Discovery of Phosphodiesterase 10A (PDE10A) PET Tracer AMG 580 to Support Clinical Studies.

We report the discovery of PDE10A PET tracer AMG 580 developed to support proof of concept studies with PDE10A inhibitors in the clinic. To find a tracer with higher binding potential (BPND) in NHP than our previously reported tracer 1, we implemented a surface plasmon resonance assay to measure the binding off-rate to identify candidates with slower washout rate in vivo. Five candidates (2-6) from two structurally distinct scaffolds were identified that possessed both the in vitro characteristics that would favor central penetration and the structural features necessary for PET isotope radiolabeling. Two cinnolines (2, 3) and one keto-benzimidazole (5) exhibited PDE10A target specificity and brain uptake comparable to or better than 1 in the in vivo LC-MS/MS kinetics distribution study in SD rats. In NHP PET imaging study, [(18)F]-5 produced a significantly improved BPND of 3.1 and was nominated as PDE10A PET tracer clinical candidate for further studies.

Radiosynthesis and initial characterization of a PDE10A specific PET tracer [18F]AMG 580 in non-human primates.

INTRODUCTION: Phosphodiesterase 10A (PDE10A) is an intracellular enzyme responsible for the breakdown of cyclic nucleotides which are important second messengers for neurotransmission. Inhibition of PDE10A has been identified as a potential target for treatment of various neuropsychiatric disorders. To assist drug development, we have identified a selective PDE10A positron emission tomography (PET) tracer, AMG 580. We describe here the radiosynthesis of [(18)F]AMG 580 and in vitro and in vivo characterization results. METHODS: The potency and selectivity were determined by in vitro assay using [(3)H]AMG 580 and baboon brain tissues. [(18)F]AMG 580 was prepared by a 1-step [(18)F]fluorination procedure. Dynamic brain PET scans were performed in non-human primates. Regions-of-interest were defined on individuals' MRIs and transferred to the co-registered PET images. Data were analyzed using two tissue compartment analysis (2TC), Logan graphical (Logan) analysis with metabolite-corrected input function and the simplified reference tissue model (SRTM) method. A PDE10A inhibitor and unlabeled AMG 580 were used to demonstrate the PDE10A specificity. KD was estimated by Scatchard analysis of high and low affinity PET scans. RESULTS: AMG 580 has an in vitro KD of 71.9 pM. Autoradiography showed specific uptake in striatum. Mean activity of 121 ± 18 MBq was used in PET studies. In Rhesus, the baseline BPND for putamen and caudate was 3.38 and 2.34, respectively, via 2TC, and 3.16, 2.34 via Logan, and 2.92, and 2.01 via SRTM. A dose dependent decrease of BPND was observed by the pre-treatment with a PDE10A inhibitor. In baboons, 0.24 mg/kg dose of AMG 580 resulted in about 70% decrease of BPND. The in vivo KD of [(18)F]AMG 580 was estimated to be around 0.44 nM in baboons. CONCLUSION: [(18)F]AMG 580 is a selective and potent PDE10A PET tracer with excellent specific striatal binding in non-human primates. It warrants further evaluation in humans.

Initial characterization of a PDE10A selective positron emission tomography tracer [11C]AMG 7980 in non-human primates.

INTRODUCTION: Phosphodiesterase 10A (PDE10A) is an intracellular enzyme responsible for the breakdown of cyclic nucleotides which are important secondary messengers in the central nervous system. Inhibition of PDE10A has been identified as a potential therapeutic target for treatment of various neuropsychiatric disorders. To assist the drug development program, we have identified a selective PDE10A PET tracer, [(11)C]AMG 7980, for imaging PDE10A distribution using positron emission tomography. METHODS: [(11)C]AMG 7980 was prepared in a one-pot, two-step reaction. Dynamic PET scans were performed in non-human primates following a bolus or bolus plus constant infusion tracer injection paradigm. Regions-of-interest were defined on individuals' MRIs and transferred to the co-registered PET images. Data were analyzed using Logan graphical analysis with metabolite-corrected input function, the simplified reference tissue model (SRTM) method and occupancy plots. A benchmark PDE10A inhibitor was used to demonstrate PDE10A-specific binding. RESULTS: [(11)C]AMG 7980 was prepared with a mean specific activity of 99 ± 74 GBq/µmol (n=10) and a synthesis time of 45 min. Specific binding of the tracer was localized to the striatum and globus pallidus (GP) and low in other brain regions. Thalamus was used as the reference tissue to derive binding potentials (BPND). The BPND for caudate, putamen, and GP were 0.23, 0.65, 0.51, respectively by the graphical method, and 0.42, 0.76, and 0.75 from the SRTM method. A dose dependent decrease of BPND was observed with the pre-treatment of a PDE10A inhibitor. A bolus plus infusion injection paradigm yielded similar results. CONCLUSION: [(11)C]AMG 7980 has been successfully used for imaging PDE10A in non-human primate brain. Despite the fast brain kinetics it can be used to measure target occupancy of PDE10A inhibitors in non-human primates and potentially applicable to humans.

