Synaptic density in aging mice measured by [18F]SynVesT-1 PET.

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and in aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [18F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57BL/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 7)) were PET scanned with [18F]SynVesT-1. Brain retention of [18F]SynVesT-1 expressed as the volume of distribution (VIDIF) was calculated using an image-derived input function. Estimates of VIDIF were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [18F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [18F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [18F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.

Radiosynthesis and preclinical evaluation of [11 C]Cimbi-701 - Towards the imaging of cerebral 5-HT7 receptors.

The serotonin 7 (5-HT7 ) receptor is suggested to be involved in a broad variety of CNS disorders, but very few in vivo tools exist to study this important target. Molecular imaging with positron emission tomography (PET) would enable an in vivo characterization of the 5-HT7 receptor. However, no clinical PET radiotracer exists for this receptor, and thus we aimed to develop such a tracer. In this study, we present the preclinical evaluation of [11 C]Cimbi-701. Cimbi-701 was synthesized in a one-step procedure starting from SB-269970. Its selectivity profile was determined using an academic screening platform (NIMH Psychoactive Drug Screening Program). Successful radiolabeling of [11 C]Cimbi-701 and subsequent in vivo evaluation was conducted in rats, pigs and baboon. In vivo specificity was investigated by 5-HT7 and σ receptor blocking studies. P-gp efflux transporter dependency was investigated using elacridar. [11 C]Cimbi-701 could successfully be synthesized. Selectivity profiling revealed high affinity for the 5-HT7 (Ki = 18 nM), σ-1 (Ki = 9.2 nM) and σ-2 (Ki = 1.6 nM) receptors. In rats, [11 C]Cimbi-701 acted as a strong P-gp substrate. After P-gp inhibition, rat brain uptake could specifically be blocked by 5-HT7 and σ receptor ligands. In pig, high brain uptake and specific 5-HT7 and σ-receptor binding was found for [11 C]Cimbi-701 without P-gp inhibition. Finally, low brain uptake was found in baboons. Both the specific σ-receptor binding and the low brain uptake of [11 C]Cimbi-701 displayed in baboon discouraged further translation to humans. Instead, we suggest exploration of this structural class as results indicate that selective 5-HT7 receptor imaging might be possible when more selective non-P-gp substrates are identified.

In Vitro and In Vivo Characterization of Dibenzothiophene Derivatives [125I]Iodo-ASEM and [18F]ASEM as Radiotracers of Homo- and Heteromeric α7 Nicotinic Acetylcholine Receptors.

The α7 nicotinic acetylcholine receptor (α7 nAChR) is involved in several cognitive and physiologic processes; its expression levels and patterns change in neurologic and psychiatric diseases, such as schizophrenia and Alzheimer's disease, which makes it a relevant drug target. Development of selective radioligands is important for defining binding properties and occupancy of novel molecules targeting the receptor. We tested the in vitro binding properties of [125I]Iodo-ASEM [(3-(1,4-diazabycyclo[3.2.2]nonan-4-yl)-6-(125I-iododibenzo[b,d]thiopentene 5,5-dioxide)] in the mouse, rat and pig brain using autoradiography. The in vivo binding properties of [18F]ASEM were investigated using positron emission tomography (PET) in the pig brain. [125I]Iodo-ASEM showed specific and displaceable high affinity (~1 nM) binding in mouse, rat, and pig brain. Binding pattern overlapped with [125I]α-bungarotoxin, specific binding was absent in α7 nAChR gene-deficient mice and binding was blocked by a range of α7 nAChR orthosteric modulators in an affinity-dependent order in the pig brain. Interestingly, relative to the wild-type, binding in ß2 nAChR gene-deficient mice was lower for [125I]Iodo-ASEM (58% ± 2.7%) than [125I]α-bungarotoxin (23% ± 0.2%), potentially indicating different binding properties to heteromeric α7ß2 nAChR. [18F]ASEM PET in the pig showed high brain uptake and reversible tracer kinetics with a similar spatial distribution as previously reported for α7 nAChR. Blocking with SSR-180,711 resulted in a significant decrease in [18F]ASEM binding. Our findings indicate that [125I]Iodo-ASEM allows sensitive and selective imaging of α7 nAChR in vitro, with better signal-to-noise ratio than previous tracers. Preliminary data of [18F]ASEM in the pig brain demonstrated principal suitable kinetic properties for in vivo quantification of α7 nAChR, comparable to previously published data.

Migraine is associated with high brain 5-HT levels as indexed by 5-HT4 receptor binding.

