride is the first PET radioligand that allows measurement of the effects of amphetamine on D2/D3 ligand binding in striatum and extra-striatal brain regions in a single scanning session following amphetamine. We scanned 15 healthy volunteer subjects with [(18)F] fallypride at baseline and following amphetamine (0.3 mg/kg) using arterial plasma input-based modeling as well as reference region methods. We found that amphetamine effect was robustly detected in ventral striatum, globus pallidus, and posterior putamen, and with slightly higher variability in other striatal subregions. However, the observed effect sizes in striatum were less than those observed in previous studies in our laboratory using [(11)C] raclopride. Robust effect was also detected in limbic extra-striatal regions (hippocampus, amygdala) and substantia nigra, but the signal-to-noise ratio was too low to allow accurate measurement in cortical regions. We conclude that [(18)F] fallypride is a suitable ligand for measuring amphetamine effect in striatum and limbic regions, but it is not suitable for measuring the effect in cortical regions and may not provide the most powerful way to measure the effect in striatum.

Within-subject comparison of striatal D2 receptor occupancy measurements using [123I]IBZM SPECT and [11C]Raclopride PET.

Antipsychotic-induced D2 receptor occupancy values tend to be lower when measured with [(123)I]IBZM SPECT than with [(11)C]Raclopride PET. To clarify this issue, D2 receptor occupancy was measured in the same subjects using both techniques. Twenty patients with schizophrenia on monotherapy with risperidone (n=7; 3-9 mg/d), olanzapine (n=5; 5-20 mg/d) or clozapine (n=8; 150-450 mg/d) at stable doses, and ten healthy volunteers (HV) underwent both a [(123)I]IBZM SPECT and a [(11)C]Raclopride PET examinations in random order on different days within a week. Patients with schizophrenia were scanned at a fixed interval after last dose administration. Quantification of receptor availability was performed using the most conventional methods from the literature: the tissue ratio derived specific uptake ratios (SUR) were used for SPECT, and simplified reference tissue model (SRTM) derived binding potentials (BP(ND)) for PET. Analysis was performed using both occipital cortex and cerebellum as reference regions for both modalities. Striatal D2 receptor occupancy was measured as the percentage reduction of [(123)I]IBZM SUR or [(11)C]Raclopride BP(ND) compared to the population average measured in HV using the same modality. Occupancy values measured by SPECT were lower than those measured with PET, by 12.4% and 13.8% when occipital cortex and cerebellum were used as reference regions. This difference should be taken in consideration when interpreting reported antipsychotic striatal D2 receptor occupancy values from the literature.

Kinetic modeling of 11C-SB207145 binding to 5-HT4 receptors in the human brain in vivo.

UNLABELLED: The serotonin 4 receptor (5-HT(4) receptor) is known to be involved in learning and memory. We evaluated for the first time the quantification of a novel 5-HT(4) receptor radioligand, (11)C-SB207145, for in vivo brain imaging with PET in humans. METHODS: For evaluation of reproducibility, 6 subjects were scanned twice with (11)C-SB207145 on the same day. A further 2 subjects were scanned before and after blocking with the selective 5-HT(4) receptor inverse agonist piboserod (SB207266). Arterial blood samples were drawn for the calculation of metabolite-corrected arterial input functions. Regions of interest were delineated automatically on the individual's MR images coregistered to the PET images, and regional time-activity curves were extracted. Quantitative tracer kinetic modeling was investigated with 1- and 2-tissue-compartment models using plasma input functions and the simplified reference tissue model (SRTM). RESULTS: (11)C-SB207145 readily entered the brain and showed a distribution consistent with the known localization of the 5-HT(4) receptor. Using plasma input models, the time-activity data were well described by the 2-tissue-compartment model in all regions and allowed for the estimate of binding potentials relative to the reference region (BP(ND): striatum, 3.38 +/- 0.72; hippocampus, 0.82 +/- 0.19; parietal cortex, 0.30 +/- 0.08). Quantification with the 1-tissue-compartment model, 2-tissue-compartment model, and SRTM were associated with good test-retest reproducibility and time stability. However, the SRTM-generated BP(ND) values in the striatum were underestimated by 20%-43% in comparison to the 2-tissue-compartment model. The blocking study with piboserod confirmed that the radioligand was selective for the 5-HT(4) receptor, that the cerebellum was a suitable reference region devoid of specific binding, and that nonspecific binding was constant across brain regions. CONCLUSION: In vivo imaging of cerebral 5-HT(4) receptors can be determined reliably using (11)C-207145 PET with arterial input in humans. SRTM showed high reproducibility and reliability but bias in the striatum, and therefore, the use of SRTM should be considered carefully for individual applications.

Positron emission tomography imaging of amphetamine-induced dopamine release in the human cortex: a comparative evaluation of the high affinity dopamine D2/3 radiotracers [11C]FLB 457 and [11C]fallypride.

