Characterization and Reliability of [18F]2FNQ1P in Cynomolgus Monkeys as a PET Radiotracer for Serotonin 5-HT6 Receptors

SB-258585 reduces food motivation while blocking 5-HT6 receptors in the non-human primate striatum

The interest in new 5-HT₆ agents stems from their ability to modulate cognition processing, food motivation and anxiety-like behaviors. While these findings come primarily from rodent studies, no studies on primates have been published. Furthermore, our understanding of where and how they act in the brain remains limited. Although the striatum is involved in all of these processes and expresses the highest levels of 5-HT₆ receptors, few studies have focused on it. We thus hypothesized that 5-HT6 receptor blockade would influence food motivation and modulate behavioral expression in non-human primates through striatal 5-HT6 receptors. This study thus aimed to determine the effects of acute administration of the SB-258585 selective 5-HT6 receptor antagonist on the feeding motivation and behaviors of six male macaques. Additionally, we investigated potential 5-HT6 targets using PET imaging to measure 5-HT6 receptor occupancy throughout the brain and striatal subregions. We used a food-choice task paired with spontaneous behavioral observations, checking 5-HT6 receptor occupancy with the specific PET imaging [18F]2FNQ1P radioligand. We demonstrated, for the first time in non-human primates, that modulation of 5-HT6 transmission, most likely through the striatum (the putamen and caudate nucleus), significantly reduces food motivation while exhibiting variable, weaker effects on behavior. While these results are consistent with the literature showing a decrease in food intake in rodents and proposing that 5-HT6 receptor antagonists can be used in obesity treatment, they question the antagonists' anxiolytic potential.

Recent applications of positron emission tomographic (PET) imaging in psychiatric drug discovery

INTRODUCTION

Psychiatry is one of the medical disciplines that suffers most from a lack of innovation in its therapeutic arsenal. Many failures in drug candidate trials can be explained by pharmacological properties that have been poorly assessed upstream, in terms of brain passage, brain target binding and clinical outcomes. Positron emission tomography can provide pharmacokinetic and pharmacodynamic data to help select candidate-molecules for further clinical trials.

AREAS COVERED

This review aims to explain and discuss the various methods using positron-emitting radiolabeled molecules to trace the cerebral distribution of the drug-candidate or indirectly measure binding to its therapeutic target. More than an exhaustive review of PET studies in psychopharmacology, this article highlights the contributions this technology can make in drug discovery applied to psychiatry.

EXPERT OPINION

PET neuroimaging is the only technological approach that can, in vivo in humans, measure cerebral delivery of a drug candidate, percentage and duration of target binding, and even the pharmacological effects. PET studies in a small number of subjects in the early stages of the development of a psychotropic drug can therefore provide the pharmacokinetic/pharmacodynamic data required for subsequent clinical evaluation. While PET technology is demanding in terms of radiochemical, radiopharmacological and nuclear medicine expertise, its integration into the development process of new drugs for psychiatry has great added value.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

There is a need to disentangle the etiological puzzle of age-related neurodegenerative diseases, whose clinical phenotypes arise from known, and as yet unknown, pathways that can act distinctly or in concert. Enhanced sub-phenotyping and the identification of in vivo biomarker-driven signature profiles could improve the stratification of patients into clinical trials and, potentially, help to drive the treatment landscape towards the precision medicine paradigm. The rapidly growing field of neuroimaging offers valuable tools to investigate disease pathophysiology and molecular pathways in humans, with the potential to capture the whole disease course starting from preclinical stages. Positron emission tomography (PET) combines the advantages of a versatile imaging technique with the ability to quantify, to nanomolar sensitivity, molecular targets in vivo. This review will discuss current research and available imaging biomarkers evaluating dysregulation of the main molecular pathways across age-related neurodegenerative diseases. The molecular pathways focused on in this review involve mitochondrial dysfunction and energy dysregulation; neuroinflammation; protein misfolding; aggregation and the concepts of pathobiology, synaptic dysfunction, neurotransmitter dysregulation and dysfunction of the glymphatic system. The use of PET imaging to dissect these molecular pathways and the potential to aid sub-phenotyping will be discussed, with a focus on novel PET biomarkers.

Change in Expression of 5-HT6 Receptor at Different Stages of Alzheimer's Disease: A Postmortem Study with the PET Radiopharmaceutical [18F]2FNQ1P

BACKGROUND

The 5-HT6 receptor is one of the most recently identified serotonin receptors in the central nervous system. Because of its role in memory and cognitive process, this receptor might be implicated in Alzheimer's disease (AD) and associated disorders.

OBJECTIVE

The aim of this study was to investigate the binding of [18F]2FNQ1P, a new specific radiotracer of 5-HT6 receptors, and to quantify 5-HT6 receptor density in caudate nucleus in a population of patients with different AD stages.

METHODS

Patients were classified according to the "ABC" NIA-AA classification. In vitro binding assays were performed in postmortem brain tissue from the healthy control (HC; n = 8) and severe AD ("High"; n = 8) groups. In vitro quantitative autoradiography was performed in human brain tissue (caudate nucleus) from patients with different stages of AD: HC (n = 15), "Low" (n = 18), "Int" (n = 20), and "High" (n = 15).

