Synthesis and in vitro affinities of various MDL 100907 derivatives as potential 18F-radioligands for 5-HT2A receptor imaging with PET

A pilot study of cerebral metabolism and serotonin 5-HT2A receptor occupancy in rats treated with the psychedelic tryptamine DMT in conjunction with the MAO inhibitor harmine

Rationale: The psychedelic effects of the traditional Amazonian botanical decoction known as ayahuasca are often attributed to agonism at brain serotonin 5-HT2A receptors by N,N-dimethyltryptamine (DMT). To reduce first pass metabolism of oral DMT, ayahuasca preparations additionally contain reversible monoamine oxidase A (MAO-A) inhibitors, namely β-carboline alkaloids such as harmine. However, there is lacking biochemical evidence to substantiate this pharmacokinetic potentiation of DMT in brain via systemic MAO-A inhibition. Objectives: We measured the pharmacokinetic profile of harmine and/or DMT in rat brain, and tested for pharmacodynamic effects on brain glucose metabolism and DMT occupancy at brain serotonin 5-HT2A receptors. Methods: We first measured brain concentrations of harmine and DMT after treatment with harmine and/or DMT at low sub-cutaneous doses (1 mg/kg each) or harmine plus DMT at moderate doses (3 mg/kg each). In the same groups of rats, we also measured ex vivo the effects of these treatments on the availability of serotonin 5-HT2A receptors in frontal cortex. Finally, we explored effects of DMT and/or harmine (1 mg/kg each) on brain glucose metabolism with [18F]FDG-PET. Results: Results confirmed that co-administration of harmine inhibited the formation of the DMT metabolite indole-3-acetic acid (3-IAA) in brain, while correspondingly increasing the cerebral availability of DMT. However, we were unable to detect any significant occupancy by DMT at 5-HT2A receptors measured ex vivo, despite brain DMT concentrations as high as 11.3 µM. We did not observe significant effects of low dose DMT and/or harmine on cerebral [18F]FDG-PET uptake. Conclusion: These preliminary results call for further experiments to establish the dose-dependent effects of harmine/DMT on serotonin receptor occupancy and cerebral metabolism.

Repeated low doses of psilocybin increase resilience to stress, lower compulsive actions, and strengthen cortical connections to the paraventricular thalamic nucleus in rats

Psilocybin (a classic serotonergic psychedelic drug) has received appraisal for use in psychedelic-assisted therapy of several psychiatric disorders. A less explored topic concerns the use of repeated low doses of psychedelics, at a dose that is well below the psychedelic dose used in psychedelic-assisted therapy and often referred to as microdosing. Psilocybin microdose users frequently report increases in mental health, yet such reports are often highly biased and vulnerable to placebo effects. Here we establish and validate a psilocybin microdose-like regimen in rats with repeated low doses of psilocybin administration at a dose derived from occupancy at rat brain 5-HT2A receptors in vivo. The rats tolerated the repeated low doses of psilocybin well and did not manifest signs of anhedonia, anxiety, or altered locomotor activity. There were no deficits in pre-pulse inhibition of the startle reflex, nor did the treatment downregulate or desensitize the 5-HT2A receptors. However, the repeated low doses of psilocybin imparted resilience against the stress of multiple subcutaneous injections, and reduced the frequency of self-grooming, a proxy for human compulsive actions, while also increasing 5-HT7 receptor expression and synaptic density in the paraventricular nucleus of the thalamus. These results establish a well-validated regimen for further experiments probing the effects of repeated low doses of psilocybin. Results further substantiate anecdotal reports of the benefits of psilocybin microdosing as a therapeutic intervention, while pointing to a possible physiological mechanism.

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.

Design, Synthesis, and Pharmacological Characterization of Heterobivalent Ligands for the Putative 5-HT2A/mGlu2 Receptor Complex

