Svante Nyberg

Detection of amyloid in Alzheimer’s disease with positron emission tomography using [11C]AZD2184

PurposeCurrent positron emission tomography (PET) radioligands for detection of Aβ amyloid in Alzheimer’s disease (AD) are not ideal for quantification. To improve the signal to noise ratio we have developed the radioligand [11C]AZD2184 and report here the first clinical evaluation.MethodsEight AD patients and four younger control subjects underwent 93-min PET measurements with [11C]AZD2184. A ratio approach using the cerebellum as reference region was applied to determine binding parameters.ResultsBrain uptake of [11C]AZD2184 peaked within 1 min at 3–4% of injected radioactivity. AD patients had high radioactivity in cortical regions while controls had uniformly low radioactivity uptake. Specific binding peaked within 30 min at which time standardized uptake value ratios (SUVR) ranged between 1.19 and 2.57.Conclusion[11C]AZD2184 is a promising radioligand for detailed mapping of Aβ amyloid depositions in Alzheimer’s disease, due to low non-specific binding, high signal to background ratio and reversible binding as evident from early peak equilibrium.

[11C]AZ10419369: a selective 5-HT1B receptor radioligand suitable for positron emission tomography (PET). Characterization in the primate brain.

The 5-HT1B receptor has been implicated in several psychiatric disorders and is a potential pharmacological target in the treatment of depression. Here we report the synthesis of a novel PET radioligand, [11C]AZ10419369 (5-methyl-8-(4-methyl-piperazin-1-yl)-4-oxo-4H-chromene-2-carboxylic acid (4-morpholin-4-yl-phenyl)-amide), for in vivo visualization of 5-HT1B receptors in the brains of macaques and humans subjects. [11C]AZ10419369 was prepared by N-methylation of (8-(1-piperazinyl)-5-methylchrom-2-en-4-one-2-(4-morpholinophenyl) carboxamide, using carbon-11 methyl triflate. Regional brain uptake patterns of [11C]AZ10419369 were characterized by PET measurements in two macaques and a preliminary study in two human subjects. In addition, AZ10419369 was prepared in tritium labeled form for in vitro autoradiography studies in macaque brain tissue sections. The radiochemical purity of [11C]AZ10419369 was >99% and specific radioactivity was >3600 Ci/mmol. After iv injection of [11C]AZ10419369, 3-4% was in brain after 7.5 min. The regional brain distribution of radioactivity was similar in humans and macaques showing the highest uptake of radioactivity in the occipital cortex and the basal ganglia, in accord with autoradiographic studies performed using [3H]AZ10419369. Uptake was moderate in the temporal and frontal cortical regions, lower in the thalamus and lowest in the cerebellum. In macaques pre-treated with the selective 5-HT1B receptor antagonist, AR-A000002, binding was reduced in a dose-dependent manner, consistent with specific binding to 5-HT1B receptors. These data support [11C]AZ10419369 as a suitable radioligand for labeling 5-HT1B receptors in the primate brain. This radioligand may be useful in future studies evaluating drug-induced receptor occupancy and measurement of brain 5-HT1B receptor levels in patients with psychiatric disorders.

Quantitative analysis of [11C]AZ10419369 binding to 5-HT1B receptors in human brain

A novel radioligand for positron emission tomography (PET) imaging of serotonin 5-HT1B receptors, [11C]AZ10419369, has been recently described. In this study, the potential for quantitative analysis of [11C]AZ10419369 binding to central 5-HT1B receptors was evaluated in human subjects. PET measurements were performed after injection of [11C]AZ10419369 in 10 subjects. Data were analyzed with kinetic modeling and linear graphical analysis using the arterial plasma as input function, and with reference tissue models using cerebellar cortex as the reference region. Binding of [11C]AZ10419369 was highest in pallidum, ventral striatum, and occipital cortex and lowest in cerebellum. The percentage of unchanged radioligand in plasma was 97% to 99%, indicating that no significant amounts of radioactive metabolites were formed during the time of analysis. Time–activity curves of [11C]AZ10419369 could be described with both one-tissue compartment (1-TC) and two-tissue compartment (2-TC) models in the majority of subjects. The 2-TC model failed to deliver reasonable estimates of the kinetic parameters. However, stable estimates of binding potential (BPND) were obtained by constraining K1/k2 to the distribution volume obtained with the 1-TC model in the cerebellar cortex. BPND values estimated with reference tissue models were correlated with the corresponding values obtained with kinetic modeling. The findings support the use of reference tissue models in applied clinical studies with [11C]AZ10419369.

High 5HT2A receptor occupancy in M100907-treated schizophrenic patients

Abstract Rationale: Selective drugs are required to test the hypothesis whether antipsychotic effects may be induced or modulated by 5HT2A receptor antagonism. M100907 (previously known as MDL 100,907) is a highly selective 5HT2A antagonist in clinical development. Objective: To test if the suggested clinical dose of 20 mg M100907 daily induces high 5HT2A receptor occupancy in patients with schizophrenia. Methods: The 5HT2A receptor occupancy was determined in two patients with schizophrenia treated with M100907, 20 mg once a day. Positron emission tomography (PET) with 11C-labeled M100907, was performed prestudy and under steady state conditions. Clinical ratings were performed weekly. Results: Clinical treatment with M100907, 20 mg daily induced a very high 5HT2A receptor occupancy in the frontal cortex of both patients (>90%). M100907 was well tolerated. One patient improved minimally and one patient became minimally worse during treatment. Conclusions: The results confirm that an oral dose of 20 mg per day ensures adequate 5HT2A receptor occupancy for clinical proof of concept. The sample is too small to allow conclusions about the clinical effect.