INTRODUCTION: Serotonin (5-HT) plays a role in migraine pathophysiology, but whether brain 5-HT is involved in the conversion from episodic to chronic migraine is unknown. Here, we investigated brain 5-HT levels, as indexed by 5-HT4 receptor binding, in chronic migraine patients and evaluated whether these were associated with migraine frequency. METHODS: Sixteen chronic migraine patients underwent a dynamic PET scan after injection of [11C]SB207145, a specific 5-HT4 receptor radioligand. Data from 15 episodic migraine patients and 16 controls were included for comparison. Quantification of 5-HT4 receptor binding was used as a proxy for brain 5-HT levels, since 5-HT4 receptor binding is inversely related to brain 5-HT levels. RESULTS: Chronic migraine patients had 9.1% (95% CI: [-17%; -1.0%]) lower 5-HT4 receptor binding compared to controls ( p = 0.039). There was no difference in 5-HT4 receptor binding between chronic and episodic migraine patients ( p = 0.48) and no association between number of monthly migraine days and 5-HT4 receptor binding (slope estimate 0.003, 95% CI: [-0.004; 0.715], p = 0.39). CONCLUSION: The finding of low 5-HT4 receptor binding suggests that cerebral levels of 5-HT are elevated in chronic migraine patients. This is in line with observations made in patients with episodic migraine. Elevated brain 5-HT levels may thus be an inherent trait of the migraine brain rather than a risk factor for conversion from episodic to chronic migraine.

Improved radiosynthesis and preliminary in vivo evaluation of the 11C-labeled tetrazine [11C]AE-1 for pretargeted PET imaging.

Pretargeted nuclear imaging based on the ligation between tetrazines and nano-sized targeting agents functionalized with trans-cyclooctene (TCO) has recently been shown to improve both imaging contrast and dosimetry in nuclear imaging of nanomedicines. Herein, we describe the improved radiosynthesis of a 11C-labeled tetrazine ([11C]AE-1) and its preliminary evaluation in both mice and pigs. Pretargeted imaging in mice was carried out using both a new TCO-functionalized polyglutamic acid and a previously reported TCO-functionalized bisphosphonate system as targeting agents. Unfortunately, pretargeted imaging was not successful using these targeting agents in pair with [11C]AE-1. However, brain imaging in pig indicated that the tracer crossed the blood-brain-barrier. Hence, we suggest that this tetrazine scaffold could be used as a starting point for the development of pretargeted brain imaging, which has so far been a challenging task.

Evaluation of [18 F]2FP3 in pigs and non-human primates.

So far, no suitable 5-HT7 R radioligand exists for clinical positron emission tomography (PET) imaging. [18 F]2FP3 was first tested in vivo in cats, and the results were promising for further evaluations. Here, we evaluate the radioligand in pigs and non-human primates (NHPs). Furthermore, we investigate species differences in 5-HT7 R binding with [3 H]SB-269970 autoradiography in post-mortem pig, NHP, and human brain tissue. Specific binding of [18 F]2FP3 was investigated by intravenous administration of the 5-HT7 R specific antagonist SB-269970. [3 H]SB-269970 autoradiography was performed as previously described. [18 F]2FP3 was synthesized in an overall yield of 35% to 45%. High brain uptake of the tracer was found in both pigs and NHPs; however, pretreatment with SB-269970 only resulted in decreased binding of 20% in the thalamus, a 5-HT7 R-rich region. Autoradiography on post-mortem pig, NHP, and human tissues revealed that specific binding of [3 H]SB-269970 was comparable in the thalamus of pig and NHP. Despite the high uptake of [18 F]2FP3 in both species, the binding could only be blocked to a limited degree with the 5-HT7 R antagonists. We speculate that the affinity of the radioligand is too low for imaging the 5-HT7 Rs in vivo and that part of the PET signal arises from targets other than the 5-HT7 R.

Functional Characterization of 5-HT1B Receptor Drugs in Nonhuman Primates Using Simultaneous PET-MR.

