Recent Advances in the Development of Sigma Receptor (Radio)Ligands and Their Application in Tumors

Abstract

Cancer ranks among the top triumvirate leading causes of human deaths worldwide. The pathological mechanisms are notably intricate, demonstrating proliferative and metastatic capabilities, which complicate therapeutic interventions. The sigma-1 receptor (σ₁R) plays a crucial role in tumor survival and migration, while the sigma-2 receptor (σ₂R) is intimately associated with tumor proliferation. This review encapsulated the investigation concerning σ₁R and σ₂R in neoplasms and rigorously summarized the ligands and radio-ligands development and their tumor applications, such as antitumor cell proliferation and PET/SPECT imaging in tumors. A comprehensive classification discussion was undertaken regarding the chemical structures and emphasized the possibility of dual/multitargeted ligands. Ultimately, we discussed the effects of chiral structures and the pharmacological characteristics of ligands on affinity and pharmacokinetic features in vivo, particularly concerning radiopharmaceuticals. This review functions as a beneficial resource, fostering ligand deployment and stimulating the generation of innovative ideas for developing innovative radiopharmaceuticals.

Cancer constitutes a systemic malignancy and represents a significant global public health challenge, profoundly impacting individuals’ physical health as well as societal and economic progression. According to the data from the International Agency for Research on Cancer (IARC), in 2022, there were approximately 20 million new cancer cases and 9.7 million cancer deaths worldwide, respectively.¹ In 2022, the National Cancer Center (NCC) of China reported approximately 4.82 million incidences of newly diagnosed cancer cases and alongside 2.57 million cancer-related fatalities.² The five leading primary cancer forms—lung, colorectal, thyroid, liver, and gastric—constitute over 57% of total cases.² These data indicate that the worldwide burden of cancer is substantially severe.^1,2 Thus, prompt diagnosis, intervention, and management of tumors are crucial for enhancing patient survival and prognosis.³

The sigma receptors (σRs) are located in mitochondria-associated endoplasmic reticulum membranes (MAMs)⁴ and endoplasmic reticulum (ER) resident membranes,⁵ respectively, that play a variety of important roles in the cell.⁶ They comprise two subtype categories: sigma-1 receptor (σ₁R) and sigma-2 receptor (σ₂R).⁷ Research indicated that σ₁R and σ₂R are genetically unrelated but share a connection with enzymes with similar functions. The closest homologue of the σ₁R is the yeast C8–C7 sterol isomerase ERG2p, whereas the σ₂R is associated with emopamil binding protein (EBP), a mammalian C8–C7 sterol isomerase.^8,9 Therefore, although σ₁R and σ₂R are not genetically related, their similar pharmacological properties may be the result of convergent evolution.⁶

Cancer is characterized chiefly by the accelerated proliferation, growth, and metastatic spread of tumor cells. Both σ₁R and σ₂R play important roles in the pathological process of these tumors. The σ₁R plays a critical role in tumor survival and migration,¹⁰ whereas σ₂R is implicated in rapid proliferation of tumor cell and serves as a novel tumor biomarker^11,12 (Figure 1). Numerous developed σRs-targeted ligands, particularly σ₁R antagonists¹³ and σ₂R agonists,^14,15 are employed in the treatment of various cancers with high σRs expression,¹⁶ including colorectal, breast, pancreatic cancers, etc. In summary, σRs are increasingly recognized as a promising target for advancing cancer diagnosis and treatment strategies.

Figure 1.

Primary functions of σ₁R and σ₂R in tumor progression.

Radiopharmaceutical-based molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), can offer precise and dependable visual data for the early detection and prognostic assessment of tumors.¹⁷ Employing PET and SPECT imaging with σRs-targeted radioactive probes (radiopharmaceuticals) enables visualization of the pathological processes of tumors, such as proliferation and metastatic. Currently, clinical research lacks σ₁R tumor molecular probes, with only the σ₂R-targeted radioligand [¹⁸F]ISO-1 advancing to clinical trials (phase I, NCT02284919 and NCT03057743) in primary breast cancer imaging.^18,19 This review summarizes recent advancements in the σ₁R- and σ₂R-targeted ligands and radioligands/radiopharmaceuticals development, emphasizing the link between chemical structure and biological activity. That serves as a guide for the future σ₁R- and σ₂R-targeted ligands and radiopharmaceuticals/radioligands development for tumor diagnosis and therapy and not just for tumors.

The Sigma Receptors

The σRs are a relatively novel class of receptors that has yet to be completely understood, and it has been less than 50 years since it was discovered. Initially, Martin and co-workers identified and categorized σRs as “opioid receptors” in 1976;²⁰ however, a few years later, Vaupel et al. revealed that σRs and opioid receptors had distinct binding sites, resulting in an entirely new kind of receptor protein.²¹ In 1992, Quirion et al. employed the radioligands (+)-[³H]pentazocine and [³H]1,3-di(2-tolyl) guanidine ([³H]DTG) to define the binding site of σRs. Based on the differences in ligand-binding profile, it was divided into the σ₁R and σ₂R subtypes⁷ without any homology with opioid receptors and N-methyl-D-aspartate (NMDA) receptors.⁸

The σ₁R is a distinct “ligand-regulated receptor chaperone”, possessing a trimeric form and a solitary transmembrane domain.^4,22 In 2016, the protein was structurally characterized as a trimer, each protomer comprising a single transmembrane domain and a cupin fold within the C-terminal domain.²³ The σ₂R was identified as ER-resident transmembrane protein 97 (TMEM97) also named meningioma-related protein 30 (MAC30) in 2017.⁵ In 2021, Alon et al. accomplished the structural elucidation of bovine σ₂R/TMEM97 as a closely linked homodimer, comprising four kinked transmembrane helices per protomer.²⁴ Despite significant advancements in σRs research for nearly five decades, considerable knowledge gaps remain to be addressed.

Both σ₁R and σ₂R/TMEM97 are widely distributed in the central nervous system (CNS) and peripheral tissues, and they play a crucial role in variety of physiological disorders such as CNS diseases (e.g., Alzheimer’s disease—AD,²⁵⁻²⁷ Parkinson’s disease—PD,²⁸⁻³¹ Major Depressive Disorder—MDD^28,32), cancers (such as, breast,³³⁻³⁵ lung,^16,35,36 and prostate^37,38), diabetes,³⁹ inflammation,⁶ etc. Consequently, increasing research efforts are directed toward σRs to identify novel diagnostic and therapeutic strategies for CNS⁴⁰ and cancer diseases.¹⁶ Presently, nine σRs nonradioligands are utilized in clinical trials, comprising seven targeting the σ₁R and two for the σ₂R (the structure of ligands see our previous review,²⁶ except chlorpromazine). Among them, the σ₁R antagonist MR309/E-52862 was in phase II clinical trials for the treatment of chemotherapy-induced neuropathic pain.⁴¹ Additionally, Pridopidine, a σ₁R agonist, was investigated in a phase II/III for Huntington’s disease (HD) (NCT02494778) and Amyotrophic Lateral Sclerosis (ALS) (NCT04615923, NCT06069934). Dextromethorphan (AVP-923), a σ₁R agonist, has been shown in phase II clinical trials (NCT01324232) for central neuralgia in multiple sclerosis patients.⁴² ANAVEX2–73, a σ₁R agonist, was undergoing phase II trials for AD,^43,44 while Cutamesine (SA4503) was in phase II for ischemic stroke (NCT00639249)⁴⁵ and MDD (NCT00551109).⁴⁶ Chlorpromazine, an unknown σ₁R antagonist or agonist, is used against acute respiratory infections caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (NCT04366739, NCT04354805).⁴⁷ In addition, both σ₂R antagonists, Roluperidone (MIN-101) and (Elayta) CT1812, were used in clinical trials of AD.^26,48

Pathophysiological Role of σRs in Tumor Progression

The σRs are implicated in various cellular processes critical to tumor physiology, including cell proliferation, survival, and apoptosis.^10−12,49 Exploring and understanding the pathophysiology of σRs in tumors reveal their intricate functions in cancer progression. Unraveling the role and mechanisms of σRs in tumors is crucial for cancer elucidation and the creation of novel anticancer therapies.

