Selective D1- and D2-dopamine receptor blockade both induces akathisia in humans — a PET study with [11C]SCH 23390 and [11C]raclopride

Pharmacological effects were recorded and time course for receptor binding in brain was followed by positron emission tomography after IV injection of the selective D₁-dopamine receptor antagonist SCH 23390 in four healthy subjects in doses of 310–810 µg. Akathisia, the syndrome of motor restlessness, appeared after the three highest doses. The akathisia was transient and occurred only when [¹¹C]SCH 23390 binding in the basal ganglia was at a high level with a central D₁-dopamine receptor occupancy of 45–59%. The D₂-dopamine receptor antagonist [¹¹C]raclopride was injected IV into 20 healthy subjects and 13 schizophrenic patients. Akathisia appeared in 14 healthy subjects and 7 patients and coincided with maximal [¹¹C]raclopride binding in the basal ganglia. The findings for [¹¹C]raclopride and [¹¹C]SCH 23390 are the first demonstration of a relationship between time courses for radioligand binding in the human brain and simultaneously induced pharmacological effects.     

PET examination of [11C]NNC 687 and [11C]NNC 756 as new radioligands for the D1-dopamine receptor

The benzazepines NNC 687 and NNC 756 have in animal studies been described as selective D₁-dopamine receptor antagonists. Both compounds have been labeled with¹¹C for examination by positron emission tomography (PET). In the present study central receptor binding was studied in monkeys and healthy men. After IV injection of both radioligands in Cynomolgus monkeys radioactivity accumulated markedly in the striatum, a region with a high density of D₁-dopamine receptors. This striatal uptake was displaced by high doses of the selective D₁-antagonist SCH 23390 (2 mg/kg) but not by the 5HT₂-antagonist ketanserin (1.5 mg/kg) or the selective D₂-antagonist raclopride (3 mg/kg). The cortical uptake after injection of [¹¹C]NNC 687 was not reduced in displacement experiments with ketanserin. The cortical uptake of [¹¹C]NNC 756 was reduced in displacement and protection experiments with ketanserin by 24–28% (1.5 mg/kg), whereas no reduction could be demonstrated on striatal uptake. In healthy males both compounds accumulated markedly in the striatum. For [¹¹C]NNC 687 the ratio of radioactivity in the putamen to cerebellum was about 1.5. For [¹¹C]NNC 756 the ratio was about 5. This ratio of 5 for [¹¹C]NNC 756 is the highest obtained so far for PET radioligands for the D₁-dopamine receptor.     

Prefrontal dopamine D1 receptors and working memory in schizotypal personality disorder: a PET study with [11C]NNC112

Rationale

Schizotypal personality disorder (SPD) is associated with working memory (WM) impairments that are similar to those observed in schizophrenia. Imaging studies have suggested that schizophrenia is associated with alterations in dopamine D1 receptor availability in the prefrontal cortex (PFC) that may be related to the WM impairments that characterize this disorder.

Objectives

The aim of this study was to characterize prefrontal D1 receptor availability and its relation to WM performance in SPD.

Methods

We used positron emission tomography (PET) and the radiotracer [¹¹C]NNC112 with 18 unmedicated SPD and 21 healthy control participants; as an index of D1 receptor availability, binding potential (BP) measures (BP_F, BP_ND, and BP_P) were calculated for prefrontal and striatal subregions. To assess WM, SPD participants completed the 2-back and Paced Auditory Serial Addition Test (PASAT).

Results

There were no significant group differences in PFC BP. BP_F and BP_P in the medial PFC were significantly negatively related to PASAT performance (r _s ?=??0.551, p?=?.022 and r _s ?=??0.488, p?=?.047, respectively), but BP was not related to 2-back performance.

Conclusions

In contrast to what has been found in schizophrenia, SPD was not associated with significant alterations in prefrontal D1 receptor availability. Similar to previous schizophrenia findings, however, higher prefrontal D1 receptor availability was associated with poorer WM performance (as measured by the PASAT) in SPD. These findings suggest that schizophrenia and SPD may share a common pathophysiological feature related to prefrontal dopamine functioning that contributes to WM dysfunction, but that in SPD, alterations in D1 may occur only in a subset of individuals and/or to an extent that is minor relative to what occurs in schizophrenia.     