In the present study, we used a simultaneous PET-MR experimental design to investigate the effects of functionally different compounds (agonist, partial agonist, and antagonist) on 5-HT1B receptor (5-HT1BR) occupancy and the associated hemodynamic responses. In anesthetized male nonhuman primates (n = 3), we used positron emission tomography (PET) imaging with the radioligand [11C]AZ10419369 administered as a bolus followed by constant infusion to measure changes in 5-HT1BR occupancy. Simultaneously, we measured changes in cerebral blood volume (CBV) as a proxy of drug effects on neuronal activity. The 5-HT1BR partial agonist AZ10419369 elicited a dose-dependent biphasic hemodynamic response that was related to the 5-HT1BR occupancy. The magnitude of the response was spatially overlapping with high cerebral 5-HT1BR densities. High doses of AZ10419369 exerted an extracranial tissue vasoconstriction that was comparable to the less blood-brain barrier-permeable 5-HT1BR agonist sumatriptan. By contrast, injection of the antagonist GR127935 did not elicit significant hemodynamic responses, even at a 5-HT1BR cerebral occupancy similar to the one obtained with a high dose of AZ10419369. Given the knowledge we have of the 5-HT1BR and its function and distribution in the brain, the hemodynamic response informs us about the functionality of the given drug: changes in CBV are only produced when the receptor is stimulated by the partial agonist AZ10419369 and not by the antagonist GR127935, consistent with low basal occupancy by endogenous serotonin.SIGNIFICANCE STATEMENT We here show that combined simultaneous positron emission tomography and magnetic resonance imaging uniquely enables the assessment of CNS active compounds. We conducted a series of pharmacological interventions to interrogate 5-HT1B receptor binding and function and determined blood-brain barrier passage of drugs and demonstrate target involvement. Importantly, we show how the spatial and temporal effects on brain hemodynamics provide information about pharmacologically driven downstream CNS drug effects; the brain hemodynamic response shows characteristic dose-related effects that differ depending on agonistic or antagonistic drug characteristics and on local 5-HT1B receptor density. The technique lends itself to a comprehensive in vivo investigation and understanding of drugs' effects in the brain.

Low 5-HT1B receptor binding in the migraine brain: A PET study.

Background The pathophysiology of migraine may involve dysfunction of serotonergic signaling. In particular, the 5-HT1B receptor is considered a key player due to the efficacy of 5-HT1B receptor agonists for treatment of migraine attacks. Aim To examine the cerebral 5-HT1B receptor binding in interictal migraine patients without aura compared to controls. Methods Eighteen migraine patients, who had been migraine free for >48 hours, and 16 controls were scanned after injection of the 5-HT1B receptor specific radioligand [11C]AZ10419369 for quantification of cerebral 5-HT1B receptor binding. Patients who reported migraine <48 hours after the PET examination were excluded from the final analysis. We defined seven brain regions involved in pain modulation as regions of interest and applied a latent variable model (LVM) to assess the group effect on binding across these regions. Results Our data support a model wherein group status predicts the latent variable ( p = 0.038), with migraine patients having lower 5-HT1B receptor binding across regions compared to controls. Further, in a whole-brain voxel-based analysis, time since last migraine attack correlated positively with 5-HT1B receptor binding in the dorsal raphe and in the midbrain. Conclusion We report here for the first time that migraine patients have low 5-HT1B receptor binding in pain modulating regions, reflecting decreased receptor density. This is either a primary constitutive trait of the migraine brain or secondary to repeated exposure to migraine attacks. We also provide indirect support for the dorsal raphe 5-HT1B receptors being temporarily downregulated during the migraine attack, presumably in response to higher cerebral serotonin levels in the ictal phase.

Synthesis, radiofluorination, and preliminary evaluation of the potential 5-HT2A receptor agonists [18 F]Cimbi-92 and [18 F]Cimbi-150.

An agonist PET tracer is of key interest for the imaging of the 5-HT2A receptor, as exemplified by the previously reported success of [11 C]Cimbi-36. Fluorine-18 holds several advantages over carbon-11, making it the radionuclide of choice for clinical purposes. In this respect, an 18 F-labelled agonist 5-HT2A receptor (5-HT2A R) tracer is highly sought after. Herein, we report a 2-step, 1-pot labelling methodology of 2 tracer candidates. Both ligands display high in vitro affinities for the 5-HT2A R. The compounds were synthesised from easily accessible labelling precursors, and radiolabelled in acceptable radiochemical yields, sufficient for in vivo studies in domestic pigs. PET images partially conformed to the expected brain distribution of the 5-HT2A R; a notable exception however being significant uptake in the striatum and thalamus. Additionally, a within-scan displacement challenge with a 5-HT2A R antagonist was unsuccessful, indicating that the tracers cannot be considered optimal for neuroimaging of the 5-HT2A R.

A regularized full reference tissue model for PET neuroreceptor mapping.