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.

Rapid identification of a novel small molecule phosphodiesterase 10A (PDE10A) tracer.

A radiolabeled tracer for imaging therapeutic targets in the brain is a valuable tool for lead optimization in CNS drug discovery and for dose selection in clinical development. We report the rapid identification of a novel phosphodiesterase 10A (PDE10A) tracer candidate using a LC-MS/MS technology. This structurally distinct PDE10A tracer, AMG-7980 (5), has been shown to have good uptake in the striatum (1.2% ID/g tissue), high specificity (striatum/thalamus ratio of 10), and saturable binding in vivo. The PDE10A affinity (K(D)) and PDE10A target density (B(max)) were determined to be 0.94 nM and 2.3 pmol/mg protein, respectively, using [(3)H]5 on rat striatum homogenate. Autoradiography on rat brain sections indicated that the tracer signal was consistent with known PDE10A expression pattern. The specific binding of [(3)H]5 to rat brain was blocked by another structurally distinct, published PDE10A inhibitor, MP-10. Lastly, our tracer was used to measure in vivo PDE10A target occupancy of a PDE10A inhibitor in rats using LC-MS/MS technology.

Increased prefrontal cortical D1 receptors in drug naive patients with schizophrenia: a PET study with [¹¹C]NNC112.

D1 receptors are the main mediators of dopamine transmission in the cortex and subserve cognitive functions that are affected in patients with schizophrenia. Prior imaging studies have suggested abnormalities in the expression of these receptors in schizophrenia, but no conclusive picture has emerged yet. One source of discrepancy may have been prior antipsychotic exposure. We used positron emission tomography (PET) and a D1 radiotracer, [¹¹C]NNC112, in drug naïve (DN, n = 12) and drug free (DF, n = 13) patients with schizophrenia and 40 healthy control subjects (HC, n = 40 total, n = 24 per comparison group) matched for age, gender, ethnicity, parental socioeconomic status and cigarette smoking. We measured the binding potential BPP, corrected for partial volume effects. The outcome measure was obtained in cortical and striatal subregions outlined on coregistered individual MRIs. Partial volume effect corrected BPP measures were significantly higher in DN vs controls in cortical regions. No such increases were found in the DF versus controls comparison. Furthermore, in the DF group, DF interval correlated positively with cortical BPP. We conclude that upregulation of D1 receptors in schizophrenia is related to the illness itself and may be corrected and normalized by chronic antipsychotic treatment.

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.

Characterization of 64Cu-DOTA-conatumumab: a PET tracer for in vivo imaging of death receptor 5.