Versatile Regulator of in Tumor Cell Signal Transduction: σ₁R

While σ₁R is predominantly studied within neuropharmacology,⁵⁰ increasing evidence points toward its involvement in tumor biology. Studies have shown that σ₁R plays a significant role in the genesis, progression, survival and energy metabolism of tumors, positioning it as a novel therapeutic target for treatment.^51,52 The σ₁R exhibits pronounced expression and even overexpression in several cancers, including those of the breast, colorectal, prostate, brain, as well as in chronic myeloid leukemia cell lines.^51,53,54 In small cell lung cancer and T-cell leukemia, the σ₁R prevents apoptosis by blocking volume-regulated chloride channels (VRCCs), influencing the cell cycle.⁵⁵ Highly selective σ₁R ligands, such as DTG and igmesine, can impede these chloride (Cl^–) channels, demonstrating the critical functional coupling between σ₁R and these channels in cancer cell regulation.⁵⁵ The σ₁R is significantly overexpressed in human breast cancer epithelial cells. Its antagonists, such as haloperidol or reduced haloperidol, demonstrated a dose-dependent inhibition of cell proliferation at elevated concentrations.⁵⁶ What is more important, the σ₁R modulates the adherence of breast cancer cell lines, influencing their metastatic properties. Specifically, silencing the σ₁R gene markedly decreases adhesion in cancer cells by 30–41%, a property not observed in normal cells.⁵³ The σ₁R is associated with tumor cell status, notably its overexpression in esophageal squamous cell carcinoma (ESCC), where it shows a positive correlation with tumor severity according to the pathological tumor-node-metastasis (TNM/pTNM) classification and lymph node metastasis.⁵⁷ Despite the potential of σ₁R in oncological research, contradictions persist, notably its role in promoting tumor growth.^58,59

In tumor cells, σ₁R regulates voltage-gated ion channels such as K_v11.1, Na_v1.5, and VRAC (a Cl^– channels), as well as the PI3K-Akt signaling pathway, thereby leading to programmed apoptosis.¹⁰ Upregulation of σ₁R in breast and colorectal cancer enhances cell migration, invasion, and survival by increasing intracellular SK3-dependent calcium ion (Ca²⁺) concentration and regulating membrane potential activity.⁶⁰ In leukemia and small cell lung cancer cells, σ₁R modulates volume-regulated Cl^– channels, enhancing resistance to apoptotic signals.⁵⁵ Additionally, the σ₁R modulates the membrane expression of voltage-dependent potassium (K⁺) channels and the human ether-à-go-go-related gene (hERG) in myeloid leukemia and colorectal cancer cell lines.⁶¹ This regulation enhances cell motility, vascular endothelial growth factor (VEGF) secretion, and tumor cell aggressiveness by promoting invasion and angiogenesis, resulting in poorer survival outcomes.⁶¹

Since the concomitant activity of σ₁R can be modulated by exogenous ligands, it represents a potential therapeutic target for specifically altering ion channel activity in cancers. The σ₁R ligands hold potential as anticancer agents, either as monotherapy or in combination treatment with others.¹⁶ Research indicates that exogenous σ₁R ligands exert cytotoxic or antiproliferative effects on tumor cell lines, demonstrating growth inhibition and induction of cell death, in vivo and in vitro.¹⁶ The σ₁R ligands act as regulators of protein synthesis (p70S6K, S6, and 4E-BP1) in tumor cells, such as the antagonists (e.g., haloperidol), which attenuate the phosphorylation of translational regulatory proteins in breast and prostate cancer cell lines.⁶² The σ₁R antagonist Rimcazole elevates hypoxia-inducible factor-1 alpha (HIF-1α) protein levels in colorectal (HCT-116) and breast cancer (MDA-MB-231) cells, thereby promoting apoptosis.¹³ Additionally, σ₁R binds to interleukin-24 (IL-24) and form an IL-24-σ₁R complex, which boosts IL-24-induced endoplasmic reticulum stress and reduces reactive oxygen species (ROS) production and calcium signal (Ca²⁺) transmission, ultimately inducing apoptosis. This IL-24-induced apoptosis could be blocked by σ₁R agonists such as (+)-SKF10047.⁶³ Studies indicated that σ₁R plays a critical role in the tumor immune response targeting PD-1/PD-L1, and ligand (e.g., IPAG) inhibition of σ₁R decreases PD-L1 immune checkpoint function and triggers autophagic degradation.⁶⁴ The σ₁R ligands have been demonstrated to induce apoptosis via the NF-κB pathway in tumor cells (lung, Hodgkin’s lymphoma, breast cancers, etc.)⁶² by modulating ER stress, stimulating ROS production, caspase-3 activation,^65,66 and so on. Furthermore, these ligands have shown potential for antiproliferation and antiangiogenesis mechanisms in breast cancer xenografts.⁶⁵

Therefore, σ₁R combines with multiple types of proteins and signaling pathways in tumor cells, playing various roles in cell signal transduction. It was regarded as a multifunctional regulator in tumor biology.

Emerging Tumor Biomarker: σ₂R

The σ₂R/TMEM97 significantly influences lipid metabolism and cholesterol regulation, crucial factors in cancer development and advancement.⁶⁷⁻⁷⁰ It was variably expressed across multiple tumors types and plays a crucial role in tumor cell proliferation, differentiation, and apoptosis.⁷¹⁻⁷³ In numerous cancer cell lines, the σ₂R overexpression was observed, exhibiting levels approximately 10-fold higher than those in quiescent states.^18,74−76 Agonistic ligands, such as Siramesine, PB28, SV119, CB-64D, and CB-184, exhibit potency in inhibiting the proliferation of a spectrum of tumors, notably within breast, cervical, lung, prostate, and connective tissues,^{14,16,73,74,77−79} presenting a promising avenue for therapeutic intervention. A positive correlation exists between σ₂R and Ki-67 protein expression, a biomarker of tumor cell proliferation. Consequently, σ₂R may serve as an alternative biomarker for tumor proliferation imaging, potentially replacing Ki-67.^11,80 Thus, σ₂R was hailed as a novel tumor proliferation biomarker.

In human Hela cells, the σ₂R, in conjunction with progesterone receptor membrane component 1 (PGRMC1) and the low-density lipoprotein receptor (LDLR), forms a compact ternary complex PGRMC1-LDLR-σ₂R/TMEM97 that accelerates low-density lipoprotein (LDL) internalization in proliferating tumor cells compared to their quiescent counterparts.⁸¹ This specific trimer could be crucial to the accelerated intake of cholesterol and elevation of free cholesterol levels.^81,82 This sequence may trigger ER stress, leading to the unfolded protein response and activation of sterol regulatory element-binding protein 2 (SREBP2), serving as an adaptive mechanism in tumor cells.^82,83 The use of σ₂R ligands, alone (C10-SMAC or C6-Erastin) or in combination (with FDA-approved simvastatin), can expose and trigger this process of free cholesterol—ER-stress—SREBP2-activation, thereby disrupting this adaptive process and enhancing tumor cell sensitivity to cell death.⁸² These discoveries pave the way for developing highly efficient therapeutic approaches across a broad range of human cancers.

Lung cancer sample analysis revealed that increased σ₂R production supports squamous cell survival, highlighting its critical correlation between σ₂R and cancer cell survival.⁸⁴ In human SK-N-SH neuroblastoma cells, σ₂R significantly influences ion channels like the Ca²⁺ signaling pathway.^85,86 Agonists of σ₂R, such as PB28 and F281, have the ability to impair the release of Ca²⁺ from mitochondria or the endoplasmic reticulum via blocking inositol 1,4,5-trisphosphate (InsP₃) receptors.^87,88 This process decreases the tumoral metabolic activity, lowers intracellular ATP levels, and ultimately induces cell death.

The σ₂R markedly overexpressed in estrogen receptor-positive breast cancer cells, which are the primary cause of tumor development and progression.^89,90 While closely linked to estrogen and progesterone receptors, σ₂R was not associated with human epidermal growth factor receptor 2 (HER2) status.³⁴ Elevated σ₂R expression confers a survival advantage to breast cancer cells, whereas its suppression hampers cellular proliferation. Specifically, σ₂R modulates the resistance and sensitivity of breast cancer cells, like MCF-7 and T-47D, to tamoxifen through the regulation of phosphorylated estrogen receptor α (ERα) levels and influencing the mTOR/S6K1 signaling pathway.³⁴ The novel σ₂R-targeted fluorescent ligand NO1 significantly inhibited proliferation and induced apoptosis in triple-negative breast cancer (TNBC) cell lines MDA-MB-231 and MDA-MB-468, demonstrating potential therapeutic efficacy.⁹¹

A011, a novel σ₂R ligand, effectively induces apoptosis in MCF-7 cells and reversal of doxorubicin and paclitaxel resistance by modulating ATP-binding cassette subfamily B member 1 (ABCB1, also called P-glycoprotein, P-gp) and ABCG2 transporter activity.⁹² Additionally, A011 significantly elevates intracellular Ca²⁺ and reactive oxygen species levels, triggering autophagy.⁹³ It further induces endoplasmic reticulum stress, activates the PERK-eIF2α-CHOP, inhibits PI3K-Akt-mTOR pathway activation, and culminates in cell apoptosis.⁹³ These observations suggest that endoplasmic reticulum stress might be central to the anticancer capabilities of σ₂R ligands. Thus, σ₂R emerges as a novel regulator of breast cancer cell proliferation, apoptosis, and multidrug resistance (MDR).

