Current radiosynthesis strategies for 5-HT2Areceptor PET tracers

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

The role of neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects millions of people worldwide. Two hallmarks of PD are the accumulation of alpha-synuclein and the loss of dopaminergic neurons in the brain. There is no cure for PD, and all existing treatments focus on alleviating the symptoms. PD diagnosis is also based on the symptoms, such as abnormalities of movement, mood, and cognition observed in the patients. Molecular imaging methods such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) can detect objective alterations in the neurochemical machinery of the brain and help diagnose and study neurodegenerative diseases. This review addresses the application of functional MRI, PET, and SPECT in PD patients. We provide an overview of the imaging targets, discuss the rationale behind target selection, the agents (tracers) with which the imaging can be performed, and the main findings regarding each target's state in PD. Molecular imaging has proven itself effective in supporting clinical diagnosis of PD and has helped reveal that PD is a heterogeneous disorder, which has important implications for the development of future therapies. However, the application of molecular imaging for early diagnosis of PD or for differentiation between PD and atypical parkinsonisms has remained challenging. The final section of the review is dedicated to new imaging targets with which one can detect the PD-related pathological changes upstream from dopaminergic degeneration. The foremost of those targets is alpha-synuclein. We discuss the progress of tracer development achieved so far and challenges on the path toward alpha-synuclein imaging in humans.

Machine learning vs. field 3D-QSAR models for serotonin 2A receptor psychoactive substances identification

Serotonergic psychedelics, substances exerting their effects primarily through the serotonin 2A receptor (5HT2AR), continue to comprise a substantial portion of reported new psychoactive substances (NPS). In this paper five quantitative structure-activity relationship (QSAR) models for predicting the affinity of 5-HT2AR ligands have been developed. The resulting models, exploiting the accessibility of the QSAR equations, generate a useful tool for the investigation and identification of unclassified molecules. The models have been built using a set of 375 molecules using Forge software, and the quality was confirmed by statistical analysis, resulting in effective tools with respect to their predictive and descriptive capabilities. The best performing algorithm among the machine learning approaches and the classical field 3D-QSAR model were then combined to produce a consensus model and were exploited, together with a pharmacophorefilter, to explore the 5-HT2AR activity of 523 105 natural products, to classify a set of recently reported 5-HT2AR NPS and to design new potential active molecules. The findings of this study should facilitate the identification and classification of emerging 5-HT2AR ligands including NPS.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Kinetic analysis of [18F] altanserin bolus injection in the canine brain using PET imaging

BACKGROUND

Currently, [¹⁸F] altanserin is the most frequently used PET-radioligand for serotonin_2A (5-HT_2A) receptor imaging in the human brain but has never been validated in dogs. In vivo imaging of this receptor in the canine brain could improve diagnosis and therapy of several behavioural disorders in dogs. Furthermore, since dogs are considered as a valuable animal model for human psychiatric disorders, the ability to image this receptor in dogs could help to increase our understanding of the pathophysiology of these diseases. Therefore, five healthy laboratory beagles underwent a 90-min dynamic PET scan with arterial blood sampling after [¹⁸F] altanserin bolus injection. Compartmental modelling using metabolite corrected arterial input functions was compared with reference tissue modelling with the cerebellum as reference region.

RESULTS

The distribution of [¹⁸F] altanserin in the canine brain corresponded well to the distribution of 5-HT_2A receptors in human and rodent studies. The kinetics could be best described by a 2-Tissue compartment (2-TC) model. All reference tissue models were highly correlated with the 2-TC model, indicating compartmental modelling can be replaced by reference tissue models to avoid arterial blood sampling.

CONCLUSIONS

This study demonstrates that [¹⁸F] altanserin PET is a reliable tool to visualize and quantify the 5-HT_2A receptor in the canine brain.

