Kinetic models for estimating occupancy from single-scan PET displacement studies.

The traditional design of PET target engagement studies is based on a baseline scan and one or more scans after drug administration. We here evaluate an alternative design in which the drug is administered during an on-going scan (i.e., a displacement study). This approach results both in lower radiation exposure and lower costs. Existing kinetic models assume steady state. This condition is not present during a drug displacement and consequently, our aim here was to develop kinetic models for analysing PET displacement data. We modified existing compartment models to accommodate a time-variant increase in occupancy following the pharmacological in-scan intervention. Since this implies the use of differential equations that cannot be solved analytically, we developed instead one approximate and one numerical solution. Through simulations, we show that if the occupancy is relatively high, it can be estimated without bias and with good accuracy. The models were applied to PET data from six pigs where [11C]UCB-J was displaced by intravenous brivaracetam. The dose-occupancy relationship estimated from these scans showed good agreement with occupancies calculated with Lassen plot applied to baseline-block scans of two pigs. In summary, the proposed models provide a framework to determine target occupancy from a single displacement scan.

Evaluation of the α-synuclein PET radiotracer (d3)-[11C]MODAG-001 in pigs.

BACKGROUND: A positron emission tomography (PET) radiotracer to neuroimage α-synuclein aggregates would be a crucial addition for early diagnosis and treatment development in disorders such as Parkinson's disease, where elevated aggregate levels are a histopathological hallmark. The radiotracer (d3)-[11C]MODAG-001 has recently shown promise for visualization of α-synuclein pre-formed fibrils (α-PFF) in rodents. We here test the radiotracer in a pig model where proteins are intracerebrally injected immediately before scanning. Four pigs were injected in one hemisphere with 150 µg α-PFF, and in the other hemisphere, either 75 µg α-PFF or human brain homogenate from either dementia with Lewy bodies (DLB) or Alzheimer's disease (AD) was injected. All pigs underwent one or two (d3)-[11C]MODAG-001 PET scans, quantified with the non-invasive Logan graphical analysis using the occipital cortex as a reference region. RESULTS: The α-PFF and AD homogenate injected brain regions had high uptake of (d3)-[11C]MODAG-001 compared to the occipital cortex or cerebellum. BPND values in 150 µg α-PFF injected regions was 0.78, and in the AD homogenate injected regions was 0.73. By contrast, the DLB homogenate injected region did not differ in uptake and clearance compared to the reference regions. The time-activity curves and BPND values in the 150 µg and 75 µg injected regions of α-PFFs show a dose-dependent effect, and the PET signal could be blocked by pretreatment with unlabeled MODAG-001. CONCLUSION: We find that both α-PFF and AD brain homogenates give rise to increased binding of (d3)-[11C]MODAG-001 when injected into the pig brain. Despite its limited specificity for cerebral α-synuclein pathology, (d3)-[11C]MODAG-001 shows promise as a lead tracer for future radiotracer development.

A Single Dose of Psilocybin Increases Synaptic Density and Decreases 5-HT2A Receptor Density in the Pig Brain.

A single dose of psilocybin, a psychedelic and serotonin 2A receptor (5-HT2AR) agonist, may be associated with antidepressant effects. The mechanism behind its antidepressive action is unknown but could be linked to increased synaptogenesis and down-regulation of cerebral 5-HT2AR. Here, we investigate if a single psychedelic dose of psilocybin changes synaptic vesicle protein 2A (SV2A) and 5-HT2AR density in the pig brain. Twenty-four awake pigs received either 0.08 mg/kg psilocybin or saline intravenously. Twelve pigs (n = 6/intervention) were euthanized one day post-injection, while the remaining twelve pigs were euthanized seven days post-injection (n = 6/intervention). We performed autoradiography on hippocampus and prefrontal cortex (PFC) sections with [3H]UCB-J (SV2A), [3H]MDL100907 (5-HT2AR antagonist) and [3H]Cimbi-36 (5-HT2AR agonist). One day post psilocybin injection, we observed 4.42% higher hippocampal SV2A density and lowered hippocampal and PFC 5-HT2AR density (-15.21% to -50.19%). These differences were statistically significant in the hippocampus for all radioligands and in the PFC for [3H]Cimbi-36 only. Seven days post-intervention, there was still significantly higher SV2A density in the hippocampus (+9.24%) and the PFC (+6.10%), whereas there were no longer any differences in 5-HT2AR density. Our findings suggest that psilocybin causes increased persistent synaptogenesis and an acute decrease in 5-HT2AR density, which may play a role in psilocybin's antidepressive effects.