UNLABELLED: Conatumumab is a fully human monoclonal antibody that binds to and activates human death receptor 5 (DR5; also known as TRAIL receptor 2). The purpose of this study was to characterize (64)Cu-labeled conatumumab as a PET tracer for imaging DR5 in tumors. METHODS: DOTA-conatumumab was synthesized by incubating conatumumab with 2,2',2″-(10-(2-(2,5-dioxopyrrolidin-1-yloxy)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (DOTA-NHS). The absolute numbers of DOTA molecules per conatumumab molecules were determined by matrix-assisted laser desorption ionization mass spectrometry and electrospray ionization quadrupole time-of-flight mass spectrometry. (64)Cu-DOTA-conatumumab was prepared by incubating (64)CuCl(2) (33-222 MBq) with DOTA-conatumumab at 37°C for 1 h. Binding of conatumumab and DOTA-conatumumab to Fc-coupled human DR5 (huTR2-Fc) was tested in a kinetic analysis assay, and the biologic activity of copper-DOTA-conatumumab was measured using a caspase-3/7 luminescent assay. In vivo evaluation of DOTA-conatumumab and copper-DOTA-conatumumab was done in severe combined immunodeficiency mice bearing Colo205 xenografts: tissue uptake was determined with biodistribution studies, and small-animal PET and autoradiography were used to determine the uptake of (64)Cu-DOTA conatumumab into tumors and other tissues. RESULTS: DOTA-conatumumab was prepared with an average of 5 DOTA molecules per conatumumab molecule. The in vitro median effective concentration required to induce a 50% effect of DOTA-conatumumab and conatumumab from the assay were 389 and 320 pM, respectively. The median effective dose (±SD) of DOTA-conatumumab and conatumumab via the caspase assay was 135 ± 31 and 128 ± 30 pM, respectively. In female CB17 severe combined immunodeficiency mice bearing Colo205 xenografts, DOTA-conatumumab and conatumumab inhibited tumor growth to the same extent. Small-animal PET studies showed tumor uptake at 24 h after injection of the tracer, with a mean standardized uptake value of 3.16 (n = 2). Tumor uptake was decreased by the coadministration of 400 µg of unlabeled conatumumab (mean standardized uptake value, 1.55; n = 2), suggesting saturable uptake. Tissue uptake determined by biodistribution studies was in agreement with the small-animal PET findings. CONCLUSION: These results suggest that (64)Cu-DOTA-conatumumab is a potential PET tracer for imaging DR5 in tumors and may be useful for measuring on-target occupancy by conatumumab.

Measurement of the serotonin 1A receptor availability in patients with schizophrenia during treatment with the antipsychotic medication ziprasidone.

The aim of this study was to compare 5-HT(1A) availability in vivo in individuals with schizophrenia before and during treatment with the atypical antipsychotic ziprasidone. Six individuals with schizophrenia underwent two PET scans with [(11)C]WAY 100635; the first while medication-free (baseline) and the second while taking the atypical antipsychotic ziprasidone (on-medication). Regional volumes of distribution (V(T), mL g(-1)) were derived using a two-tissue compartment kinetic model. Outcome measures included binding potential relative to the plasma (BP(P), mL g(-1)) and the binding potential relative to the nonspecific distribution volume (BP(ND), unitless). No significant differences were observed in regional BP(P) or BP(ND) with ziprasidone treatment. A significant correlation was noted between BP(P) measured in the orbitofrontal cortex during the on-medication condition and degree of improvement in negative symptoms with treatment (r = 0.96, p = 0.004). Consistent with the published literature of changes in 5-HT(1A) binding during treatment with 5-HT(1A) receptor agonists, this study did not detect a significant reduction in 5-HT(1A) binding with ziprasidone. The finding of a relationship between 5-HT(1A) binding and the degree of improvement in negative symptoms provides further support for the role of the 5-HT(1A) receptor in the pathophysiology and treatment of this symptom domain.

Increased synaptic dopamine function in associative regions of the striatum in schizophrenia.

