Sigma-2 receptor ligands and their perspectives in cancer diagnosis and therapy

Structure-Affinity relationships of novel σ2R/TMEM97 ligands

The sigma 2 receptor (σ2R), which was recently identified as the transmembrane protein 97 (TMEM97), is increasingly attracting interest as a possible therapeutic target for indications in neuroscience. Toward identifying novel modulators of σ2R/TMEM97, we prepared a collection of benzoxazocine, benzomorphan, and methanobenzazepine ligands related to the known bioactive norbenzomorphans DKR-1677, FEM-1689, and EES-1686 and determined their Ki values for σ2R/TMEM97 and the sigma 1 receptor (σ1R). The σ2R/TMEM97 binding affinities and selectivities relative to σ1R of these new benzoxazocine, benzomorphan, and methanobenzazepine analogs are lower, often significantly lower, than their respective norbenzomorphan counterparts, suggesting the spatial orientation of pharmacophoric substituents is critical for binding to the two proteins. The benzoxazocine, benzomorphan, and methanobenzazepine congeners of DKR-1677 and FEM-1689 tend to be weakly selective for σ2R/TMEM97 versus σ1R, whereas EES-1686 derivatives exhibit the greatest selectivity, suggesting the size and/or nature of the substituent on the nitrogen atom of the scaffold may be important for selectivity. Computational docking studies were performed for the 1S,5R-and 1R,5S-enantiomers of DKR-1677, FEM-1689, and EES-1686 and their benzoxazocine, benzomorphan, and methanobenzazepine counterparts. These computations predict that the protonated amino group of each ligand forms a highly conserved salt bridge and a H-bonding interaction with Asp29 as well as a cation-π interaction with Tyr150 of σ2R/TMEM97. These electrostatic interactions are major driving forces for binding to σ2R/TMEM97 and are similar, though not identical, for each ligand. Other interactions within the well-defined binding pocket also tend to be comparable, but there are some major differences in how the hydrophobic aryl groups of various ligands interact with the protein surface external to the binding pocket. Overall, these studies show that the orientations of aryl and N-substituents on the norbenzomorphan and related scaffolds are important determinants of binding affinity of σ2R/TMEM97 ligands, and small changes can have significant effects upon binding profiles.

Glitches in the brain: the dangerous relationship between radiotherapy and brain fog

Up to approximately 70% of cancer survivors report persistent deficits in memory, attention, speed of information processing, multi-tasking, and mental health functioning, a series of symptoms known as "brain fog." The severity and duration of such effects can vary depending on age, cancer type, and treatment regimens. In particular, every year, hundreds of thousands of patients worldwide undergo radiotherapy (RT) for primary brain tumors and brain metastases originating from extracranial tumors. Besides its potential benefits in the control of tumor progression, recent studies indicate that RT reprograms the brain tumor microenvironment inducing increased activation of microglia and astrocytes and a consequent general condition of neuroinflammation that in case it becomes chronic could lead to a cognitive decline. Furthermore, radiation can induce endothelium reticulum (ER) stress directly or indirectly by generating reactive oxygen species (ROS) activating compensatory survival signaling pathways in the RT-surviving fraction of healthy neuronal and glial cells. In particular, the anomalous accumulation of misfolding proteins in neuronal cells exposed to radiation as a consequence of excessive activation of unfolded protein response (UPR) could pave the way to neurodegenerative disorders. Moreover, exposure of cells to ionizing radiation was also shown to affect the normal proteasome activity, slowing the degradation rate of misfolded proteins, and further exacerbating ER-stress conditions. This compromises several neuronal functions, with neuronal accumulation of ubiquitinated proteins with a consequent switch from proteasome to immunoproteasome that increases neuroinflammation, a crucial risk factor for neurodegeneration. The etiology of brain fog remains elusive and can arise not only during treatment but can also persist for an extended period after the end of RT. In this review, we will focus on the molecular pathways triggered by radiation therapy affecting cognitive functions and potentially at the origin of so-called "brain fog" symptomatology, with the aim to define novel therapeutic strategies to preserve healthy brain tissue from cognitive decline.

Design, development and bio-evaluation of a novel radio-ligand 99mTc-THQ-DTPA as a sigma 2 receptor specific breast tumor imaging agent

Over-expression of sigma-2 receptor in cancer cells provides an opportunity to develop molecular probes for diagnosis, even for non-receptor specific malignancies like triple negative breast cancers. In this work, a novel sigma-2 receptor ligand [THQ-DTPA] has been synthesized and characterized using 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (THQ) and diethylenetriaminepentaacetic acid (DTPA). The ligand is further chelated with 99mTc for application as metal based radiotracer [99mTc-THQ-DTPA]. Radiolabelling with 99mTc was achieved in an excellent yield of 98.0 ± 0.5% using stannous chloride as a reducing agent. The radioligand was found to be stable in human serum up-to 24 h, bio-compatible with less than 4% hemolysis, and exhibited high binding with sigma receptors isolated from rat liver membrane (Kd of 16.32 ± 4.93 nM and Bmax of 0.5232 ± 0.06 pmol/mg). Bio-distribution studies in triple-negative breast tumor bearing nude mice showed high tumor uptake after 30 min of injection with tumor/muscle (T/M) ratio of 3.58 ± 0.09. At 240 min, the T/M ratio (2.84 ± 0.20) decreased by 35% when administered in sigma blocked tumor bearing mice (1.81 ± 0.16) suggesting the selectivity of the ligand. Tumor imaging in gamma camera indicated a contrast of 3.56 at 30 min p.i. The above findings indicate that the ligand 99mTc-THQ-DTPA binds to sigma-2 receptors with high affinity and has potential for triple-negative breast tumor imaging.

Highly specific σ2R/TMEM97 ligand FEM-1689 alleviates neuropathic pain and inhibits the integrated stress response

The sigma 2 receptor (σ2R) was described pharmacologically more than three decades ago, but its molecular identity remained obscure until recently when it was identified as transmembrane protein 97 (TMEM97). We and others have shown that σ2R/TMEM97 ligands alleviate mechanical hypersensitivity in mouse neuropathic pain models with a time course wherein maximal antinociceptive effect is approximately 24 h following dosing. We sought to understand this unique antineuropathic pain effect by addressing two key questions: do these σ2R/TMEM97 compounds act selectively via the receptor, and what is their downstream mechanism on nociceptive neurons? Using male and female conventional knockout mice for Tmem97, we find that a σ2R/TMEM97 binding compound, FEM-1689, requires the presence of the gene to produce antinociception in the spared nerve injury model in mice. Using primary mouse dorsal root ganglion neurons, we demonstrate that FEM-1689 inhibits the integrated stress response (ISR) and promotes neurite outgrowth via a σ2R/TMEM97-specific action. We extend the clinical translational value of these findings by showing that FEM-1689 reduces ISR and p-eIF2α levels in human sensory neurons and that it alleviates the pathogenic engagement of ISR by methylglyoxal. We also demonstrate that σ2R/TMEM97 is expressed in human nociceptors and satellite glial cells. These results validate σ2R/TMEM97 as a promising target for further development for the treatment of neuropathic pain.

3,3-Disubstituted 3,4-Dihydro-1,2,4-benzotriazines: Chemistry, Biological Activity, and Affinity to Sigma Receptors

By reducing the 2-nitrophenylhydrazone of cyclohexanone with sodium dithionite, an unexpected yellow compound was obtained instead of the corresponding colorless amino derivative. Many years later, the structure of this compound, namely, cyclohexane-3-spiro-3,4-dihydro-1,2,4-benzotriazine, was demonstrated. From that time, the reduction of 2-nitrophenylhydrazones of different kinds of ketones, followed by air oxidation of the initially formed amino compounds, has represented a general way to synthesize a variety of 3,3-disubstituted 3,4-dihydro-1,2,4-benzotriazines. Many derivatives have been obtained so far by a single research group, and most of them have demonstrated interesting pharmacological activities, mainly antihypertensive, anti-inflammatory, and diuretic effects and other activities with lower diffusion. Moreover, 3,3-disubstituted 3,4-dihydro-1,2,4-benzotriazines represent a novel class of ligands for sigma receptors, with nanomolar affinity to the σ1 subtype. This property might promote the development of agents for cardiovascular, neurodegenerative, and proliferative pathologies. The present commentary, by collecting compounds and biological results obtained so far, intends to celebrate the centennial of the discovery of the first member of this class of compounds and to promote further investigation in the field.

