Synthesis and Structure-Activity Relationships of 2,5-Dimethoxy-4-Substituted Phenethylamines and the Discovery of CYB210010: A Potent, Orally Bioavailable and Long-Acting Serotonin 5-HT2 Receptor Agonist

Structure-activity studies of 4-substituted-2,5-dimethoxyphenethylamines led to the discovery of 2,5-dimethoxy-4-thiotrifluoromethylphenethylamines, including CYB210010, a potent and long-acting serotonin 5-HT2 receptor agonist. CYB210010 exhibited high agonist potency at 5-HT2A and 5-HT2C receptors, modest selectivity over 5-HT2B, 5-HT1A, 5-HT6, and adrenergic α2A receptors, and lacked activity at monoamine transporters and over 70 other proteins. CYB210010 (0.1-3 mg/kg) elicited a head-twitch response (HTR) and could be administered subchronically at threshold doses without behavioral tolerance. CYB210010 was orally bioavailable in three species, readily and preferentially crossed into the CNS, engaged frontal cortex 5-HT2A receptors, and increased the expression of genes involved in neuroplasticity in the frontal cortex. CYB210010 represents a new tool molecule for investigating the therapeutic potential of 5-HT2 receptor activation. In addition, several other compounds with high 5-HT2A receptor potency, yet with little or no HTR activity, were discovered, providing the groundwork for the development of nonpsychedelic 5-HT2A receptor ligands.

Desorption Electrospray Ionization Mass Spectrometry Imaging of Cimbi-36, a 5-HT2A Receptor Agonist, with Direct Comparison to Autoradiography and Positron Emission Tomography

PURPOSE

The study demonstrates the use of Desorption Electrospray Ionization mass spectrometry imaging (DESI-MSI) for imaging of the PET tracer compound Cimbi-36 in brain tissue and compares imaging by DESI-MSI to imaging by autoradiography and PET.

PROCEDURES

Rats were dosed intraperitoneally with 3 mg/kg of Cimbi-36 and euthanized at t = 5, 10, 15, 30, 60 and 120 min post-injection. The brains were removed, frozen and sectioned, and sagittal sections were imaged by DESI-MSI in positive ion mode. Additionally, brain sections from a non-dosed animal were incubated with 14C-labelled Cimbi-36 and imaged by autoradiography. Finally, PET images were acquired from an animal dosed with 11C-labelled Cimbi-36.

RESULTS

DESI-MSI and autoradiography images of a sagittal brain sections showed similar distributions of Cimbi-36, with increased abundance in the frontal cortex and choroid plexus, regions which are high in 5-HT2A and 5-HT2C receptors. The PET image also showed increased abundance in cortex, but the spatial resolution was clearly inferior to DESI-MSI and autoradiography. The DESI-MSI results showed increased abundance of Cimbi-36 in brain tissue until 15 min, after which the abundance was declining. The PET-tracer was still clearly detectable at t = 120 min. Similar imaging of the kidneys showed the abundance of Cimbi-36 peaking at 30 min. Cimbi-36 was quantified in a t = 15 min brain section by quantitative DESI-MSI, resulting in tissue concentrations of 19.8 μg/g in cortex, 15.4 μg/g in cerebellum and 12.5 μg/g in whole brain.

CONCLUSIONS

DESI imaging from an in vivo dosing experiment showed distribution of the PET tracer remarkably similar to what was obtained by autoradiography of an in vitro incubation experiment, indicating that the obtained results represent actual binding to certain receptors in the brain. DESI-MSI is suggested as a cost-effective screening tool, which does not rely on labelling of compounds.