Towards selective CNS PET imaging of the 5-HT7 receptor system: Past, present and future

Since its discovery in 1993, the serotonin receptor subtype 7 (5-HT₇) has attracted significant attention as a potential drug target; due to its elucidated roles in conditions such as insomnia, schizophrenia, and more. Therefore, it is unsurprising that there has been relatively early efforts undertaken to develop a positron emission tomography (PET) imaging agent for said receptor system. PET can be clinically used to probe receptor systems in vivo, permitting information such as a drug's occupancy against this system to be investigated. This review focuses on the efforts towards the development of a 5-HT₇R selective PET CNS tracer over the last 20 years, critically reflecting on applied strategies and commonly employed chemical frameworks and suggests future considerations that are needed to successfully develop a PET tracer for this clinically relevant target. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Characterization of the serotonin 2A receptor selective PET tracer (R)-[18F]MH.MZ in the human brain

PURPOSE

The serotonin receptor subtype 2A antagonist (5-HT_2AR) (R)-[¹⁸F]MH.MZ has in preclinical studies been identified as a promising PET imaging agent for quantification of cerebral 5-HT_2ARs. It displays a very similar selectivity profile as [¹¹C]MDL 100907, one of the most selective compounds identified thus far for the 5-HT_2AR. As [¹¹C]MDL 100907, (R)-[¹⁸F]MH.MZ also displays slow brain kinetics in various animal models; however, the half-life of fluorine-18 allows for long scan times and consequently, a more precise determination of 5-HT_2AR binding could still be feasible. In this study, we aimed to evaluate the potential of (R)-[¹⁸F]MH.MZ PET to image and quantify the 5-HT_2AR in the human brain in vivo.

METHODS

Nine healthy volunteers underwent (R)-[¹⁸F]MH.MZ PET at baseline and four out of these also received a second PET scan, after ketanserin pretreatment. Regional time-activity curves of 17 brain regions were analyzed before and after pretreatment. We also investigated radiometabolism, time-dependent stability of outcomes measures, specificity of (R)-[¹⁸F]MH.MZ 5-HT_2AR binding, and performance of different kinetic modeling approaches.

RESULTS

Highest uptake was determined in 5-HT_2AR rich regions with a BP_ND of approximately 1.5 in cortex regions. No radiometabolism was observed. 1TCM and 2TCM resulted in similar outcome measure, whereas reference tissue models resulted in a small, but predictable bias. (R)-[¹⁸F]MH.MZ binding conformed to the known distribution of 5-HT_2AR and could be blocked by pretreatment with ketanserin. Moreover, outcomes measures were stable after 100-110 min.

CONCLUSION

(R)-[¹⁸F]MH.MZ is a suitable PET tracer to image and quantify the 5-HT_2AR system in humans. In comparison with [¹¹C]MDL 100907, faster and more precise outcome measure could be obtained using (R)-[¹⁸F]MH.MZ. We believe that (R)-[¹⁸F]MH.MZ has the potential to become the antagonist radiotracer of choice to investigate the human 5-HT_2AR system.

In vivo PET Imaging of [11C]CIMBI-5, a 5-HT2AR Agonist Radiotracer in Nonhuman Primates

PURPOSE

5-HT2AR exists in high and low affinity states. Agonist PET tracers measure binding to the active high affinity site and thus provide a functionally relevant measure of the receptor. Limited in vivo data have been reported so far for a comparison of agonist versus antagonist tracers for 5-HT2AR used as a proof of principle for measurement of high and low affinity states of this receptor. We compared the in vivo binding of [11C]CIMBI-5, a 5-HT2AR agonist, and of the antagonist [11C]M100907, in monkeys and baboons.

METHODS

[11C]CIMBI-5 and [11C]M100907 baseline PET scans were performed in anesthetized male baboons (n=2) and male vervet monkeys (n=2) with an ECAT EXACT HR+ and GE 64-slice PET/CT Discovery VCT scanners. Blocking studies were performed in vervet monkeys by pretreatment with MDL100907 (0.5 mg/kg, i.v.) 60 minutes prior to the scan. Regional distribution volumes and binding potentials were calculated for each ROI using the likelihood estimation in graphical analysis and Logan plot, with either plasma input function or reference region as input, and simplified reference tissue model approaches.