Highly specific σ2R/TMEM97 ligand alleviates neuropathic pain and inhibits the integrated stress response

The Sigma 2 receptor (σ2R) was described pharmacologically more than three decades ago, but its molecular identity remained obscure until recently when it was identified as transmembrane protein 97 (TMEM97). We and others have shown that σ2R/TMEM97 ligands alleviate mechanical hypersensitivity in mouse neuropathic pain models with a time course wherein maximal anti-nociceptive effect is approximately 24 hours following dosing. We sought to understand this unique anti-neuropathic pain effect by addressing two key questions: do these σ2R/TMEM97 compounds act selectively via the receptor, and what is their downstream mechanism on nociceptive neurons? Using male and female conventional knockout (KO) mice for Tmem97, we find that a new σ2R/TMEM97 binding compound, FEM-1689, requires the presence of the gene to produce anti-nociception in the spared nerve injury model in mice. Using primary mouse dorsal root ganglion (DRG) neurons, we demonstrate that FEM-1689 inhibits the integrated stress response (ISR) and promotes neurite outgrowth via a σ2R/TMEM97-specific action. We extend the clinical translational value of these findings by showing that FEM-1689 reduces ISR and p-eIF2α levels in human sensory neurons and that it alleviates the pathogenic engagement of ISR by methylglyoxal. We also demonstrate that σ2R/TMEM97 is expressed in human nociceptors and satellite glial cells. These results validate σ2R/TMEM97 as a promising target for further development for the treatment of neuropathic pain.

Development of a Highly Specific 18F-Labeled Radioligand for Imaging of the Sigma-2 Receptor in Brain Tumors

Novel ligands with the 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline or 5,6-dimethoxyisoindoline pharmacophore were designed and synthesized for evaluation of their structure-activity relationship to the sigma-2 (σ2) receptor and developed as suitable PET radioligands. Compound 1 was found to possess nanomolar affinity (Ki(σ1) = 2.57 nM) for the σ2 receptor, high subtype selectivity (>2000-fold), and high selectivity over 40 other receptors and transporters. Radioligand [18F]1 was prepared with radiochemical yield of 37-54%, > 99% radiochemical purity, and molar activity of 107-189 GBq/μmol. Biodistribution and blocking studies in mice and micro-PET/CT imaging of [18F]1 in rats indicated excellent binding specificity to the σ2 receptors in vivo. Micro-PET/CT imaging of [18F]1 in the U87MG glioma xenograft model demonstrated clear tumor visualization with high tumor uptake and tumor-to-background ratio. Co-injection with CM398 (5 μmol/kg) led to a remarkable reduction of tumor uptake (80%, 60-70 min), indicating high specific binding of [18F]1 in U87MG glioma xenografts.

Discovery of WLB-89462, a New Drug-like and Highly Selective σ2 Receptor Ligand with Neuroprotective Properties

The synthesis and pharmacological activity of a new series of isoxazolylpyrimidines as sigma-2 receptor (σ2R) ligands are reported. Modification of a new hit retrieved in an HTS campaign allowed the identification of the compound WLB-89462 (20c) with good σ2R affinity (Ki = 13 nM) and high selectivity vs both the σ1R (Ki = 1777 nM) and a general panel of 180 targets. It represents one of the first σ2R ligands with drug-like properties, linked to a good physicochemical and ADMET profile (good solubility, no CYP inhibition, good metabolic stability, high permeability, brain penetration, and high oral exposure in rodents). Compound 20c shows neuroprotective activity in vitro and improves short-term memory impairment induced by hippocampal injection of amyloid β peptide in rats. Together with the promising effects in the chronic models where 20c is currently being evaluated, these results pave the way toward its clinical development as a neuroprotective agent.

Structure-affinity relationships of stereoisomers of norbenzomorphan-derived σ2R/TMEM97 modulators

The sigma 2 receptor (σ₂R), which is identical to transmembrane protein 97 (TMEM97), is attracting increasing interest as a possible therapeutic target for various indications in neuroscience. In continuation of a program to identify novel compounds that bind with high affinity and selectivity to σ₂R/TMEM97, we performed structure-affinity-relationship (SAfiR) studies of several sets of σ₂R/TMEM97 ligands having a B-norbenzomorphan ring core. Binding data for σ₂R/TMEM97 and σ₁R of several enantiomeric pairs of piperazine-substituted norbenzomorphans show the (1S,5R)-enantiomers have affinities (K_i = 9-75 nM) for σ₂R/TMEM97 that are 2-3-fold higher than their enantiomorphic (1R,5S)-analogs; however, there is no clear trend for selectivity for σ₂R/TMEM97 vs σ₁R. A series of N-alkyl piperazino (1S,5R)-norbenzomorphans was then evaluated, and with the exception of compounds having N-alkyl groups substituted with oxygen or amino groups at C (2) of an ethylene chain, K_i values for σ₂R/TMEM97 are less than 25 nM, and several compounds have good selectivities (ca 7-16-fold) for σ₂R/TMEM97 vs σ₁R. Mono-substituted carbobenzyloxy analogs have K_i values for σ₂R/TMEM97 comparable to the unsubstituted parent (K_i = ca 7-27 nM), but replacing the N-acyloxy group with N-acyl or N-arylsulfonyl groups provides analogs having lower affinity and selectivity. Some congeners with bioisosteric replacements of the piperazine group on the (1S,5R)-norbenzomorphan core have high affinity (K_i = <30 nM) for σ₂R/TMEM97, but selectivities are modest. Computational docking studies for racemic pairs of piperazino norbenzomorphans show that individual (1S,5R)- and (1R,5S)-enantiomers adopt distinct poses upon binding to σ₂R/TMEM97, whereas ligands belongingto the same enantiomeric series adopt closely similar binding poses. The protonated amino group in each of the enantiomorphic ligands engages in highly conserved salt bridges with Asp29 and cation-π interactions with Tyr150 that are the primary determinants of binding affinity. There is no correlation between any of the computational parameter outputs and K_i values, but this is unsurprising given the small energetic differences involved. Modeling also suggest sthat some compounds can extend deeper into σ₂R/TMEM97 binding pocket forming salt bridges with Glu73.

Targeting Sigma Receptors for the Treatment of Neurodegenerative and Neurodevelopmental Disorders

The sigma-1 receptor is a 223 amino acid-long protein with a recently identified structure. The sigma-2 receptor is a genetically unrelated protein with a similarly shaped binding pocket and acts to influence cellular activities similar to the sigma-1 receptor. Both proteins are highly expressed in neuronal tissues. As such, they have become targets for treating neurological diseases, including Alzheimer's disease (AD), Huntington's disease (HD), Parkinson's disease (PD), multiple sclerosis (MS), Rett syndrome (RS), developmental and epileptic encephalopathies (DEE), and motor neuron disease/amyotrophic lateral sclerosis (MND/ALS). In recent years, there have been many pre-clinical and clinical studies of sigma receptor (1 and 2) ligands for treating neurological disease. Drugs such as blarcamesine, dextromethorphan and pridopidine, which have sigma-1 receptor activity as part of their pharmacological profile, are effective in treating multiple aspects of several neurological diseases. Furthermore, several sigma-2 receptor ligands are under investigation, including CT1812, rivastigmine and SAS0132. This review aims to provide a current and up-to-date analysis of the current clinical and pre-clinical data of drugs with sigma receptor activities for treating neurological disease.