The full reference tissue model (FRTM) is a PET analysis framework that includes both free and specifically bound compartments within tissues, together with rate constants defining association and dissociation from the specifically bound compartment. The simplified reference tissue model (SRTM) assumes instantaneous exchange between tissue compartments, and this "1-tissue" approximation reduces the number of parameters and enables more robust mapping of non-displaceable binding potentials. Simulations based upon FRTM have shown that SRTM exhibits biases that are spatially dependent, because biases depend upon binding potentials. In this work, we describe a regularized model (rFRTM) that employs a global estimate of the dissociation rate constant from the specifically bound compartment (k4). The model provides an internal calibration for optimizing k4 through the reference-region outflow rate k2', a model parameter that should be a global constant but varies regionally in SRTM. Estimates of k4 by rFRTM are presented for four PET radioligands. We show that SRTM introduces bias in parameter estimates by assuming an infinite value for k4, and that rFRTM ameliorates bias with an appropriate choice of k4. Theoretical considerations and simulations demonstrate that rFRTM reduces bias in non-displaceable binding potentials. A two-parameter reduction of the model (rFRTM2) provides robust mapping at a voxel-wise level. With a structure similar to SRTM, the model is easily implemented and can be applied as a PET reference region analysis that reduces parameter bias without substantially altering parameter variance.

The Center for Integrated Molecular Brain Imaging (Cimbi) database.

We here describe a multimodality neuroimaging containing data from healthy volunteers and patients, acquired within the Lundbeck Foundation Center for Integrated Molecular Brain Imaging (Cimbi) in Copenhagen, Denmark. The data is of particular relevance for neurobiological research questions related to the serotonergic transmitter system with its normative data on the serotonergic subtype receptors 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT4 and the 5-HT transporter (5-HTT), but can easily serve other purposes. The Cimbi database and Cimbi biobank were formally established in 2008 with the purpose to store the wealth of Cimbi-acquired data in a highly structured and standardized manner in accordance with the regulations issued by the Danish Data Protection Agency as well as to provide a quality-controlled resource for future hypothesis-generating and hypothesis-driven studies. The Cimbi database currently comprises a total of 1100 PET and 1000 structural and functional MRI scans and it holds a multitude of additional data, such as genetic and biochemical data, and scores from 17 self-reported questionnaires and from 11 neuropsychological paper/computer tests. The database associated Cimbi biobank currently contains blood and in some instances saliva samples from about 500 healthy volunteers and 300 patients with e.g., major depression, dementia, substance abuse, obesity, and impulsive aggression. Data continue to be added to the Cimbi database and biobank.

Metabolic Fate of Hallucinogenic NBOMes.

2,5-Dimethoxy-N-benzylphenethylamines (NBOMes) are very potent 5-HT2AR agonists. Illicit use of these psychedelic compounds has emerged in recent years, and several fatalities have been linked to their recreational use. In its [(11)C]-labeled form, one NBOMe (25B-NBOMe) was recently developed as a PET-ligand for clinical investigations of 5HT2AR ([(11)C]Cimbi-36). Herein, we have identified the phase I and phase II metabolites of 25B-NBOMe in pigs as well as in humans. We find that the primary route of metabolism is 5'-demethylation, followed by conjugation to glucuronic acid. Carbon-11 labeling of 25B-NBOMe in three different positions followed by in vivo evaluation in pigs and humans corroborated these findings.

(11)C-labeling and preliminary evaluation of pimavanserin as a 5-HT2A receptor PET-radioligand.

Pimavanserin is a selective serotonin 2A receptor (5-HT2AR) inverse agonist that has shown promise for treatment of psychotic symptoms in patients with Parkinson's disease. Here, we detail the (11)C-labeling and subsequently evaluate pimavanserin as a PET-radioligand in pigs. [(11)C]Pimavanserin was obtained by N-methylation of an appropriate precursor using [(11)C]MeOTf in acetone at 60°C giving radiochemical yields in the range of 1-1.7GBq (n=4). In Danish Landrace pigs the radio ligand readily entered the brain and displayed binding in the cortex in accordance with the distribution of 5-HT2ARs. However, this binding could not be blocked by either ketanserin or pimavanserin itself, indicating high nonspecific binding. The lack of displacement by the 5-HT2R antagonist and binding in the thalamus suggests that [(11)C]pimavanserin is not selective for the 5-HT2AR in pigs.

Labeling and preliminary in vivo evaluation of the 5-HT(7) receptor selective agonist [(11)C]E-55888.