Metabolism of the PET ligand [11C]SCH 23390. Identification of two radiolabelled metabolites with HPLC

[¹¹C]SCH 23390 is a radioligand used for PET determination of the D₁-dopamine receptor in the living human brain. An HPLC procedure was developed for analysis of [¹¹C]SCH 23390 and its radiolabelled metabolites in plasma after injection of the radioligand into human subjects in realtion to PET studies. After i.v. injection [¹¹C]SCH 23390 rapidly declined from plasma, about 50 per cent of the radioligand remaining unchanged after 4 min and about 10 per cent after 40 min. The decline followed a biexponential time course. Two of the probable metabolites, the O-sulphate and the O-glucuronide of SCH 23390 were synthesized and characterized. Two radioactive substances in plasma co-migrated with these reference compounds.     

Decreased thalamic D2/D3 receptor binding in drug-naive patients with schizophrenia: a PET study with [11C]FLB 457

The thalamus is a neuroanatomic structure that has reciprocal connections with several brain regions suggested to be involved in the pathophysiology of schizophrenia. Recent studies have reported structural as well as functional abnormalities of the thalamus in schizophrenia. The aim of the present exploratory study was to examine D2/D3 dopamine receptors in the thalamus as well as the anterior cingulate and the frontal and temporal cortices by using the high-affinity radioligand [11C]FLB 457 and positron emission tomography (3D PET) and to explore the data in relation to disease, age and psychopathology. Nine drug-naive patients with schizophrenia and eight control subjects were examined. Regional binding potential (BP) values were calculated using the simplified reference tissue model. The D2/D3 receptor binding was significantly lower in the right medial thalamus in the schizophrenia patients compared to control subjects. In addition, we found a significant negative age effect on the D2/D3 BP in the frontal and temporal cortex for both groups. There was no significant age effect on the D2/D3 BP in the thalamus or in the anterior cingulate. The result provides additional support to the view that the age effect on D2/D3 receptors differ between brain regions. The preliminary finding of low thalamic D2/D3 BP in patients strengthens the hypothesis that the thalamus is a key region in the pathophysiology of schizophrenia.     

Effects of the substituted (S)-3-phenylpiperidine (-)-OSU6162 on PET measurements of [11C]SCH23390 and [11C]raclopride binding in primate brains

The substituted (S)-3-phenylpiperidine (-)-OSU6162 belongs to a novel class of functional modulators of dopaminergic systems. In vivo, (-)-OSU6162 has a unique stabilising profile on dopaminergic functions. In vitro this compound exhibits low affinity for the dopamine D2 receptor, but due to its similarity to neuroleptics on brain dopaminergic neurochemistry and different postsynaptic effects it has been characterised as a preferential dopamine autoreceptor antagonist. To further clarify the effects of (-)-OSU6162 on the postjunctional nigrostriatal dopaminergic system, dopamine receptor binding was measured in rhesus monkeys (Macaca mulatta) by positron emission tomography (PET) using the D1 and D2 dopamine receptor radioligands [11C]SCH23390 and [11C]raclopride respectively, before and during continuous intravenous infusions of(-)-OSU6162. Additionally, the test-retest variability of sequential [11C]SCH23390 scans was estimated. Following the administration of (-)-OSU6162, [11C]raclopride binding in striatum was dose-dependently decreased with a 76% reduction occurring after 3.0 mg/kg per h continuous infusion. Whereas (-)-OSU6162 in the lower doses had no effect on [11C]SCH23390 binding, the highest dose, 3.0 mg/kg per h, increased [11C]SCH23390 binding, which may indicate a potentiating effect on D1 dopamine receptor mediated functions. Thus, in contrast to the conditions in vitro, (-)-OSU6162 produces a high displacement of raclopride from D2 receptors in vivo.     

The effect of benzodiazepines on the binding of [3H]SCH 23390 in vivo.