From Cell to Symptoms: The Role of SARS-CoV-2 Cytopathic Effects in the Pathogenesis of COVID-19 and Long COVID

Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2) infection triggers various events from molecular to tissue level, which in turn is given by the intrinsic characteristics of each patient. Given the molecular diversity characteristic of each cellular phenotype, the possible cytopathic, tissue and clinical effects are difficult to predict, which determines the heterogeneity of COVID-19 symptoms. The purpose of this article is to provide a comprehensive review of the cytopathic effects of SARS-CoV-2 on various cell types, focusing on the development of COVID-19, which in turn may lead, in some patients, to a persistence of symptoms after recovery from the disease, a condition known as long COVID. We describe the molecular mechanisms underlying virus-host interactions, including alterations in protein expression, intracellular signaling pathways, and immune responses. In particular, the article highlights the potential impact of these cytopathies on cellular function and clinical outcomes, such as immune dysregulation, neuropsychiatric disorders, and organ damage. The article concludes by discussing future directions for research and implications for the management and treatment of COVID-19 and long COVID.

The Sigma Enigma: A Narrative Review of Sigma Receptors

The sigma-1 and sigma-2 receptors were first discovered in the 1960s and were thought to be a form of opioid receptors initially. Over time, more was gradually learned about these receptors, which are actually protein chaperones, and many of their unique or unusual properties can contribute to a range of important new therapeutic applications. These sigma receptors translocate in the body and regulate calcium homeostasis and mitochondrial bioenergetics and they also have neuroprotective effects. The ligands to which these sigma receptors respond are several and dissimilar, including neurosteroids, neuroleptics, and cocaine. There is controversy as to their endogenous ligands. Sigma receptors are also involved in the complex processes of cholesterol homeostasis and protein folding. While previous work on this topic has been limited, research has been conducted in multiple disease states, such as addiction, aging. Alzheimer's disease, cancer, psychiatric disorders, pain and neuropathic pain, Parkinson's disease, and others. There is currently increasing interest in sigma-1 and sigma-2 receptors as they provide potential therapeutic targets for many disease indications.

Design and Synthesis of Orally Active Quinolyl Pyrazinamides as Sigma 2 Receptor Ligands for the Treatment of Pancreatic Cancer

Sigma 2 receptor (σ2R) is overexpressed in select cancers and is regarded as a biomarker for tumor proliferation. σ2R ligands are emerging as promising theranostics for cancer and neurodegenerative diseases. Herein, we describe the design and synthesis of a series of novel quinolyl pyrazinamides as selective and potent σ2R ligands that show sub-micromolar potency in pancreatic cancer cell lines. Compounds 14 (JR1-157) and 17 (JR2-298) bind σ2R with K_i of 47 and 10 nM, respectively. Importantly, compound 14 has an oral bioavailability of 60% and shows significant in vivo efficacy without obvious toxicity in a syngeneic model of pancreatic cancer. The cytotoxicity of the quinolyl pyrazinamides significantly enhanced in the presence of copper and diminished in the presence of the copper-chelator tetrathiomolybdate. In conclusion, compound 14 is water-soluble, metabolically stable, orally active, and increases the expression of the autophagy marker LC3B and warrants further development for the treatment of pancreatic cancer.

An Evaluation of the Anticancer Properties of SYA014, a Homopiperazine-Oxime Analog of Haloperidol in Triple Negative Breast Cancer Cells

Triple negative breast cancer (TNBC) is a type of breast cancer associated with early metastasis, poor prognosis, high relapse rates, and mortality. Previously, we demonstrated that SYA013, a selective σ2RL, could inhibit cell proliferation, suppress migration, reduce invasion, and induce mitochondria-mediated apoptosis in MDA-MB-231 cell lines, although we were unable to demonstrate the direct involvement of sigma receptors. This study aimed to determine the anticancer properties and mechanisms of action of SYA014, [4-(4-(4-chlorophenyl)-1,4-diazepan-1-yl)-1-(4-fluorophenyl)butan-1-one oxime], an oxime analogue of SYA013, the contribution of its sigma-2 receptor (σ2R) binding, and its possible synergistic use with cisplatin to improve anticancer properties in two TNBC cell lines, MDA-MB-231 (Caucasian) and MDA-MB-468 (Black). In the present investigation, we have shown that SYA014 displays anticancer properties against cell proliferation, survival, metastasis and apoptosis in the two TNBC cell lines. Furthermore, a mechanistic investigation was conducted to identify the apoptotic pathway by which SYA014 induces cell death in MDA-MB-231 cells. Since SYA014 has a higher binding affinity for σ2R compared to σ1R, we tested the role of σ2R on the antiproliferative property of SYA014 with a σ2R blockade. We also attempted to evaluate the combination effect of SYA014 with cisplatin in TNBC cells.

Neuroprotective Effects of σ2R/TMEM97 Receptor Modulators in the Neuronal Model of Huntington's Disease

Huntington's disease (HD) is a genetic neurodegenerative disease caused by an expanded CAG repeat in the Huntingtin (HTT) gene that encodes for an expanded polyglutamine (polyQ) repeat in exon-1 of the human mutant huntingtin (mHTT) protein. The presence of this polyQ repeat results in neuronal degeneration, for which there is no cure or treatment that modifies disease progression. In previous studies, we have shown that small molecules that bind selectively to σ₂R/TMEM97 can have significant neuroprotective effects in models of Alzheimer's disease, traumatic brain injury, and several other neurodegenerative diseases. In the present work, we extend these investigations and show that certain σ₂R/TMEM97-selective ligands decrease mHTT-induced neuronal toxicity. We first synthesized a set of compounds designed to bind to σ₂R/TMEM97 and determined their binding profiles (K_i values) for σ₂R/TMEM97 and other proteins in the central nervous system. Modulators with high affinity and selectivity for σ₂R/TMEM97 were then tested in our HD cell model. Primary cortical neurons were cultured in vitro for 7 days and then co-transfected with either a normal HTT construct (Htt N-586-22Q/GFP) or the mHTT construct Htt-N586-82Q/GFP. Transfected neurons were treated with either σ₂R/TMEM97 or σ₁R modulators for 48 h. After treatment, neurons were fixed and stained with Hoechst, and condensed nuclei were quantified to assess cell death in the transfected neurons. Significantly, σ₂R/TMEM97 modulators reduce the neuronal toxicity induced by mHTT, and their neuroprotective effects are not blocked by NE-100, a selective σ₁R antagonist known to block neuroprotection by σ₁R ligands. These results indicate for the first time that σ₂R/TMEM97 modulators can protect neurons from mHTT-induced neuronal toxicity, suggesting that targeting σ₂R/TMEM97 may lead to a novel therapeutic approach to treat patients with HD.

Personalized Targeted Therapeutic Strategies against Oral Squamous Cell Carcinoma. An Evidence-Based Review of Literature

Oral squamous cell carcinoma (OSCC) is the most common type of malignant tumor in the head and neck, with a poor prognosis mainly due to recurrence and metastasis. Classical treatment modalities for OSCC like surgery and radiotherapy have difficulties in dealing with metastatic tumors, and together with chemotherapy, they have major problems related to non-specific cell death. Molecular targeted therapies offer solutions to these problems through not only potentially maximizing the anticancer efficacy but also minimizing the treatment-related toxicity. Among them, the receptor-mediated targeted delivery of anticancer therapeutics remains the most promising one. As OSCC exhibits a heterogeneous nature, selecting the appropriate receptors for targeting is the prerequisite. Hence, we reviewed the OSCC-associated receptors previously used in targeted therapy, focused on their biochemical characteristics and expression patterns, and discussed the application potential in personalized targeted therapy of OSCC. We hope that a better comprehension of this subject will help to provide the fundamental information for OSCC personalized therapeutic planning.