Furthermore, the administration of σ₂R agonist PB221 at a tolerated dose can decelerate brain tumor progression and extend the survival of ALTS1C1 tumor-bearing mice, suggesting its viability as a treatment for brain tumors.⁹⁴ The agonist PB28 targets the σ₂R, attenuating phosphorylation within the PI3K-AKT-mTOR signaling pathway, thus impeding renal cancer cell proliferation, migration, invasion, and increasing cisplatin susceptibility in these cells.⁹⁵ The σ₂R ligand SV119 significantly amplified the chemotherapeutic efficacy of paclitaxel and gemcitabine in adjuvant pancreatic cancer treatment (dose-dependent fashion) in murine (Panc-02) and human (AsPC-1, CFPAC-1, and Panc-1) pancreatic adenocarcinoma cell lines, presenting no discernible adverse effects.⁷⁷ The σ₂R ligand/modulator and fluorescent probe, BS148, demonstrated antiproliferative effects on SK-MEL-2 melanoma cells through activation of ER-stress responses and cholesterol-mediated MAPK (mitogen-activated protein kinase) signaling pathways.⁹⁶ Additionally, it significantly inhibited proliferation and migration in melanoma patient-derived xenograft (PDX) cells.⁹⁶ These further underscores the significance of σ₂R-mediated cholesterol inhibition in tumor proliferation,⁸² highlighting its promising implications for cancer therapy. In addition to the aforementioned, recent developments in σ₂R ligands have catalyzed their application in research and therapeutic strategies targeting apoptosis in tumor cells, including those from pancreatic, breast, and lung cancers.^35,97−100

In summary, σ₁R and σ₂R significantly influence tumor dynamics, including proliferation, metastasis, apoptosis, and chemoresistance. Ligands targeting these receptors can suppress tumor growth, underscoring the critical role of σR subtypes in oncogenesis.

Multitarget Approach Focus on σRs in Cancer

Cancer is a complex disease that involves multiple cell types, requiring a multipharmaceutical approach as it affects multiple cell populations. The use of multitarget directed ligands (MTDLs), single-molecule drugs capable of interacting with multiple pharmacological targets, offers a promising approach to overcoming the pharmacokinetic challenges of combining multiple drug administrations. Owing to their significance in the tumor pathological process, novel dual/multitargeting σ₁R and σ₂R ligands (even other tumor biological targets), also named pan-σR ligands, are valuable for improving tumor diagnostics and treatment.^101,102 Haloperidol metabolite II (HP II) is one of the earliest pan-σR ligands (K_i(σ1) = 2.3 nM and K_i(σ2) = 2.0 nM), as well as a metabolic product derived from the reduction of haloperidol, which can trigger apoptosis of colon, breast, and glioma cancer cells.^103,104 Furthermore, the pan-σR ligand RC-106 (K_i(σ1) = 12 nM and K_i(σ2) = 22 nM)¹⁰⁵ has been demonstrated to have potent antiproliferative and pro-apoptotic effects in a variety of cancer cells, particularly the pancreatic PaCa3 cells, which abundantly express both σ₁R and σ₂R.^106,107

Up to now, many MTDLs or prodrugs acting at the σ₁R and σ₂R have been developed. In addition to the involvement of σ₁R and σ₂R in the pathological processes of these diseases, such as CNS and tumors, there is additional regulation of targets such as opioids, ROS, NMDA, dopamine, TNFα, histamine H₃ (H₃R), and cholinergic receptors, which has promoted the development of σ₁R- and σ₂R-based MTDLs for the diagnosis and treatment of challenging and multifactorial diseases.^108−110 For a detailed introduction to MTDLs in recent years, see Section Dual/Multitargeted σRs Ligands.

The σR-Targeted Ligands and Applications

Given the diverse nature of tumor diseases, tailored treatment strategies necessitate a deep comprehension of their distinct pathological molecular mechanisms and involved pathways. Sigma receptors (σRs), implicated in various tumor pathologies, present novel avenues for personalized therapeutic interventions. Thus, utilizing diagnostic radioligands/radiopharmaceuticals to image σRs could enhance our comprehension of tumor biology and pathological roles of those receptors and support the creation of innovative cancer therapies. Additionally, therapeutic radioligands and radiopharmaceuticals can precisely target σRs to effectively eliminate tumors. Typical medical radionuclides encompass diagnostic isotopes utilized in PET (e.g., ¹⁸F, ¹¹C, and ⁶⁸Ga) and SPECT (e.g., ^99mTc, ¹²³I, and ¹¹¹In) imaging, alongside therapeutic isotopes (e.g., ¹³¹I, ¹²⁵I, ⁸⁹Sr, ²¹¹At, and ¹⁷⁷Lu). Currently reported σR radioligands commonly utilize radionuclide labels such as ¹⁸F, ¹¹C, ^99mTc, ¹³¹I, and ²¹¹At, reflecting the diversity of their application and detection methodologies.

Selective Ligands for σ₁R Targeting

In 1994 and 2000, the Glennon pharmacophore model (Figure 2) elucidated σ₁R receptor–ligand binding, offering a reasonable explanation that gained broad acceptance.^111,112 In addition, research indicates the capacity of σ₁R to bind with ligands devoid of basic nitrogen atoms, such as neurosteroids.^113,114 The varying bonding behavior between the σ₁R and certain ligands under protonated conditions¹¹⁵ may arise from the unique interaction mode of the Glu172 amino acid residue (critical for receptor–ligand affinity),^116,117 suggesting a multifaceted binding mechanism. Predominantly, σ₁R ligands reported in the literature exhibit structures incorporating piperidine-, spiropiperidine-, and piperazine-related frameworks featuring aromatic moieties. Universally, these compounds adhere to the Glennon model (Figure 2), centered around a basic nitrogen (N) atom.

Figure 2.

Schematic of σ₁R pharmacophores of the Glennon model.

Well-prepared and curated reviews of the latest σ₁R ligands and radiopharmaceuticals in regards to tumors, neurodegenerative diseases, psychiatric disorders, etc.^100,118,119 have been recently conducted in 2020 and 2021. Moreover, in 2024, Fabio and Anna reviewed the chemistry and biological activity of benzotriazines σ₁R ligands, as well as its potential therapeutic applications, such as antihypertensive, anti-inflammatory, and antitumor effects.¹²⁰ Omitting those, in this review, we mainly discussed the ligands and radiopharmaceuticals over 5 years.

Haloperidol (1, Figure 3) is a prototypical example within the category of σ₁R ligands, illustrating the characteristic features of phenylpiperidines.^121,122 It was discovered as one of the earliest ligands to show significant binding specificity for σ₁R, although it notably lacks good subtype-selectivity for σ₂R (K_i(σ1) = 0.90–6.60 nM; K_i(σ2) = 7.93–125 nM; K_i(σ2)/K_i(σ1) = 4–30).^123−127 Haloperidol (1) can inhibit glioblastoma cell migration, reduce stem cell frequency and viability in a dose-dependent manner, and be used in combination with alternative chemotherapy (modazolomide) and radiation treatment to increase programmed cell death.¹²⁸ Research has investigated the treatment and prevention of various tumor cells such as solid Ehrlich tumors,¹²⁹ endometrial cancer,^130,131 prostate cancer,¹³² etc.

Figure 3.

Chemical structure of haloperidol and its derivatives.

Reduced haloperidol analogue ((±)-2, Figure 3) is a potent σ₁R agonist (K_i(σ1) = 1.8 nM; K_i(σ2) = 32 nM; K_i(σ2)/K_i(σ1) < 2), capable of increasing brain-derived neurotrophic factor (BDNF) release from brain astrocytes, which potentially makes it a suitable medication for cognitive disorders.¹³³ The isoforms (S)-(+)-2 and (R)-(−)-2 exhibit 2- and 3-fold lower σ₁R affinities than (±)-2 with 3.6 and 5.3 nM, respectively. These compounds display properties akin to haloperidol. They exhibit moderate affinity for σ₂R (30.9 nM for (S)-(+)-2 and 24.1 nM for (R)-(−)-2) and poor selectivity (K_i(σ2)/K_i(σ1) < 10).¹³³ Moreover, haloperidol derivative 3 (Figure 3) has multiple stereocenters in contrast with comparable ligand 2. Tropane-based four chirality σ₁R antagonists ((R, Z)-3, (R, E)-3, (S, Z)-3, and (S, E)-3) possessing antiallodynic properties have been identified.¹³⁴ In the study by Elena et al., the four stereoisomers exhibited distinct affinities for the σ₁R ((R, E)-3 and (S, Z)-3 showed subnanomolar affinity for σ₁R in 0.8 and 0.45 nM, ((R, Z) and (S, E)-3 showed high affinity for σ₁R in 18 and 6.6 nM).¹³⁴ These chirality ligands exhibit moderate affinity for σ₂R (K_i(σ2) > 10 nM) and moderate selectivity (K_i(σ2)/K_i(σ1) = 10–25).¹³⁴ Despite possessing psychoactive agents, haloperidol-based ligands on the skeleton display low subtype selectivity, so it may not be preferable for selecting highly selective ligands development.

