Radiosynthesis and Evaluation of 11C-CIMBI-5 as a 5-HT2A Receptor Agonist Radioligand for PET

25X-NBOMe compounds - chemistry, pharmacology and toxicology. A comprehensive review

Recently, a growing number of reports have indicated a positive effect of hallucinogenic-based therapies in different neuropsychiatric disorders. However, hallucinogens belonging to the group of new psychoactive substances (NPS) may produce high toxicity. NPS, due to their multi-receptors affinity, are extremely dangerous for the human body and mental health. An example of hallucinogens that have been lately responsible for many severe intoxications and deaths are 25X-NBOMes - N-(2-methoxybenzyl)-2,5-dimethoxy-4-substituted phenethylamines, synthetic compounds with strong hallucinogenic properties. 25X-NBOMes exhibit a high binding affinity to serotonin receptors but also to dopamine, adrenergic and histamine receptors. Apart from their influence on perception, many case reports point out systemic and neurological poisoning with these compounds. In humans, the most frequent side effects are tachycardia, anxiety, hypertension and seizures. Moreover, preclinical studies confirm that 25X-NBOMes cause developmental impairments, cytotoxicity, cardiovascular toxicity and changes in behavior of animals. Metabolism of NBOMes seems to be very complex and involves many metabolic pathways. This fact may explain the observed high toxicity. In addition, many analytical methods have been applied in order to identify these compounds and their metabolites. The presented review summarized the current knowledge about 25X-NBOMes, especially in the context of toxicity.

30 years of [11C]methyl triflate

Some scientific discoveries are well known only to a core group of researchers working on technical subjects. Nevertheless, they open new research directions, allow existing knowledge to be viewed in entirely new and useful ways, or provide a way to make something that was hard or impossible to make before. Carbon-11 methyl triflate ([¹¹C]MeOTf) is one such advance, facilitating the synthesis of many carbon-11 radio tracers and broadening the range of applications of carbon-11 radiochemistry. The year 2022 marked the 30th anniversary of the original paper in Applied Radiation and Isotopes introducing a simple synthesis of [¹¹C]MeOTf from carbon-11 methyl iodide ([¹¹C]MeI) and it also marked the end of the fruitful career and life of the researcher who developed it, Douglas Jewett. It seems fitting to say a few words on how it came to be and how it has helped advance carbon-11 radiochemistry.

4-EA-NBOMe, an amphetamine derivative, alters glutamatergic synaptic transmission through 5-HT1A receptors on cortical neurons from SpragueDawley rat and pyramidal neurons from C57BL/6 mouse

New psychoactive substances (NPSs) are compounds designed to mimic illegal recreational drugs. In particular, there are difficulties in legal restrictions because there is no fast NPS detection method to suppress the initial spread of NPS with criminal records; thus, they expose the public to serious health threats, including toxicity and dependence. However, the effects of NPSs on the brain and the related cellular mechanisms are well unknown. One of the recently emerging drugs is 4-ethylamphetamine-NBOMe (4-EA-NBOMe), a member of the 2 C phenylalanine family with a similar structure to methamphetamine (methA). In this study, we tested the effect of methA analogs on the glutamatergic synaptic transmission on primary cultured cortical neurons of SpragueDawley (SD) rats and C57BL/6 mice, and also layer 2/3 pyramidal neurons of the medial prefrontal cortex (mPFC) of C57BL/6 mice. We found that acute treatment with 4-EA-NBOMe inhibits spontaneous excitatory postsynaptic currents (EPSCs) and that withdrawal after chronic inhibition by 4-EA-NBOMe augments glutamatergic synaptic transmission. These modifications of synaptic responses are mediated by 5-HT_1A receptors. These findings suggest that 4-EA-NBOMe directly affects the central nervous system by changing the efficacy of glutamatergic synaptic transmission.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Neurotoxicological profile of the hallucinogenic compound 25I-NBOMe

