Despite irreversible binding, PET tracer [11C]-SA5845 is suitable for imaging of drug competition at sigma receptors—The cases of ketamine and haloperidol

Repurposing of CNS drugs to treat COVID-19 infection: targeting the sigma-1 receptor

The novel coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The escalating number of SARS-CoV-2-infected individuals has conferred the viral spread with the status of global pandemic. However, there are no prophylactic or therapeutic drugs available on the market to treat COVID-19, although several drugs have been approved. Recently, two articles using the comparative viral-human protein-protein interaction map revealed that the sigma-1 receptor in the endoplasmic reticulum plays an important role in SARS-CoV-2 replication in cells. Knockout and knockdown of SIGMAR1 (sigma-1 receptor, encoded by SIGMAR1) caused robust reductions in SARS-CoV-2 replication, which indicates that the sigma-1 receptor is a key therapeutic target for SARS-CoV-2 replication. Interestingly, a recent clinical trial demonstrated that treatment with the antidepressant fluvoxamine, which has a high affinity at the sigma-1 receptor, could prevent clinical deterioration in adult outpatients infected with SARS-CoV-2. In this review, we discuss the brief history of the sigma-1 receptor and its role in SARS-CoV-2 replication in cells. Here, we propose repurposing of traditional central nervous system (CNS) drugs that have a high affinity at the sigma-1 receptor (i.e., fluvoxamine, donepezil, ifenprodil) for the treatment of SARS-CoV-2-infected patients. Finally, we discussed the potential of other CNS candidates such as cutamesine and arketamine.

The neurophysiology of ketamine: an integrative review

The drug ketamine has been extensively studied due to its use in anaesthesia, as a model of psychosis and, most recently, its antidepressant properties. Understanding the physiology of ketamine is complex due to its rich pharmacology with multiple potential sites at clinically relevant doses. In this review of the neurophysiology of ketamine, we focus on the acute effects of ketamine in the resting brain. We ascend through spatial scales starting with a complete review of the pharmacology of ketamine and then cover its effects on in vitro and in vivo electrophysiology. We then summarise and critically evaluate studies using EEG/MEG and neuroimaging measures (MRI and PET), integrating across scales where possible. While a complicated and, at times, confusing picture of ketamine’s effects are revealed, we stress that much of this might be caused by use of different species, doses, and analytical methodologies and suggest strategies that future work could use to answer these problems.

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine's pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

Evaluation of the Enantiomer Specific Biokinetics and Radiation Doses of [(18)F]Fluspidine-A New Tracer in Clinical Translation for Imaging of σ₁ Receptors

The enantiomers of [(18)F]fluspidine, recently developed for imaging of σ₁ receptors, possess distinct pharmacokinetics facilitating their use in different clinical settings. To support their translational potential, we estimated the human radiation dose of (S)-(-)-[(18)F]fluspidine and (R)-(+)-[(18)F]fluspidine from ex vivo biodistribution and PET/MRI data in mice after extrapolation to the human scale. In addition, we validated the preclinical results by performing a first-in-human PET/CT study using (S)-(-)-[(18)F]fluspidine. Based on the respective time-activity curves, we calculated using OLINDA the particular organ doses (ODs) and effective doses (EDs). The ED values of (S)-(-)-[(18)F]fluspidine and (R)-(+)-[(18)F]fluspidine differed significantly with image-derived values obtained in mice with 12.9 μSv/MBq and 14.0 μSv/MBq (p < 0.025), respectively. A comparable ratio was estimated from the biodistribution data. In the human study, the ED of (S)-(-)-[(18)F]fluspidine was calculated as 21.0 μSv/MBq. Altogether, the ED values for both [(18)F]fluspidine enantiomers determined from the preclinical studies are comparable with other (18)F-labeled PET imaging agents. In addition, the first-in-human study confirmed that the radiation risk of (S)-(-)-[(18)F]fluspidine imaging is within acceptable limits. However, as already shown for other PET tracers, the actual ED of (S)-(-)-[(18)F]fluspidine in humans was underestimated by preclinical imaging which needs to be considered in other first-in-human studies.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

