Preclinical efficacy profiles of the sigma-1 modulator E1R and of fenfluramine in two chronic mouse epilepsy models.

OBJECTIVE: Given its key homeostatic role affecting mitochondria, ionotropic and metabotropic receptors, and voltage-gated ion channels, sigma-1 receptor (Sig1R) represents an interesting target for epilepsy management. Antiseizure effects of the positive allosteric modulator E1R have already been reported in acute seizure models. Although modulation of serotonergic neurotransmission is considered the main mechanism of action of fenfluramine, its interaction with Sig1R may be of additional relevance. METHODS: To further explore the potential of Sig1R as a target, we assessed the efficacy and tolerability of E1R and fenfluramine in two chronic mouse models, including an amygdala kindling paradigm and the intrahippocampal kainate model. The relative contribution of the interaction with Sig1R was analyzed using combination experiments with the Sig1R antagonist NE-100. RESULTS: Whereas E1R exerted pronounced dose-dependent antiseizure effects at well-tolerated doses in fully kindled mice, only limited effects were observed in response to fenfluramine, without a clear dose dependency. In the intrahippocampal kainate model, E1R failed to influence electrographic seizure activity. In contrast, fenfluramine significantly reduced the frequency of electrographic seizure events and their cumulative duration. Pretreatment with NE-100 reduced the effects of E1R and fenfluramine in the kindling model. Surprisingly, pre-exposure to NE-100 in the intrahippocampal kainate model rather enhanced and prolonged fenfluramine's antiseizure effects. SIGNIFICANCE: In conclusion, the kindling data further support Sig1R as an interesting target for novel antiseizure medications. However, it is necessary to further explore the preclinical profile of E1R in chronic epilepsy models with spontaneous seizures. Despite the rather limited effects in the kindling paradigm, the findings from the intrahippocampal kainate model suggest that it is of interest to further assess a possible broad-spectrum potential of fenfluramine.

Sigma Receptor Ligands Are Potent Antiprion Compounds that Act Independently of Sigma Receptor Binding.

Prion diseases are invariably fatal neurodegenerative diseases of humans and other animals for which there are no effective treatment options. Previous work from our laboratory identified phenethylpiperidines as a novel class of anti-prion compounds. While working to identify the molecular target(s) of these molecules, we unexpectedly discovered ten novel antiprion compounds based on their known ability to bind to the sigma receptors, σ1R and σ2R, which are currently being tested as therapeutic or diagnostic targets for cancer and neuropsychiatric disorders. Surprisingly, however, knockout of the respective genes encoding σ1R and σ2R (Sigmar1 and Tmem97) in prion-infected N2a cells did not alter the antiprion activity of these compounds, demonstrating that these receptors are not the direct targets responsible for the antiprion effects of their ligands. Further investigation of the most potent molecules established that they are efficacious against multiple prion strains and protect against downstream prion-mediated synaptotoxicity. While the precise details of the mechanism of action of these molecules remain to be determined, the present work forms the basis for further investigation of these compounds in preclinical studies. Given the therapeutic utility of several of the tested compounds, including rimcazole and haloperidol for neuropsychiatric conditions, (+)-pentazocine for neuropathic pain, and the ongoing clinical trials of SA 4503 and ANAVEX2-73 for ischemic stroke and Alzheimer's disease, respectively, this work has immediate implications for the treatment of human prion disease.

Allosteric Modulation of the Sigma-1 Receptor Elicits Antipsychotic-like Effects.