CONTEXT: A long-standing version of the dopamine hypothesis of schizophrenia postulates that hyperactivity of dopaminergic transmission at D(2) receptors in the limbic striatum is associated with the illness and that blockade of mesolimbic D(2) receptors is responsible for the antipsychotic action of D(2) receptor antagonists. OBJECTIVE: To localize dopaminergic hyperactivity within the striatum in schizophrenia. DESIGN: Case-control study. SETTING: Inpatient research unit. PARTICIPANTS: Eighteen untreated patients with schizophrenia and 18 healthy control subjects matched for age, sex, ethnicity, parental socioeconomic status, cigarette smoking, and weight. MAIN OUTCOME MEASURES: Percentage change in dopamine D(2) receptor availability in striatal subregions within each subject measured by positron emission tomography with carbon 11-labeled raclopride before and during pharmacologically induced dopamine depletion. RESULTS: In the associative striatum, acute dopamine depletion resulted in a larger increase in D(2) receptor availability in patients with schizophrenia (mean [SD], 15% [7%]) than in control subjects (10% [7%], P = .045), suggesting higher synaptic dopamine concentration. Within the associative striatum, this effect was most pronounced in the precommissural dorsal caudate (15% [8%] in patients vs 9% [8%] in controls, P = .03). No between-group differences were observed in the limbic and sensorimotor striatum. CONCLUSIONS: These findings suggest that schizophrenia is associated with elevated dopamine function in associative regions of the striatum. Because the precommissural dorsal caudate processes information from the dorsolateral prefrontal cortex, this observation also suggests that elevated subcortical dopamine function might adversely affect performance of the dorsolateral prefrontal cortex in schizophrenia. On the other hand, the absence of a group difference in the limbic striatum brings into question the therapeutic relevance of the mesolimbic selectivity of second-generation antipsychotic drugs.

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 D1 receptors in cocaine dependence measured with PET and the choice to self-administer cocaine.

The goal of this study was to determine D(1) receptor availability in human cocaine-dependent (CD) subjects and matched healthy controls (HCs). In addition, the CD subjects performed cocaine self-administration sessions in order to explore the association between D(1) receptor availability and cocaine-seeking behavior. Twenty-five CD subjects (40+/-4 years, 19M/6 F) and 23 matched HCs (38+/-4 years, 19M/4F) were scanned with PET and the radiotracer [(11)C]NNC 112. During the cocaine self-administration sessions, CD volunteers were given the choice to self-administer cocaine (0, 6, and 12 mg) or to receive a monetary voucher worth $5. D(1) receptor availability was measured in the limbic, associative, and sensori-motor striatum in addition to cortical brain regions. No difference in D(1) receptor availability was seen between the two groups. A negative association was seen between D(1) receptor BP(ND) in the limbic striatum and the choice for the 6 mg dose of cocaine (r=-0.47, p=0.02, corrected for age). These results do not support the hypothesis that cocaine dependence is associated with a reduction in D(1) receptor availability in the striatum. However, within the CD subjects, low D(1) receptor availability in the ventral striatum was associated with the choice to self-administer cocaine, suggesting that low D(1) receptor availability may be associated with an increased risk of relapse in cocaine dependence.

Positron emission tomography imaging of the serotonin transporter and 5-HT(1A) receptor in alcohol dependence.

BACKGROUND: Rodent models as well as studies in humans have suggested alterations in serotonin (5-HT) innervation and transmission in early-onset genetically determined or type II alcoholism. This study examines two indices of serotonergic transmission, 5-HT transporter levels and 5-HT(1A) availability, in vivo, in type II alcoholism. This is the first report of combined tracers for pre- and postsynaptic serotonergic transmission in the same alcoholic subjects and the first study of 5-HT(1A) receptors in alcoholism. METHODS: Fourteen alcohol-dependent subjects were scanned (11 with both tracers, 1 with [(11)C]DASB only, and two with [(11)C]WAY100635 only). Twelve healthy control subjects (HC) subjects were scanned with [(11)C]DASB, and another 13 were scanned with [(11)C]WAY100635. Binding potential (BP(p), mL/cm(3)) and the specific to nonspecific partition coefficient (BP(ND), unitless) were derived for both tracers using a two-tissue compartment model and compared with control subjects across different brain regions. Relationships to severity of alcoholism were assessed. RESULTS: No significant differences were observed in regional BP(p) or BP(ND) between patients and control subjects in any of the regions examined. No significant relationships were observed between regional 5-HT transporter availability, 5-HT(1A) availability, and disease severity, with the exception of a significant negative correlation between 5-HT transporters and years of dependence in amygdala and insula. CONCLUSION: This study did not find alterations in measures of 5-HT(1A) or 5-HT transporter levels in patients with type II alcoholism.

In vivo DA D(1) receptor selectivity of NNC 112 and SCH 23390.