The effect of flunitrazepam upon the binding of [3H]SCH 23390 in vivo was investigated. Acute treatment with flunitrazepam decreased the binding of [3H]SCH 23390 in the striatum in a dose-dependent manner. The time course of radioactivity in the striatum, cerebral cortex and cerebellum in controls and flunitrazepam (1 mg/kg)-treated mice was measured after intravenous injection of [3H]SCH 23390. The binding kinetics were calculated, using the cerebellum as a reference region for the estimation of the amount of free ligand in the brain. Flunitrazepam significantly decreased the input rate constant to the receptor compartment and the dissociation rate constant in vivo. An in vivo displacement study, using carrier SCH 23390, also showed significant reduction in the dissociation rate constant of [3H]SCH 23390 in vivo. The drug Ro 15-1788 reversed the effect of flunitrazepam, suggesting that this reduction in binding of [3H]SCH 23390 was mediated by benzodiazepine receptors. To evaluate the relationship between the reduction in binding of [3H]SCH 23390 in vivo and in vivo occupancy of benzodiazepine receptors, in vivo occupancy of benzodiazepine receptors was measured using [3H]Ro 15-1788. A non-linear relationship was found between the reduction in dopamine D1 receptor binding in vivo and the occupancy of benzodiazepine receptors in vivo, indicating that benzodiazepines exerted the maximum change in dopamine receptor binding at a low fractional occupancy of receptors.     

Absence of [3H]SCH 23390 binding sites in the rat adrenal gland

The binding of D2-dopamine receptor ligand [3H]spiperone and selective D1-ligand [3H]SCH 23390 to the rat adrenal gland and striatum has been compared. [3H]Spiperone showed specific binding in both tissues revealing a Bmax of 887 fmol mg-1 protein and KD of 0.38 nM, and B of 34 fmol mg-1 protein and KD of 0.66 nM in the striatum and adrenal gland, respectively. On the other hand, [3H]SCH 23390 showed a specific binding to the striatal tissue with Bmax of 747 fmol mg-1 protein and KD of 0.70 nM, while in the adrenal tissue no specific binding was observed. These results apparently indicated only D2-dopamine receptor binding sites being present in the rat adrenal gland.     

Evaluation of EMD 128 130 occupancy of the 5-HT1A and the D2 receptor: a human PET study with [11C]WAY-100635 and [11C]raclopride

The use of so-called, atypical antipsychotic medication is becoming more widespread in the treatment of psychotic disorders. EMD 128 130 is a novel compound acting as an agonist at the 5-HT1A receptor, and as an antagonist at the dopamine-2 (D2) receptor. This dual action may confer additional benefits over selective D2 antagonists in the treatment of psychotic disorders. In this study, we investigated the occupancy of EMD 128 130 in vivo at the human D2 and 5-HT1A receptors with positron emission tomography using the radiotracers [11C]raclopride and [11C]WAY-100635. Seven healthy volunteers were examined before and after 5 days of treatment with EMD 128 130, administered in an incremental dose building up to 50 mg, b.d. A significant occupancy was demonstrated at the human D2 receptor (40% following a dose of 50 mg, b.d.) while there was no consistent effect observed at the 5-HT1A receptor, despite a similar affinity of EMD 128 130 for cloned human D2 and 5-HT1A receptors, and the presence of typical, central 5-HT1A agonist side-effects. The differential effects of EMD 128 130 at the D2 and the 5-HT1A receptor (antagonist at D2 receptor, agonist at the 5-HTIA receptor) may explain the differences in occupancy observed.     

Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding to dopamine receptors in vivo in mouse brain