Accurate prediction of virus-host protein-protein interactions via a Siamese neural network using deep protein sequence embeddings

Prediction and understanding of virus-host protein-protein interactions (PPIs) have relevance for the development of novel therapeutic interventions. In addition, virus-like particles open novel opportunities to deliver therapeutics to targeted cell types and tissues. Given our incomplete knowledge of PPIs on the one hand and the cost and time associated with experimental procedures on the other, we here propose a deep learning approach to predict virus-host PPIs. Our method (Siamese Tailored deep sequence Embedding of Proteins [STEP]) is based on recent deep protein sequence embedding techniques, which we integrate into a Siamese neural network. After showing the state-of-the-art performance of STEP on external datasets, we apply it to two use cases, severe acute respiratory syndrome coronavirus 2 and John Cunningham polyomavirus, to predict virus-host PPIs. Altogether our work highlights the potential of deep sequence embedding techniques originating from the field of NLP as well as explainable artificial intelligence methods for the analysis of biological sequences.

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Deep learning approach (STEP) predicts virus protein to human host protein interactions

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It is based on recent deep protein sequence embeddings and Siamese neural network

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Prediction of PPIs of the JCV VP1 protein and of the SARS-CoV-2 spike protein

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Identify parts of sequences that most likely contribute to the PPI using explainable AI

The development of novel cell and tissue-specific therapies requires a profound knowledge about protein-protein interactions (PPIs). Identifying these PPIs with experimental approaches such as biochemical assays or yeast two-hybrid screens is cumbersome, costly, and at the same time difficult to scale. Computational approaches can help to prioritize huge amounts of possible PPIs by learning from biological sequences plus already known PPIs. In this work, we developed an approach that is based on recent deep protein sequence embedding techniques, which we integrate into a Siamese neural network architecture. We use this approach to train models by using protein sequence information and known PPIs. We apply the models to two use cases to predict virus protein to human host interactions. Altogether our work highlights the potential of deep sequence embedding techniques as well as explainable artificial intelligence methods for the analysis of biological sequence data.

Novel Sigma-2 receptor ligand A011 overcomes MDR in adriamycin-resistant human breast cancer cells by modulating ABCB1 and ABCG2 transporter function

Multidrug resistance (MDR) is thought to be one of the main reasons for the failure of chemotherapy in cancers. ATP-binding cassette subfamily B member 1 (ABCB1) or P-glycoprotein (P-gp) and ATP-binding cassette subfamily G member 2 (ABCG2) play indispensable roles in cancer cell MDR. Sigma-2 (σ2) receptor is considered to be a cancer biomarker and a potential therapeutic target due to its high expression in various proliferative tumors. Recently, σ2 receptor ligands have been shown to have promising cytotoxic effects against cancer cells and to modulate the activity of P-glycoprotein (ABCB1) in vitro experiments, but their specific effects and mechanisms remain to be elucidated. We found that A011, a σ2 receptor ligand with the structure of 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline, showed promising cytotoxicity against breast cancer MCF-7 and adriamycin-resistant MCF-7 (MCF-7/ADR), induced apoptosis, and reversed adriamycin (ADR) and paclitaxel resistance in MCF-7/ADR cells. Furthermore, we demonstrated that A011 increased the accumulation of rhodamine 123 and mitoxantrone in MCF-7/ADR cells. A011 significantly decreased the ATPase activity of the ABCB1 and down-regulated ABCG2 protein expression. In addition, A011, administered alone or in combination with ADR, significantly inhibited tumor growth in the MCF-7/ADR tumor-bearing nude mouse model. A011 may be a potential therapeutic agent for the treatment of tumor resistance.

Development of tumor-targeting aza-vesamicol derivatives with high affinity for sigma receptors for cancer theranostics

As sigma receptors are highly expressed on various cancer cells, radiolabeled sigma receptor ligands have been developed as imaging and therapeutic probes for cancer. Previously, we synthesized and evaluated a radioiodinated vesamicol derivative, 2-(4-[¹²⁵I](4-iodophenyl)piperidine)cyclohexanol ((+)-[¹²⁵I]pIV), and a radioiodinated aza-vesamicol derivative, trans-2-(4-(3-[¹²⁵I](4-iodophenyl)propyl)piperazin-1-yl)cyclohexan-1-ol ([¹²⁵I]2), as sigma-1 receptor-targeting probes. In order to obtain sigma receptor-targeting probes with superior biodistribution characteristics, we firstly synthesized twelve bromine-containing aza-vesamicol derivatives and evaluated their affinity for sigma receptors. One such derivative exhibited high selectivity for the sigma-1 receptor and another exhibited high affinity for both the sigma-1 and sigma-2 receptors. Thus, their halogen-substituted iodine- and radioiodine-containing compounds were prepared. The ¹²⁵I-labeled compounds exhibited high uptake in tumor and lower uptake in non-target tissues than the two previously developed and evaluated ¹²⁵I-labeled sigma receptor-targeting probes, [¹²⁵I]pIV and [¹²⁵I]2. Therefore, these novel radioiodine-labeled compounds should be promising as sigma receptor-targeting probes.

A011, a novel small-molecule ligand of σ2 receptor, potently suppresses breast cancer progression via endoplasmic reticulum stress and autophagy

Breast cancer has surpassed lung cancer to become the most commonly diagnosed cancer in women worldwide. Sigma-2 (σ₂) receptor is considered to be a potential therapeutic target for breast cancer because of its high expression in breast cancer cells and low expression in normal breast cells. Many σ₂ ligands have been reported to have excellent anticancer activity, but their mechanism of action has not been fully elucidated. We discovered that A011 had high affinity and selectivity for σ₂ receptor, reduced proliferation in five cancer cell lines, and significantly inhibited the monoclonal formation ability of MCF-7 cells. Furthermore, A011 rapidly increased the levels of intracellular Ca²⁺ and reactive oxygen species and induced autophagy. Molecular pharmacology studies revealed that A011 induced endoplasmic reticulum stress, activated the PERK-eIF2α-CHOP pathway and inhibited the activation of the PI3K-Akt-mTOR pathway, leading to cell apoptosis. In an in vivo tumor model, A011 showed obvious anti-tumor activity and no significant toxicity. More importantly, our study demonstrated for the first time that endoplasmic reticulum stress is the main mechanism of anti-cancer effects for σ₂ ligands, at least for A011. A011 may potentially be useful as a therapeutic agent for treating breast cancer.

Characterization of CM-398, a Novel Selective Sigma-2 Receptor Ligand, as a Potential Therapeutic for Neuropathic Pain

Sigma receptors modulate nociception, offering a potential therapeutic target to treat pain, but relatively little is known regarding the role of sigma-2 receptors (S2R) in nociception. The purpose of this study was to investigate the in vivo analgesic and anti-allodynic activity and liabilities of a novel S2R selective ligand, 1-[4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)butyl]-3-methyl-1,3-dihydro-1,3-benzimidazol-2-one (CM-398). The inhibition of thermal, induced chemical, or inflammatory pain as well as the allodynia resulting from chronic nerve constriction injury (CCI) model of neuropathic pain were assessed in male mice. CM-398 dose-dependently (10–45 mg/kg i.p.) reduced mechanical allodynia in the CCI neuropathic pain model, equivalent at the higher dose to the effect of the control analgesic gabapentin (50 mg/kg i.p.). Likewise, pretreatment (i.p.) with CM-398 dose-dependently produced antinociception in the acetic acid writhing test (ED50 (and 95% C.I.) = 14.7 (10.6–20) mg/kg, i.p.) and the formalin assay (ED50 (and 95% C.I.) = 0.86 (0.44–1.81) mg/kg, i.p.) but was without effect in the 55 °C warm-water tail-withdrawal assay. A high dose of CM-398 (45 mg/kg, i.p.) exhibited modest locomotor impairment in a rotarod assay and conditioned place aversion, potentially complicating the interpretation of nociceptive testing. However, in an operant pain model resistant to these confounds, mice experiencing CCI and treated with CM-398 demonstrated robust conditioned place preference. Overall, these results demonstrate the S2R selective antagonist CM-398 produces antinociception and anti-allodynia with fewer liabilities than established therapeutics, adding to emerging data suggesting possible mediation of nociception by S2R, and the development of S2R ligands as potential treatments for chronic pain.