Low Basicity as a Characteristic for Atypical Ligands of Serotonin Receptor 5-HT2

Serotonin receptors are extensively examined by academic and industrial researchers, due to their vital roles, which they play in the organism and constituting therefore important drug targets. Up to very recently, it was assumed that the basic nitrogen in compound structure is a necessary component to make it active within this receptor system. Such nitrogen interacts in its protonated form with the aspartic acid from the third transmembrane helix (D3x32) forming a hydrogen bond tightly fitting the ligand in the protein binding site. However, there are several recent studies that report strong serotonin receptor affinity also for compounds without a basic moiety in their structures. In the study, we carried out a comprehensive in silico analysis of the low-basicity phenomenon of the selected serotonin receptor ligands. We focused on the crystallized representatives of the proteins of 5-HT_1B, 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors, and examined the problem both from the ligand- and structure-based perspectives. The study was performed for the native proteins, and for D3x32A mutants. The investigation resulted in the determination of nonstandard structural requirements for activity towards serotonin receptors, which can be used in the design of new nonbasic ligands.

Mechanisms of Action of Cassiae Semen for Weight Management: A Computational Molecular Docking Study of Serotonin Receptor 5-HT2C

Overweight and obesity is a growing global health concern. Current management of obesity includes lifestyle intervention, bariatric surgery and medication. The serotonin receptor, 5-HT2C, is known to mediate satiety, appetite and consumption behaviour. Lorcaserin, an appetite control drug, has demonstrated efficacy in appetite control by targeting 5-HT2C but causes undesirable side effects. This study aimed to explore the potential usage of Cassiae semen (CS), a well-known traditional Chinese medicine used to treat obesity. A computational molecular docking study was performed to determine the binding mechanism of CS compounds to the 5-HT2C receptors in both active, agonist-bound and inactive, antagonist-bound conformations. By comparing binding poses and predicted relative binding affinities towards the active or inactive forms of the receptor, we hypothesise that two of the CS compounds studied may be potent agonists which may mimic the appetite suppression effects of lorcaserin: obtusifoliol and cassiaside B2. Furthermore, two ligands, beta-sitosterol and juglanin, were predicted to bind favourably to 5-HT2C outside of the known agonist binding pocket in the active receptor, suggesting that such ligands may serve as positive allosteric modulators of 5-HT2C receptor function. Overall, this study proposed several CS compounds which may be responsible for exerting anti-obesity effects via appetite suppression by 5-HT2C receptor activation.

Synthesis and Biological Evaluation of Disubstituted Pyrimidines as Selective 5-HT2C Agonists

Here, we describe the synthesis of disubstituted pyrimidine derivatives and their biological evaluation as selective 5-HT_2C agonists. To improve selectivity for 5-HT_2C over other subtypes, we synthesized two series of disubstituted pyrimidines with fluorophenylalkoxy groups at either the 5-position or 4-position and varying cyclic amines at the 2-position. The in vitro cell-based assay and binding assay identified compounds 10a and 10f as potent 5-HT_2C agonists. Further studies on selectivity to 5-HT subtypes and drug-like properties indicated that 2,4-disubstituted pyrimidine 10a showed a highly agonistic effect on the 5-HT_2C receptor, with excellent selectivity, as well as exceptional drug-like properties, including high plasma and microsomal stability, along with low CYP inhibition. Thus, pyrimidine 10a could be considered a viable lead compound as a 5-HT_2C selective agonist.

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.

Neurochemical pharmacology of psychoactive substituted N-benzylphenethylamines: High potency agonists at 5-HT2A receptors