We report the synthesis of the first series of heterobivalent ligands targeting the putative heteromeric 5-HT_2A/mGlu₂ receptor complex, based on the 5-HT_2A antagonist MDL-100,907 and the mGlu₂ ago-PAM JNJ-42491293. The functional properties of monovalent and heterobivalent ligands were characterized in 5-HT_2A-, mGlu₂/Gqo5-, 5-HT_2A/mGlu₂-, and 5-HT_2A/mGlu₂/Gqo5-expressing HEK293 cells using a Ca²⁺ imaging assay and a [³H]ketanserin binding assay. Pronounced functional crosstalk was observed between the two receptors in 5-HT_2A/mGlu₂ and 5-HT_2A/mGlu₂/Gqo5 cells. While the synthesized monovalent ligands retained the 5-HT_2A antagonist and mGlu₂ ago-PAM functionalities, the seven bivalent ligands inhibited 5-HT-induced responses in 5-HT_2A/mGlu₂ cells and both 5-HT- and Glu-induced responses in 5-HT_2A/mGlu₂/Gqo5 cells. However, no definitive correlation between the functional potency and spacer length of the ligands was observed, an observation substantiated by the binding affinities exhibited by the compounds in 5-HT_2A, 5-HT_2A/mGlu₂, and 5-HT_2A/mGlu₂/Gqo5 cells. In conclusion, while functional crosstalk between 5-HT2A and mGlu2 was demonstrated, it remains unclear how these heterobivalent ligands interact with the putative receptor complex.

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.

Novel Bivalent 5-HT2A Receptor Antagonists Exhibit High Affinity and Potency in Vitro and Efficacy in Vivo

The 5-HT_2A receptor (5-HT_2AR) plays an important role in various neuropsychiatric disorders, including substance use disorder and schizophrenia. Homodimerization of this receptor has been suggested, but tools that allow direct assessment of the relevance of the 5-HT_2AR:5-HT_2AR homodimer in these disorders are necessary. We chemically modified the selective 5-HT_2AR antagonist M100907 to synthesize a series of homobivalent ligands connected by ethylene glycol linkers of varying lengths that may be useful tools for probing 5-HT_2AR:5-HT_2AR homodimer function. We tested these molecules for 5-HT_2AR antagonist activity in a cell line stably expressing the functional 5-HT_2AR and quantified a downstream signaling target, activation (phosphorylation) of extracellular regulated kinases 1/2 (ERK_1/2), in comparison to in vivo efficacy of altering spontaneous or cocaine-evoked locomotor activity in rats. All of the synthetic compounds inhibited 5-HT-mediated phosphorylation of ERK_1/2 in the cellular signaling assay; the potency of the bivalent ligands varied as a function of linker length, with the intermediate linker lengths being the most potent. The K_i values for the binding of bivalent ligands to 5-HT_2AR were only slightly lower than the values for the parent (+)-M100907 compound, but significant selectivity for 5-HT_2AR over 5-HT_2BR or 5-HT_2CR binding was retained. In addition, the 11-atom-linked bivalent 5-HT_2AR antagonist (2 mg/kg, intraperitoneally) demonstrated efficacy on par with that of (+)-M100907 in inhibiting cocaine-evoked hyperactivity. As we develop further strategies for ligand-evoked receptor assembly and analyses of diverse signaling and functional roles, these novel homobivalent 5-HT_2AR antagonist ligands will serve as useful in vitro and in vivo probes of 5-HT_2AR structure and function.

Sex differences and serotonergic mechanisms in the behavioural effects of psilocin

Psilocybin has recently attracted a great deal of attention as a clinical research and therapeutic tool. The aim of this paper is to bridge two major knowledge gaps regarding its behavioural pharmacology - sex differences and the underlying receptor mechanisms. We used psilocin (0.25, 1 and 4 mg/kg), an active metabolite of psilocybin, in two behavioural paradigms - the open-field test and prepulse inhibition (PPI) of the acoustic startle reaction. Sex differences were evaluated with respect to the phase of the female cycle. The contribution of serotonin receptors in the behavioural action was tested in male rats with selective serotonin receptor antagonists: 5-HT1A receptor antagonist (WAY100635 1 mg/kg), 5-HT2A receptor antagonist (MDL100907 0.5 mg/kg), 5-HT2B receptor antagonist (SB215505 1 mg/kg) and 5-HT2C receptor antagonist (SB242084 1 mg/kg). Psilocin induced dose-dependent inhibition of locomotion and suppression of normal behaviour in rats (behavioural serotonin syndrome, impaired PPI). The effects were more pronounced in male rats than in females. The inhibition of locomotion was normalized by 5-HT1A and 5-HT2B/C antagonists; however, PPI was not affected significantly by these antagonists. Our findings highlight an important issue of sex-specific reactions to psilocin and that apart from 5-HT2A-mediated effects 5-HT1A and 5-HT2C/B receptors also play an important role. These findings have implications for recent clinical trials.