Discovery and development of brain-penetrant 18F-labeled radioligands for neuroimaging of the sigma-2 receptors

We have discovered and synthesized a series of indole-based derivatives as novel sigma-2 (σ ₂) receptor ligands. Two ligands with high σ ₂ receptor affinity and subtype selectivity were then radiolabeled with F-18 in good radiochemical yields and purities, and evaluated in rodents. In biodistribution studies in male ICR mice, radioligand [¹⁸F]9, or 1-(4-(5,6-dimethoxyisoindolin-2-yl)butyl)-4-(2-[¹⁸F]fluoroethoxy)-1H-indole, was found to display high brain uptake and high brain-to-blood ratio. Pretreatment of animals with the selective σ ₂ receptor ligand CM398 led to significant reductions in both brain uptake (29%-54%) and brain-to-blood ratio (60%-88%) of the radioligand in a dose-dependent manner, indicating high and saturable specific binding of [¹⁸F]9 to σ ₂ receptors in the brain. Further, ex vivo autoradiography in male ICR mice demonstrated regionally heterogeneous specific binding of [¹⁸F]9 in the brain that is consistent with the distribution pattern of σ ₂ receptors. Dynamic positron emission tomography imaging confirmed regionally distinct distribution and high levels of specific binding for [¹⁸F]9 in the rat brain, along with appropriate tissue kinetics. Taken together, results from our current study indicated the novel radioligand [¹⁸F]9 as the first highly specific and promising imaging agent for σ ₂ receptors in the brain.

Brain-derived neurotrophic factor for high-throughput evaluation of selective Sigma-1 receptor ligands

The Sigma-1 receptor (S1R) is an endoplasmic reticulum (ER) chaperone protein that has been implicated in attenuating inflammatory stress-mediated brain injuries. Selective S1R agonists represent a new class of therapeutic agent for treating neuropsychiatric and neurodegenerative disorders, however, to date, no S1R ligand has been approved for therapeutic purposes. We used three potential methods on known and potential S1R ligands to develop an unambiguous high-throughput cell screen for S1R activity. We screened known and potential S1R ligands using radioligand binding and previously reported markers of S1R activity including BDNF release, modulation of IP3 mediated calcium release, and modulation of NGF-induced neurite sprouting. Here, we present results several prototypical S1R compounds and some compounds with the potential for drug repurposing. Using an in-situ ELISA approach we demonstrated that these compounds could stimulate S1R-mediated BDNF release, which is a valuable therapeutic property since BDNF plays a critical role in neuronal support. These compounds were classified as S1R agonists because the BDNF response was comparable to the prototypical agonist 4-PPBP and because it could be reversed by a S1R selective concentration of the antagonist BD1063. When modulation of IP3 mediated calcium response and NGF-induced neurite sprouting were used as a measure of S1R activation, we were unable to reproduce the published results and determined that they are not reliable measures for evaluating functional properties of S1R ligands.

Multifunctional thiosemicarbazones targeting sigma receptors: in vitro and in vivo antitumor activities in pancreatic cancer models

PURPOSE

Association of the metal chelating portion of thiosemicarbazone with the cytotoxic activity of sigma-2 receptors appears a promising strategy for the treatment of pancreatic tumors. Here, we developed a novel sigma-2 receptor targeting thiosemicarbazone (FA4) that incorporates a moiety associated with lysosome destabilization and ROS increase in order to design more efficient antitumor agents.

METHODS

The density of sigma receptors in pancreatic cancer cells was evaluated by flow cytometry. In these cells, cytotoxicity (MTT assay) and activation of ER- and mitochondria-dependent cell death pathways (mRNA expression of GRP78, ATF6, IRE1, PERK; ROS levels by MitoSOX and DCFDA-AM; JC-1 staining) induced by the thiosemicarbazones FA4, MLP44, PS3 and ACthio-1, were evaluated. The expression of autophagic proteins (ATG5, ATG7, ATG12, beclin, p62 and LC3-I) was also studied. In addition, the in vivo effect of FA4 in xenograft models with and without gemcitabine challenge was investigated.

RESULTS

We found that FA4 exerted a more potent cytotoxicity than previously studied thiosemicarbazones (MLP44, PS3 and ACthio-1), which were found to display variable effects on the ER or the mitochondria-dependent pro-apoptotic axis. By contrast, FA4 activated pro-apoptotic pathways and decreased autophagy, except in MiaPaCa2 cells, in which autophagic proteins were expressed at lower levels and remained unmodified by FA4. FA4 treatment of PANC-1 xenografted mouse models, poorly responsive to conventional chemotherapy, significantly reduced tumor volumes and increased intratumor apoptosis compared to gemcitabine, with no signs of toxicity.

CONCLUSIONS

Our data indicate that FA4 exhibits encouraging activity in pancreatic cancer cells unresponsive to gemcitabine. These results warrant further investigation in patient-derived pancreatic cancers, and hold promise for the development of therapies that can more efficiently target the specific characteristics of individual tumor types.

Current development of sigma-2 receptor radioligands as potential tumor imaging agents

Sigma receptors are transmembrane proteins with two different subtypes: σ₁ and σ₂. Because of its overexpression in tumors, the σ₂ receptor (σ₂R) is a well-known biomarker for cancer cells. A large number of small-molecule ligands for the σ₂Rs have been identified and tested for imaging the proliferative status of tumors using single photon emission computed tomography (SPECT) and positron emission tomography (PET). These small molecules include derivatives of bicyclic amines, indoles, cyclohexylpiperazines and tetrahydroisoquinolines. This review discusses various aspects of small molecule ligands, such as chemical composition, labeling strategy, affinity for σ₂Rs, and in vitro/in vivo investigations. The recent studies described here could be useful for the development of σ₂R radioligands as potential tumor imaging agents.

Development of New Benzylpiperazine Derivatives as σ1 Receptor Ligands with in Vivo Antinociceptive and Anti-Allodynic Effects

σ-1 receptors (σ1R) modulate nociceptive signaling, driving the search for selective antagonists to take advantage of this promising target to treat pain. In this study, a new series of benzylpiperazinyl derivatives has been designed, synthesized, and characterized for their affinities toward σ1R and selectivity over the σ-2 receptor (σ2R). Notably, 3-cyclohexyl-1-{4-[(4-methoxyphenyl)methyl]piperazin-1-yl}propan-1-one (15) showed the highest σ1R receptor affinity (Ki σ1 = 1.6 nM) among the series with a significant improvement of the σ1R selectivity (Ki σ2/Ki σ1= 886) compared to the lead compound 8 (Ki σ2/Ki σ1= 432). Compound 15 was further tested in a mouse formalin assay of inflammatory pain and chronic nerve constriction injury (CCI) of neuropathic pain, where it produced dose-dependent (3-60 mg/kg, i.p.) antinociception and anti-allodynic effects. Moreover, compound 15 demonstrated no significant effects in a rotarod assay, suggesting that this σ1R antagonist did not produce sedation or impair locomotor responses. Overall, these results encourage the further development of our benzylpiperazine-based σ1R antagonists as potential therapeutics for chronic pain.

Sigma ligands as potent inhibitors of Aβ and AβOs in neurons and promising therapeutic agents of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease and characterized by dementia, memory decline, loss of learning and cognitive disorder. The main pathological features of AD are the deposition of amyloid plaques and the formation of neurofibrillary tangles (NFTs) in the brain. The current anti-AD drugs have shown unsatisfactory therapeutic results. Due to the complications and unclear pathogenesis, AD is still irreversible and incurable. Among several hypotheses proposed by the academic community, the amyloid cascade is widely recognized by scholars and supported by a large amount of evidences. However, controversy over pathogenic factors has also been ongoing. Increasing evidence has shown that amyloid-β (Aβ) and especially amyloid-β oligomers (AβOs) are highly neurotoxic and pathogenic agents that damage neurons, mediate various receptors in the downstream pathways, and ultimately lead to learning and cognitive dysfunction. However, efforts in developing inhibitors of Aβ or amyloid-β precursor protein (APP) have all failed to yield good clinical results. More recently, it has been demonstrated that sigma receptors, including sigma-1 and sigma-2 subtypes, may play critical roles in the regulation of binding and metabolism of AβOs in neuron cells and the pathophysiology of AD. Thus, sigma receptor ligands are being recognized as promising therapeutic agents for treating or ameliorating AD. This article will review the pathophysiology of AD and highlight the sigma ligands that display the capability of preventing or even reversing Aβ- and AβOs-induced neurotoxicity and blocking the signal transduction caused by AβOs.