In vivo binding of the benzamide derivative [3H]raclopride was studied in mouse brain. The binding was saturable, reversible and stereospecific. Non-specific binding was 5-15% of the total binding. Pharmacological characterization of the binding indicated labelling of dopamine D2 receptors since the binding was potently inhibited by compounds with high affinity for this receptor in vitro. On the other hand, compounds with low affinity in vitro i.e., dopamine D1-selective compounds were weak or inactive as inhibitors of [3H]raclopride binding. A comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo indicated that compounds with selectivity in vitro retained this selectivity in vivo. Thus, spiroperidol, haloperidol, 1-sulpiride, clebopride, LY 171555 and (-)-NPA ((-)-N-propyl-norapomorphine) were D2 selective while SCH 23390, SKF 38393 and SKF 75670 were D1 selective. Clozapine, tilozepine, cis-flupentixol, chlorpromazine and butaclamol were non-selective both in vitro and in vivo. However, a few compounds changed profile in vivo compared to in vitro. Thus, fluperlapine and fluphenazine had a dual D1-D2 receptor profile in vitro but were D1- or D2-selective in vivo, respectively. Pergolide and molindone which were D2-selective in vitro both had a dual D1-D2 receptor profile in vivo. In conclusion, [3H]raclopride, in vivo, selectively labels the dopamine D2 receptor. Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo supported the that the dopamine D1 receptor is an important target for a variety of neuroleptics, especially of the clozapine type. This may indicate that blockade of the dopamine D1 receptor conveys antipsychotic action.     

3H]SCH 23390 labels dopamine D-1 receptor sites in the rat kidney

The binding of [3H]SCH 23390, a selective dopamine D-1 receptor radioligand, to rat kidney cortical membranes was studied. Scatchard analysis revealed a single class of binding sites. Of dopamine, noradrenaline and serotonin, dopamine was the most potent of these to displace [3H]SCH 23390 binding. The selective D-1 ligands SCH 23390 and SK & F 38393 were more potent to displace [3H]SCH 23390 than the selective D-2 ligands S-sulpiride and LY 171555, thus indicating that [3H]SCH 23390 binds predominantly to dopamine D-1 receptor sites.     

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 [¹¹C]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.Dr. Lombardo participated in this work while at Columbia University but subsequently has moved to Pfizer Inc., 235 east 42nd Street MS 10/33, New York, NY 10017, USA.     

Chronic imipramine reduces [3H]SCH 23390 binding and DA-sensitive adenylate cyclase in the limbic system

[3H]SCH 23390 binding and dopamine (DA)-stimulated adenylate cyclase activity were measured in brain membrane preparations from rats chronically treated with imipramine (10 mg/kg twice daily for 14 days). [3H]SCH 23390 binding sites were decreased by 27% in the limbic system but only 14% in the striatum. The responsiveness of adenylate cyclase to DA was reduced by 38% in the limbic system but was not modified in the striatum. Concomitant treatment with alpha-methyltyrosine (alpha-MPT) (50 mg/kg daily for 14 days) prevented the imipramine-induced reduction in both [3H]SCH 23390 binding sites and the responsiveness of adenylate cyclase to DA.     

Serotonin 5-HT1A receptor binding potential declines with age as measured by [11C]WAY-100635 and PET.

Positron emission tomography (PET) and [11C]WAY-100635 were used to examine the effect of age on serotonin-1A (5-HT1A) receptor binding potential (BP) in 19 healthy subjects. Regions of interest (ROI) were drawn on the co-registered magnetic resonance imaging (MRI) in orbitofrontal (OFC), dorsolateral prefrontal (DLPFC), anterior cingulate (ACC), lateral (LTC), and mediotemporal (MTC), parietal, occipital and cerebellar cortex, and the raphe nuclei. BP values were calculated using a simplified reference tissue method. In addition, a voxelwise analysis was performed using SPM99. Voxelwise analysis revealed a significant global decrease of 5-HT(1A) BP with age (set level <.001). ROI analysis revealed significant age-related 5-HT(1A) BP decreases in DLPFC (r = -0.56), ACC (r = -0.44), OFC (r = -0.42), LTC (r = -0.40), parietal (r = -0.65), and occipital cortex (r = -0.43), but not in MTC or raphe nuclei. Overall, cortical 5-HT(1A) BP declined by approximately 10% per decade, except for the MTC, where we did not find a significant age effect. Hence, careful age matching may be recommended for future studies using PET and [11C]WAY-100635 to examine 5-HT1A receptors.     