RESULTS

In vitro binding assays did not show significant differences for the KD and Bmax parameters between "High" and HC groups. In vitro quantitative autoradiography showed a significant difference between the "High" and HC groups (p = 0.0025). We also showed a progressive diminution in [18F]2FNQ1P specific binding, which parallels 5-HT6 receptors expression, according to increasing AD stage. Significant differences were observed between the HC group and all AD stages combined ("Low", "Intermediate", and "High") (p = 0.011).

CONCLUSION

This study confirms the interest of investigating the role of 5-HT6 receptors in AD and related disorders. [18F]2FNQ1P demonstrated specific binding to 5-HT6 receptors.

Preclinical validation of [18F]2FNQ1P as a specific PET radiotracer of 5-HT6 receptors in rat, pig, non-human primate and human brain tissue

INTRODUCTION

The aim of this study was to perform in-vitro and in-vivo radiopharmacological characterizations of [¹⁸F]2FNQ1P, a new PET radiotracer of 5-HT₆ receptors, in rat, pig, non-human primate and human tissues. The 5-HT₆ receptor is one of the more recently identified serotonin receptors in central nervous system and, because of its role in memory and cognitive processes, is considered as a promising therapeutic target.

METHODS

In-vitro autoradiography and saturation binding assays were performed in postmortem brain tissues from rat, pig, non-human primate and human caudate nucleus, completed by serum stability assessment in all species and cerebral radiometabolite and biodistribution studies in rat.

RESULTS

In all species, autoradiography data revealed high binding levels of [¹⁸F]2FNQ1P in cerebral regions with high 5-HT₆ receptor density. Binding was blocked by addition of SB258585 as a specific antagonist. Binding assays provided K_D and B_max values of respectively 1.34 nM and 0.03 pmol·mg^-1 in rat, 0.60 nM and 0.04 pmol·mg^-1 in pig, 1.38 nM and 0.07 pmol·mg^-1 in non-human primate, and 1.39 nM and 0.15 pmol·mg^-1 in human caudate nucleus. In rat brain, the proportion of unmetabolized [¹⁸F]2FNQ1P was >99% 5 min after iv injection and 89% at 40 min. The biodistribution studies found maximal radioactivity in lungs and kidneys (3.5 ± 1.2% ID/g and 2.0 ± 0.7% ID/g, respectively, 15 min post-injection).

CONCLUSION

These radiopharmacological data confirm that [¹⁸F]2FNQ1P is a specific radiotracer for molecular imaging of 5-HT₆ receptors and suggest that it could be used as a radiopharmaceutical in humans.

Virtual screening-driven discovery of dual 5-HT6/5-HT2A receptor ligands with pro-cognitive properties

A virtual screening campaign aimed at finding structurally new compounds active at 5-HT₆R provided a set of candidates. Among those, one structure, 4-(5-{[(2-{5-fluoro-1H-pyrrolo[2,3-b]pyridin-3-yl}ethyl)amino]methyl}furan-2-yl)phenol (1, 5-HT₆R K_i = 91 nM), was selected as a hit for further optimization. As expected, the chemical scaffold of selected compound was significantly different from all the serotonin receptor ligands published to date. Synthetic efforts, supported by molecular modelling, provided 43 compounds representing different substitution patterns. The derivative 42, 4-(5-{[(2-{5-fluoro-1H-pyrrolo[2,3-b]pyridin-3-yl}ethyl)amino]methyl}furan-2-yl)phenol (5-HT₆R K_i = 25, 5-HT_2AR K_i = 32 nM), was selected as a lead and showed a good brain/plasma concentration profile, and it reversed phencyclidine-induced memory impairment. Considering the unique activity profile, the obtained series might be a good starting point for the development of a novel antipsychotic or antidepressant with pro-cognitive properties.

The role of 5 HT6-receptor antagonists in Alzheimer's disease: an update

INTRODUCTION

Despite recent advances in Alzheimer's disease (AD) research, no breakthrough treatments have been discovered. Cholinesterase inhibitors and the NMDA-receptor antagonist memantine are currently the two approved symptomatic treatments for AD. 5-HT6 receptor antagonism has recently emerged as a promising treatment strategy to improve cognition in AD, with a modest side-effect profile.

AREAS COVERED

5-HT6 receptors, exclusively found in the central nervous system, modulate primarily GABA and glutamate levels, facilitating the secondary release of other neurotransmitters including dopamine, noradrenaline, and acetylcholine, all of which are compromised in AD. This review discusses findings of preclinical and phase I-III clinical trials conducted with three major 5-HT6 receptor antagonists: idalopirdine, intepirdine, and SUVN-502, in the field of AD.

EXPERT OPINION

Despite early positive findings, larger phase-III trials have failed to demonstrate any statistically significant impact on cognition for both idalopirdine and intepirdine, as adjunct to cholinesterase inhibitors. Paradoxically, 5-HT6 receptor agonists have also been shown to have cognitive enhancing properties. Thus, a better understanding of the mechanism of action of the 5-HT6 receptor and its ligands is warranted. Investigating 5-HT6 receptor partial or inverse agonists may be promising in future AD trials.