1,2,3,4-Tetrahydroisoquinoline (THIQ) as privileged scaffold for anticancer de novo drug design

Introduction: Cancer is a dreadful disorder that is emerging as one of the leading causes of mortality across the globe. The complex tumor environment, supplemented with drawbacks of the existing drugs, has made it a global health concern. The Tetrahydroisoquinoline (THIQ) ring holds an important position in medicinal chemistry due to its wide range of pharmacological properties. Several THIQ based natural products have been previously explored for their antitumor properties, making it a vital scaffold for anticancer drug design.Areas covered: This review article addresses the potential of THIQ as anticancer agents. Various medicinal chemistry strategies employed for the design and development of THIQ analogs as inhibitors or modulators of relevant anticancer targets have been discussed in detail. Moreover, the common strategies employed for the synthesis of the core scaffold are also highlighted.Expert opinion: Evidently, THIQs have tremendous potential in anticancer drug design. Some of these analogs exhibited potent activity against various cancer molecular targets. However, there are some drawbacks, such as selectivity that need addressing. The synthetic ease for constructing the core scaffold complimented with its reactivity makes it ideal for further structure-activity relationship studies. For these reasons, THIQ is a privileged scaffold for the design and development of novel anticancer agents.

Proteome-wide data analysis reveals tissue-specific network associated with SARS-CoV-2 infection

For patients with COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the damages to multiple organs have been clinically observed. Since most of current investigations for virus–host interaction are based on cell level, there is an urgent demand to probe tissue-specific features associated with SARS-CoV-2 infection. Based on collected proteomic datasets from human lung, colon, kidney, liver, and heart, we constructed a virus-receptor network, a virus-interaction network, and a virus-perturbation network. In the tissue-specific networks associated with virus–host crosstalk, both common and different key hubs are revealed in diverse tissues. Ubiquitous hubs in multiple tissues such as BRD4 and RIPK1 would be promising drug targets to rescue multi-organ injury and deal with inflammation. Certain tissue-unique hubs such as REEP5 might mediate specific olfactory dysfunction. The present analysis implies that SARS-CoV-2 could affect multi-targets in diverse host tissues, and the treatment of COVID-19 would be a complex task.

Synthesis, binding, and functional properties of tetrahydroisoquinolino-2-alkyl phenones as selective σ2R/TMEM97 ligands

Sigma-2 receptor (σ2R/TMEM97) has been implicated to play important roles in multiple cellular dysfunctions, such as cell neoplastic proliferation, neuro-inflammation, neurodegeneration, etc. Selective σ2 ligands are believed to be promising pharmacological tools to regulate or diagnose various disorders. As an ongoing effort of discovery of new and selective σ2 ligands, we have synthesized a series of tetrahydroisoquinolino-2-alkyl phenone analogs and identified that 10 of them have moderate to potent affinity and selectivity for σ2R/TMEM97. Especially, 4 analogs showed Ki values ranging from 0.38 to 5.1 nM for σ2R/TMEM97 with no or low affinity for sigma-1 receptor (σ1R). Functional assays indicated that these 4 most potent analogs had no effects on intracellular calcium concentration and were classified as putative σ2R/TMEM97 antagonists according to current understanding. The σ2R/TMEM97 has been suggested to play important roles in the central nervous system. Based on published pharmacological and clinical results from several regarded σ2R/TMEM97 antagonists, these analogs may potentially be useful for the treatment of various neurodegenerative diseases.

The Sigma-2 receptor / transmembrane protein 97 (σ2R/TMEM97) modulator JVW-1034 reduces heavy alcohol drinking and associated pain states in male mice

Alcohol Use Disorder (AUD) is a chronic relapsing disorder characterized by compulsive alcohol intake, loss of control over alcohol intake, and a negative emotional state when access to alcohol is prevented. AUD is also closely tied to pain, as repeated alcohol drinking leads to increased pain sensitivity during withdrawal. The sigma-2 receptor, recently identified as transmembrane protein 97 (σ2R/TMEM97), is an integral membrane protein involved in cholesterol homeostasis and lipid metabolism. Selective σ2R/Tmem97 modulators have been recently shown to relieve mechanical hypersensitivity in animal models of neuropathic pain as well as to attenuate alcohol withdrawal signs in C. elegans and to reduce alcohol drinking in rats, suggesting a potential key role for this protein in alcohol-related behaviors. In this study, we tested the effects of a potent and selective σ2R/TMEM97 ligand, JVW-1034, on heavy alcohol drinking and alcohol-induced heightened pain states in mice using an intermittent access model. Administration of JVW-1034 decreased both ethanol intake and preference for ethanol, without affecting water intake, total fluid intake, or food intake. Notably, this effect was specific for alcohol, as JVW-1034 had no effect on sucrose intake. Furthermore, JVW-1034 reduced both thermal hyperalgesia and mechanical hypersensitivity in ethanol withdrawn mice. Our data provide important evidence that modulation of σ2R/TMEM97 with small molecules can mediate heavy alcohol drinking as well as chronic alcohol-induced heightened pain sensitivity, thereby identifying a promising novel pharmacological target for AUD and associated pain states.

Fluorinated 2-Arylcyclopropan-1-amines - A new class of sigma receptor ligands

Stereoisomeric 2-aryl-2-fluoro-cyclopropan-1-amines have been discovered as a new class of σ receptor ligands showing different selectivity for the two subtypes of the receptor. Generally, compounds substituted in 4-position are much more active than corresponding 3-substituted isomers. trans-2-Fluoro-2-(4-methoxyphenyl)cyclopropan-1-amine (19a) was the most potent (K_i = 4.8 nM) σ₁ receptor ligand, while cis-2-fluoro-2-(4-trifluoromethylphenyl)cyclopropan-1-amine (20b) was the most potent (K_i = 95 nM) σ₂ receptor ligand.

The Sigma 2 receptor promotes and the Sigma 1 receptor inhibits mu-opioid receptor-mediated antinociception

The Sigma-1 receptor (σ1R) has emerged as an interesting pharmacological target because it inhibits analgesia mediated by mu-opioid receptors (MOR), and also facilitates the development of neuropathic pain. Based on these findings, the recent cloning of the Sigma-2 receptor (σ2R) led us to investigate its potential role as a regulator of opioid analgesia and of pain hypersensitivity in σ2R knockout mice. In contrast to σ1R deficient mice, σ2R knockout mice developed mechanical allodynia following establishment of chronic constriction injury-induced neuropathic pain, which was alleviated by the σ1R antagonist S1RA. The analgesic effects of morphine, [D-Ala, N-MePhe, Gly-ol]-encephalin (DAMGO) and β-endorphin increased in σ1R-/- mice and diminished in σ2R-/- mice. The analgesic effect of morphine was increased in σ2R-/- mice by treatment with S1RA. However, σ2R-/- mice and wild-type mice exhibited comparable antinociceptive responses to the delta receptor agonist [D-Pen2,5]-encephalin (DPDPE), the cannabinoid type 1 receptor agonist WIN55,212-2 and the α2-adrenergic receptor agonist clonidine. Therefore, while σR1 inhibits and σ2R facilitates MOR-mediated analgesia these receptors exchange their roles when regulating neuropathic pain perception. Our study may help identify new pharmacological targets for diminishing pain perception and improving opioid detoxification therapies.