Effects of a single dose of psilocybin on behaviour, brain 5-HT2A receptor occupancy and gene expression in the pig.

Psilocybin has in some studies shown promise as treatment of major depressive disorder and psilocybin therapy was in 2019 twice designated as breakthrough therapy by the U.S. Food and Drug Administration (FDA). A very particular feature is that ingestion of just a single dose of psilocybin is associated with lasting changes in personality and mood. The underlying molecular mechanism behind its effect is, however, unknown. In a translational pig model, we here present the effects of a single dose of psilocybin on pig behaviour, receptor occupancy and gene expression in the brain. An acute i.v. injection of 0.08 mg/kg psilocybin to awake female pigs induced characteristic behavioural changes in terms of headshakes, scratching and rubbing, lasting around 20 min. A similar dose was associated with a cerebral 5-HT2A receptor occupancy of 67%, as determined by positron emission tomography, and plasma psilocin levels were comparable to what in humans is associated with an intense psychedelic experience. We found that 19 genes were differentially expressed in prefrontal cortex one day after psilocybin injection, and 3 genes after 1 week. Gene Set Enrichment Analysis demonstrated that multiple immunological pathways were regulated 1 week after psilocybin exposure. This provides a framework for future investigations of the lasting molecular mechanisms induced by a single dose of psilocybin. In the light of an ongoing debate as to whether psilocybin is a safe treatment for depression and other mental illnesses, it is reassuring that our data suggest that any effects on gene expression are very modest.

Visual stimuli induce serotonin release in occipital cortex: A simultaneous positron emission tomography/magnetic resonance imaging study.

Endogenous serotonin (5-HT) release can be measured noninvasively using positron emission tomography (PET) imaging in combination with certain serotonergic radiotracers. This allows us to investigate effects of pharmacological and nonpharmacological interventions on brain 5-HT levels in living humans. Here, we study the neural responses to a visual stimulus using simultaneous PET/MRI. In a cross-over design, 11 healthy individuals were PET/MRI scanned with the 5-HT1B receptor radioligand [11 C]AZ10419369, which is sensitive to changes in endogenous 5-HT. During the last part of the scan, participants either viewed autobiographical images with positive valence (n = 11) or kept their eyes closed (n = 7). The visual stimuli increased cerebral blood flow (CBF) in the occipital cortex, as measured with pseudo-continuous arterial spin labeling. Simultaneously, we found decreased 5-HT1B receptor binding in the occipital cortex (-3.6 ± 3.6%), indicating synaptic 5-HT release. Using a linear regression model, we found that the change in 5-HT1B receptor binding was significantly negatively associated with change in CBF in the occipital cortex (p = .004). For the first time, we here demonstrate how cerebral 5-HT levels change in response to nonpharmacological stimuli in humans, as measured with PET. Our findings more directly support a link between 5-HT signaling and visual processing and/or visual attention.

Association Between Sumatriptan Treatment During a Migraine Attack and Central 5-HT1B Receptor Binding.