Phenylethylamine and phenoxyamine derivatives (last 5 years) display more variety and might not even adhere to the Glennon model in terms of chemical structures (e.g., 4–5, 9–12, Figure 4).^135−137 The structures of the compounds 4–5 and 9–12 (Figure 4) contain only one aromatic hydrophobic region as well as a basic nitrogen atom. Ligand compounds 4 and 5 exhibited potent binding to σ₁R (K_i(σ1) = 4.8 and 13 nM, respectively), however, the selectivity for σ₂R was contrasting with compound 4 showing high affinity and selectivity (replacement of 32% at concentration of 1 μM) and compound 5 showing medium affinity (K_i(σ2) = 95 nM) and low selectivity (K_i(σ2)/K_i(σ1) = 7).¹³⁵ Compounds 9 and 10 possess nanomolar affinities for σ₁R (K_i(σ1) = 19.6 and 27.2 nM, respectively) and differ nearly 5-fold selectivity for σ₂R (K_i(σ2) = 2630 and 750 nM and K_i(σ2)/K_i(σ1) = 130 and 28, respectively) due to differences in linker length.¹³⁶ Compounds 11 and 12 demonstrated subnanomolar affinity for σ₁R (K_i(σ1) = 0.86 and 0.89 nM, respectively) and are also highly selective toward σ₂R (K_i(σ2) = 239 and 170 nM; K_i(σ2)/K_i(σ1) = 278 and 191, respectively).¹³⁷ The electron withdrawing (−Cl) and donating (−OCH₃) groups on the benzene ring did not impact the affinity and selectivity in compounds 11 and 12. Additionally, they have been determined to be σ₁R agonists while also possessing powerful antiamnestic effects in neurological models.¹³⁷

Figure 4.

Phenethylamine and phenoxyamine-based σ₁R ligands.

In addition, compounds 6–8 and 13 (Figure 4) have structural congruence with the Glennon model. These ligands contain chiral centers, and their different isomers have widely varied σ₁R and σ₂R affinities. Compound 6 was comprised of a phenylethylamine group and an isoquinoline unit at opposing ends of its nitrogen (N) atom. The enantiomer (S)-6 shown high affinity for σ₁R and moderate affinity for σ₂R (K_i(σ1) = 11 nM; K_i(σ2) = 169 nM; K_i(σ2)/K_i(σ1) = 15), while only the enantiomer (R)-6 displayed no selectivity for both receptors with a converse and moderate affinity (K_i(σ1) = 252 nM; K_i(σ2) = 94 nM; K_i(σ2)/K_i(σ1) = 0.4). Similar to ligand 6, compounds 7 and 13 also exhibited stark differences in σ₁R and σ₂R affinity among their stereoisomers with moderate affinity¹¹⁷ (Table 1). Compound 8 was characterized by the presence of two chiral centers at positions 2 and 6, providing enantiomers (2S,6R)-8 and (2R,6S)-8 that demonstrate significant σ₁R affinity (K_i(σ1) = 1.6 and 5.4 nM, respectively) and high selectivity for σ₂R (K_i(σ2) = 378 and 426 nM; K_i(σ2)/K_i(σ1) = 236 and 79, respectively).¹³⁸ The phenylethylamine group in the structure of compound 8 was substituted with a cyclohexyl group to produce compound 14 (Figure 4), characterized as a phenoxyamine derivative.¹³⁸ The isomer (2S,6R)-14 behaved properties for a high affinity for σ₁R and selectivity for σ₂R (K_i(σ1) = 0.94 nM; K_i(σ2) = 57 nM; K_i(σ2)/K_i(σ1) = 60).¹³⁸ Another isomer, (2R,6S)-14, exhibited a high σ₁R affinity (2.4 nM) and moderate affinity for σ₂R (36 nM), which means it was less subtype selective (about 15).

Table 1. Binding Affinity and Subtype Selectivity of σ₁R Ligands^a.

compd.	σ1R	σ2R	selectivity	action	ref
1 (Haloperidol)	0.90–6.60	7.93–125	4–30	antagonist	(123−127,133)
(±)-2	1.8	32	2	against	(133)
(S)-(+)-2	3.6	30.9	9	ndb	(133)
(R)-(−)-2	5.3	24.1	5	nd	(133)
(R,Z)-3	18	29	2	antagonist	(134)
(R,E)-3	0.8	10	13	antagonist	(134)
(S,Z)-3	0.45	11	24	antagonist	(134)
(S,E)-3	6.6	27	4	antagonist	(134)
4	4.8	32%c	ndb	nd	(135)
5	13	95	7	nd	(135)
(S)-6	11	169	15	nd	(117)
(R)-6	252	94	0.4	nd	(117)
(S)-7	81	5108	63	nd	(117)
(R)-7	699	920	1.3	nd	(117)
(2S,6R)-8	1.6	378	236	antagonist	(138)
(2R,6S)-8	5.4	426	79	antagonist	(138)
9	19.6	2630	130	against	(136)
10	27.2	750	28	against	(136)
11	0.86	239	278	against	(137)
12	0.89	170	191	against	(137)
(S)-13	132	463	3.5	nd	(117)
(R)-13	58	176	3	nd	(117)
(2S,6R)-14	0.95	57	60	antagonist	(138)
(2R,6S)-14	2.4	36	15	antagonist	(138)
15d	11.0	6.33	nd	antagonist	(139)
16d	10.9	6.09	nd	antagonist	(139)
17	7.9	483	61	antagonist	(140)
18	27	1600	59	antagonist	(140)
19	nd	nd	nd	antagonist	(141)
20	0.93	72	77	agonist	(142)
21	1.10	88	80	agonist	(142)
22 (LY-0919)	175.1	38200	218	agonist	(127)
23	1.2	36	30	antagonist	(143)
24	8.3	166	20	agonist	(144)
25	3.1	70	23	agonist	(144)
(S)-26	2.4	123	51	agonist	(144)
(R)-26	8.8	206	23	agonist	(144)
(1RS,6SR)-27	3.6	177	49	nd	(145)
(1RS,6RS)-27	28	425	15	nd	(145)
28	1.6	1418	886	antagonist	(146)
29	8.8	3253	370	nd	(146)
30	6.1	2583	423	nd	(146)
31	0.74	35	42	agonist	(147)
32	1.3	91	70	agonist	(147)
33	2.1	90	43	nd	(147)
34	5.4	182	36	nd	(147)
(S)-35 ((S)-FBFP)	2.26	299	127	agonist	(26,151)
(R)-35 ((R)-FBFP)	1.61	246	152	antagonist	(26,151)
36	95.5	1107	12	nd	(154)
37	7.2	440	61	nd	(155)
(R)-38 (WLB-87848)	9	>1000	>110	agonist	(156)
39	51	>10000	>196	antagonist	(159)
40	63	>10000	>159	antagonist	(159)

^aK_i values (nM);

^bnd = no date or not determined;

^cdisplacement of radioligand (%);

^dreported pK_i values.

Donepezil and Cutamesine (SA4503) are the representative Benzylpiperidine- (Figure 5A) and benzylpiperazine-based (Figure 5B) ligands targeting the σ₁R, respectively. Compounds 15 (pK_i(σ1) = 11.0 nM; pK_i(σ2) = 6.33) and 16 (pK_i(σ1) = 10.9 nM; pK_i(σ2) = 6.09)¹³⁹ fall under a grouping of spirocyclic piperidine derivatives (well-known Fluspidine), which has been documented in numerous studies and reviews.^100,118,119 As σ₁R antagonists, ligands 15 and 16 have been used to treat eating disorders in rats.¹³⁹ Compounds 17 (K_i(σ1) = 7.9 nM; K_i(σ2) = 483 nM; K_i(σ2)/K_i(σ1) = 61) and 18 (K_i(σ1) = 27 nM; K_i(σ2) = 1600 nM; K_i(σ2)/K_i(σ1) = 59) are phenylpiperidine σ₁R antagonists that suppress the growth of human prostate cancer DU145.¹⁴⁰ The σ₁R antagonist 19, without σ₁R affinity data reported, a novel developed targeted therapeutic ligand suitable for relieving symptoms of neuropathic pain by reversing mechanical allodynia in mice, restored mechanical thresholds to near-paclitaxel treatment levels.¹⁴¹ The σ₁R agonists 20 (K_i(σ1) = 0.93 nM; K_i(σ2) = 72 nM; K_i(σ2)/K_i(σ1) = 77) and 21 (K_i(σ1) = 1.1 nM; K_i(σ2) = 88 nM; K_i(σ2)/K_i(σ1) = 80) promote nerve growth factor (NGF)-triggered neurite outgrowth in PC12 cells and protect against NMDA-mediated toxic damage in SH-SY5Y cells. They also demonstrate promising results and safety in vitro in human cancer cell lines including A549, LoVo, and Panc-1.¹⁴²

Figure 5.