4-Iodo-2,5-dimethoxy-N-(2-methoxybenzyl)phenethylamine (25I-NBOMe) is a new psychoactive substance with strong hallucinogenic properties. Our previous data reported increased release of dopamine, serotonin, and glutamate after acute injections and a tolerance development in the neurotransmitters release and rats’ behavior after chronic treatment with 25I-NBOMe. The recreational use of 25I-NBOMe is associated with severe intoxication and deaths in humans. There is no data about 25I-NBOMe in vivo toxicity towards the brain tissue. In this article 25I-NBOMe-crossing through the blood–brain barrier (BBB), the impact on DNA damage, apoptosis induction, and changes in the number of cortical and hippocampal cells were studied. The presence of 25I-NBOMe in several brain regions shortly after the drug administration and its accumulation after multiple injections was found. The DNA damage was detected 72 h after the chronic treatment. On the contrary, at the same time point apoptotic signal was not identified. A decrease in the number of glial but not in neural cells in the frontal (FC) and medial prefrontal cortex (mPFC) was observed. The obtained data indicate that 25I-NBOMe passes easily across the BBB and accumulates in the brain tissue. Observed oxidative DNA damage may lead to the glial cells’ death.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT_2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca²⁺ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

NBOMes-Highly Potent and Toxic Alternatives of LSD

Recently, a new class of psychedelic compounds named NBOMe (or 25X-NBOMe) has appeared on the illegal drug market. NBOMes are analogs of the 2C family of phenethylamine drugs, originally synthesized by Alexander Shulgin, that contain a N-(2-methoxy)benzyl substituent. The most frequently reported drugs from this group are 25I-NBOMe, 25B-NBOMe, and 25C-NBOMe. NBOMe compounds are ultrapotent and highly efficacious agonists of serotonin 5-HT_2A and 5-HT_2C receptors (Ki values in low nanomolar range) with more than 1000-fold selectivity for 5-HT_2A compared with 5-HT_1A. They display higher affinity for 5-HT_2A receptors than their 2C counterparts and have markedly lower affinity, potency, and efficacy at the 5-HT_2B receptor compared to 5-HT_2A or 5-HT_2C. The drugs are sold as blotter papers, or in powder, liquid, or tablet form, and they are administered sublingually/buccally, intravenously, via nasal insufflations, or by smoking. Since their introduction in the early 2010s, numerous reports have been published on clinical intoxications and fatalities resulting from the consumption of NBOMe compounds. Commonly observed adverse effects include visual and auditory hallucinations, confusion, anxiety, panic and fear, agitation, uncontrollable violent behavior, seizures, excited delirium, and sympathomimetic signs such mydriasis, tachycardia, hypertension, hyperthermia, and diaphoresis. Rhabdomyolysis, disseminated intravascular coagulation, hypoglycemia, metabolic acidosis, and multiorgan failure were also reported. This survey provides an updated overview of the pharmacological properties, pattern of use, metabolism, and desired effects associated with NBOMe use. Special emphasis is given to cases of non-fatal and lethal intoxication involving these compounds. As the analysis of NBOMes in biological materials can be challenging even for laboratories applying modern sensitive techniques, this paper also presents the analytical methods most commonly used for detection and identification of NBOMes and their metabolites.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

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.

Seizures, Systemic Inflammatory Response, and Rhabdomyolysis Associated With Laboratory-Confirmed 2C-I and 25-I Exposure