Allosteric modulation represents an important approach in drug discovery because of its advantages in safety and selectivity. SOMCL-668 is the first selective and potent sigma-1 receptor allosteric modulator, discovered in our laboratory. The present work investigates the potential therapeutic effects of SOMCL-668 on phencyclidine (PCP)-induced schizophrenia-related behavior in mice and further elucidates underlying mechanisms for its antipsychotic-like effects. SOMCL-668 not only attenuated acute PCP-induced hyperactivity and PPI disruption, but also ameliorated social deficits and cognitive impairment induced by chronic PCP treatment. Pretreatment with the selective sigma-1 receptor antagonist BD1047 blocked the effects of SOMCL-668, indicating sigma-1 receptor-mediated responses. This was confirmed using sigma-1 receptor knockout mice, in which SOMCL-668 failed to ameliorate PPI disruption and hyperactivity induced by acute PCP and social deficits and cognitive impairment induced by chronic PCP treatment. Additionally, in vitro SOMCL-668 exerted positive modulation of sigma-1 receptor agonist-induced intrinsic plasticity in brain slices recorded by patch-clamp. Furthermore, in vivo lower dose of SOMCL-668 exerted positive modulation of improvement in social deficits and cognitive impairment induced by the selective sigma-1 agonist PRE084. Also, SOMCL-668 reversed chronic PCP-induced down-regulation in expression of frontal cortical p-AKT/AKT, p-CREB/CREB and BDNF in wide-type but not sigma-1 knockout mice. Moreover, administration of the PI3K/AKT inhibitor LY294002 abolished amelioration by SOMCL-668 of chronic PCP-induced schizophrenia-related behaviors by inhibition of BDNF expression. The present data provide initial, proof-of-concept evidence that allosteric modulation of the sigma-1 receptor may be a novel approach for the treatment of psychotic illness.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors.

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Pridopidine reduces mutant huntingtin-induced endoplasmic reticulum stress by modulation of the Sigma-1 receptor.

The endoplasmic reticulum (ER)-localized Sigma-1 receptor (S1R) is neuroprotective in models of neurodegenerative diseases, among them Huntington disease (HD). Recent clinical trials in HD patients and preclinical studies in cellular and mouse HD models suggest a therapeutic potential for the high-affinity S1R agonist pridopidine. However, the molecular mechanisms of the cytoprotective effect are unclear. We have previously reported strong induction of ER stress by toxic mutant huntingtin (mHtt) oligomers, which is reduced upon sequestration of these mHtt oligomers into large aggregates. Here, we show that pridopidine significantly ameliorates mHtt-induced ER stress in cellular HD models, starting at low nanomolar concentrations. Pridopidine reduced the levels of markers of the three branches of the unfolded protein response (UPR), showing the strongest effects on the PKR-like endoplasmic reticulum kinase (PERK) branch. The effect is S1R-dependent, as it is abolished in cells expressing mHtt in which the S1R was deleted using CRISPR/Cas9 technology. mHtt increased the level of the detergent-insoluble fraction of S1R, suggesting a compensatory cellular mechanism that responds to increased ER stress. Pridopidine further enhanced the levels of insoluble S1R, suggesting the stabilization of activated S1R oligomers. These S1R oligomeric species appeared in ER-localized patches, and not in the mitochondria-associated membranes nor the ER-derived quality control compartment. The colocalization of S1R with the chaperone BiP was significantly reduced by mHtt, and pridopidine restored this colocalization to normal, unstressed levels. Pridopidine increased toxic oligomeric mHtt recruitment into less toxic large sodium dodecyl sulfate-insoluble aggregates, suggesting that this in turn reduces ER stress and cytotoxicity.

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.

Distinct Regulation of σ 1 Receptor Multimerization by Its Agonists and Antagonists in Transfected Cells and Rat Liver Membranes.

Extensive studies have shown that the σ 1 receptor (σ 1R) interacts with and modulates the activity of multiple proteins with important biological functions. Recent crystal structures of σ 1R as a homotrimer differ from a dimer-tetramer model postulated earlier. It remains inconclusive whether ligand binding regulates σ 1R oligomerization. Here, novel nondenaturing gel methods and mutational analysis were used to examine σ 1R oligomerization. In transfected cells, σ 1R exhibited as multimers, dimers, and monomers. Overall, σ 1R agonists decreased, whereas σ 1R antagonists increased σ 1R multimers, suggesting that agonists and antagonists differentially affect the stability of σ 1R multimers. Endogenous σ 1R in rat liver membranes also showed similar regulation of oligomerization as in cells. Mutations at key residues lining the trimerization interface (Arg119, Asp195, Phe191, Trp136, and Gly91) abolished multimerization without disrupting dimerization. Intriguingly, truncation of the N terminus reduced σ 1R to apparent monomer. These results demonstrate that multiple domains play crucial roles in coordinating high-order quaternary organization of σ 1R. The E102Q σ 1R mutant implicated in juvenile amyotrophic lateral sclerosis formed dimers only, suggesting that dysregulation of σ 1R multimeric assembly may impair its function. Interestingly, oligomerization of σ 1R was pH-dependent and correlated with changes in [3H](+)-pentazocine binding affinity and Bmax Combined with mutational analysis, it is reasoned that σ 1R multimers possess high-affinity and high-capacity [3H](+)-pentazocine binding, whereas monomers likely lack binding. These results suggest that σ 1R may exist in interconvertible oligomeric states in a dynamic equilibrium. Further exploration of ligand-regulated σ 1R multimerization may provide novel approaches to modulate the function of σ 1R and its interacting proteins. SIGNIFICANCE STATEMENT: The σ 1 receptor (σ 1R) modulates the activities of various partner proteins. Recently, crystal structures of σ 1R were elucidated as homotrimers. This study used novel nondenaturing gel methods to examine σ1R oligomerization in transfected cells and rat liver membranes. Overall, agonist binding decreased, whereas antagonist binding increased σ 1R multimers, which comprised trimers and larger units. σ 1R multimers were shown to bind [3H](+)-pentazocine with high affinity and high capacity. Furthermore, mutational analysis revealed a crucial role of its N-terminal domain in σ 1R multimerization.