[11C]Mirtazapine binding in depressed antidepressant nonresponders studied by PET neuroimaging

Rationale  Lack of benefit from antidepressant drug therapy is a major source of human suffering, affecting at least 25% of people with major depressive disorder. We want to know whether nonresponse to antidepressants can be linked to aberrant neuroreceptor binding. Objective  This study aims to assess the antidepressant binding in brain regions of depressed nonresponders compared with healthy controls. Materials and methods  Healthy volunteers and depressed subjects who had failed to benefit from at least 2 antidepressant treatments were recruited by newspaper advertisements. All subjects had received no antidepressant medication for at least 2 months before positron emission tomography (PET) that was carried out with [¹¹C]mirtazapine. Kinetic parameters of [¹¹C]mirtazapine were determined from PET data in selected brain regions by the simplified reference tissue model. Results  Binding potentials of [¹¹C]mirtazapine in cerebral cortical regions were lower in depressed nonresponders than in healthy controls. Removal rates of [¹¹C]mirtazapine were higher in diencephalic regions of depressed nonresponders than in healthy controls. Conclusions  PET neuroimaging with [¹¹C]mirtazapine showed aberrant neuroreceptor binding in brain regions of depressed subjects who had failed to benefit from treatment with antidepressant drugs.     

2-11C]isopropyl-, [1-11C]ethyl-, and [11C]methyl-labeled phenoxyphenyl acetamide derivatives as positron emission tomography ligands for the peripheral benzodiazepine receptor: radiosynthesis, uptake, and in vivo binding in brain

The peripheral benzodiazepine receptor (PBR) is widely expressed in peripheral tissues, blood cells, and in glia cells in the brain. We have previously developed two positron emission tomography (PET) ligands, N-(2-[(11)C],5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([(11)C]2) and its [(18)F]fluoroethyl analogue ([(18)F]6), for the current investigation of PBR in the human brain. The aim of this study was to label the potent PBR agonist N-(4-chloro-2-phenoxyphenyl)-N-(isopropoxybenzyl)acetamide (3) and its ethyl (7) and methyl (8) homologues with (11)C and to evaluate them as PET ligands for PBR with mice, rats, and monkeys. Ligands [(11)C]3, [(11)C]7, and [(11)C]8 were synthesized by alkylation of phenol precursor 9 with 2-[2-(11)C]iodopropane ([(11)C]10), [1-(11)C]iodoethane ([(11)C]11), and [(11)C]iodomethane ([(11)C]12), respectively. The alkylating agent [(11)C]10 or [(11)C]11 was prepared by reacting CH(3)MgBr with [(11)C]CO(2), followed by reduction with LiAlH(4) and iodination with HI. In vitro quantitative autoradiography determined that 3, 7, and 8 had potent binding affinities (K(i) = 0.07-0.19 nM) for PBR in the rat brain. These [(11)C]ligands could pass across the blood-brain barrier and enter the rat brain (0.17-0.32% of injected dose per gram wet tissue). Ex vivo autoradiography showed that the [(11)C]ligands preferably distributed in the olfactory bulb and cerebellum, two regions with richer PBR density in the rat brain. The co-injection of PBR-selective 2 reduced the [(11)C]ligand binding in the two regions, suggesting that binding in the rat brain was specific to PBR. PET study determined that the [(11)C]ligands preferably accumulate in the occipital cortex of the monkey brain, a region with a high density of PBR in the primate brain. Moreover, in vivo binding of the methyl homologue [(11)C]8 in the monkey brain could be inhibited by PBR-selective 2 or 1, indicating that some of the [(11)C]8 binding was due to PBR. Metabolite analysis demonstrated that these [(11)C]ligands were metabolized by debenzylation to polar products mainly in the plasma.     

In vivo measurement of [11C]verapamil kinetics in human tissues

OBJECTIVE: To evaluate [11C]verapamil kinetics in humans. METHODS: After intravenous injection of [11C]verapamil (370 MBq, specific activity >1,600 GBq/mmol), kinetics were evaluated in five cancer patients using positron emission tomography (PET). RESULTS: One hour after injection, accumulation of [11C] in lungs, heart and tumour was 43.0%, 1.3% and 0.9% of the injected verapamil dose, respectively. Half-lives of [11C]verapamil in these tissues were 46.2 min, 73.8 min and 23.7 min, respectively. CONCLUSIONS: Intravenously administered [11C] was mainly extracted by the lungs. Transport of a [11C]verapamil bolus out of solid tumour tissue is relatively fast.