E-55888 has been identified as a selective serotonin 7 (5-HT7) receptor agonist. In this study, we describe the synthesis, radiolabeling and in vivo evaluation of [(11)C]E-55888 as a radioligand for positron emission tomography (PET) imaging. [(11)C]E-55888 was obtained by N-methylation of an appropriate precursor using [(11)C]MeOTf in acetone at 60 °C giving isolated quantities in the range of 1.7-2.4 GBq. Studies in Danish Landrace pigs demonstrated that [(11)C]E-55888 has good brain uptake, however, the distribution in the brain tissue was dominated by non-specific binding, as binding could neither be displaced by the structurally different 5-HT7 receptor ligand SB-269970 nor by self-block with unlabeled E-55888. Based on these data, [(11)C]E-55888 does not show promise as a PET radioligand for imaging the 5-HT7 receptor in vivo.

(11)C-labeling and preliminary evaluation of vortioxetine as a PET radioligand.

Vortioxetine is a new multi-modal drug against major depressive disorder with high affinity for a range of different serotonergic targets in the CNS. We report the (11)C-labeling of vortioxetine with [(11)C]MeI using a Suzuki-protocol that allows for the presence of an unprotected amine. Preliminary evaluation of [(11)C]vortioxetine in a Danish Landrace pig showed rapid brain uptake and brain distribution in accordance with the pharmacological profile, all though an unexpected high binding in cerebellum was also observed. [(11)C]vortioxetine displayed slow tracer kinetics with peak uptake after 60 min and with limited wash-out from the brain. Further studies are needed but this radioligand may prove to be a valuable tool in unraveling the clinical effects of vortioxetine.

Design, synthesis, radiolabeling and in vivo evaluation of potential positron emission tomography (PET) radioligands for brain imaging of the 5-HT7 receptor.

Here we describe the design, synthesis, and pharmacological evaluation of a set of compounds structurally related to the high affinity serotonin 5-HT7 receptor agonist N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (6, LP-211). Specific structural modifications were performed in order to maintain affinity for the target receptor and to improve the selectivity over 5-HT1A and adrenergic α1 receptors. The synthesized compounds have chemical features that could enable labeling with a positron emitter radioisotope (carbon-11 or fluorine-18) and lipophilicity within the range considered optimal for brain penetration and low non-specific binding. 4-[2-(4-Methoxyphenyl)phenyl]-N-(pyridin-4-ylmethyl)piperazinehexanamide (23a) and N-pyridin-4-ylmethyl-3-[4-[2-(4-methoxyphenyl)phenyl]piperazin-1-yl]ethoxy]propanamide (26a) were radiolabeled on the methoxy group with carbon-11. Positron emission tomography (PET) analysis revealed that [(11)C]-23a and [(11)C]-26a were P-glycoprotein (P-gp) substrates and rapidly metabolized, resulting in poor brain uptake. These features were not predicted by in vitro tests.

Acute Effects of Hallucinogens on Functional Connectivity: Psilocybin and Salvinorin-A.

The extent of changes in functional connectivity (FC) within functional networks as a common feature across hallucinogenic drug classes is under-explored. This work utilized fMRI to assess the dissociative hallucinogens Psilocybin, a classical serotonergic psychedelic, and Salvinorin-A, a kappa-opioid receptor (KOR) agonist, on resting-state FC in nonhuman primates. We highlight overlapping and differing influence of these substances on FC relative to the thalamus, claustrum, prefrontal cortex (PFC), default mode network (DMN), and DMN subcomponents. Analysis was conducted on a within-subject basis. Findings support the cortico-claustro-cortical network model for probing functional effects of hallucinogens regardless of serotonergic potential, with a potential key paradigm centered around the claustrum, PFC, anterior cingulate cortices (ACC), and angular gyrus relationship. Thalamo-cortical networks are implicated but appear dependent on 5-HT2AR activation. Acute desynchronization relative to the DMN for both drugs was also shown. Our findings provide a framework to understand broader mechanisms at which hallucinogens in differing classes may impact subjects regardless of the target receptor.

Direct comparison of [(18) F]MH.MZ and [(18) F] altanserin for 5-HT2A receptor imaging with PET.