The 2C drugs are hallucinogenic phenethylamines. They and their n-benzyloxymethyl analogs have become popular as "legal highs," and significant toxicity has been attributed to their use. We report on a case of seizures, systemic inflammatory response, and rhabdomyolysis associated with laboratory-confirmed 4-iodo-2,5-dimethoxyphenethylamine and 4-iodo-2,5-dimethoxy-N-(2-methoxybenzyl) phenethylamine exposure. A 17-year-old male teenager developed seizures after taking "2 strips of acid." The seizures resolved with midazolam, but he became apneic and was intubated. His head computed tomography was unremarkable. Initial creatinine level was 1.5 mg/dL, with a creatine kinase of 112 U/L. His urine immunoassay drug screen was negative. He was extubated within 12 hours but had elevated temperatures for 48 hours. He was treated with antibiotics, but no source of infection was identified. His creatinine level peaked at 2.46 mg/dL. His creatine kinase peaked 72 hours later at 14579 U/L. He was treated with intravenous fluids and did not require renal replacement therapy. He recovered fully and was discharged after 5 days. Serum and urine samples were tested using liquid chromatography time-of-flight mass spectrometry. We detected 4-iodo-2,5-dimethoxyphenethylamine and 4-iodo-2,5-dimethoxy-N-(2-methoxybenzyl) phenethylamine in both serum and urine. No other substances were detected. The 2C drugs and their n-benzyloxymethyl analogs are potent serotonergic agents. Their use has been associated with multiple adverse effects including seizures, rhabdomyolysis, and death. They should be considered in differential diagnosis for drug-induced seizures and as a cause for systemic inflammatory response. This case highlights the significant toxicity seen with these compounds.

From simultaneous to synergistic MR-PET brain imaging: A review of hybrid MR-PET imaging methodologies

Simultaneous Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) scanning is a recent major development in biomedical imaging. The full integration of the PET detector ring and electronics within the MR system has been a technologically challenging design to develop but provides capacity for simultaneous imaging and the potential for new diagnostic and research capability. This article reviews state-of-the-art MR-PET hardware and software, and discusses future developments focusing on neuroimaging methodologies for MR-PET scanning. We particularly focus on the methodologies that lead to an improved synergy between MRI and PET, including optimal data acquisition, PET attenuation and motion correction, and joint image reconstruction and processing methods based on the underlying complementary and mutual information. We further review the current and potential future applications of simultaneous MR-PET in both systems neuroscience and clinical neuroimaging research. We demonstrate a simultaneous data acquisition protocol to highlight new applications of MR-PET neuroimaging research studies.

Chemistry and Structure-Activity Relationships of Psychedelics

This chapter will summarize structure-activity relationships (SAR) that are known for the classic serotonergic hallucinogens (aka psychedelics), focusing on the three chemical types: tryptamines, ergolines, and phenethylamines. In the brain, the serotonin 5-HT_2A receptor plays a key role in regulation of cortical function and cognition, and also appears to be the principal target for hallucinogenic/psychedelic drugs such as LSD. It is one of the most extensively studied of the 14 known types of serotonin receptors. Important structural features will be identified for activity and, where possible, those that the psychedelics have in common will be discussed. Because activation of the 5-HT_2A receptor is the principal mechanism of action for psychedelics, compounds with 5-HT_2A agonist activity generally are quickly discarded by the pharmaceutical industry. Thus, most of the research on psychedelics can be related to activation of 5-HT_2A receptors. Therefore, much of the discussion will include not only clinical or anecdotal studies, but also will consider data from animal models as well as a certain amount of molecular pharmacology where it is known.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

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.

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.