The development of radiotracers for imaging sigma (σ) receptors in the central nervous system (CNS) using positron emission tomography (PET)

Sigma (σ) receptors are unique mammalian proteins, distributed in the central nervous system and elsewhere, which are increasingly implicated in the pathophysiology of virtually all major central nervous system disorders. The heterogeneous but wide distribution of σ1 in the brain has prompted the development of selective radiotracers for imaging these sites using positron emission tomography (PET). To date, some 50 carbon-11-labelled and fluorine-18-labelled candidate PET radioligands targeting σ receptors have been reported. The historical development of selective σ1 receptor ligands as potential PET imaging agents, as well as the radiochemistry and application of the most recently developed examples, is described herein.

Radiosynthesis and in vivo evaluation of a novel σ1 selective PET ligand

The σ₁ receptor is an important target for CNS disorders. We previously identified a σ₁ ligand TZ3108 having highly potent (K_i-σ1 = 0.48 nM) and selective affinity for σ₁ versus σ₂ receptors. TZ3108 was ¹⁸F-labeled with F-18 for in vivo evaluation. Biodistribution and blocking studies of [¹⁸F]TZ3108 in male Sprague-Dawley rats demonstrated high brain uptake, which was σ₁-specific with no in vivo defluorination. MicroPET studies in cynomolgus macaques showed high brain penetration of [¹⁸F]TZ3108; the regional brain distribution was consistent with that of the σ₁ receptor. Pseudo-equilibrium in the brain was reached ~ 45 min post-injection. Metabolite analysis of [¹⁸F]TZ3108 in NHP blood and rodent blood and brain revealed that ~ 70% parent remained in the plasma of NHPs 60 min post-injection and the major radiometabolite did not cross the blood-brain barrier in rats. In summary, the potent, selective and metabolically stable σ₁ specific radioligand [¹⁸F]TZ3108 represents a potentially useful PET radioligand for quantifying the σ₁ receptor in the brain.

Evaluation of sigma (σ) receptors in the antidepressant-like effects of ketamine in vitro and in vivo

Ketamine is an NMDA antagonist and dissociative anesthetic that has been shown to display rapid acting and prolonged antidepressant activity in small-scale human clinical trials. Ketamine also binds to σ receptors, which are believed to be protein targets for a potential new class of antidepressant medications. The purpose of this study was to determine the involvement of σ receptors in the antidepressant-like actions of ketamine. Competition binding assays were performed to assess the affinity of ketamine for σ(1) and σ(2) receptors. The antidepressant-like effects of ketamine were assessed in vitro using a neurite outgrowth model and PC12 cells, and in vivo using the forced swim test. The σ receptor antagonists, NE-100 and BD1047, were evaluated in conjunction with ketamine in these assays to determine the involvement of σ receptors in the antidepressant-like effects of ketamine. Ketamine bound to both σ(1) and σ(2) receptors with μM affinities. Additionally, ketamine potentiated NGF-induced neurite outgrowth in PC12 cells and this effect was attenuated in the presence of NE-100. Ketamine also displayed antidepressant-like effects in the forced swim test; however, these effects were not attenuated by pretreatment with NE-100 or BD1047. Taken together, these data suggest that σ receptor-mediated neuronal remodeling may contribute to the antidepressant effects of ketamine.

Enantioselective σ1 receptor binding and biotransformation of the spirocyclic PET tracer 1'-benzyl-3-(3-fluoropropyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]

It was shown that racemic (±)-2 [1'-benzyl-3-(3-fluoropropyl)-3H-spiro[[2]benzofuran-1,4'-piperidine], WMS-1813] represents a promising positron emission tomography (PET) tracer for the investigation of centrally located σ(1) receptors. To study the pharmacological activity of the enantiomers of 2, a preparative HPLC separation of (R)-2 and (S)-2 was performed. The absolute configuration of the enantiomers was determined by CD-spectroscopy together with theoretical calculations of the CD-spectrum of a model compound. In receptor binding studies with the radioligand [(3)H]-(+)-pentazocine, (S)-2 was thrice more potent than its (R)-configured enantiomer (R)-2. The metabolic degradation of the more potent (S)-enantiomer was considerably slower than the metabolism of (R)-2. The structures of the main metabolites of both enantiomers were elucidated by determination of the exact mass using an Orbitrap-LC-MS system. These experiments showed a stereoselective biotransformation of the enantiomers of 2.