Assessing the role of serotonergic receptors in cannabidiol's anticonvulsant efficacy

Cannabidiol (CBD) is a phytocannabinoid that has demonstrated anticonvulsant efficacy in several animal models of seizure. The current experiment validated CBD's anticonvulsant effect using the acute pentylenetetrazol (PTZ) model. Furthermore, it tested whether CBD reduces seizure activity by interacting with either the serotonergic 5HT1A or 5HT2A receptor. 120 male adolescent Wistar-Kyoto rats were randomly assigned to 8 treatment groups in two consecutive experiments. In both experiments, subjects received either CBD (100mg/kg) or vehicle 60min prior to seizure testing. In Experiment 1, subjects received either WAY-100635 (1mg/kg), a 5HT1A antagonist, or saline vehicle injection 80min prior to seizure testing. In Experiment 2, subjects received either MDL-100907 (0.3mg/kg), a specific 5HT2A antagonist, or 40% DMSO vehicle 80min prior to seizure testing. 85mg/kg of PTZ was administered to induce seizure, and behavior was recorded for 30min. Seizure behaviors were subsequently coded using a 5-point scale of severity. Across both experiments, subjects in the vehicle control groups exhibited high levels of seizure activity and mortality. In both experiments, CBD treatment significantly attenuated seizure activity. Pre-treatment with either WAY-100635 or MDL-100907 did not block CBD's anticonvulsant effect. WAY-100635 administration, by itself, also led to a significant attenuation of seizure activity. These results do not support the hypothesis that CBD attenuates seizure activity through activation of the 5HT1A or 5HT2A receptor. While this work further confirms the anticonvulsant efficacy of CBD and supports its application in the treatment of human seizure disorders, additional research on CBD's mechanism of action must be conducted.

5HT2A receptor blockade in dorsomedial striatum reduces repetitive behaviors in BTBR mice

Restricted and repetitive behaviors are a defining feature of autism, which can be expressed as a cognitive flexibility deficit or stereotyped, motor behaviors. There is limited knowledge about the underlying neuropathophysiology contributing to these behaviors. Previous findings suggest that central 5HT2A receptor activity is altered in autism, while recent work indicates that systemic 5HT2A receptor antagonist treatment reduces repetitive behaviors in an idiopathic model of autism. 5HT2A receptors are expressed in the orbitofrontal cortex and striatum. These two regions have been shown to be altered in autism. The present study investigated whether 5HT2A receptor blockade in the dorsomedial striatum or orbitofrontal cortex in the BTBR mouse strain, an idiopathic model of autism, affects the phenotype related to restricted and repetitive behaviors. Microinfusion of the 5HT2A receptor antagonist, M100907 into the dorsomedial striatum alleviated a reversal learning impairment and attenuated grooming behavior. M100907 infusion into the orbitofrontal cortex increased perseveration during reversal learning and potentiated grooming. These findings suggest that increased 5HT2A receptor activity in the dorsomedial striatum may contribute to behavioral inflexibility and stereotyped behaviors in the BTBR mouse. 5HT2A receptor signaling in the orbitofrontal cortex may be critical for inhibiting a previously learned response during reversal learning and expression of stereotyped behavior. The present results suggest which brain areas exhibit abnormalities underlying repetitive behaviors in an idiopathic mouse model of autism, as well as which brain areas systemic treatment with M100907 may principally act on in BTBR mice to attenuate repetitive behaviors.

Serotonin 2A receptor agonist binding in the human brain with [(11)C]Cimbi-36: Test-retest reproducibility and head-to-head comparison with the antagonist [(18)F]altanserin

INTRODUCTION

[(11)C]Cimbi-36 is a recently developed serotonin 2A (5-HT2A) receptor agonist positron emission tomography (PET) radioligand that has been successfully applied for human neuroimaging. Here, we investigate the test-retest variability of cerebral [(11)C]Cimbi-36 PET and compare [(11)C]Cimbi-36 and the 5-HT2A receptor antagonist [(18)F]altanserin.

METHODS

Sixteen healthy volunteers (mean age 23.9 ± 6.4years, 6 males) were scanned twice with a high resolution research tomography PET scanner. All subjects were scanned after a bolus of [(11)C]Cimbi-36; eight were scanned twice to determine test-retest variability in [(11)C]Cimbi-36 binding measures, and another eight were scanned after a bolus plus constant infusion with [(18)F]altanserin. Regional differences in the brain distribution of [(11)C]Cimbi-36 and [(18)F]altanserin were assessed with a correlation of regional binding measures and with voxel-based analysis.