PURPOSE: [(11)C]NNC 112 and [(11)C]SCH 23390 are selective positron emission tomography (PET) tracers for visualizing dopamine D(1) receptors. It is known that both have some affinity for serotonin 2A receptors, but previous studies have suggested this is negligible compared to D(1) affinity. We sought to verify this property in vivo. PROCEDURES: Two baboons were scanned to measure the selectivity of both tracers with a displacement paradigm. Four baboons were scanned to directly assess [(11)C] NNC 112 affinity for both receptors. RESULTS: In vivo, D(1) to 5-HT(2A) selectivity is six to fourteenfold, not 100-fold as previously reported by other investigators. CONCLUSION: We conclude that about 1/4 of the cortical signal of both [(11)C]NNC 112 and [(11)C]SCH 23390 is due to binding to 5-HT(2A) receptors. If confirmed in humans, this suggests caution should be exercised when drawing conclusions from studies using either tracer. These results also indicate the need for more selective tracers for the D(1) receptor.

Amphetamine-induced dopamine release: markedly blunted in cocaine dependence and predictive of the choice to self-administer cocaine.

OBJECTIVE: Dopamine is an important mediator of the reinforcing effects of cocaine, and alterations in dopamine function might be involved in cocaine dependence. The goals of the present study were to characterize pre- and postsynaptic dopamine function in recently detoxified cocaine-dependent subjects. Specifically, dopamine response to an acute amphetamine challenge was assessed in striatal subregions in cocaine-dependent and healthy comparison participants using positron emission tomography (PET). Furthermore, the relationship between this dopamine response and the choice to self-administer cocaine in a laboratory model of relapse was investigated. METHOD: Twenty-four cocaine-dependent participants and 24 matched healthy subjects underwent [(11)C]raclopride scans under a baseline condition and following intravenous amphetamine administration (0.3 mg/kg). Cocaine-dependent participants also completed cocaine self-administration sessions in which a priming dose of cocaine was followed by the choice to either self-administer subsequent cocaine doses or receive a monetary reward. RESULTS: Cocaine dependence was associated with a marked reduction in amphetamine-induced dopamine release in each of the functional subregions of the striatum (limbic striatum: -1.2% in cocaine-dependent participants versus -12.4% in healthy subjects; associative striatum: -2.6% versus -6.7%, respectively; sensorimotor striatum: -4.3% versus -14.1%). Blunted dopamine transmission in the ventral striatum and anterior caudate was predictive of the choice for cocaine over money. CONCLUSIONS: Cocaine dependence is associated with impairment of dopamine function, and this impairment appears to play a critical role in relapse.

Dopamine (D2/3) receptor agonist positron emission tomography radiotracer [11C]-(+)-PHNO is a D3 receptor preferring agonist in vivo.

[11C]PHNO is a recently introduced agonist to image DA D2-like receptors with Positron Emission Tomography (PET). In cats and humans, [11C]PHNO revealed an atypical distribution compared to radiolabeled D2-like antagonists (such as [11C]raclopride) or other D2-like agonists (such as [11C]NPA), as it displayed unusual high binding in the globus pallidus (GP). The goal of this study was to assess the pharmacological nature of the binding of [11C]PHNO in the GP in nonhuman primates. As previously reported in humans, [11C]PHNO equilibrium specific to nonspecific equilibrium partition coefficients (V3'') in baboons was much higher in GP (3.88 +/- 1.15) than in the dorsal striatum (DST, 2.07 +/- 0.43), whereas the reverse was true for [11C]raclopride (1.48 +/- 0.41 in GP, 2.56 +/- 0.91 in DST) and [11C]NPA (0.87 +/- 0.19 in GP, 1.02 +/- 0.13 in DST). Administration of unlabeled raclopride resulted in similar reductions of [11C] PHNO V3'' and [11C]raclopride V3'' in both the GP and the DST. This observation demonstrated that the [11C]PHNO binding in the GP was specific to D2-like receptors. To evaluate the respective contribution of D3 and D2 receptors to the binding potential (BP) of [11C]PHNO and [11C]raclopride, experiments were carried out with the selective D3 partial agonist 1-(4(2-Napthoylamino)butyl)-4-(2-methoxyphenyl)-1A-piperazine HCL (BP897). BP897 reduced [11C]raclopride V3'' by 29% +/- 9%, 19% +/- 8%, and 10% +/- 7% in GP, VST, and DST, respectively, a result consistent with expectation from postmortem studies (D3/D2 ratio in GP > VST > DST). BP897 reduced [11C]PHNO V3'' by 57% +/- 11%, 30% +/- 11%, and 13% +/- 8% in GP, VST, and DST, respectively, indicating that the D3 receptor contribution to [11C]PHNO signal is higher than that of [11C]raclopride. From these experiments we conclude that [11C]PHNO is a D3 preferring agonist, and that this property explains the high GP signal not observed with [11C]raclopride or [11C]NPA. This property might contribute to its higher vulnerability to endogenous DA compared to [11C]raclopride and [11C]NPA.