Mechanism of Anesthetic Toxicity: Metabolism, Reactive Oxygen Species, Oxidative Stress, and Electron Transfer

There is much literature on the toxic effects of anesthetics. This paper deals with both the volatiles and locals. Adverse effects appear to be multifaceted, with the focus on radicals, oxidative stress (OS), and electron transfer (ET). ET functionalities involved are quinone, iminoquinone, conjugated iminium, and nitrone. The non-ET routes involving radicals and OS apparently pertain to haloalkanes and ethers. Beneficial effects of antioxidants, evidently countering OS, are reported. Knowledge at the molecular level should aid in devising strategies to combat the adverse effects.

Evaluation of spirocyclic 3-(3-fluoropropyl)-2-benzofurans as sigma1 receptor ligands for neuroimaging with positron emission tomography

A series of various N-substituted 3-(3-fluoropropyl)-3H-spiro[[2]benzofuran-1,4'-piperidines] (7) has been synthesized. In receptor binding studies, the N-benzyl derivative 7a (WMS-1813) revealed extraordinarily high sigma(1) receptor affinity (K(i) = 1.4 nM) and excellent sigma(1)/sigma(2) selectivity (>600 fold). In vitro biotransformation of 7a with rat liver microsomes led to three main metabolites. N-Debenzylation was inhibited by introduction of an N-phenylethyl residue (7 g). The PET tracer [(18)F]7a was synthesized by nucleophilic substitution of the tosylate 13 with K[(18)F]F-K222-carbonate complex. The decay corrected radiochemical yield of [(18)F]7a was 35-48% with a radiochemical purity of >99.5% and a specific activity of 150-238 GBq/micromol. The radiotracer properties were evaluated in female CD-1 mice by organ distribution and ex vivo brain autoradiography. The radiotracer uptake in the brain was fast and sufficient, with values of approximately 4% injected dose per gram. Target specificity of [(18)F]7a was validated in blocking studies by preapplication of haloperidol, and significant reduction in the uptake of radioactivity was observed in the brain and peripheral organs expressing sigma(1) receptors.

The consolidation of neuroleptic therapy: Janssen, the discovery of haloperidol and its introduction into clinical practice

The discovery of haloperidol at the end of the 1950s constitutes one of the greatest advances of 20th century psychiatry. This antipsychotic drug has their origin in the research process of central analgesic molecules derived from pethidine and methadone, carried out by the Belgian company Janssen Phamaceutica. After the synthesis of phenoperidine, numerous analogues of this compound were studied, and chemists at Janssen took the decision to substitute the propiophenone group for a butyrophenone group. One of these compounds went the R-1625, a stronger agent with specifically neuroleptic properties but lacking morphine-like activity. This substance was synthesized on the 11th February 1958 and received the generic name of haloperidol because of the two halogenated substitutes incorporated into the molecule. Clinical development of haloperidol was conducted, primarily, by psychiatric research team at the University of Liège that confirmed its efficacy in the treatment of various psychiatric disorders such as acute and chronic paranoid psychosis, mania, or chronic treatment-resistant schizophrenia. Under the brand name Haldol((R)), haloperidol was licensed and marketed in Belgium in October 1959. The direct and differed consequences of its introduction into the psychiatric practice have been multiple, involving different areas of socio-sanitary reality. Moreover, haloperidol has contributed substantially to the development of biological psychiatry and currently neuroscience, because it made possible the development of new experimental models for predicting the effects of antipsychotics, and allowed the postulate of the firsts biological hypotheses about the schizophrenia etiology. Haloperidol has been included in the World Health Organisation's list of essential medicines.