RESULTS

Test-retest variability of [(11)C]Cimbi-36 non-displaceable binding potential (BPND) was consistently <5% in high-binding regions and lower for reference tissue models as compared to a 2-tissue compartment model. We found a highly significant correlation between regional BPNDs measured with [(11)C]Cimbi-36 and [(18)F]altanserin (mean Pearson's r: 0.95 ± 0.04) suggesting similar cortical binding of the radioligands. Relatively higher binding with [(11)C]Cimbi-36 as compared to [(18)F]altanserin was found in the choroid plexus and hippocampus in the human brain.

CONCLUSIONS

Excellent test-retest reproducibility highlights the potential of [(11)C]Cimbi-36 for PET imaging of 5-HT2A receptor agonist binding in vivo. Our data suggest that Cimbi-36 and altanserin both bind to 5-HT2A receptors, but in regions with high 5-HT2C receptor density, choroid plexus and hippocampus, the [(11)C]Cimbi-36 binding likely represents binding to both 5-HT2A and 5-HT2C receptors.

Neocortical serotonin2A receptor binding predicts quetiapine associated weight gain in antipsychotic-naive first-episode schizophrenia patients

Antipsychotic-induced weight gain is of major clinical importance since it is associated with severe metabolic complications and increased mortality. The serotonin2A receptor system has been suggested to be implicated in weight gain and obesity. However, no previous in vivo imaging data have related serotonin2A receptor binding to weight gain before and after antipsychotic monotherapy. Fifteen antipsychotic-naive first-episode schizophrenia patients were included and investigated before and after six months of quetiapine treatment. We examined the relationship between serotonin2A receptor binding as measured with positron emission tomography (PET) and [18F]altanserin and change in body mass index (BMI). Quetiapine was chosen because it is characterized by a moderately high affinity for the serotonin2A receptor and a fast dissociation rate from the dopamine D2 receptor. At baseline the mean BMI was 24.2 kg/m2, range 18-36 kg/m2. After six months of quetiapine treatment (mean dose: 383 mg/day) the BMI had, on average, increased by 6.7%, corresponding to an average weight gain of 5.0 kg. We found a significant positive correlation both between neocortical serotonin2A receptor binding prior to treatment and subsequent increase in BMI (rho=0.59, p=0.022). At follow-up, the serotonin2A receptor occupancy was positively correlated with BMI increase (rho=0.54, p=0.038). To our knowledge, these are the first in vivo receptor imaging data in initially antipsychotic-naive first-episode schizophrenia patients to show that the cerebral serotonin2A receptor is associated with antipsychotic-induced weight gain.

Pharmacology of serotonin and female sexual behavior

In this review, first a historical perspective of serotonin's (5-HT) involvement in female sexual behavior is presented. Then an overview of studies implicating 5-HT is presented. The effect of drugs that increase or decrease CNS levels of 5-HT is reviewed. Evidence is presented that drugs which increase 5-HT have negative effects on female sexual behavior while a decrease in 5-HT is associated with facilitation of sexual behavior. Studies with compounds that act on 5-HT₁, 5-HT₂ or 5-HT₃ receptors are discussed. Most evidence indicates that 5-HT₁A receptor agonists inhibit sexual behavior while 5-HT₂ or 5-HT₃ receptors may exert a positive influence. There is substantial evidence to support a role for 5-HT in the modulation of female consummatory sexual behavior, but studies on the role of 5-HT in other elements of female sexual behavior (e.g. desire, motivation, sexual appetite) are few. Future studies should be directed at determining if these additional components of female sexual behavior are also modulated by 5-HT.

Fluorination of aryl boronic acids using acetyl hypofluorite made directly from diluted fluorine

Aryl boronic acids or pinacol esters containing EDG were converted in good yields and fast reactions to the corresponding aryl fluorides using the readily obtainable solutions of AcOF. In reactions with aryl boronic acids containing EWG at the para position, there are two competing forces: one directing the fluorination to take place ortho to the boronic acid and the other, toward an ipso substitution. With EWG meta to the boronic acid, substitution ipso to the boron moiety takes place in good yields.