RESULTS

PET imaging of [11C]CIMBI-5 in baboons and monkeys showed the highest binding in 5-HT2AR-rich cortical regions, while the lowest binding was observed in cerebellum, consistent with the expected distribution of 5-HT2AR. Very low free fractions and rapid metabolism were observed for [11C]CIMBI-5 in baboon plasma. Binding potential values for [11C]CIMBI-5 were 25-33% lower than those for [11C]MDL100907 in the considered brain regions.

CONCLUSION

The lower binding potential of [11C]CIMBI-5 in comparison to [11C]MDL100907 is likely due to the preferential binding of the former to the high affinity site in vivo in contrast to the antagonist,  [11C]MDL100907, which binds to both high and low affinity sites.

Classics in Neuroimaging: The Serotonergic 2A Receptor System-from Discovery to Modern Molecular Imaging

Already in 1953, Woolley and Shaw speculated that serotonin could be involved in a range of central nervous system (CNS) disorders. Lysergic acid diethylamide (LSD) displayed an important role in this respect. It was used not only to antagonize biological effects of serotonin and to study the system itself, but also to identify serotonergic subtype receptors. The 5-HT_2A receptor was discovered in the 1970s and identified as the responsible receptor mediating psychedelic effects of LSD. The development of positron emission tomography (PET) allowed to study this receptor system in vivo. Parameters such as abundance of 5-HT_2A neuroreceptors or receptor occupancy can be determined using PET. As such, the development of 5-HT_2A receptor tracers started immediately after the introduction of PET in the mid-1970s. In this Viewpoint, we provide a historical overview from the discovery of serotonin to the identification of the 5-HT_2A receptor subtype and the subsequent development of 5-HT_2A receptor subtype specific PET tracers over the last four decades. We emphasize the interplay between pharmacology, medicinal chemistry, radiochemistry, and nuclear medicine that is important while developing a PET tracer. Moreover, we highlight selected examples applying 5-HT_2A receptor PET tracers within neurological diseases and drug occupancy studies.

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-HT_2A 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-HT_2A receptor (5-HT_2A 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-HT_2A 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-HT_2A R; a notable exception however being significant uptake in the striatum and thalamus. Additionally, a within-scan displacement challenge with a 5-HT_2A R antagonist was unsuccessful, indicating that the tracers cannot be considered optimal for neuroimaging of the 5-HT_2A R.

Small Molecule Receptor Ligands for PET Studies of the Central Nervous System-Focus on G Protein Coupled Receptors

G protein-coupled receptors (GPRCs) are a class of proteins that are expressed in high abundance and are responsible for numerous signal transduction pathways in the central nervous system. Consequently, alterations in GPRC function have been associated with a wide variety of neurologic and neuropsychiatric disorders. The development of PET probes for imaging GPRCs has served as a major emphasis of PET radiotracer development and PET imaging studies over the past 30 years. In this review, a basic description of the biology of G proteins and GPRCs is provided. This includes recent evidence of the existence of dimeric and multimeric species of GPRCs that have been termed "receptor mosaics," with an emphasis on the different GPRCs that form complexes with the dopamine D2 receptor. An overview of the different PET radiotracers for imaging the component GPRC within these different multimeric complexes of the D2 receptor is also provided.

In vivo evaluation of [18F]FECIMBI-36, an agonist 5-HT2A/2C receptor PET radioligand in nonhuman primate

We recently reported the radiosynthesis and in vitro evaluation of [18F]-2-(4-bromo-2,5-dimethoxyphenyl)-N-(2-(2-fluoroethoxy)benzyl)ethanamine, ([18F]FECIMBI-36) or ([18F]1), an agonist radioligand for 5HT2A/2C receptors in postmortem samples of human brain. Herein we describe the in vivo evaluation of [18F]FECIMBI-36 in vervet/African green monkeys by PET imaging. PET images show that [18F]FECIMBI-36 penetrates the blood-brain barrier and a low retention of radioactivity is observed in monkey brain. Although the time activity curves indicate a somehow heterogeneous distribution of the radioligand in the brain, the low level of [18F]FECIMBI-36 in brain may limit the use of this tracer for quantification of 5-HT2A/2C receptors by PET.