Pharmacology and Toxicology of N-Benzylphenethylamine ("NBOMe") Hallucinogens

Serotonergic hallucinogens induce profound changes in perception and cognition. The characteristic effects of hallucinogens are mediated by 5-HT2A receptor activation. One class of hallucinogens are 2,5-dimethoxy-substituted phenethylamines, such as the so-called 2C-X compounds 2,5-dimethoxy-4-bromophenethylamine (2C-B) and 2,5-dimethoxy-4-iodophenethylamine (2C-I). Addition of an N-benzyl group to phenethylamine hallucinogens produces a marked increase in 5-HT2A-binding affinity and hallucinogenic potency. N-benzylphenethylamines ("NBOMes") such as N-(2-methoxybenzyl)-2,5-dimethoxy-4-iodophenethylamine (25I-NBOMe) show subnanomolar affinity for the 5-HT2A receptor and are reportedly highly potent in humans. Several NBOMEs have been available from online vendors since 2010, resulting in numerous cases of toxicity and multiple fatalities. This chapter reviews the structure-activity relationships, behavioral pharmacology, metabolism, and toxicity of members of the NBOMe hallucinogen class. Based on a review of 51 cases of NBOMe toxicity reported in the literature, it appears that rhabdomyolysis is a relatively common complication of severe NBOMe toxicity, an effect that may be linked to NBOMe-induced seizures, hyperthermia, and vasoconstriction.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Repurposing the Microsoft Kinect for Windows v2 for external head motion tracking for brain PET

Medical imaging systems such as those used in positron emission tomography (PET) are capable of spatial resolutions that enable the imaging of small, functionally important brain structures. However, the quality of data from PET brain studies is often limited by subject motion during acquisition. This is particularly challenging for patients with neurological disorders or with dynamic research studies that can last 90 min or more. Restraining head movement during the scan does not eliminate motion entirely and can be unpleasant for the subject. Head motion can be detected and measured using a variety of techniques that either use the PET data itself or an external tracking system. Advances in computer vision arising from the video gaming industry could offer significant benefits when re-purposed for medical applications. A method for measuring rigid body type head motion using the Microsoft Kinect v2 is described with results presenting  ⩽0.5 mm spatial accuracy. Motion data is measured in real-time at 30 Hz using the KinectFusion algorithm. Non-rigid motion is detected using the residual alignment energy data of the KinectFusion algorithm allowing for unreliable motion to be discarded. Motion data is aligned to PET listmode data using injected pulse sequences into the PET/CT gantry allowing for correction of rigid body motion. Pilot data from a clinical dynamic PET/CT examination is shown.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

Receptor interaction profiles of novel N-2-methoxybenzyl (NBOMe) derivatives of 2,5-dimethoxy-substituted phenethylamines (2C drugs)

BACKGROUND

N-2-methoxybenzyl-phenethylamines (NBOMe drugs) are newly used psychoactive substances with poorly defined pharmacological properties. The aim of the present study was to characterize the receptor binding profiles of a series of NBOMe drugs compared with their 2,5-dimethoxy-phenethylamine analogs (2C drugs) and lysergic acid diethylamide (LSD) in vitro.

METHODS

We investigated the binding affinities of 2C drugs (2C-B, 2C-C, 2C-D, 2C-E, 2C-H, 2C-I, 2C-N, 2C-P, 2C-T-2, 2C-T-4, 2C-T-7, and mescaline), their NBOMe analogs, and LSD at monoamine receptors and determined functional 5-hydroxytryptamine-2A (5-HT2A) and 5-HT2B receptor activation. Binding at and the inhibition of monoamine uptake transporters were also determined. Human cells that were transfected with the respective human receptors or transporters were used (with the exception of trace amine-associated receptor-1 [TAAR1], in which rat/mouse receptors were used).

RESULTS

All of the compounds potently interacted with serotonergic 5-HT2A, 5-HT2B, 5-HT2C receptors and rat TAAR1 (most Ki and EC50: <1 μM). The N-2-methoxybenzyl substitution of 2C drugs increased the binding affinity at serotonergic 5-HT2A, 5-HT2C, adrenergic α1, dopaminergic D1-3, and histaminergic H1 receptors and monoamine transporters but reduced binding to 5-HT1A receptors and TAAR1. As a result, NBOMe drugs were very potent 5-HT2A receptor agonists (EC50: 0.04-0.5 μM) with high 5-HT2A/5-HT1A selectivity and affinity for adrenergic α1 receptors (Ki: 0.3-0.9 μM) and TAAR1 (Ki: 0.06-2.2 μM), similar to LSD, but not dopaminergic D1-3 receptors (most Ki:>1 μM), unlike LSD.