Ketamine: The final frontier or another depressing end?

Two decades ago, the observation of a rapid and sustained antidepressant response after ketamine administration provided an exciting new avenue in the search for more effective therapeutics for the treatment of clinical depression. Research elucidating the mechanism(s) underlying ketamine's antidepressant properties has led to the development of several hypotheses, including that of disinhibition of excitatory glutamate neurons via blockade of N-methyl-d-aspartate (NMDA) receptors. Although the prominent understanding has been that ketamine's mode of action is mediated solely via the NMDA receptor, this view has been challenged by reports implicating other glutamate receptors such as AMPA, and other neurotransmitter systems such as serotonin and opioids in the antidepressant response. The recent approval of esketamine (Spravato™) for the treatment of depression has sparked a resurgence of interest for a deeper understanding of the mechanism(s) underlying ketamine's actions and safe therapeutic use. This review aims to present our current knowledge on both NMDA and non-NMDA mechanisms implicated in ketamine's response, and addresses the controversy surrounding the antidepressant role and potency of its stereoisomers and metabolites. There is much that remains to be known about our understanding of ketamine's antidepressant properties; and although the arrival of esketamine has been received with great enthusiasm, it is now more important than ever that its mechanisms of action be fully delineated, and both the short- and long-term neurobiological/functional consequences of its treatment be thoroughly characterized.

Environmental Control of Astrocyte Pathogenic Activities in CNS Inflammation.

Genome-wide studies have identified genetic variants linked to neurologic diseases. Environmental factors also play important roles, but no methods are available for their comprehensive investigation. We developed an approach that combines genomic data, screens in a novel zebrafish model, computational modeling, perturbation studies, and multiple sclerosis (MS) patient samples to evaluate the effects of environmental exposure on CNS inflammation. We found that the herbicide linuron amplifies astrocyte pro-inflammatory activities by activating signaling via sigma receptor 1, inositol-requiring enzyme-1α (IRE1α), and X-box binding protein 1 (XBP1). Indeed, astrocyte-specific shRNA- and CRISPR/Cas9-driven gene inactivation combined with RNA-seq, ATAC-seq, ChIP-seq, and study of patient samples suggest that IRE1α-XBP1 signaling promotes CNS inflammation in experimental autoimmune encephalomyelitis (EAE) and, potentially, MS. In summary, these studies define environmental mechanisms that control astrocyte pathogenic activities and establish a multidisciplinary approach for the systematic investigation of the effects of environmental exposure in neurologic disorders.

Neuroprotective Efficacy of a Sigma 2 Receptor/TMEM97 Modulator (DKR-1677) after Traumatic Brain Injury.