The use of PET and SPECT endogenous competition binding techniques has contributed to the understanding of the role of dopamine in several neuropsychiatric disorders. An important limitation of these imaging studies is the fact that measurements of acute changes in synaptic dopamine have been restricted to the striatum. The ligands previously used, such as [(11)C]raclopride and [(123)I]IBZM, do not provide sufficient signal to noise ratio to quantify D(2) receptors in extrastriatal areas, such as cortex, where the concentration of D(2) receptors is much lower than in the striatum. Given the importance of cortical DA function in cognition, a method to measure cortical dopamine function in humans would be highly desirable. The goal of this study was to compare the ability of two high affinity DA D(2) radioligands [(11)C]FLB 457 and [(11)C]fallypride to measure amphetamine-induced changes in DA transmission in the human cortex. D(2) receptor availability was measured in the cortical regions of interest with PET in 12 healthy volunteers under control and postamphetamine conditions (0.5 mg kg(-1), oral), using both [(11)C]FLB 457 and [(11)C]fallypride (four scans per subjects). Kinetic modeling with an arterial input function was used to derive the binding potential (BP(ND)) in eight cortical regions. Under controlled conditions, [(11)C]FLB 457 BP(ND) was 30-70% higher compared with [(11)C]fallypride BP(ND) in cortical regions. Amphetamine induced DA release led to a significant decrease in [(11)C]FLB 457 BP(ND) in five out the eight cortical regions evaluated. In contrast, no significant decrease in [(11)C]fallypride BP(ND) was detected in cortex following amphetamine. The difference between [(11)C]FLB 457 and [(11)C]fallypride ability to detect changes in the cortical D(2) receptor availability following amphetamine is related to the higher signal to noise ratio provided by [(11)C]FLB 457. These findings suggest that [(11)C]FLB 457 is superior to [(11)C]fallypride for measurement of changes in cortical synaptic dopamine.

In vivo quantification of regional dopamine-D3 receptor binding potential of (+)-PHNO: Studies in non-human primates and transgenic mice.

Examination of dopamine-D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine-D2 (D2) receptors. The two types co-localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [(11)C]-(+)-PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB-277011 to dissect regional fractions of the [(11)C]-(+)-PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with (3)H-(+)-PHNO in wild-type, D2-knock-out, and D3-knock-out mice examined at baseline and following administration of SB-277011. Both sets of results converged to indicate a predominant D3-related component to (+)-PHNO binding in extra-striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [(11)C]-(+)-PHNO PET in vivo.

Dopamine transporters, D2 receptors, and dopamine release in generalized social anxiety disorder.

BACKGROUND: Dopamine D2 receptor and dopamine transporter (DAT) availability in the striatum (STR) have each been reported abnormal in generalized social anxiety disorder (GSAD) in studies using single photon emission computerized tomography (SPECT). D2 receptors and DAT have not previously been studied within the same GSAD subjects, however, and prior GSAD studies have not assessed dopamine release or subdivided the STR into functional subregions. METHODS: Unmedicated adults with GSAD (N=17) and matched healthy comparison (HC) subjects (N=13) participated in this study. Of these, 15 GSAD and 13 HC subjects completed baseline assessment of D2 receptor availability using positron emission tomography (PET) with the radiotracer [11C]raclopride. Twelve GSAD and 13 HC subjects completed a repeat scan after intravenous administration of d-amphetamine to study dopamine release. Twelve of the GSAD subjects and 10 of the HC subjects also completed SPECT with the radiotracer [123I] methyl 3beta-(4-iodophenyl) tropane-2beta-carboxylate ([123I]beta-CIT) to assess DAT availability. RESULTS: GSAD and HC groups did not differ significantly in striatal DAT availability, the overall striatal or striatal subregion D2 receptor availability at baseline, or change in D(2) receptor availability after d-amphetamine. Receptor availability and change after d-amphetamine were not significantly associated with severity of social anxiety or trait detachment. CONCLUSIONS: These findings do not replicate previous findings of altered striatal DAT and D2 receptor availability in GSAD subjects assessed with SPECT. The differences from results of prior studies may be due to differences in imaging methods or characteristics of samples.

Dopamine D3 receptor antagonists: the quest for a potentially selective PET ligand. Part 3: Radiosynthesis and in vivo studies.

Compound 1 is a potent and selective antagonist of the dopamine D(3) receptor. With the aim of developing a carbon-11 labeled ligand for the dopamine D(3) receptor, 1 was selected as a potential PET probe. [(11)C]1 was obtained by palladium catalyzed cross coupling using [(11)C]cyanide and 4 with a specific activity of 55.5+/-25.9GBq/micromol (1.5+/-0.7Ci/micromol). [(11)C]1 was tested in porcine and non-human primate models to assess its potential as a radioligand for PET imaging of the dopamine D(3) receptor. We conclude that in both species and despite appropriate in vitro properties, [(11)C]1 does not show any specific signal for the dopamine D(3) receptor.

D2 receptor occupancy of olanzapine pamoate depot using positron emission tomography: an open-label study in patients with schizophrenia.

A long-acting depot formulation of olanzapine that sustains plasma olanzapine concentrations for over a month after a single injection is currently under development. This multicenter, open-label study explored D(2) receptor occupancy of a fixed dose of olanzapine pamoate (OP) depot given every 4 weeks. Patients (nine male, five female) with schizophrenia or schizoaffective disorder previously stabilized on oral olanzapine were switched to OP depot 300 mg by intramuscular injection every 4 weeks for 6 months. No visitwise within-group significant changes were found in Brief Psychiatric Rating Scale Total or Clinical Global Impressions-Severity of Illness scores, although seven patients received oral olanzapine supplementation during the first four injection cycles. To minimize impact on D(2) occupancy, positron emission tomography (PET) scans were not completed during injection cycles that required supplemental oral olanzapine. Two patients reported transient injection site adverse events, which did not result in discontinuation. The most frequently reported treatment-emergent adverse events were insomnia, aggravated psychosis, and anxiety. Mean striatal D(2) receptor occupancy, as measured by [(11)C]-raclopride PET, was 69% on oral olanzapine (5-20 mg/day) and 50% (trough) on OP depot at steady state. Following an initial decline, occupancy returned to 84% of baseline oral olanzapine occupancy after six injections. Over the study period, D(2) receptor occupancy and plasma olanzapine concentrations were significantly correlated (r=0.76, P