A positron emission tomography study in healthy volunteers to estimate mGluR5 receptor occupancy of AZD2066 — Estimating occupancy in the absence of a reference region

AZD2066 is a new chemical entity pharmacologically characterized as a selective, negative allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5). Antagonism of mGluR5 has been implicated in relation to various diseases such as anxiety, depression, and pain disorders. To support translation from preclinical results and previous experiences with this target in man, a positron emission tomography study was performed to estimate the relationship between AZD2066 plasma concentrations and receptor occupancy in the human brain, using the mGluR5 radioligand [(11)C]-ABP688. The study involved PET scans on 4 occasions in 6 healthy volunteers. The radioligand was given as a tracer dose alone and following oral treatment with different doses of AZD2066. The analysis was based on the total volume of distribution derived from each PET-assessment. A non-linear mixed effects model was developed where ten delineated brain regions of interest from all PET scans were included in one simultaneous fit. For comparison the analysis was also performed according to a method described previously by Lassen et al. (1995). The results of the analysis showed that the total volume of distribution decreased with increasing drug concentrations in all regions with an estimated Kipl of 1170 nM. Variability between individuals and occasions in non-displaceable volume of distribution could explain most of the variability in the total volume of distribution. The Lassen approach provided a similar estimate for Kipl, but the variability was exaggerated and difficult to interpret.

Comparison of D2 dopamine receptor occupancy after oral administration of quetiapine fumarate immediate-release and extended-release formulations in healthy subjects

Quetiapine is an established drug for treatment of schizophrenia, bipolar disorder, and major depressive disorder. While initially manufactured as an immediate-release (IR) formulation, an extended-release (XR) formulation has recently been introduced. Pharmacokinetic studies show that quetiapine XR provides a lower peak and more stable plasma concentration than the IR formulation. This study investigated if the pharmacokinetic differences translate into different time curves for central D2 dopamine receptor occupancy. Eleven control subjects were examined with positron emission tomography (PET) and the radioligand [11C]raclopride. Eight subjects underwent all of the scheduled PET measurements. After baseline examination, quetiapine XR was administered once-daily for 8 d titrated to 300 mg/d on days 5–8, followed by 300 mg/d quetiapine IR on days 9–12. PET measurements were repeated after the last doses of quetiapine XR and IR at predicted times of peak and trough plasma concentrations. Striatal D2 receptor occupancy was calculated using the simplified reference tissue model. Peak D2 receptor occupancy was significantly higher with quetiapine IR than XR in all subjects (50±4% and 32±11%, respectively), consistent with lower peak plasma concentrations for the XR formulation. Trough D2 receptor occupancy was similarly low for both formulations (IR 7±7%, XR 8±6%). The lower peak receptor occupancy associated with quetiapine XR may explain observed pharmacodynamic differences between the formulations. Assuming that our findings in control subjects are valid for patients with schizophrenia, the study supports the view that quetiapine, like the prototype atypical antipsychotic clozapine, may show antipsychotic effect at lower D2 receptor occupancy than typical antipsychotics.

Radiosynthesis of the candidate β‐amyloid radioligand [11C]AZD2184: Positron emission tomography examination and metabolite analysis in cynomolgus monkeys

β‐Amyloid accumulation is associated with the pathogenesis of Alzheimers disease (AD). AZD2184, a new radioligand for high‐contrast positron emission tomography (PET) imaging of Aβ‐deposits, has recently been developed and characterized in vitro and in rodents ex vivo. The objective of this study was to label AZD2184 with carbon‐11, to perform in vivo characterization of [11C]AZD2184 ([11C]5) in the cynomolgus monkey brain as well as whole‐body dosimetry, and to examine the metabolism of the labeled radioligand. [11C]5 was prepared by a two‐step radiosynthesis starting with the reaction of 5‐(6‐(tert‐butyldimethylsilyloxy)benzo[d]thiazol‐2‐yl)pyridin‐2‐amine with [11C]methyl iodide followed by deprotection using water. Four brain PET measurements in two cynomolgus monkeys and one whole‐body PET measurement were performed with [11C]5. There was a high and rapid brain uptake (2.2–3.4% of injected dose at 2 min). The distribution of brain radioactivity was fairly uniform, with early to late‐brain concentration ratios (peak vs. 60 min) higher for [11C]5 than ratios previously reported for [11C]PIB (8.2 and 4.6, respectively). Based on the whole‐body data, it was estimated that an effective dose in an adult male would be 6.2 μSv/MBq and thus would be safe from a radiation point of view for multiple scans within the same year. [11C]5 shows binding characteristics, suggesting low levels of white‐matter retention, and may thus provide improved contrast when compared with currently used PET radioligands for visualization of Aβ‐deposits. On the basis of the labeling chemistry and the results of the biological evaluation, we conclude that [11C]5 should be useful for routine clinical studies. Synapse 64:733–741, 2010.