CONCLUSION

The binding profile of NBOMe drugs predicts strong hallucinogenic effects, similar to LSD, but possibly more stimulant properties because of α1 receptor interactions.

Synthesis and in vitro evaluation of [18F]FECIMBI-36: A potential agonist PET ligand for 5-HT2A/2C receptors

Radiosynthesis and in vitro evaluation of [(18)F]-2-(4-bromo-2,5-dimethoxyphenyl)-N-(2-(2-fluoroethoxy)benzyl)ethanamine, ([(18)F]FECIMBI-36) or ([(18)F]1), a potential agonist PET imaging agent for 5-HT2A/2C receptors is described. Syntheses of reference standard 1 and the corresponding des-fluoroethyl radiolabeling precursor (2) were achieved with 75% and 65% yields, respectively. In vitro pharmacology assay of FECIMBI-36 by [(3)H]-ketanserin competition binding assay obtained from NIMH-PDSP showed high affinities to 5-HT2AR (Ki = 1nM) and 5-HT2CR (Ki=1.7 nM). Radiolabeling of FECIMBI-36 was achieved from the boc-protected precursor 2 using [(18)F]-fluoroethyltosylate in presence of Cs2CO3 in DMSO followed by removal of the protective group. [(18)F]1 was isolated using RP-HPLC in 25 ± 5% yield, purity > 95% and specific activity 1-2Ci/μmol (N = 6). In vitro autoradiography studies demonstrate that [(18)F]1 selectively label 5-HT2A and 5-HT2C receptors in slide-mounted sections of postmortem human brain using phosphor imaging. Our results indicate the potential of [(18)F]1 for imaging 5-HT2A/2C receptors in the high affinity state in vivo using PET imaging.

Preclinical safety assessment of the 5-HT2A receptor agonist PET radioligand [ 11C]Cimbi-36

PURPOSE

[11C]Cimbi-36 was recently developed as an agonist radioligand for brain imaging of serotonin 2A receptors (5-HT2A) with positron emission tomography (PET). This may be used to quantify the high-affinity state of 5-HT2A receptors and may have the potential to quantify changes in cerebral 5-HT levels in vivo. We here investigated safety aspects related to clinical use of [11C]Cimbi-36, including radiation dosimetry and in vivo pharmacology.

PROCEDURES

[11C]Cimbi-36 was injected in rats or pigs, and radiation dosimetry was examined by ex vivo dissection or with PET scanning, respectively. Based on animal data, the Organ Level INternal Dose Assessment software was used to estimate extrapolated human dosimetry for [11C]Cimbi-36. The 5-HT2A receptor agonist actions of [11C]Cimbi-36 in vivo pharmacological effects in mice elicited by increasing doses of Cimbi-36 were assessed with the head-twitch response (HTR).

RESULTS

The effective dose as extrapolated from both rat and pig data was low, 7.67 and 4.88 μSv/MBq, respectively. In addition, the estimated absorbed radiation dose to human target organs did not exceed safety levels. Administration of 0.5 mg/kg Cimbi-36 leads to significant HTR compared to saline, whereas 0.05 mg/kg Cimbi-36 (doses much larger than those given in conjunction with a PET scan) did not elicit a significant HTR.

CONCLUSIONS

Administration of tracer doses of [11C]Cimbi-36 does not seem to be associated with unusual radiation burden or adverse clinical effects.