The use of new psychoactive substituted 2,5-dimethoxy-N-benzylphenethylamines is associated with abuse and toxicity in the United States and elsewhere and their pharmacology is not well known. This study compares the mechanisms of action of 2-(2,5-dimethoxy-4-methylphenyl)-N-(2-methoxybenzyl)ethanamine (25D-NBOMe), 2-(4-ethyl-2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine (25E-NBOMe), 2-(2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine (25H-NBOMe), 2-(((4-iodo-2,5-dimethoxyphenethyl)amino)methyl)phenol (25I-NBOH); and 2-(2,5-dimethoxy-4-nitrophenyl)-N-(2-methoxybenzyl)ethanamine) (25N-NBOMe) with hallucinogens and stimulants. Mammalian cells heterologously expressing 5-HT1A, 5-HT2A, 5-HT2B or 5-HT2C receptors, or dopamine, serotonin or norepinephrine transporters (DAT, SERT and NET, respectively) were used to assess drug affinities at radioligand binding sites. Potencies and efficacies were determined using [35S]GTPγS binding assays (5-HT1A), inositol-phosphate accumulation assays (5-HT2A, 5-HT2B and 5-HT2C), and uptake and release assays (transporters). The substituted phenethylamines were very low potency and low efficacy agonists at the 5-HT1A receptor. 25D-NBOMe, 25E-NBOMe, 25H-NBOMe, 25I-NBOH and 25N-NBOMe had very high affinity for, and full efficacy at, 5-HT2A and 5-HT2C receptors. In the 5-HT2A receptor functional assay, 25D-NBOMe, 25E-NBOMe, 25I-NBOH and 25N-NBOMe had subnanomolar to low nanomolar potencies similar to (+)lysergic acid diethylamide (LSD) while 25H-NBOMe had lower potency, similar to serotonin. At the 5-HT2C receptor, four had very high potencies, similar to LSD and serotonin, while 25H-NBOMe had lower potency. At the 5-HT2B receptor, the compounds had lower affinity, potency and efficacy compared to 5-HT2A or 5-HT2C. The phenethylamines had low to mid micromolar affinities and potencies at the transporters. These results demonstrate that these -NBOMe and -NBOH substituted phenethylamines have a biochemical pharmacology consistent with hallucinogenic activity, with little psychostimulant activity.

Return of the lysergamides. Part IV: Analytical and pharmacological characterization of lysergic acid morpholide (LSM-775)

Lysergic acid diethylamide (LSD) is perhaps one of the best-known psychoactive substances and many structural modifications of this prototypical lysergamide have been investigated. Several lysergamides were recently encountered as 'research chemicals' or new psychoactive substances (NPS). Although lysergic acid morpholide (LSM-775) appeared on the NPS market in 2013, there is disagreement in the literature regarding the potency and psychoactive properties of LSM-775 in humans. The present investigation attempts to address the gap of information that exists regarding the analytical profile and pharmacological effects of LSM-775. A powdered sample of LSM-775 was characterized by X-ray crystallography, nuclear magnetic resonance spectroscopy (NMR), gas chromatography mass spectrometry (GC-MS), high mass accuracy electrospray MS/MS, high performance liquid chromatography (HPLC) diode array detection, HPLC quadrupole MS, and GC solid-state infrared analysis. Screening for receptor affinity and functional efficacy revealed that LSM-775 acts as a nonselective agonist at 5-HT1A and 5-HT2A receptors. Head twitch studies were conducted in C57BL/6J mice to determine whether LSM-775 activates 5-HT2A receptors and produces hallucinogen-like effects in vivo. LSM-775 did not induce the head twitch response unless 5-HT1A receptors were blocked by pretreatment with the antagonist WAY-100,635 (1 mg/kg, subcutaneous). These findings suggest that 5-HT1A activation by LSM-775 masks its ability to induce the head twitch response, which is potentially consistent with reports in the literature indicating that LSM-775 is only capable of producing weak LSD-like effects in humans.

5-HT2C Receptor Structures Reveal the Structural Basis of GPCR Polypharmacology

Drugs frequently require interactions with multiple targets-via a process known as polypharmacology-to achieve their therapeutic actions. Currently, drugs targeting several serotonin receptors, including the 5-HT_2C receptor, are useful for treating obesity, drug abuse, and schizophrenia. The competing challenges of developing selective 5-HT_2C receptor ligands or creating drugs with a defined polypharmacological profile, especially aimed at G protein-coupled receptors (GPCRs), remain extremely difficult. Here, we solved two structures of the 5-HT_2C receptor in complex with the highly promiscuous agonist ergotamine and the 5-HT_2A-C receptor-selective inverse agonist ritanserin at resolutions of 3.0 Å and 2.7 Å, respectively. We analyzed their respective binding poses to provide mechanistic insights into their receptor recognition and opposing pharmacological actions. This study investigates the structural basis of polypharmacology at canonical GPCRs and illustrates how understanding characteristic patterns of ligand-receptor interaction and activation may ultimately facilitate drug design at multiple GPCRs.