Benzylpiperidine- (A) and benzylpiperazine-based (B) σ₁R ligands.

Several high-affinity and high-selectivity σ₁R ligands have potential benefits for a variety of painful conditions, such as chemotherapy-induced neuropathy and postoperative pain.¹⁴³ Antagonist 23 is an antiallodynic agent with high affinity for the σ₁R (1.2 nM) and moderate σ₂R affinity (36 nM) for pain relief.¹⁴³ In recent years, the σ₁R has become an appealing target for novel antidepressant drugs.³² The σ₁R agonist Hypidone hydrochloride (YL-0919, 22), a promising candidate as a fast-onset antidepressant through activation of σ₁R, has shown moderate affinity (175.1 nM) for the σ₁R, and has high selectivity for the σ₂R (K_i(σ2) = 38200 nM and K_i(σ2)/K_i(σ1) = 218).¹²⁷ Furthermore, there are efforts to discover new σ₁R ligands. While there are no diseases applications, many of these attempts have resulted in a range of high-affinity agents (such as high-affinity σ₁R ligands 24–27), which provides a large number of optional ligands for future research^144,145 (σ₁R and σ₂R affinities are shown in Table 1).

The σ₁R ligands containing a piperazine moiety as a basic center possess a relatively low lipid solubility, thus minimizing drug off-target distribution. For example, Cutamesine (SA4503), a σ₁R agonist that is extensively studied in diverse diseases, is examined in phase II clinical trials for the management of ischemic stroke⁴⁵ and depression.⁴⁶ Next, we present a number of highly prospective σ₁R agonist and/or antagonist ligands (Figure 5B, 28–35) that were developed within the last 5 years and explore their applications.

Benzylpiperazine-derived σ₁R antagonist 28 is a potential pain treatment agent with central antinociceptive and antiallodynic effects.¹⁴⁶ It has demonstrated a high affinity for σ₁R (K_i(σ1) = 1.6 nM) and subtype selectivity for σ₂R (K_i(σ2) = 1418 nM and K_i(σ2)/K_i(σ1) = 886),¹⁴⁶ making it a promising candidate for CNS drug development. Meanwhile, σ₁R antagonist 28 analogs 29 (substituting benzene ring for cyclohexane) and 30 (changing the linker length of 29, n = 2–4) showed excellent affinity (K_i(σ1) = 8.8 and 6.1 nM, respectively) in the nanomolar and also demonstrated high selectivity (K_i(σ2) = 3253 and 2583 nM; K_i(σ2)/K_i(σ1) = 370 and 432, respectively).¹⁴⁶ Innovatively designed by Franchini and colleagues, compounds 28–35 (K_i(σ1) = 0.74–5.4 nM; K_i(σ2) = 35–182 nM; and K_i(σ2)/K_i(σ1) = 42–70) stand out among a broad category that are structurally unique benzylpiperazine derivatives with O and N heterocycles.¹⁴⁷ These σ₁R agonists demonstrated protective properties against NMDA-mediated toxicity in SH-SY5Y cells similar to the innovative σ₁R agonists 20 and 21.¹⁴²

Our team has developed a unique, novel σ₁R ligand named FBFP applied to CNS PET imaging.^148,149 In 2022, the S- and R- isomers of FBFP were investigated. Although both isomers ((S)-(+)-35 and (R)-(−)-35) demonstrated high affinity for σ₁R (K_i(σ1) = 2.26 and 1.61 nM, respectively) and σ₂R subtype selectivity (K_i(σ2) = 299 and 246 nM, and K_i(σ2)/K_i(σ1) = 127 and 152, respectively),¹⁵⁰ pharmacological and pharmacokinetic profiles shifted dramatically in vivo (S-FBFP as an agonist, R-FBFP as an antagonist).^26,151 These particular benzylpiperazine-based σ₁R ligands discussed herein are widely employed in the investigation of CNS diseases owing to their low lipid solubility.

Thiazolidinone-based ligands, such as FTC-146 (K_i(σ1) = 0.0025 nM)¹⁵² and SN56 (K_i(σ1) = 0.56 nM),¹⁵³ are often found to possess high specificity and potency toward σ₁R. Thiazolidine-2,4-dione nucleus compound 36 (Figure 6) has a different central structure from FTC-146 and SN56 and demonstrates promising high affinity for σ₁R (K_i(σ1) = 95.5 nM) and subtype selectivity for σ₂R (K_i(σ2) = 1107 nM).¹⁵⁴ Compound 37 (benzo[d]thiazol-2(3H) one-based, Figure 6) is an analog of FTC-146 and SN56, substituting an adamantane moiety for an azepane ring group, and their core heterocyclic structures are consistent.¹⁵⁵ It displayed a low nanomolar affinity for σ₁R (K_i(σ1) = 7.2 nM), along with an appropriate selectivity over σ₂R (K_i(σ2) = 440 nM and K_i(σ2)/K_i(σ1) = 61).¹⁵⁵ The core of WLB-87848 ((R)-38) comprises a fused, heterocyclic, six-membered aromatic system with four carbon atoms and one nitrogen atom,¹⁵⁶ akin to the high-affinity σ₁R antagonists PD144418 (K_i(σ1) = 0.08 nM)¹⁵⁷ and E-52862 (K_i(σ1) = 17 nM).¹⁵⁸

Figure 6.

Other kinds of heterocyclic core derivatives employed as σ₁R ligands.

The enantiopure compound (R)-38 (WLB-87848, Figure 6), developed by Almansa C.’s team, a high affinity and selective σ₁R agonist (K_i(σ1) = 9 nM, K_i(σ2) > 1000 nM and K_i(σ2)/K_i(σ1) > 110), acts as a neuroprotective agent, presents a peculiar structure (unusual edgewise NH moiety).¹⁵⁶ Additionally, in 2021, Almansa C.’s team reported the preparation and biological evaluation of novel tricyclic triazoles compounds 39 and 40 with moderate affinity (K_i(σ1) = 51 and 63 nM, respectively) and high selectivity σ₁R antagonists (both K_i(σ2) > 10⁴ and K_i(σ2)/K_i(σ1) > 150, respectively), which have carried out analgesia exploration.¹⁵⁹ The distinctive feature of compounds 38–40 (Figure 6) is the presence of isomers, each of which exhibits distinct characteristics in terms of affinity profiles and both in vitro and in vivo applications. For instance, the S-isomer of 38 was not preferred by investigators, suggesting an increased potential for (R)-38 compared to that of its predecessor. Therefore, in the development of σ₁R ligands, the chemical property of chirality cannot be neglected.

The chemical structures of compounds 1–40 is shown in Figures 3–6. In Table 1, we summarized the activity profiles of compounds 1–40 across σ₁R and σ₂R subtypes, including affinity, selectivity, and functional effects.

Selective Ligands for σ₂R Targeting

In 2020, Alamri and Afzal utilized almost 20 structure diversity compounds (subnanomolar affinity to σ₂R, K_i < 1 nM) and pharmacophore-based virtual screening (PBVS) to propose a σ₂R receptor–ligand pharmacophore model based on common structural features among all training set molecules.¹⁶⁰ In 2021, the crystal structure of σ₂R was resolved by Alon et al., which provided strong support for σ₂R and its ligands development.²⁴ Various σ₂R ligands produced in recent decades have been employed in investigating a variety of human disorders. This review classifies most of these compounds into five categories, namely, nitrogen-bridging ring/morphan, cyclohexyl piperazine, indole, benzimidazolone, and isoquinoline/isoindoline analogs.