Synthesis and Pharmacological Evaluation of σ2 Receptor Ligands Based on a 3-Alkoxyisoxazole Scaffold: Potential Antitumor Effects against Osteosarcoma

Since its initial discovery as the basis for nicotinic acetylcholine receptor ligands, the 3-alkoxyisoxazole scaffold has been shown to be a versatile platform for the development of potent σ1 and σ2 receptor ligands. Herein we report a further SAR exploration of the 3-alkoxyisoxazole scaffold with the aim of obtaining potent σ2 receptor ligands. Various substitutions on the benzene ring and at the basic amino regions resulted in a total of 21 compounds that were tested for their binding affinities for the σ2 receptor. In particular, compound 51 [(2S)-1-(4-ammoniobutyl)-2-(((5-((3,4-dichlorophenoxy)methyl)isoxazol-3-yl)oxy)methyl)pyrrolidin-1-ium chloride] was identified as one of the most potent σ2 ligands within the series, with a K_i value of 7.9 nM. It demonstrated potent antiproliferative effects on both osteosarcoma cell lines 143B and MOS-J (IC₅₀ values of 0.89 and 0.71 μM, respectively), relative to siramesine (IC₅₀ values of 1.81 and 2.01 μM). Moreover, compound 51 inhibited clonal formation of osteosarcoma 143B cells at 1 μM, corresponding to half the dose required of siramesine for similar effects. The general cytotoxicity profile of compound 51 was assessed in a number of normal cell lines, including HaCaT, HAF, and LO2 cells. Furthermore, FACS analysis showed that compound 51 likely inhibits osteosarcoma cell growth by disruption of the cell cycle and promotion of apoptosis.

Cannabinoid Receptor Subtype 2 (CB2R) in a Multitarget Approach: Perspective of an Innovative Strategy in Cancer and Neurodegeneration

The cannabinoid receptor subtype 2 (CB2R) represents an interesting and new therapeutic target for its involvement in the first steps of neurodegeneration as well as in cancer onset and progression. Several studies, focused on different types of tumors, report a promising anticancer activity induced by CB2R agonists due to their ability to reduce inflammation and cell proliferation. Moreover, in neuroinflammation, the stimulation of CB2R, overexpressed in microglial cells, exerts beneficial effects in neurodegenerative disorders. With the aim to overcome current treatment limitations, new drugs can be developed by specifically modulating, together with CB2R, other targets involved in such multifactorial disorders. Building on successful case studies of already developed multitarget strategies involving CB2R, in this Perspective we aim at prompting the scientific community to consider new promising target associations involving HDACs (histone deacetylases) and σ receptors by employing modern approaches based on molecular hybridization, computational polypharmacology, and machine learning algorithms.

Novel 1,2,4-Oxadiazole Derivatives in Drug Discovery

Five-membered 1,2,4-oxadiazole heterocyclic ring has received considerable attentionbecause of its unique bioisosteric properties and an unusually wide spectrum of biological activities.Thus, it is a perfect framework for the novel drug development. After a century since the1,2,4-oxadiazole have been discovered, the uncommon potential attracted medicinal chemists'attention, leading to the discovery of a few presently accessible drugs containing 1,2,4-oxadiazoleunit. It is worth noting that the interest in a 1,2,4-oxadiazoles' biological application has been doubledin the last fifteen years. Herein, after a concise historical introduction, we present a comprehensiveoverview of the recent achievements in the synthesis of 1,2,4-oxadiazole-based compounds and themajor advances in their biological applications in the period of the last five years as well as briefremarks on prospects for further development.

Sigma Receptor Ligands Carrying a Nitric Oxide Donor Nitrate Moiety: Synthesis, In Silico, and Biological Evaluation

We report the development of molecular hybrids in which a nitrate group serving as nitric oxide (NO) donor is covalently joined to σ receptor ligands to give candidates for double-targeted cancer therapy. The compounds have been evaluated in radioligand binding assay at both σ receptors and selected compounds tested for NO release. Compounds 9, 15, 18, 19, and 21 were subjected to MTT test. Compound 15 produced a significant reduction of MCF-7 and Caco-2 cellular viability with comparable IC₅₀ as doxorubicin, being also not toxic for fibroblast HFF-1 cells. Compound 15 has shown a σ₁ receptor antagonist/σ₂ receptor agonist profile. Two derivatives of compound 15 lacking the nitrate group did not induce a reduction of MCF-7 cellular viability, suggesting a potential synergistic effect between the σ receptors and the NO-mediated events. Overall, the combination of NO donor and σ receptors ligands provided compounds with beneficial effects for the treatment of cancer.

Heterotargeted Nanococktail with Traceless Linkers for Eradicating Cancer

Clinical application of drug cocktails for cancer therapy is limited by their severe systemic toxicity. To solve a catch-22 dilemma between safety and efficacy for drug cocktails, a hetero-targeted nano-cocktail (PPPDMA) with traceless linkers has been developed. In the PPPDMA nanogel, a hetero-targeting strategy is employed to improve its tumor selective targeting efficacy by overcoming the cancer cell mono-ligand density limitation. Benefit from its glutathione and reactive oxygen species responsiveness, the loaded paclitaxel and doxorubicin can be quickly and tracelessly released into the cytoplasm in their original form, which bestows PPPDMA nanogels the capability to overwhelm the processing capacity of cancer cell's P-glycoprotein efflux pump allows, and ultimately kill them without inducing side effects. The PPPDMA treatment reduced its tumor burden over 99% (in tumor weight) and 96% (in tumor number). Most importantly, no detectable tumor in more than half of the PPPDMA treated mice. We conclude that traceless linker and hetero-targeted nano-cocktail strategy could be a safe and effective approach for cancer treatment.

Sigma-2 receptor/TMEM97 agonist PB221 as an alternative drug for brain tumor

Background

There are limited effective drugs that can reach the brain to target brain tumors, in particular glioblastoma, which is one of the most difficult cancers to be cured from. Because the overexpression of the sigma-2 receptor is frequently reported in glioma clinical samples and associated with poor prognosis and malignancy, we herein studied the anti-tumor effect of the sigma-2 receptor agonist PB221 (4-cyclohexyl-1-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperidine) on an anaplastic astrocytoma tumor model based on previous encouraging results in pancreatic cancer and neuroblastoma SK-N-SH cells.

Methods

The expression of the sigma-2 receptor, transmembrane protein 97 (TMEM97), in ALTS1C1 and UN-KC6141 cell lines was measured by RT-PCR and quantitative RT-PCR. The binding of sigma-2 receptor fluorescent ligands PB385 (6-[5-[3-(4-cyclohexylpiperazin-1-yl)propyl]-5,6,7,8-tetrahydronaphthalen-5-yloxy]-N-(7-nitro-2,1,3-benzoxadiazol-4-yl)hexanamine) and NO1 (2-{6-[2-(3-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)propyl)-3,4-dihydroisoquinolin-1(2H)-one-5-yloxy]hexyl}-5-(dimethylamino)isoindoline-1,3-dione) was examined by flow cytometry and the fluorescent plate reader. The antitumor activity of PB221 was initially examined in the murine brain tumor cell line ALTS1C1 and then in the murine pancreatic cell line UN-KC6141. The potential therapeutic efficacy of PB221 for murine brain tumors was examined by in vitro migration and invasion assays and in vivo ectopic and orthotopic ALTS1C1 tumor models.

Results: The IC₅₀ of PB221 for ALTS1C1 and UN-KC6141 cell lines was 10.61 ± 0.96 and 13.13 ± 1.15 μM, respectively. A low dose of PB221 (1 μM) significantly repressed the migration and invasion of ALTS1C1 cells, and a high dose of PB221 (20 μM) resulted in the apoptotic cell death of ALTS1C1 cells. These effects were reduced by the lipid antioxidant α-tocopherol, but not by the hydrophilic N-acetylcysteine, suggesting mitochondrial oxidative stress is involved. The in vivo study revealed that PB221 effectively retarded tumor growth to 36% of the control tumor volume in the ectopic intramuscular tumor model and increased the overall survival time by 20% (from 26 to 31 days) in the orthotopic intracerebral tumor model.