Importance: Triptans, the most efficient acute treatment for migraine attacks, are 5-HT1B/1D receptor agonists, but their precise mechanism of action is not completely understood. The extent to which triptans enter the central nervous system and bind to 5-HT1B receptors in the brain is unknown. Objectives: To determine the occupancy of sumatriptan to central 5-HT1B receptors, and to investigate changes in brain serotonin levels during migraine attacks. Design, Setting, and Participants: This study of 8 patients in Denmark used a within-participant design and was conducted from April 20, 2015, to December 5, 2016. Participants were otherwise healthy patients with untreated episodic migraine without aura, aged between 18 and 65 years, and recruited from the general community. Data analysis was performed from January 2017 to April 2018. Interventions: All participants underwent positron emission tomographic scans after injection of [11C]AZ10419369, a specific 5-HT1B receptor radiotracer. All participants were scanned 3 times: (1) during an experimentally induced migraine attack, (2) after a subcutaneous injection of 6-mg subcutaneous sumatriptan, and (3) on a migraine attack-free day. Scans 1 and 2 were conducted on the same study day. Each scan lasted for 90 minutes. Main Outcome and Measure: The primary outcome was the nondisplaceable binding potential of [11C]AZ10419369 across 7 brain regions involved in pain modulation. The binding potential reflects receptor density, and changes in binding potential reflects displacement of the radiotracer. The occupancy of sumatriptan was estimated from the 2 scans before and after sumatriptan administration. Results: Eight patients with migraine were included in the study; of these participants, 7 (87%) were women. The mean (SD) age of participants on study day 1 was 29.5 (9.2) years and on study day 2 was 30.0 (8.9) years. Sumatriptan was associated with statistically significantly reduced 5-HT1B receptor binding across pain-modulating regions (mean [SD] binding potential, 1.20 [0.20] vs 1.02 [0.22]; P = .001), corresponding to a mean (SD) drug occupancy rate of 16.0% (5.3%). Furthermore, during migraine attacks, as compared with outside of attacks, 5-HT1B receptor binding was statistically significantly associated with reduced in pain-modulating regions (mean [SD] binding potential, 1.36 [0.22] vs 1.20 [0.20]; P = .02). Conclusions and Relevance: Treatment with sumatriptan during migraine attacks appeared to be associated with a decrease in 5-HT1B receptor binding, a finding that is most likely associated with the binding of sumatriptan to central 5-HT1B receptors, but the contribution of ongoing cerebral serotonin release to the lower binding cannot be excluded; the migraine attack-associated decrease in binding could indicate that migraine attacks are associated with increases in endogenous serotonin.

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

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

Serotonergic mechanisms in the migraine brain - a systematic review.

Background Migraine is one of the most common and disabling of all medical conditions, affecting 16% of the general population, causing huge socioeconomic costs globally. Current available treatment options are inadequate. Serotonin is a key molecule in the neurobiology of migraine, but the exact role of brain serotonergic mechanisms remains a matter of controversy. Methods We systematically searched PubMed for studies investigating the serotonergic system in the migraine brain by either molecular neuroimaging or electrophysiological methods. Results The literature search resulted in 59 papers, of which 13 were eligible for review. The reviewed papers collectively support the notion that migraine patients have alterations in serotonergic neurotransmission. Most likely, migraine patients have a low cerebral serotonin level between attacks, which elevates during a migraine attack. Conclusion This review suggests that novel methods of investigating the serotonergic system in the migraine brain are warranted. Uncovering the serotonergic mechanisms in migraine pathophysiology could prove useful for the development of future migraine drugs.

Convergent 18F-labeling and evaluation of N-benzyl-phenethylamines as 5-HT2A receptor PET ligands.

Positron emission tomography (PET) investigations of the 5-HT2A receptor (5-HT2AR) system can be used as a research tool in diseases such as depression, Alzheimer's disease and schizophrenia. We have previously developed a 11C-labeled agonist PET ligand ([11C]Cimbi-36), and the aim of this study was to identify a 18F-labeled analogue of this PET-ligand. Thus, we developed a convergent radiochemical approach giving easy access to 5 different 18F-labeled ligands structurally related to Cimbi-36 from a common 18F-labeled intermediate. After intravenous injection, all ligands entered the pig brain. However, since within-scan intervention with ketanserin, a known orthosteric 5-HT2A receptor antagonist, did not result in significant blocking, the radioligands seem unsuitable for neuroimaging of the 5-HT2AR in vivo.

Synthesis and evaluation of (18)F-labeled 5-HT2A receptor agonists as PET ligands.