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.

Development of Fluorescent Probes that Target Serotonin 5-HT2B Receptors

Some 5-HT2B fluorescent probes were obtained by tagging 1-(2,5-dimethoxy-4-iodophenyl)-propan-2-amine (DOI) with a subset of fluorescent amines. Some of the resulting fluorescent ligands showed excellent affinity and selectivity profiles at the 5-HT2B receptors (e.g. 12b), while retain the agonistic functional behaviour of the model ligand (DOI). The study highlighted the most salient features of the structure-activity relationship in this series and these were substantiated by a molecular modelling study based on a receptor-driven docking model constructed on the basis of the crystal structure of the human 5-HT2B receptor. One of the fluorescent ligands developed in this work, compound 12i, specifically labelled CHO-K1 cells expressing 5-HT2B receptors and not parental CHO-K1 cells in a concentration-dependent manner. 12i enables imaging and quantification of specific 5-HT2B receptor labelling in live cells by automated fluorescence microscopy as well as quantification by measurements of fluorescence intensity using a fluorescence plate reader.

Identification of Optically Active Pyrimidine Derivatives as Selective 5-HT2C Modulators

A series of pyrimidine derivatives 4a-i were synthesized and evaluated for their binding affinities towards 5-HT_2C receptors. With regard to designed molecules 4a-i, the influence of the size of alkyl ether and the absolute configuration of a stereogenic center on the 5-HT_2C binding affinity and selectivity was studied. The most promising diasteromeric mixtures 4d and 4e were selected in the initial radioligand binding assay and they were further synthesized as optically active forms starting from optically active alcohols 5d and 5e, prepared by an enzymatic kinetic resolution. Pyrimidine analogue (R,R)-4e displayed an excellent 5-HT_2C binding affinity with good selectivity values against a broad range of other 5-HT receptor subtypes.

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.

Return of the lysergamides. Part II: Analytical and behavioural characterization of N6 -allyl-6-norlysergic acid diethylamide (AL-LAD) and (2'S,4'S)-lysergic acid 2,4-dimethylazetidide (LSZ)

Lysergic acid N,N-diethylamide (LSD) is perhaps one of the most intriguing psychoactive substances known and numerous analogs have been explored to varying extents in previous decades. In 2013, N6 -allyl-6-norlysergic acid diethylamide (AL-LAD) and (2'S,4'S)-lysergic acid 2,4-dimethylazetidide (LSZ) appeared on the 'research chemicals'/new psychoactive substances (NPS) market in both powdered and blotter form. This study reports the analytical characterization of powdered AL-LAD and LSZ tartrate samples and their semi-quantitative determination on blotter paper. Included in this study was the use of nuclear magnetic resonance (NMR) spectroscopy, gas chromatography-mass spectrometry (GC-MS), low and high mass accuracy electrospray MS(/MS), high performance liquid chromatography diode array detection and GC solid-state infrared analysis. One feature shared by serotonergic psychedelics, such as LSD, is the ability to mediate behavioural responses via activation of 5-HT2A receptors. Both AL-LAD and LSZ displayed LSD-like responses in male C57BL/6 J mice when employing the head-twitch response (HTR) assay. AL-LAD and LSZ produced nearly identical inverted-U-shaped dose-dependent effects, with the maximal responses occurring at 200 µg/kg. Analysis of the dose responses by nonlinear regression confirmed that LSZ (ED50  = 114.2 nmol/kg) was equipotent to LSD (ED50  = 132.8 nmol/kg) in mice, whereas AL-LAD was slightly less potent (ED50  = 174.9 nmol/kg). The extent to which a comparison in potency can be translated directly to humans requires further investigation. Chemical and pharmacological data obtained from NPS may assist research communities that are interested in various aspects related to substance use and forensic identification. Copyright © 2016 John Wiley & Sons, Ltd.