Development of novel chiral capillary electrophoresis methods for the serotonin receptor (5-HT2A) antagonist MDL 100,907 (volinanserin) and for its key intermediate compound

Enantioselective capillary electrophoretic methods were elaborated for the determination of the enantiomeric purity of (R)-MDL 100,907 and its preparatively resolved key intermediate compound during the synthesis route. The pKa values of the intermediate compound and the end product determined by CE were 10.5±0.1 and 9.0±0.1, respectively. The enantiopurity of the intermediate compound can be monitored in fully protonated state by applying 15mM sulfobutylether-β-cyclodextrin at pH 5 when the peak belonging to the impurity migrates before the main component. The fact that the consecutive steps of the synthesis do not affect the enantiomeric purity was verified by the other, newly developed CE method. The enantiomers of rac-MDL 100,907 were resolved by 15mM carboxymethyl-γ-cyclodextrin at pH 3. The applicability (selectivity, LOD, LOQ, repeatability, precision and accuracy) of the methods was studied as well.

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.

Synergism between a serotonin 5-HT2A receptor (5-HT2AR) antagonist and 5-HT2CR agonist suggests new pharmacotherapeutics for cocaine addiction

Relapse to cocaine dependence, even after extended abstinence, involves a number of liability factors including impulsivity (predisposition toward rapid, unplanned reactions to stimuli without regard to negative consequences) and cue reactivity (sensitivity to cues associated with cocaine-taking which can promote cocaine-seeking). These factors have been mechanistically linked to serotonin (5-hydroxytryptamine, 5-HT) signaling through the 5-HT(2A) receptor (5-HT(2A)R) and 5-HT(2C)R; either a selective 5-HT(2A)R antagonist or a 5-HT(2C)R agonist suppresses impulsivity and cocaine-seeking in preclinical models. We conducted proof-of-concept analyses to evaluate whether a combination of 5-HT(2A)R antagonist plus 5-HT(2C)R agonist would have synergistic effects over these liability factors for relapse as measured in a 1-choice serial reaction time task and cocaine self-administration/reinstatement assay. Combined administration of a dose of the selective 5-HT(2A)R antagonist M100907 plus the 5-HT(2C)R agonist WAY163909, each ineffective alone, synergistically suppressed cocaine-induced hyperactivity, inherent and cocaine-evoked impulsive action, as well as cue- and cocaine-primed reinstatement of cocaine-seeking behavior. The identification of synergism between a 5-HT(2A)R antagonist plus a 5-HT(2C)R agonist to attenuate these factors important in relapse indicates the promise of a bifunctional ligand as an anti-addiction pharmacotherapeutic, setting the stage to develop new ligands with improved efficacy, potency, selectivity, and in vivo profiles over the individual molecules.

Evaluation of dopamine D₂/D₃ and serotonin 5-HT₂A receptor occupancy for a novel antipsychotic, lurasidone, in conscious common marmosets using small-animal positron emission tomography

RATIONALE

Lurasidone is a novel antipsychotic drug with potent binding affinity for dopamine D(2) and serotonin (5-hydroxytryptamine, 5-HT)(2A), 5-HT(7), and 5-HT(1A) receptors. Previous pharmacological studies have revealed that lurasidone exhibits a preferable profile (potent antipsychotic activity and lower incidence of catalepsy) to other antipsychotic drugs, although the contribution of receptor subtypes to this profile remains unclear.

OBJECTIVES

To compare target engagements of lurasidone with those of an atypical antipsychotic, olanzapine, we performed evaluation of dopamine D(2)/D(3) and serotonin 5-HT(2A) receptor occupancy in vivo by positron emission tomography (PET) with conscious common marmosets.

METHODS

We measured brain receptor occupancies in conscious common marmosets after oral administrations of lurasidone or olanzapine by PET with [(11)C]raclopride and [(11)C]R-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine methanol (MDL 100907) for D(2)/D(3) and 5-HT(2A) receptors, respectively.

RESULTS

Increases in brain D(2)/D(3) receptor occupancies of both lurasidone and olanzapine, which reached >80 % at maximum, were observed in the striatum with significant correlations to plasma drug levels. However, lurasidone showed lower 5-HT(2A) receptor occupancy in the frontal cortex within the same dose range, while olanzapine showed broadly comparable 5-HT(2A) and D(2)/D(3) receptor occupancies.

CONCLUSIONS

Compared with olanzapine, lurasidone preferentially binds to D(2)/D(3) receptors rather than 5-HT(2A) receptors in common marmosets. These results suggest that the contribution of in vivo 5-HT(2A) receptor blocking activity to the pharmacological profile of lurasidone might differ from olanzapine in terms of the low risk of extrapyramidal syndrome and efficacy against negative symptoms.