Compounds targeting the sigma 2 receptor, which we recently cloned and showed to be identical with transmembrane protein 97 (σ2R/TMEM97), are broadly applicable therapeutic agents currently in clinical trials for imaging in breast cancer and for treatment of Alzheimer's disease and schizophrenia. These promising applications coupled with our previous observation that the σ2R/TMEM97 modulator SAS-0132 has neuroprotective attributes and improves cognition in wild-type mice suggests that modulating σ2R/TMEM97 may also have therapeutic benefits in other neurodegenerative conditions such as traumatic brain injury (TBI). Herein, we report that DKR-1677, a novel derivative of SAS-0132 with increased affinity and selectivity for σ2R/Tmem97 ( Ki = 5.1 nM), is neuroprotective after blast-induced and controlled cortical impact (CCI) TBI in mice. Specifically, we discovered that treatment with DKR-1677 decreases axonal degeneration after blast-induced TBI and enhances survival of cortical neurons and oligodendrocytes after CCI injury. Furthermore, treatment with DKR-1677 preserves cognition in the Morris water maze after blast TBI. Our results support an increasingly broad role for σ2R/Tmem97 modulation in neuroprotection and suggest a new approach for treating patients suffering from TBI.

New piperidine-based derivatives as sigma receptor ligands. Synthesis and pharmacological evaluation.

The sigma receptor (σR) family has been considered mysterious for a long time. In fact, the σ2R subtype has been cloned only recently, revealing its identity as TMEM97, a NPC1-binding protein involved in cholesterol biosynthesis and implicated in the pathogenesis of cancer and neurologic disorders. With the aim of developing new chemical entities gifted with σR affinity, herein we report the design and synthesis of new piperidine-based alkylacetamide derivatives with mixed affinity towards both σ1 and σ2R subtypes.

AF710B, an M1/sigma-1 receptor agonist with long-lasting disease-modifying properties in a transgenic rat model of Alzheimer's disease.

INTRODUCTION: AF710B (aka ANAVEX 3-71) is a novel selective allosteric M1 muscarinic and sigma-1 receptor agonist. In 3×Tg-AD mice, AF710B attenuates cognitive deficits and decreases Alzheimer-like hallmarks. We now report on the long-lasting disease-modifying properties of AF710B in McGill-R-Thy1-APP transgenic (Tg) rats. METHODS: Chronic treatment with AF710B (10 µg/kg) was initiated in postplaque 13-month-old Tg rats. Drug or vehicle was administered orally daily for 4.5 months and interrupted 5 weeks before behavioral testing. RESULTS: AF710B long-term treatment reverted the cognitive deficits associated with advanced Alzheimer-like amyloid neuropathology in Tg rats. These effects were accompanied by reductions in amyloid pathology and markers of neuroinflammation and increases in amyloid cerebrospinal fluid clearance and levels of a synaptic marker. Importantly, these effects were maintained following a 5-week interruption of the treatment. DISCUSSION: With M1/sigma-1 activity and long-lasting disease-modifying properties at low dose, AF710B is a promising novel therapeutic agent for treating Alzheimer's disease.

Effects of Long-Term Alcohol Drinking on the Dopamine D2 Receptor: Gene Expression and Heteroreceptor Complexes in the Striatum in Rats.

BACKGROUND: Reduced dopamine D2 receptor (D2R) ligand binding has repeatedly been demonstrated in the striatum of humans with alcohol use disorder (AUD). The attenuated D2R binding has been suggested to reflect a reduced D2R density, which in turn has been proposed to drive craving and relapse. However, results from rodent studies addressing the effects of alcohol drinking on D2R density have been inconsistent. METHODS: A validated alcohol drinking model (intermittent access to 20% alcohol) in Wistar rats was used to study the effects of voluntary alcohol drinking (at least 12 weeks) on the D2R in the striatum compared to age-matched alcohol-naïve control rats. Reverse transcriptase quantitative PCR was used to quantify isoform-specific Drd2 gene expression levels. Using bisulfite pyrosequencing, DNA methylation levels of a regulatory region of the Drd2 gene were determined. In situ proximity ligation assay was used to measure densities of D2R receptor complexes: D2R-D2R, adenosine A2A receptor (A2AR)-D2R, and sigma1 receptor (sigma1R)-D2R. RESULTS: Long-term voluntary alcohol drinking significantly reduced mRNA levels of the long D2R isoform in the nucleus accumbens (NAc) but did not alter CpG methylation levels in the analyzed sequence of the Drd2 gene. Alcohol drinking also reduced the striatal density of D2R-D2R homoreceptor complexes, increased the density of A2AR-D2R heteroreceptor complexes in the NAc shell and the dorsal striatum, and decreased the density of sigma1R-D2R heteroreceptor complexes in the dorsal striatum. CONCLUSIONS: The present results on long-term alcohol drinking might reflect reduced D2R levels through reductions in D2R-D2R homoreceptor complexes and gene expression. Furthermore, based on antagonistic interactions between A2AR and D2R, an increased density of A2AR-D2R heteroreceptor complexes might indicate a reduced affinity and signaling of the D2R population within the complex. Hence, both reduced striatal D2R levels and reduced D2R protomer affinity within the striatal A2AR-D2R complex might underlie reduced D2R radioligand binding in humans with AUD. This supports the hypothesis of a hypodopaminergic system in AUD and suggests the A2AR-D2R heteroreceptor complex as a potential novel treatment target.