Effects of the hallucinogen 2,5-dimethoxy-4-iodophenethylamine (2C-I) and superpotent N-benzyl derivatives on the head twitch response

N-benzyl substitution markedly enhances the affinity of phenethylamine hallucinogens at the 5-HT(2A) receptor. N-benzyl substituted derivatives of 2,5-dimethoxy-4-iodophenethylamine (2C-I), such as N-(2-methoxybenzyl)-2,5-dimethoxy-4-iodophenethylamine (25I-NBOMe) and N-(2,3-methylenedioxybenzyl)-2,5-dimethoxy-4-iodophenethylamine (25I-NBMD), have appeared recently as designer drugs, but have not been characterized behaviorally. The head twitch response (HTR) is induced by 5-HT(2A) receptor activation in rats and mice, and is widely used as a behavioral proxy for hallucinogen effects in humans. Nevertheless, it is not clear whether phenethylamine hallucinogens reliably provoke this behavior. Hence, we investigated whether 2C-I, 25I-NBOMe and 25I-NBMD induce head twitches in C57BL/6J mice. The HTR was assessed using a head-mounted magnet and a magnetometer coil. 2C-I (1-10 mg/kg SC), 25I-NBOMe (0.1-1 mg/kg SC), and 25I-NBMD (1-10 mg/kg SC) induced the HTR. 25I-NBOMe displayed 14-fold higher potency than 2C-I, and the selective 5-HT(2A) antagonist M100,907 completely blocked the HTR induced by all three compounds. These findings show that phenethylamine hallucinogens induce the HTR by activating 5-HT(2A) receptors. Our results demonstrate that 25I-NBOMe is a highly potent derivative of 2C-I, confirming previous in vitro findings that N-benzyl substitution increases 5-HT(2A) affinity. Given the high potency and ease of synthesis of N-benzylphenethylamines, it is likely that the recreational use of these hallucinogens will become more widespread in the future.

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.

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.

Extended characterisation of the serotonin 2A (5-HT2A) receptor-selective PET radiotracer 11C-MDL100907 in humans: quantitative analysis, test-retest reproducibility, and vulnerability to endogenous 5-HT tone

INTRODUCTION

Scanning properties and analytic methodology of the 5-HT2A receptor-selective positron emission tomography (PET) tracer 11C-MDL100907 have been partially characterised in previous reports. We present an extended characterisation in healthy human subjects.

METHODS

64 11C-MDL100907 PET scans with metabolite-corrected arterial input function were performed in 39 healthy adults (18-55 years). 12 subjects were scanned twice (duration 150 min) to provide data on plasma analysis, model order estimation, and stability and test-retest characteristics of outcome measures. All other scans were 90 min duration. 3 subjects completed scanning at baseline and following 5-HT2A receptor antagonist medication (risperidone or ciproheptadine) to provide definitive data on the suitability of the cerebellum as reference region. 10 subjects were scanned under reduced 5-HT and control conditions using rapid tryptophan depletion to investigate vulnerability to competition with endogenous 5-HT. 13 subjects were scanned as controls in clinical protocols. Pooled data were used to analyse the relationship between tracer injected mass and receptor occupancy, and age-related decline in 5-HT2A receptors.

RESULTS

Optimum analytic method was a 2-tissue compartment model with arterial input function. However, basis function implementation of SRTM may be suitable for measuring between-group differences non-invasively and warrants further investigation. Scan duration of 90 min achieved stable outcome measures in all cortical regions except orbitofrontal which required 120 min. Binding potential (BPP and BPND) test-retest variability was very good (7-11%) in neocortical regions other than orbitofrontal, and moderately good (14-20%) in orbitofrontal cortex and medial temporal lobe. Saturation occupancy of 5-HT2A receptors by risperidone validates the use of the cerebellum as a region devoid of specific binding for the purposes of PET. We advocate a mass limit of 4.6 μg to remain below 5% receptor occupancy. 11C-MDL100907 specific binding is not vulnerable to competition with endogenous 5-HT in humans. Paradoxical decreases in BPND were found in right prefrontal cortex following reduced 5-HT, possibly representing receptor internalisation. Mean age-related decline in brain 5-HT2A receptors was 14.0±5.0% per decade, and higher in prefrontal regions.

CONCLUSIONS

Our data confirm and extend support for 11C-MDL100907 as a PET tracer with very favourable properties for quantifying 5-HT2A receptors in the human brain.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.