Imaging the cerebral serotonin 2A (5-HT2A ) receptors with positron emission tomography (PET) has been carried out in humans with [(11) C]MDL 100907 and [(18) F]altanserin. Recently, the MDL 100907 analogue [(18) F]MH.MZ was developed combining the selectivity profile of MDL 100907 and the favourable radiophysical properties of fluorine-18. Here, we present a direct comparison of [(18) F]altanserin and [(18) F]MH.MZ. 5-HT2A receptor binding in pig cortex and cerebellum was investigated by autoradiography with [(3) H]MDL 100907, [(18) F]MH.MZ, and [(18) F]altanserin. [(18) F]MH.MZ and [(18) F]altanserin were investigated in Danish Landrace pigs by brain PET scanning at baseline and after i.v. administration of blocking doses of ketanserin. Full arterial input function and high performance liquid chromatography (HPLC) analysis allowed for tissue-compartment kinetic modeling of PET data. In vitro autoradiography showed high binding in cortical regions with both [(18) F]MH.MZ and [(18) F]altanserin. Significant 5-HT2A receptor binding was also found in the pig cerebellum, thus making this region unsuitable as a reference region for in vivo data analysis in this species. The cortical binding of [(18) F]MH.MZ and [(18) F]altanserin was blocked by ketanserin supporting that both radioligands bind to 5-HT2A receptors in the pig brain. In the HPLC analysis of pig plasma, [(18) F]MH.MZ displayed a fast and reproducible metabolism resulting in hydrophilic radiometabolites only whereas the metabolic profile of [(18) F]altanserin as expected showed lipophilic radiometabolites. Due to the slow kinetics of [(18) F]MH.MZ in high-binding regions in vivo, we suggest that [(18) F]MH.MZ will be an appropriate tracer for low binding regions where kinetics will be faster, whereas [(18) F]altanserin is a suitable tracer for high-binding regions.

No change in [¹¹C]CUMI-101 binding to 5-HT(1A) receptors after intravenous citalopram in human.

The main objective of this study was to determine the sensitivity of [¹¹C]CUMI-101 to citalopram challenge aiming at increasing extracellular 5-HT. CUMI-101 has agonistic properties in human embryonic kidney 293 cells transfected with human recombinant 5-HT(1A) receptors (Hendry et al. [2011] Nucl Med Biol 38:273-277; Kumar et al. [2006] J Med Chem 49:125-134) and has previously been demonstrated to be sensitive to bolus citalopram in monkeys (Milak et al. [2011] J Cereb Blood Flow Metab 31:243-249). We studied six healthy individuals. Two PET-scans were performed on the same day in each individual before and after constant infusion of citalopram (0.15 mg/kg). The imaging data were analyzed using two tissue compartment kinetic modeling with metabolite corrected arterial input and Simplified Reference Tissue Modeling using cerebellum as a reference region. There was no significant difference in regional distribution volume or non-displaceable binding potential values before and after citalopram infusion. The mean receptor occupancy was 0.03 (range -0.14 to 0.17). Our data imply that [¹¹C]CUMI-101 binding is not sensitive to citalopram infusion in humans.

Synthesis and evaluation of [¹¹C]Cimbi-806 as a potential PET ligand for 5-HT7 receptor imaging.

2-(2',6'-Dimethoxy-[1,1'-biphenyl]-3-yl)-N,N-dimethylethanamine has been identified as a potent ligand for the serotonin 7 (5-HT(7)) receptor. In this study, we describe the synthesis, radiolabeling and in vivo evaluation of [(11)C]2-(2',6'-dimethoxy-[1,1'-biphenyl]-3-yl)-N,N-dimethylethanamine ([(11)C]Cimbi-806) as a radioligand for imaging brain 5-HT(7) receptors with positron emission tomography (PET). Precursor and reference compound was synthesized and subsequent (11)C-labelling with [(11)C]methyltriflate produced [(11)C]Cimbi-806 in specific activities ranging from 50 to 300 GBq/µmol. Following intravenous injection, brain uptake and distribution of [(11)C]Cimbi-806 was assessed with PET in Danish Landrace pigs. The time-activity curves revealed high brain uptake in thalamic and striatal regions (SUV ∼2.5) and kinetic modeling resulted in distribution volumes (V(T)) ranging from 6 mL/cm(3) in the cerebellum to 12 mL/cm(3) in the thalamus. Pretreatment with the 5-HT(7) receptor antagonist SB-269970 did not result in any significant changes in [(11)C]Cimbi-806 binding in any of the analyzed regions. Despite the high brain uptake and relevant distribution pattern, the absence of appropriate in vivo blocking with a 5-HT(7) receptor selective compounds renders the conclusion that [(11)C]Cimbi-806 is not an appropriate PET radioligand for imaging the 5-HT(7) receptor in vivo.