Non-linear mixed effects modelling of positron emission tomography data for simultaneous estimation of radioligand kinetics and occupancy in healthy volunteers

The aim of this work was to develop a model simultaneously estimating (11)C-AZD9272 radioligand kinetics and the relationship between plasma concentration of AZD9272 and receptor occupancy in the human brain. AZD9272 is a new chemical entity pharmacologically characterised as a noncompetitive antagonist at the metabotropic glutamate receptor subtype 5 (mGluR5). Positron emission tomography (PET) was used to measure the time course of ((11)C-AZD9272) in the brain. The study included PET measurements in six healthy volunteers where the radioligand was given as a tracer dose alone as well as post oral treatment with different doses of unlabelled AZD9272. Estimation of radioligand kinetics, including saturation of receptor binding was performed by use of non-linear mixed effects modelling. Data from the regions with the highest (ventral striatum) and lowest (cerebellum) radioligand concentrations were included in the analysis. It was assumed that the extent of non-displaceable brain uptake was the same in both regions while the rate of CNS uptake and the receptor density differed. The results of the analysis showed that AZD9272 binding at the receptor is saturable with an estimated plasma concentration corresponding to 50% occupancy of approximately 200 nM. The density of the receptor binding sites was estimated to 800 nM and 200 nM in ventral striatum and cerebellum respectively. By simultaneously analysing data from several PET measurements and different brain regions in a non-linear mixed effects framework it was possible to estimate parameters of interest that would otherwise be difficult to quantify.

Role of concomitant inhibition of the norepinephrine transporter for the antipsychotic effect of quetiapine.

Quetiapine alleviates both positive and negative symptoms as well as certain cognitive impairments in schizophrenia despite a low D2 receptor occupancy and may also be used as monotherapy in bipolar and major depressive disorder. The mechanisms underlying the broad clinical utility of quetiapine remain to be clarified, but may be related to the potent inhibition of the norepinephrine transporter (NET) by norquetiapine, the major metabolite of quetiapine in humans. Since norquetiapine is not formed in rodents we here investigated in rats whether NET-inhibition may, in principle, contribute to the clinical effectiveness of quetiapine and allow for its low D2 receptor occupancy, by combining quetiapine with the selective NET-inhibitor reboxetine. Antipsychotic-like activity was assessed using the conditioned avoidance response (CAR) test, dopamine output in the medial prefrontal cortex (mPFC) and the nucleus accumbens was measured using in vivo microdialysis, and NMDA receptor-mediated transmission was measured using intracellular electrophysiological recordings in pyramidal cells of the mPFC in vitro. Adjunct reboxetine potentiated the suppression of CAR by quetiapine. Moreover, concomitant administration of quetiapine and reboxetine resulted in a synergistic increase in cortical, but not accumbal, dopamine output. The combination of low, clinically relevant concentrations of quetiapine (60 nM) and reboxetine (20 nM) markedly facilitated cortical NMDA receptor-mediated transmission in contrast to either drug alone, an effect that could be inhibited by the D₁ receptor antagonist SCH23390. We conclude that concomitant NET-inhibition by norquetiapine may contribute to the overall antipsychotic effectiveness of quetiapine in spite of its relatively low level of D₂ occupancy.

Norepinephrine transporter occupancy in the human brain after oral administration of quetiapine XR

Quetiapine, originally developed as an antipsychotic, demonstrates efficacy in clinical studies of schizophrenia, bipolar mania and depression, major depressive disorder and generalized anxiety disorder. This broad spectrum of efficacy was not predicted from the preclinical pharmacology of quetiapine. Binding studies in vitro show that quetiapine and its major active human metabolite, norquetiapine, have moderate to high affinity for dopamine D2 and serotonin 5-HT2A receptors, while norquetiapine alone has high affinity for the norepinephrine transporter (NET). This positron emission tomography (PET) study measured NET occupancy in human subjects treated with extended-release quetiapine (quetiapine XR) at doses relevant in the treatment of depression. PET measurements using the specific NET radioligand (S,S)-[(18)F]FMeNER-D2 were performed before and after quetiapine XR treatment at 150 and 300 mg/d for 6-8 d in nine healthy males (aged 21-33 yr). Regions of interest were defined for the thalamus, using the caudate as reference region. NET occupancy was calculated using a target:reference region ratio method. Plasma concentrations of quetiapine and norquetiapine were monitored during PET measurements. Following quetiapine XR treatment, the mean NET occupancy in the thalamus was 19 and 35%, respectively, at quetiapine XR doses of 150 and 300 mg/d. The estimated plasma concentration of norquetiapine corresponding to 50% NET occupancy was 161 ng/ml. This is the first demonstration of NET occupancy by an antipsychotic in the human brain. NET inhibition is accepted as a mechanism of antidepressant activity. NET occupancy may therefore contribute to the broad spectrum of efficacy of quetiapine.