Amyloid toxicity is enhanced after pharmacological or genetic invalidation of the σ1 receptor.

The sigma-1 receptor (S1R) is a molecular chaperone which activity modulates several intracellular signals including calcium mobilization at mitochondria-associated endoplasmic reticulum membranes. S1R agonists are potent neuroprotectants against neurodegenerative insults and particularly in rodent models of Alzheimer's disease (AD). We here analyzed whether S1R inactivation modifies vulnerability to amyloid toxicity in AD models. Two strategies were used: (1) amyloid ß[25-35] (Aß25-35) peptide (1, 3, 9nmol) was injected intracerebroventricularly in mice treated repeatedly with the S1R antagonist NE-100 or in S1RKO mice, and (2) WT, APPSweInd, S1RKO, and APPSweInd/S1RKO mice were created and female littermates analyzed at 8 months of age. Learning deficits, oxidative stress, Bax level and BDNF content in the hippocampus were analyzed. Aß25-35 induced learning impairment, oxidative stress, Bax induction and BDNF alteration at lower dose in NE-100-treated mice or S1RKO mice as compared to WT animals. The extent of learning deficits and biochemical alterations were also higher in APPSweInd/S1RKO mice as compared to WT, APPSweInd, and S1RKO animals. S1R inactivation or altered S1R expression augmented the pathological status in pharmacologic and genetic AD mouse models. These observations, in relation with the well-known protective effects of S1R agonists, are coherent with a role of signal amplifier in neurodegeneration and neuroprotection proposed for S1R in AD and related neurodegenerative disorders.

The sigma-1 receptor modulates methamphetamine dysregulation of dopamine neurotransmission.

Dopamine neurotransmission is highly dysregulated by the psychostimulant methamphetamine, a substrate for the dopamine transporter (DAT). Through interactions with DAT, methamphetamine increases extracellular dopamine levels in the brain, leading to its rewarding and addictive properties. Methamphetamine also interacts with the sigma-1 receptor (σ1R), an inter-organelle signaling modulator. Using complementary strategies, we identified a novel mechanism for σ1R regulation of dopamine neurotransmission in response to methamphetamine. We found that σ1R activation prevents methamphetamine-induced, DAT-mediated increases in firing activity of dopamine neurons. In vitro and in vivo amperometric measurements revealed that σ1R activation decreases methamphetamine-stimulated dopamine efflux without affecting basal dopamine neurotransmission. Consistent with these findings, σ1R activation decreases methamphetamine-induced locomotion, motivated behavior, and enhancement of brain reward function. Notably, we revealed that the σ1R interacts with DAT at or near the plasma membrane and decreases methamphetamine-induced Ca2+ signaling, providing potential mechanisms. Broadly, these data provide evidence for σ1R regulation of dopamine neurotransmission and support the σ1R as a putative target for the treatment of methamphetamine addiction.

Introduction to Sigma Proteins: Evolution of the Concept of Sigma Receptors.

For over 40 years, scientists have endeavored to understand the so-called sigma receptors. During this time, the concept of sigma receptors has continuously and significantly evolved. With thousands of publications on the subject, these proteins have been implicated in various diseases, disorders, and physiological processes. Nevertheless, we are just beginning to understand what sigma proteins do and how they work. Two subtypes have been identified, Sigma1 and Sigma2. Whereas Sigma1 (also known as sigma-1 receptor, Sig1R, σ1 receptor, and several other names) was cloned over 20 years ago, Sigma2 (sigma-2 receptor, σ2 receptor) was cloned very recently and had remained a pharmacologically defined entity. In this volume, we will focus primarily on Sigma1. We will highlight several key subject areas in which Sigma1 has been well characterized as well as (re)emerging areas of interest. Despite the large number of publications regarding Sigma1, several fundamental questions remain unanswered or only partially answered. Most of what we know about Sigma1 comes from pharmacological studies; however, a clearly defined molecular mechanism of action remains elusive. One concept has become clear; Sigma1 is not a traditional receptor. Sigma1 is now considered a unique pharmacologically regulated integral membrane chaperone or scaffolding protein. A number of landmark discoveries over the past decade have begun to reshape the concept of sigma receptors. With the rapid emergence of new information, development of new tools, and changing conceptual frameworks, the field is poised for a period of accelerated progress.