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.

Return of the lysergamides. Part I: Analytical and behavioural characterization of 1-propionyl-d-lysergic acid diethylamide (1P-LSD)

1-Propionyl-d-lysergic acid diethylamide hemitartrate (1P-LSD) has become available as a 'research chemical' in the form of blotters and powdered material. This non-controlled derivative of d-lysergic acid diethylamide (LSD) has previously not been described in the published literature despite being closely related to 1-acetyl-LSD (ALD-52), which was developed in the 1950s. This study describes the characterization of 1P-LSD in comparison with LSD using various chromatographic and mass spectrometric methods, infrared and nuclear magnetic resonance spectroscopy. An important feature common to LSD and other serotonergic hallucinogens is that they produce 5-HT2A -receptor activation and induce the head-twitch response (HTR) in rats and mice. In order to assess whether 1P-LSD displays LSD-like properties and activates the 5-HT2A receptor, male C57BL/6 J mice were injected with vehicle (saline) or 1P-LSD (0.025-0.8 mg/kg, IP) and HTR assessed for 30 min using magnetometer coil recordings. It was found that 1P-LSD produced a dose-dependent increase in HTR counts, and that it had ~38% (ED50  = 349.6 nmol/kg) of the potency of LSD (ED50  = 132.8 nmol/kg). Furthermore, HTR was abolished when 1P-LSD administration followed pretreatment with the selective 5-HT2A receptor antagonist M100907 (0.1 mg/kg, SC), which was consistent with the concept that the behavioural response was mediated by activation of the 5-HT2A receptor. These results indicate that 1P-LSD produces LSD-like effects in mice, consistent with its classification as a serotonergic hallucinogen. Nevertheless, the extent to which 1P-LSD might show psychoactive effects in humans similar to LSD remains to be investigated. Copyright © 2015 John Wiley & Sons, Ltd.

Beyond Membrane Protein Structure: Drug Discovery, Dynamics and Difficulties

Most of the previous content of this book has focused on obtaining the structures of membrane proteins. In this chapter we explore how those structures can be further used in two key ways. The first is their use in structure based drug design (SBDD) and the second is how they can be used to extend our understanding of their functional activity via the use of molecular dynamics. Both aspects now heavily rely on computations. This area is vast, and alas, too large to consider in depth in a single book chapter. Thus where appropriate we have referred the reader to recent reviews for deeper assessment of the field. We discuss progress via the use of examples from two main drug target areas; G-protein coupled receptors (GPCRs) and ion channels. We end with a discussion of some of the main challenges in the area.

We Need 2C but Not 2B: Developing Serotonin 2C (5-HT2C) Receptor Agonists for the Treatment of CNS Disorders

The serotonin 2C (5-HT2C ) receptor has been identified as a potential drug target for the treatment of a variety of central nervous system (CNS) disorders, such as obesity, substance abuse, and schizophrenia. In this Viewpoint article, recent progress in developing selective 5-HT2C agonists for use in treating these disorders is summarized, including the work of our group. Challenges in this field and the possible future directions are described. Homology modeling as a method to predict the binding modes of 5-HT2C ligands to the receptor is also discussed. Compared to known ligands, the improved pharmacological profiles of the 2-phenylcyclopropylmethylamine-based 5-HT2C agonists make them preferred candidates for further studies.

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.