Nitrogen-bridged ring derivatives, such as the morphan analog CB-184 (41) (K_i(σ1) = 7436 nM; K_i(σ2) = 13.45 nM; K_i(σ1)/K_i(σ2) = 554) and azabicyalo analog WC-59 (42) (K_i(σ1) = 1710 nM; K_i(σ2) = 0.82 nM; K_i(σ1)/K_i(σ2) = 2087), are an early class of σ₂R ligands with high affinity and selectivity.^90,161 They all function as σ₂R agonists, with CB-184 (41) being cytotoxic to breast carcinoma cells and WC-59 (42) tagged with fluorine-18 (¹⁸F) for tumor PET imaging.^90,161 Representative cyclohexyl piperazine derivatives PB28 (43) and F281 (44) demonstrate notable antiproliferative activity. PB28 (43), a unique compound that has both σ₁R antagonist and σ₂R agonist properties (K_i(σ1) = 0.38 nM; K_i(σ2) = 0.68 nM; K_i(σ1)/K_i(σ2) = 0.59),¹⁰⁰ has gained widespread attention in multiple studies, such as anti-SARS-CoV-2 effectiveness,¹⁶² and was beneficial for renal, breast, neuroblastoma, and pancreatic cancer.^79,88,95,163 Additionally, the cyclohexyl piperazine derivative F281 (44), which has a high affinity for σ₂R (K_i(σ2) = 12.6 nM) and selectivity for σ₁R (K_i(σ1) = 3450 nM and K_i(σ1)/K_i(σ2) = 274), has shown promising antiproliferative activity in renal, neuroblastoma, and pancreatic cancer.^87,100

Siramesine (45), also called Lu-28-179, is an indole derivative that was originally developed as a treatment for depression and anxiety.^164,165 It demonstrates antitumor proliferation effects^166,167 and a lack of σ₁R selectivity (Table 2) similar to PB28 (43). It is gratifying that the introduction of 5,6-dimethoxyisoindoline groups into indole-based ligands has facilitated the development of novel σ₂R ligands SYB4, SYB5, and SYB6 (46–48), in 2022.^26,168 These σ₂R ligands 46–48 offer nanomolar affinity (K_i(σ2) = 1.79, 3.27, and 2.63 nM, respectively) and a notable selectivity toward σ₁R, especially compounds 46 and 48 (K_i(σ1) = 3713.27 and 376 nM, and K_i(σ1)/K_i(σ2) = 207 and 143, respectively).^26,168 Among them, the radioligand [¹⁸F]SYB4 corresponding to SYB4 was used in PET imaging studies of CNS diseases, such as AD. It was demonstrated as the first efficient and selective σ₂R radio-probe for CNS PET imaging.^169,170

Table 2. Binding Affinity and Subtype Selectivity of σ₂R Ligands^a.

compd.	σ1R	σ2R	selectivity	action	ref
41 (CB-184)	7436	13.45	554	agonist	(90)
42 (WC-59)	1710	0.82	2087	agonist	(161)
43 (PB28)	0.38	0.68	0.59	σ2R agonist and σ1R antagonist	(100,193)
44 (F281)	3450	12.6	274	agonist	(87,100)
45 (Siramesine)	17	0.12	141.7	agonist	(194,195)
45 (Siramesine)b	10.5	12.6	0.83	agonist	(196)
45 (Siramesine)c	4.69	3.08	1.5	agonist	(197)
46 (SYB4)	371	1.79	207	ndd	(26,168)
47 (SYB5)	187	3.27	57	nd	(26,168)
48 (SYB6)	376	2.63	143	nd	(26,168)
49 (CM397)	366	2.05	179	nd	(171)
50 (CM398)	560	0.43	1302	antagonist	(171,172)
51 (ISO-1)	95–330	6.95–28.3	4–78	antagonist	(173,197,198)
52 (ISO-2/RHM-4)	2150	0.26	8269	nd	(173)
53 (JR1-157)	530	47	11.3	nd	(98)
54 (JR2-298)	165	10	16.5	nd	(98)
55 (A011)	>1000	77.1	>13	nd	(93)
56	>1000	5.1	>196	antagonist	(176)
57	160	3	53	antagonist	(176)
58	>1000	0.38	>2631	antagonist	(176)
59	>1000	1.33	>751	antagonist	(176)
60	37.7	9.2	4	nd	(177)
61	67.8	6.9	9.8	nd	(177)
62 (FEM-1689)	167	17	9.8	nd	(178)
63 (EES-1686)	102	6	17	nd	(178)
64 (JVW-1601)	200	5.5	36	nd	(180)
65 (FA4)	51.3	15.8	3.2	agonist	(182,183)
66	78%e	7.92	nd	nd	(185)
67 (WLB-89462)	1777	13	137	nd	(186)
68	789	1	789	nd	(186)
69	>1000	6	>166	nd	(186)
70	882	4	220	nd	(186)
71	765	3	255	nd	(186)
72	>1000	5	>200	nd	(186)
73	2154	1	2154	nd	(186)
ZA-I-95	>104	16	>625	nd	(188,189)
74 (MA-28C)	5169	2597	2	nd	(189)
75 (MAM03055A)	3371	55.9	60	agonist	(190)
76 (S2/IAPinh)	nd	nd	nd	nd	(191)
77	51.3	30.2	1.7	nd	(192)
78	448	51.1	8.1	nd	(192)

^aK_i value (nM);

^bfrom ref (196);

^cfrom ref (197);

^dnd = no date or not determined;

^einhibition rates on σ₁R (%).

Compound CM397 (49), with high σ₂R affinity and σ₁R selectivity (K_i(σ1) = 366 nM, K_i(σ2) = 2.05 nM, and K_i(σ1)/K_i(σ2) = 179), is a benzimidazolone derivative featuring an N-(4-fluorophenyl)-piperazine moiety. Compound CM398 (50) was obtained by replacing the N-(4-fluorophenyl)-piperazine with 6,7-dimethoxyisoquinoline, which is a potent, high affinity and selectively σ₂R ligand (K_i(σ1) = 560 nM, K_i(σ2) = 0.43 nM, and K_i(σ1)/K_i(σ2) = 1302)¹⁷¹ and is valuable for the validation of new σ₂R radioligand specificity in vivo. In addition, CM398 (50) was a potential analgesic with anti-inflammatory, analgesic, and antichronic pain properties.^171,172

Currently, the most widely employed pharmacophores for σ₂R include 6,7-dimethoxyisoquinoline and 5,6-dimethoxyisoindoline. A vast majority of σ₂R ligands contain one of these pharmacophores in their chemical structures, including compounds 46–48 and CM398 (50). The most commonly used 6,7-dimethoxyisoquinoline-based σ₂R ligand, ISO-1 (51), was noted for its ¹⁸F labeled radioligand [¹⁸F]IOS-1 (the only σ₂R-targeted radiopharmaceutical that has entered clinical trials), which is primarily used in tumor imaging.^{11,19,173,174} Subsequent investigations have revealed that ISO-1 (51) has a moderate affinity for σ₂R and low subtype selectivity for σ₁R (Table 2). Compound ISO-2 (52) is an analog of ISO-1 (51), in which the benzene ring substituent (methyl) on 2-(2-fluoroethoxy)-N-methylbenzamide was replaced by a methoxy and iodine substituent, and demonstrates subnanomolar affinity for σ₂R (K_i(σ2) = 0.26 nM) and high subtype selectivity for σ₁R (K_i(σ1) = 2150 nM and K_i(σ1)/K_i(σ2) = 8269).¹⁷³ The chemical structures and σR affinities of the above-representative σ₂R ligands (compounds 41–52) are listed in Figure 7 and Table 2, respectively.

Figure 7.

Five categories of representative σ₂R ligands 41–52.

Recently, several noteworthy and interesting σ₂R ligands (e.g., compounds 46–48 in Figure 7 and 53–73 in Figure 8) have been discovered and investigated for their potential applications in various diseases. Grafted structural compounds JR1-157 (53) and JR2-298 (54) constitute a novel ligand that selectively targets σ₂R with nanomolar affinity (K_i(σ2) = 47 and 10 nM, respectively) and >10-fold selectivity over σ₁R. Additionally, in pancreatic cancer xenograft models, ligands JR1-157 (53) and JR2-298 (54) exhibited considerable antitumor effectiveness (low micromolar cytotoxicity).⁹⁸

Figure 8.

Novel σ₂R ligands 53–73.

From the 6,7-dimethoxyisoquinoline framework, Huang and colleagues developed a variety of high affinity and selectivity σ₂R ligands, such as compounds 55–59 (Figure 8). The pyrimidine-indole derivative UM171, which is an efficient in vitro stem expansion agent, was modified by introducing the σ₂R pharmacophore 6,7-dimethoxyisoquinoline, resulting in a novel σ₂R ligand A011 (55)¹⁷⁵ with nanomolar affinity for σ₂R with 77.1 nM and >10-fold selectivity over σ₁R.⁹³ Tumor models (MCF-7, MDA-MB-231 and A549) treated with A011 (55) displayed significant antitumor effect without obvious toxicity, highlighting the ER stress pathway as the primary mechanism of action for σ₂R ligand cancer treatment for the first time.⁹³ In addition, compounds 56–59 are effective σ₂R antagonists that offer both high affinity for σ₂R (K_i(σ2) = 5.1, 3.0, 0.38, and 1.33 nM, respectively) and selectivity toward σ₁R (K_i(σ1) = 160 nM for compounds 57 and K_i(σ1) > 1000 nM for compounds 56, 58–59; K_i(σ1)/K_i(σ2) = 57 for compounds 57 and K_i(σ1)/K_i(σ2) > 196, >2631, and >751 for compounds 56, 58–59, respectively).¹⁷⁶ Analogues 56–59 were not responsible for significant cytotoxicity in MCF-7 cancer cells, which showed promise for the treatment of neurodegenerative diseases such as AD, and warranted further pharmacological evaluation.