Sigma1 Pharmacology in the Context of Cancer.

Sigma1 (also known as sigma-1 receptor, Sig1R, σ1 receptor) is a unique pharmacologically regulated integral membrane chaperone or scaffolding protein. The majority of publications on the subject have focused on the neuropharmacology of Sigma1. However, a number of publications have also suggested a role for Sigma1 in cancer. Although there is currently no clinically used anti-cancer drug that targets Sigma1, a growing body of evidence supports the potential of Sigma1 ligands as therapeutic agents to treat cancer. In preclinical models, compounds with affinity for Sigma1 have been reported to inhibit cancer cell proliferation and survival, cell adhesion and migration, tumor growth, to alleviate cancer-associated pain, and to have immunomodulatory properties. This review will highlight that although the literature supports a role for Sigma1 in cancer, several fundamental questions regarding drug mechanism of action and the physiological relevance of aberrant SIGMAR1 transcript and Sigma1 protein expression in certain cancers remain unanswered or only partially answered. However, emerging lines of evidence suggest that Sigma1 is a component of the cancer cell support machinery, that it facilitates protein interaction networks, that it allosterically modulates the activity of its associated proteins, and that Sigma1 is a selectively multifunctional drug target.

Allosteric Modulation of Opioid G-Protein Coupled Receptors by Sigma1 Receptors.

Since their proposal in 1976, the concept of sigma1 receptors has been continually evolving. Initially thought to be a member of the opioid receptor family, molecular studies have now identified its genes and established its structure crystallographically. Much effort has now revealed its importance as a chaperone in the endoplasmic reticulum, but its functions extend beyond this. Sigma1 receptors have been associated with a host of signaling systems. Evidence over the past 20 years has established the modulatory effects of sigma1 ligands on opioid systems. Despite their inability to bind directly to opioid receptors, sigma1 ligands can modulate opioid analgesia in vivo and signal transduction mechanisms in vitro. Furthermore, sigma1 receptors can physically associate with GPCRs. Together, these findings show that sigma1 ligands can function as allosteric modulators of GPCR function through their association with the sigma1 receptors, which are in direct physical association with opioid receptors, members of the G-protein coupled family of receptors.

Biodistribution and Radiation Dosimetry of 18F-FTC-146 in Humans.

The purpose of this study was to assess safety, biodistribution, and radiation dosimetry in humans for the highly selective σ-1 receptor PET agent 18F-6-(3-fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one (18F-FTC-146). Methods: Ten healthy volunteers (5 women, 5 men; age ± SD, 34.3 ± 6.5 y) were recruited, and written informed consent was obtained from all participants. Series of whole-body PET/MRI examinations were acquired for up to 3 h after injection (357.2 ± 48.8 MBq). Blood samples were collected, and standard vital signs (heart rate, pulse oximetry, and body temperature) were monitored at regular intervals. Regions of interest were delineated, time-activity curves were calculated, and organ uptake and dosimetry were estimated. Results: All subjects tolerated the PET/MRI examination well, and no adverse reactions to 18F-FTC-146 were reported. High accumulation of 18F-FTC-146 was observed in σ-1 receptor-dense organs such as the pancreas and spleen, moderate uptake in the brain and myocardium, and low uptake in bone and muscle. High uptake was also observed in the kidneys and bladder, indicating renal tracer clearance. The effective dose of 18F-FTC-146 was 0.0259 ± 0.0034 mSv/MBq (range, 0.0215-0.0301 mSv/MBq). Conclusion: First-in-human studies with clinical-grade 18F-FTC-146 were successful. Injection of 18F-FTC-146 is safe, and absorbed doses are acceptable. The potential of 18F-FTC-146 as an imaging agent for a variety of neuroinflammatory diseases is currently under investigation.