Serotonin 1A receptor availability in patients with schizophrenia and schizo-affective disorder: a positron emission tomography imaging study with [11C]WAY 100635.

BACKGROUND: Postmortem and positron emission tomography (PET) studies have reported several alterations in serotonin 1A receptor (5-HT(1A)) binding parameters in patients with schizophrenia. This study examines 5-HT(1A) availability in vivo in individuals with schizophrenia and schizo-affective disorder. MATERIALS AND METHODS: Twenty-two medication-free individuals with schizophrenia or schizo-affective disorder and 18 healthy subjects underwent PET scans with [(11)C]WAY 100635. Regional distribution volumes (V(T), in milliliters per gram) were derived using a two-tissue compartment kinetic model. Outcome measures for 5-HT(1A) availability included binding potential (BP) and the specific to nonspecific equilibrium partition coefficient (V(3)''). Eleven brain regions with high density of 5-HT(1A) were included in the analysis. RESULTS: No significant differences were observed in regional BP or V(3)'' between patients and controls. No significant relationships were observed between regional 5-HT(1A) availability and symptom severity. CONCLUSION: The postmortem literature reports increased 5-HT(1A) binding in the prefrontal cortex in schizophrenia. This study did not detect differences in 5-HT(1A) binding. Whereas in two recently published PET studies, one reports increased binding in the temporal lobe while the other reports decreased binding in the amygdala. These inconsistencies suggest that the alterations demonstrated in postmortem studies cannot be reliably detected at the resolution achieved with PET. This raises the question as to whether major changes in the level of expression of the 5-HT(1A) receptor play a role in the pathophysiology of schizophrenia.

Estimation of serotonin transporter parameters with 11C-DASB in healthy humans: reproducibility and comparison of methods.

UNLABELLED: The aim of the present study was to define the optimal analytic method to derive accurate and reliable serotonin transporter (SERT) receptor parameters with (11)C-3-amino-4-(2-[(dimethylamino)methyl]phenylthio)benzonitrile ((11)C-DASB). METHODS: Nine healthy subjects (5 females, 4 males) underwent two (11)C-DASB PET scans on the same day. Five analytic methods were used to estimate binding parameters in 10 brain regions: compartmental modeling with 1- and 2-tissue compartment models (1TC and 2TC), data-driven estimation of parametric images based on compartmental theory (DEPICT) analysis, graphical analysis, and the simplified reference tissue model (SRTM). Two variations in the fitting procedure of the SRTM method were evaluated: nonlinear optimization and basis function approach. The test/retest variability (VAR) and intraclass correlation coefficient (ICC or reliability) were assessed for 3 outcome measures: distribution volume (V(T)), binding potential (BP), and specific to nonspecific equilibrium partition coefficient (V(3)''). RESULTS: All methods gave similar values across all regions. The variability of V(T) was excellent (< or =10%) in all regions, for the 1TC, 2TC, DEPICT, and graphical approaches. The variability of BP and V(3)'' was good in regions of high SERT density and poorer in regions of moderate and lower densities. The ICC of all 3 outcome measures was excellent in all regions. The basis function implementation of SRTM demonstrated improved reliability compared with nonlinear optimization, particularly in moderate and low-binding regions. CONCLUSION: The results of this study indicate that (11)C-DASB can be used to measure SERT parameters with high reliability and low variability in receptor-rich regions of the brain, with somewhat less reliability and increased variability in regions of moderate SERT density and poor reproducibility in low-density regions.