Moreover, Martin’s group developed a collection of innovative norbenzomorphan (60–63) and phenylpiperazine (60, 61 and 64) σ₂R ligands (Figure 8), in 2022–2024, which greatly expanded the selective diversity. Compounds 60 and 61, containing norbenzomorphan and phenylpiperazine scaffolds, exhibit high affinity as σ₂R ligands (K_i(σ2) = 9.2 and 6.9 nM, respectively) and moderate affinity for σ₂R (K_i(σ1) = 37.7 and 67.8 nM, respectively), suggesting subtype selectivity is less pronounced (K_i(σ1)/K_i(σ2) = 4 and 9.8, respectively).¹⁷⁷ The compounds FEM-1689 (62) and EES-1686 (63) are norbenzomorphan derivatives. The affinity of EES-1686 (63) (K_i(σ2) = 6 nM) substituted with a hydroxyethylpiperidinamide group for σ₂R is about 3-fold higher than that of FEM-1689 (62) (K_i(σ2) = 17 nM) substituted with the propan-1-ol group. They performed moderate binding affinity to σ₁R (K_i(σ1) = 102 and 168 nM, respectively).¹⁷⁸ As a specific σ₂R ligand, FEM-1689 (62) has been employed in CNS disease, such as in alleviating neuropathic pain, suppressing the integrated stress response (ISR), and promoting neurite outgrowth.¹⁷⁹ Further, compounds 60–63 possess chiral isomers, except for the (1S,5R)-configuration, which exhibits negligible affinity for σ₂R; for this reason, they have not been included.^177,178 Compound 64 exhibits high affinity for σ₂R (K_i(σ2) = 5.5 nM) while also demonstrating 36-fold selectivity for σ₁R (K_i(σ1) = 102 nM).¹⁸⁰ It was determined as a potential CNS medication after comprehensive evaluation to interrogate the role of σ₂R in neurological diseases. Numerous analogues have also been identified with equivalent σ₂R affinity and selectivity, see ref.^180,181

Compound FA4 (65), a siramesine analogue that contains thiosemicarbazone scaffold, operates as a σ₂R agonist with moderate affinity (K_i(σ2) = 15.8 nM) and low selectivity for σ₁R (K_i(σ1) = 51.3 nM and K_i(σ1)/K_i(σ2) = 3.2).^182,183 In recent years, FA4 (65) has been progressively utilized in tumor treatment, especially shown that it accelerates cell apoptosis through an ER- and mitochondria-dependent pathway in several tumors.^35,183,184 The 3-alkoxyisoxazole scaffold-based compound 66, identified as a high-affinity σ₂R ligand (K_i(σ2) = 7.92 nM), is a potential antiproliferative drug against various osteosarcoma cells.¹⁸⁵ Compounds 67–73 are a novel class of σ₂R ligands utilizing the isoxazolylpyrimidine scaffold and were developed by Almansa and colleagues.¹⁸⁶ Among them, derivative 67, also named WLB-89462, as a representative of this kind of ligand, demonstrated a high affinity for σ₂R and subtype selectivity for σ₁R (K_i(σ1) = 1777 nM; K_i(σ2) = 13 nM; K_i(σ1)/K_i(σ2) = 137). Its effects in vitro and in vivo properties have been well investigated, and it has been employed as a neuroprotectant to improve and alleviate short-term memory impairment in the rat hippocampus induced by injection of amyloid-β (Aβ) peptide.¹⁸⁶ The chemical structures of other series isoxazolylpyrimidine-based σ₂R ligands 68–73 are listed in Figure 8, and their σ₁R and σ₂R affinity data are shown in Table 2.

Additionally, in σ₂R-positive cell lines, e.g., Panc-1 and MCF-7 cells, the combination of σ₂R ligands and chemotherapeutic drugs can stabilize tumor progression, enhance the clinical benefit of conventional chemotherapies like paclitaxel and doxorubicin, and improve survival rates.^77,187 Recently, studies have described several novel σ₂R small-molecule combinations ligands 74–78 (Figure 9) derived from conjugation or dimerization of existing pharmacophores as promising candidates for pioneering antitumor activities and specific imaging.

Figure 9.

Small-molecule combination/conjugate σ₂R ligands 74–78.

The σ₂R ligand ZA-I-95 (K_i(σ2) = 16 nM) with antitumor activity¹⁸⁸ was coupled with the chemotherapy drug doxorubicin via an amide bond under minor chemical modification conditions to obtain a new small-molecule conjugated ligand MA-28C (74). While maintaining a certain affinity for the σ₂R (K_i(σ2) = 2597 nM), it demonstrates superior antitumor properties in certain tumor cells compared to doxorubicin-only treatment.¹⁸⁹ MAM03055A (75), a dimer of CM571, was discovered by the biodegradation of CM572. Bowen and colleagues prepared the ligand by chemical synthesis. MAM03055A, while maintaining a high affinity for σ₂R, significantly enhances selectivity for σ₁R (K_i(σ1) = 3371 nM, K_i(σ2) = 55.9 nM, and K_i(σ1)/K_i(σ2) = 60). Functionally, MAM03055A, as a downregulation product of CM571, can both avoid the metabolic stimulation produced by CM571 and induce apoptosis in multiple malignancies, such as neuroblastoma, breast, and colorectal cancer cells.¹⁹⁰ The inhibitor of apoptosis protein (IAP) antagonist (IAPinh or LCL161), binds to the terminal amino group of the σ₂R ligand SW43 to produce a novel σ₂R ligand-IAPinh conjugate agent S2/IAPinh (76).¹⁹¹ It has demonstrated superior potency compared to LCL161, displaying significantly greater antiproliferative activity and increased survivability rates in pancreatic and ovarian malignancies.¹⁹¹ Moreover, the σ₂R fluorescent probes SW120 and SW116 are provided by attaching a fluorophore group to the termini of SW43⁸⁰ (no repetition here).

In 2023, the first σ₂R red-emitting fluorescent probes 77 and 78 were developed (Figure 9) by Abate et al. with nanomolar-affinity (K_i(σ2) = 30.2 and 51.5 nM, respectively).¹⁹² Although they show low σ₁R subtype selectivity (K_i(σ1) = 51.3 and 448 nM; K_i(σ1)/K_i(σ2) = 1.7 and 8.1, respectively), these probes possess superior characteristics than traditional long half-life radioligands [³H]DTG and (+)-[³H]-pentazocine in receptor–ligand affinity measuring.¹⁹² The chemical structures of these novel small-molecule combinations ligands are shown in Figure 9, and their affinities for σ₁R and σ₂R are shown in Table 2.

Dual/Multitargeted σRs Ligands

Optimal multipotent ligands concurrently influence specific groups of biological targets with high selectivity. Presently, dual/multitargeted therapies are drawing distinguished attention for their promising synergistic potential. Future efforts aimed at designing and developing such molecules hold promise for improving therapeutic outcomes and disease management. In our prior discussion (Section Multitarget Approach Focus on σRs in Cancer), we gave a brief outline of the benefits of dual/multitargeting in tumor diseases. This section displayed the representative compounds with dual or multitargeting activity across σ₁R and σ₂R that emerged in the last 5 years, exploring relevant drug discovery and applications in the CNS diseases, pain, antiobesity, and tumors.

Given that CNS disorders were characterized by several physiological dysfunctions and dysregulation of complex signaling pathway networks, effective multipotent medicines should affect particular groups of biological targets simultaneously and specifically. In 2021, Kononowicz et al. reviewed the advantages of σ₁R and histamine (H₃) receptor ligands observed in preclinical and clinical treatment of various CNS-related diseases.¹¹⁰ The potent multitargeted antiprion piperidine compound JZ107 (79) has been identified as a high-affinity pan-σR ligand capable of targeting σ₁R, σ₂R, and dopamine 3 (D₃) receptors (K_i(σ1) = 7.9 nM; K_i(σ2) = 5.1 nM; K_i(D3) = 13.7 nM).^199,200 The pan-σR ligand JZ107 (79) inhibited prion propagation in ScN2a cells and prevented prion protein (PrP)-induced synaptic toxicity and dendritic spine retraction in hippocampal neurons that were dependent on activation of the NMDA receptor/p38 MAPK-mediated signaling cascade.²⁰⁰ The dual σ₁R/H₃R-targeting ligands KSK68 (80) and E377 (81), also as piperidine derivatives, exhibit high affinity for both σ₁R (K_i(σ1) = 3.64 nM) and H₃R (K_i(H3) = 4.41 nM) and moderate affinity for σ₂R receptors (K_i(σ2) = 22.4 and 67.9 nM, respectively). KSK68 (80) and E377 (81) function as dual σ₁R and H₃R antagonists, enhancing the antinociceptive effects of loperamide, which provide new treatment strategies for neuropathic pain.²⁰¹ Compounds KSK61, KSK63, KSK73, KSK74, and KSK94 (82–86) are 1-(pyridin-4-yl)piperazine derivatives with a unique diazacyclic system (Figure 10). These compounds were developed as potential antiobesity agents since they dually target σ₂R and H₃R.^202−204 Of these, the dual σ₂R/H₃R-targeting ligands KSK63 (83) and KSK94 (86) exerted superior antiobesity activity. Simultaneously, they displayed high affinity (K_i(H3) = 3.12 nM and 7.86 nM, respectively) for H₃R and moderate affinity (K_i(σ2) = 29.2 nM and 75.2 nM, respectively) for σ₂R.^203,204 The affinities of other ligands KSK61 (82), KSK73 (84), and KSK74 (85) for σ₁R, σ₂R, and H₃R are shown in Table 3.