Conclusions

This study demonstrates that the sigma-2 receptor agonist PB221 has the potential to be an alternative chemotherapeutic drug for brain tumors with comparable side effects as the current standard-of-care drug, temozolomide.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5700-7) contains supplementary material, which is available to authorized users.

Synthesis, in vitro and in vivo characterization of new benzoxazole and benzothiazole-based sigma receptor ligands

A new set of 5-chlorobenzoxazole- and 5-chlorobenzothiazole-based derivatives containing the azepane ring as a basic moiety was designed, synthesized and evaluated through binding assays to measure their affinity and selectivity towards σ₁ and σ₂ receptors. Compounds 19, 22 and 24, with a four units spacer between the bicyclic scaffold and the azepane ring, showed nanomolar affinity towards both receptor subtype and the best K_i values (K_i σ₁ = 1.27, 2.30, and 0.78 and K_i σ₂ = 7.9, 3.8, and 7.61 nM, respectively). Evaluation of cytotoxic and apoptotic effects in MCF-7 human cancer cells was useful to assess σ₂ receptor activity, while an in vivo mice model of inflammatory pain allowed to analyze σ₁ receptor pharmacological properties. In vitro and in vivo results suggested that compound 19 is a σ₁/σ₂ agonist, compound 24 a σ₁ antagonist/σ₂ agonist, whereas compound 22 might act as σ₁ antagonist/σ₂ partial agonist. Due to their pharmacological profile, a potential therapeutic application in cancer of aforesaid novel σ₁/σ₂ receptor ligands, especially 22 and 24, is proposed.

New analogs of SYA013 as sigma-2 ligands with anticancer activity

Our previous study has revealed 4-(4-(4-chlorophenyl)-1,4-diazepan-1-yl)-1-(4-fluorophenyl)butan-1-one·2HCl (SYA013) 1 as a sigma ligand with moderate selectivity for the sigma-2 receptor. Given the overexpression of sigma receptors in solid tumors and reports of sigma ligands with anticancer activities, we selected 1 for evaluation in several solid tumor cell lines. In addition, we have synthesized new analogs of 1 and now report that several of them bind preferentially at the sigma-2 receptor and have shown inhibition of several cancer cell lines including MDA-MB-231, MDA-MB-486, A549, PC-3, MIA PaCa-2 and Panc-1 cells. In particular, compounds 1 and 12 have demonstrated sub-micromolar activity against the Panc-1 cell line. It has also been observed that several of these compounds demonstrate selective toxicity toward cancer cells, when compared to normal cells.

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 ( K_i = 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.

Synthesis and Biological Evaluation of Sigma-1 (σ1 ) Receptor Ligands Based on Phenyl-1,2,4-oxadiazole Derivatives

In this study, a series of phenyl-1,2,4-oxadiazole derivatives were synthesized and evaluated for anti-allodynic activity. Structure-activity relationship studies identified 1-{4-[3-(2,4-dichlorophenyl)-1,2,4-oxadiazol-5-yl]butyl}piperidine (39) with excellent affinity for the σ1 receptor and selectivity for the σ2 receptor, with poor activity to other central nervous system neurotransmitter receptors and transporters associated with pain. Compound 39 exhibited dose-dependent efficacy in suppressing the formalin-induced flinching and attenuating mechanical allodynia in chronic constriction injury-induced neuropathic rats. These results suggest that compound 39 exerts potent antihyperalgesic activity and could be considered as a promising candidate for treating neuropathic pain.

Synthesis, Receptor Affinity, and Antiallodynic Activity of Spirocyclic σ Receptor Ligands with Exocyclic Amino Moiety

In order to detect novel σ receptor ligands, the rigid spiro[[2]benzopyran-1,1'-cyclohexan]-4'-one was connected with amino moieties derived from σ₂ receptor preferring lead compounds resulting in mixtures of trans- and cis-configured amines 6, 18, and 27. In a four step synthesis the methyl acetals 6 were converted into fluoroethyl derivatives 13 and 30. The most promising σ₂ receptor ligand is the methyl acetal 6a bearing a 2,4-dimethylbenzylamino moiety. The fluoroethyl derivatives 13c and 13d reveal high σ₁ affinity but moderate selectivity over the σ₂ subtype. In mice 13c and 13d showed antiallodynic activity that is stronger than that of the reference σ₁ antagonist BD-1063 (34). Since the antiallodynic activity of 13c could only be partially reversed by the σ₁ agonist PRE-084 (35), it is postulated that a second mechanism contributes to its overall antiallodynic effect. In contrast, the antiallodynic effect of its diastereomer 13d can be totally explained by a σ₁ antagonism.

Sigma-2 receptor: past, present and perspectives on multiple therapeutic exploitations

Identification of sigma-2 receptor (sig-2R) has been controversial. Nevertheless, interest in sig-2R is high for its overexpression in tumors and potentials in oncology. Additionally, sig-2R antagonists inhibit Aβ binding at neurons, blocking the cognitive impairments of Alzheimer's disease. The most representative classes of sig-2R ligands are herein treated with focus on compounds that served to study sig-2R biology and to produce sig-2R: fluorescent ligands; multifunctional anticancer agents; and targeting nanoparticles. Although fluorescent ligands serve as ‘green’ pharmacological tools, sig-2R-multifunctional conjugates and sig-2R-targeted nanoparticles show how sig-2R targeting increases the activity of anticancer drugs in tumors with reduced toxicity. Altogether, this review draws a picture of the multiple approaches of sig-2R ligands in cancer therapy and as Alzheimer's disease modifying disease agents.

Benzomorphan skeleton, a versatile scaffold for different targets: A comprehensive review

Despite the fact that the benzomorphan skeleton has mainly been employed in medicinal chemistry for the development of opioid analgesics, it is a versatile structure. Its stereochemistry, as well as opportune modifications at the phenolic hydroxyl group and at the basic nitrogen, play a pivotal role addressing the benzomorphan-based compounds to a specific target. In this review, we describe the structure activity-relationships (SARs) of benzomorphan-based compounds acting at sigma 1 receptor (σ1R), sigma 2 receptor (σ2R), voltage-dependent sodium channel, N-Methyl-d-Aspartate (NMDA) receptor-channel complex and other targets. Collectively, the SARs data have highlighted that the benzomorphan nucleus could be regarded as a useful template for the synthesis of drug candidates for different targets.

High-Content Microfluidic Screening Platform Used To Identify σ2R/Tmem97 Binding Ligands that Reduce Age-Dependent Neurodegeneration in C. elegans SC_APP Model

The nematode Caenorhabditis elegans, with tractable genetics and a well-defined nervous system, provides a unique whole-animal model system to identify novel drug targets and therapies for neurodegenerative diseases. Large-scale drug or target screens in models that recapitulate the subtle age- and cell-specific aspects of neurodegenerative diseases are limited by a technological requirement for high-throughput analysis of neuronal morphology. Recently, we developed a single-copy model of amyloid precursor protein (SC_APP) induced neurodegeneration that exhibits progressive degeneration of select cholinergic neurons. Our previous work with this model suggests that small molecule ligands of the sigma 2 receptor (σ2R), which was recently cloned and identified as transmembrane protein 97 (TMEM97), are neuroprotective. To determine structure-activity relationships for unexplored chemical space in our σ2R/Tmem97 ligand collection, we developed an in vivo high-content screening (HCS) assay to identify potential drug leads. The HCS assay uses our recently developed large-scale microfluidic immobilization chip and automated imaging platform. We discovered norbenzomorphans that reduced neurodegeneration in our C. elegans model, including two compounds that demonstrated significant neuroprotective activity at multiple doses. These findings provide further evidence that σ2R/Tmem97-binding norbenzomorphans may represent a new drug class for treating neurodegenerative diseases.