INTRODUCTION: The serotonin 2A receptor (5-HT2AR) is the most abundant excitatory 5-HT receptor in the human brain and implicated in various brain disorders such as schizophrenia, depression, and Alzheimer's disease. Positron emission tomography (PET) can be used to image specific proteins and processes in the human brain and several 5-HT2AR PET antagonist radioligands are available. In contrast to an antagonist radioligand, an agonist radioligand should be able to image the population of functional receptors, i.e., those capable of inducing neuroreceptor signaling. Recently, we successfully developed and validated the first 5-HT2AR agonist PET tracer, [(11)C]Cimbi-36, for neuroimaging in humans and herein disclose some of our efforts to develop an (18)F-labeled 5-HT2AR agonist PET-ligand. METHODS AND RESULTS: Three fluorine containing derivatives of Cimbi-36 were synthesized and found to be potent 5-HT2A agonists. (18)F-labeling of the appropriate precursors was performed using [(18)F]FETos, typically yielding 0.2-2.0GBq and specific activities of 40-120GBq/µmol. PET studies in Danish landrace pigs revealed that [(18)F]1 displayed brain uptake in 5-HT2AR rich regions. However, high uptake in bone was also observed. No blocking effect was detected during a competition experiment with a 5-HT2AR selective antagonist. [(18)F]2 and [(18)F]3 showed very low brain uptake. CONCLUSION: None of the investigated (18)F-labeled Cimbi-36 derivatives [(18)F]1, [(18)F]2 and [(18)F]3 show suitable tracer characteristics for in vivo PET neuroimaging of the 5-HT2AR. Although for [(18)F]1 there was reasonable brain uptake, we suggest that a large proportion radioactivity in the brain was due to radiometabolites, which would explain why it could not be displaced by a 5-HT2AR antagonist.

Design of Infusion Schemes for Neuroreceptor Imaging: Application to [(11)C]Flumazenil-PET Steady-State Study.

This study aims at developing a simulation system that predicts the optimal study design for attaining tracer steady-state conditions in brain and blood rapidly. Tracer kinetics was determined from bolus studies and used to construct the system. Subsequently, the system was used to design inputs for bolus infusion (BI) or programmed infusion (PI) experiments. Steady-state quantitative measurements can be made with one short scan and venous blood samples. The GABAA receptor ligand [(11)C]Flumazenil (FMZ) was chosen for this purpose, as it lacks a suitable reference region. Methods. Five bolus [(11)C]FMZ-PET scans were conducted, based on which population-based PI and BI schemes were designed and tested in five additional healthy subjects. The design of a PI was assisted by an offline feedback controller. Results. The system could reproduce the measurements in blood and brain. With PI, [(11)C]FMZ steady state was attained within 40 min, which was 8 min earlier than the optimal BI (B/I ratio = 55 min). Conclusions. The system can design both BI and PI schemes to attain steady state rapidly. For example, subjects can be [(11)C]FMZ-PET scanned after 40 min of tracer infusion for 40 min with venous sampling and a straight-forward quantification. This simulation toolbox is available for other PET-tracers.

Radiolabelling and PET brain imaging of the α1-adrenoceptor antagonist Lu AE43936.

Cerebral α1-adrenoceptors are a common target for many antipsychotic drugs. Thus, access to positron emission tomography (PET) brain imaging of α1-adrenoceptors could make important contributions to the understanding of psychotic disorders as well as to the pharmacokinetics and occupancy of drugs targeting the α1-adrenoceptors. However, so far no suitable PET radioligand has been developed for brain imaging of α1-adrenoceptors. Here, we report the synthesis of both enantiomers of the desmethyl precursors of the high affinity α1-adrenoceptor ligand (1). The two enantiomers of 1 were subsequently [¹¹C] radiolabelled and evaluated for brain uptake and binding by PET imaging in Danish Landrace pigs. (S)-[¹¹C]-1 and (R)-[¹¹C]-1 showed very limited brain uptake. Pre-treatment with cyclosporine A (CsA) resulted in a large increase in brain uptake, indicating that (R)-[¹¹C]-1 is a substrate for active efflux-transporters. This was confirmed in Madin Darby canine kidney (MDCK) cells overexpressing permeability glycoprotein (Pgp). In conclusion, the limited brain uptake of both (S)-[¹¹C]-1 and (R)-[¹¹C]-1 in the pig brain necessitates the search for alternative radioligands for in vivo PET brain imaging of α1-adrenoceptors.