Synthesis and evaluation of dimeric derivatives of 5-HT(2A) receptor (5-HT(2A)R) antagonist M-100907

It is now well accepted that at least some serotonin receptors exist in dimeric and oligmeric forms. The linking of receptor ligands has been shown to have potential in the development of selective agonists and antagonists for traditionally refractive receptors. Here we report the development of a dimeric version of the known 5-HT(2A)R antagonist, M-100907. Derivatives of M-100907 were synthesized to determine an appropriate site for the linker connection. Then, homodimers with polyether linkers of different lengths were functionally tested in a bioassay to determine the optimal linker length. Attachment at the catechol of M-100907 with linkers between 12 and 18 atoms in length proved to be optimal.

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.

Blockade of 5-HT(2A) and/or 5-HT(2C) receptors modulates sevoflurane-induced immobility

PURPOSE

Blockade of 5-hydroxytryptamine (5-HT)(2A) receptors reportedly mediates or modulates the ability of isoflurane to produce immobility during noxious stimulation and would thereby influence MAC (the minimum alveolar concentration required to suppress movement in response to noxious stimulation in 50% of subjects). However, no data are yet available regarding the role of this receptor in the immobilizing action of sevoflurane. In this study, we examined how prior intraperitoneal administration of either the 5-HT(2A) receptor antagonist altanserin or the 5-HT(2C/2B) receptor antagonist SB 206553 might affect sevoflurane MAC in rats.

METHODS

Three groups of six male Wistar rats weighing 250-350 g each received one of the following drugs dissolved in dimethyl sulfoxide intraperitoneally 30 min before MAC testing: (1) altanserin 10 mg/kg; (2) SB 206553 10 mg/kg; (3) no drug (vehicle control). MAC was defined as the average of the concentrations that just prevented or just permitted movement in response to clamping the tail for 1 min.

RESULTS

The rank order of MAC values obtained after intraperitoneal drug pretreatment and sevoflurane exposure was altanserin < SB 206553 < vehicle control.

CONCLUSION

Considering the low levels of 5-HT(2B) receptors within the CNS, this result suggests that 5-HT(2A) and the 5-HT(2C) receptors are present within the neural circuitry influencing sevoflurane MAC. Blockade of 5-HT(2A) and/or 5-HT(2C) receptors may modulate the immobility produced by sevoflurane during noxious stimulation.

Novel fused pyrrole heterocyclic ring systems as structure analogs of LE 300: Synthesis and pharmacological evaluation as serotonin 5-HT(2A), dopamine and histamine H(1) receptor ligands

LE 300 represents a structurally novel type of antagonists acting preferentially at the dopamine D(1)/D(5 )receptors and the serotonin 5-HT(2A )receptor. This compound consists of a ten-membered central azecine ring fused to an indole ring on one side and a benzene moiety on the other side. To estimate the importance of the indole and / or phenyl moieties in this highly active benz-indolo-azecine, both rings were removed and replaced with a 1H-pyrrole counterpart. Accordingly, some new analogs of LE 300 namely, pyrrolo[2,3-g]indolizine, pyrrolo[3,2-a]quinolizine rings and their corresponding dimethylpyrrolo[2,3-d]azonine, and dimethylpyrrolo[2,3-d]azecine were synthesized to be evaluated for their activity at the 5-HT(2A) and dopamine D(1), D(2L), D(4), D(5) receptors in relation to LE 300. In addition, their activity at the H(1)-histamine receptors was also determined. The results suggested that the rigid pyrrolo[2,3-g]indolizine 7 and pyrrolo[3,2-a]quinolizine 8 analogs lacked biological activity in the adopted three bioassays. However, their corresponding flexible pyrrolo[2,3-d]azonine 11 and pyrrolo[2,3-d]azecine 12 derivatives revealed weak partial agonistic activity and weak antagonistic potency at the serotonin 5-HT(2A )and histamine H(1 )receptors, respectively. Meanwhile, they showed no affinity to any of the four utilized dopamine receptors. Variation in ring size did not contribute to a significant influence on the three tested bioactivities. Removal of the hydrophobic moiety (phenyl ring) and replacement of the indole moiety with a 1H-pyrrole counterpart led to a dramatic alteration in the profile of activity of such azecine-type compounds.