N-Benzyl-5-methoxytryptamines as Potent Serotonin 5-HT2 Receptor Family Agonists and Comparison with a Series of Phenethylamine Analogues

A series of N-benzylated-5-methoxytryptamine analogues was prepared and investigated, with special emphasis on substituents in the meta position of the benzyl group. A parallel series of several N-benzylated analogues of 2,5-dimethoxy-4-iodophenethylamine (2C-I) also was included for comparison of the two major templates (i.e., tryptamine and phenethylamine). A broad affinity screen at serotonin receptors showed that most of the compounds had the highest affinity at the 5-HT2 family receptors. Substitution at the para position of the benzyl group resulted in reduced affinity, whereas substitution in either the ortho or the meta position enhanced affinity. In general, introduction of a large lipophilic group improved affinity, whereas functional activity often followed the opposite trend. Tests of the compounds for functional activity utilized intracellular Ca(2+) mobilization. Function was measured at the human 5-HT2A, 5-HT2B, and 5-HT2C receptors, as well as at the rat 5-HT2A and 5-HT2C receptors. There was no general correlation between affinity and function. Several of the tryptamine congeners were very potent functionally (EC50 values from 7.6 to 63 nM), but most were partial agonists. Tests in the mouse head twitch assay revealed that many of the compounds induced the head twitch and that there was a significant correlation between this behavior and functional potency at the rat 5-HT2A receptor.

Application of palladium-mediated (18)F-fluorination to PET radiotracer development: overcoming hurdles to translation

New chemistry methods for the synthesis of radiolabeled small molecules have the potential to impact clinical positron emission tomography (PET) imaging, if they can be successfully translated. However, progression of modern reactions from the stage of synthetic chemistry development to the preparation of radiotracer doses ready for use in human PET imaging is challenging and rare. Here we describe the process of and the successful translation of a modern palladium-mediated fluorination reaction to non-human primate (NHP) baboon PET imaging–an important milestone on the path to human PET imaging. The method, which transforms [¹⁸F]fluoride into an electrophilic fluorination reagent, provides access to aryl–¹⁸F bonds that would be challenging to synthesize via conventional radiochemistry methods.

Activation of Akt through 5-HT2A receptor ameliorates serotonin-induced degradation of insulin receptor substrate-1 in adipocytes

Serotonin (5-hydroxytryptamine, 5-HT) was found to be elevated in the serum of diabetic patients. In this study, we investigate the mechanism of insulin desensitization caused by 5-HT. In 3T3-L1 adipocytes, 5-HT treatment induced the translocation of insulin receptor substrate-1 (IRS-1) from low density microsome (LDM), the important intracellular compartment for its functions, to cytosol, inducing IRS-1 ubiquitination and degradation. Moreover, inhibition of 5-HT-stimulated Akt activation by either ketanserin (a specific 5-HT2A receptor antagonist) or knocking-down the expression of 5-HT2A receptor promoted 5-HT-stimulated IRS-1 dissociation from 14-3-3β in LDM, leading to drastic ubiquitination. Interestingly, sarpogrelate, another antagonist of 5-HT2A receptor, protected IRS-1 from degradation through activation of Akt. This implicates the importance of Akt activation in extending IRS-1 life span through maintaining their optimal sub-location into adipocytes. Taken together, this study suggest that activation of Akt may be able to compensate the adverse effects of 5-HT by stabilizing IRS-1 in LDM.

Development, validation, and use of quantitative structure-activity relationship models of 5-hydroxytryptamine (2B) receptor ligands to identify novel receptor binders and putative valvulopathic compounds among common drugs

Some antipsychotic drugs are known to cause valvular heart disease by activating serotonin 5-HT(2B) receptors. We have developed and validated binary classification QSAR models capable of predicting potential 5-HT(2B) actives. The classification accuracies of the models built to discriminate 5-HT(2B) actives from the inactives were as high as 80% for the external test set. These models were used to screen in silico 59,000 compounds included in the World Drug Index, and 122 compounds were predicted as actives with high confidence. Ten of them were tested in radioligand binding assays and nine were found active, suggesting a success rate of 90%. All validated actives were then tested in functional assays, and one compound was identified as a true 5-HT(2B) agonist. We suggest that the QSAR models developed in this study could be used as reliable predictors to flag drug candidates that are likely to cause valvulopathy.