Figure 10.

Dual/Multi-Targeting σRs ligands 79–86 for CNS (A) and antiobesity (B).

Table 3. Binding Affinity of Dual- and Multitargeting σRs Ligands^a.

compd.	σ1R	σ2R	other targeting	ref
79 (JZ107)	7.9	5.1	D3 (13.7)	(199,200)
80 (KSK68)	3.64	22.4	H3 (7.7)	(201,212)
81 (E377)	4.41	67.9	H3 (6.2)	(201)
82 (KSK61)	638	108	H3 (21.1)	(203)
83 (KSK63)	726	29.24	H3 (3.12)	(203)
84 (KSK73)	408	59.7	H3 (40.5)	(203)
85 (KSK74)	38.9	65.9	H3 (38.9)	(202,203)
86 (KSK96)	2958	75.2	H3 (7.86)	(204)
87	58	ndb	μ (175)	(208)
88 (EST73502)	118	nd	μ (64)	(209)
89	5.5	18.2%c	μ (4.4)	(210)
Rec-90	16.3	14.2%c	μ (9.4)	(210)
(S)-90 (HKC-126)	19.4	23.3%c	μ (4.3)	(210)
(R)-90	24.2	nd	μ (893.2)	(210)
91 (AD164)	94	1125	H2S (38.4 μM)d	(125)
92 (Tao-191)	35.7	448.2	μ (6.5)	(211)
93	1.9	25	H3 (4.3)	(212)
94	4.8	116	H3 (5.2)	(212)
95	4.5	10	H3 (7.7)	(212)
96	64	93	NO donor	(213)
97	250	89	NO donor	(213)
98	24	19	NO donor	(213)
99	38	2917	HDACs (0.59)e	(214)
100	52	1588	HDACs (1.01)e	(214)
101	6.3	9.23	-	(215)
102	6.1	152.76	-	(215)

^aK_i value (nM);

^bnd = no date or not determined;

^c%inhibition was determined at the concentration of 10 μM;

^dthe amount of H₂S released by 91 (AD164) after 1 h incubation;

^eIC₅₀ value (μM)

Many clinical trials and preclinical studies have provided evidence that the σ₁R plays a crucial role in pain management.²⁰⁵ E-52862 (σ₁R antagonist) has shown promising therapeutic for diverse pain conditions, as evidenced by its inclusion in ongoing clinical trials.⁴¹ Therefore, dual targeting of σ₁R and μ-opioid receptor (MOR) ligands (σ₁R/MOR) is a new strategy to improve the side effects of single MOR agonists, such as respiratory depression, constipation, emesis, and tolerance.^206,207 Compounds 87–90 and 92 act as dual-targeting σ₁R/MOR ligands (Figure 11) with demonstrated mixed agonistic toward MOR and antagonistic properties toward σ₁R.^208−211 Dual-targeted phenethyl-piperidine derivatives 87 and 88 (EST73502) are derived from the same parent structure (9-phenethyl-1-oxa-4,9-diazaspiro[5.5]undecan-3-one), and both were found to possess moderate affinity for both σ₁R (K_i(σ1) = 58 nM and 118 nM, respectively) and MOR (K_i(μ) = 175 nM and 64 nM, respectively).^208,209 Both 89 and 90 are racemic compounds, with the chiral configuration of 90 (Rac-90) already determined ((S)-90 also known as HKC-126, and (R)-90). These compounds contain a 4-benzyl-piperidine structure and have high affinities for both the σ₁R (K_i(σ1) = 5.5–24.2 nM) and MOR (K_i(μ) = 4.4–9.4 nM), excluding (R)-90 (K_i(μ) = 893.2 nM).²¹⁰

Figure 11.

Dual/multitargeting σRs ligands 87–95 for pain.

Phenethyl-piperidine derivative 92 demonstrates a significantly heightened affinity for σ₁R (K_i(σ1) = 35.7 nM) and MOR (K_i(μ) = 6.5 nM) relative to comparable compounds 87 and 88, with 1.6–3.3-fold and 10–27-fold, respectively.²¹¹ The 1-benzyl-piperidine derivative AD164 (91) acts as a dual-acting σ₁R antagonist for pain treatment with nanomolar affinity (K_i(σ1) = 94 nM) and hydrogen sulfide (H₂S) releasing (38.4 μM per hour) properties in vitro.¹²⁵ The H₃R antagonist 93–95 exhibits high affinity for both σ₁R (K_i(σ1) = 1.9 nM, 4.8 nM, and 4.5 nM, respectively) and H₃R (K_i(H3) = 4.3 nM, 5.2 nM, and 7.7 nM, respectively), and was often helpful for alleviating neuropathic symptoms and nociceptive pain. Among those, compound 95 was determined to be a σ₁R antagonist with additional high-affinity interactions with the σ₂R (K_i(σ2) = 10 nM), which showed broad-spectrum analgesic efficacy in the several pain models as a σ₁R/σ₂R/H₃R triple-targeting ligand.²¹² These dual/multitargeted ligands provided many advantages and implications for several types of pain treatment, such as cancer and chemotherapy.

Additional research is needed to fully elucidate the tumorigenic potential of σ₁R and σ₂R. Alternatively, the approval of dual/multitargeted σR ligands may pave the way for novel antitumor agents, which could advance pharmacotherapy beyond its existing boundaries. Intagliata et al. (2021) reviewed the potential utility of combined σ₁R/σ₂R ligands in antiproliferative and antitumor activities,¹⁰¹ which suggested that modulation of dual/multitargeted σR ligands might represent a new direction and strategy in antitumor activities.

We describe numerous possible dual/multitargeted σR ligands 96–102 (Including benzyl-piperidine, spiro-piperidine, and benzyl-piperazine derivatives, Figure 12) developed during the five years, including σ₁R/σ₂R-mixed and combinations of σ₁R or σ₂R with additional receptors (or effectors). Both 4-benzyl-piperidine derivatives 96 and 97 exhibit moderate σ₁R (K_i(σ1) = 64 nM and 250 nM, respectively) and σ₂R (K_i(σ2) = 93 nM and 89 nM, respectively) affinity.²¹³ Minor modifications in linker length (n = 1 vs 2) between the 4-benzyl-piperidine pharmacophore and NO donor component in compounds 96 and 97 resulted in considerable alterations in σ₁R affinity (approximately 4-fold). The 1-benzyl-piperidine pharmacophore has been connected to the NO donor moiety to create the compound 98, which demonstrated increased binding affinity to both σ₁R and σ₂R (K_i(σ1) = 24 nM and K_i(σ2) = 19 nM).²¹³ Surprisingly, ligand 97, which acts as σ₁R antagonist/σ₂R agonist and is selective for σ₂R (K_i(σ1)/K_i(σ2) = 2.8), displayed excellent antitumor activity against human breast cancer cell MCF-7 (IC₅₀ = 26 μM) and human colon cancer cell Caco-2 (IC₅₀ = 28 μM), equivalent to doxorubicin.²¹³

Figure 12.

Dual/multitargeting σRs ligands 96–102 for antitumor effect.

Benzyl-piperazine derivative 99 and 1-benzyl-piperidine derivative 100 have been identified as dual-targeting ligands with nanomolar affinity for σ₁R (K_i(σ1) = 38 nM and 52 nM, respectively) and low micromolar affinity for histone deacetylase enzymes (HDACs) receptors (IC₅₀ = 0.59 μM and 1.01 μM, respectively).²¹⁴ The ligands feature a linker (−CH₂PhCH=CH– for 99 and −N(CH₃)CH₂C₆H₄– for 100) connecting the σR to the HDAC receptor binding domain (amide moiety, Figure 12). Compounds 99 and 100 both showed low micromolar affinity in varieties of tumor cells, including human gastric adenocarcinoma cell line (AGS, IC₅₀ = 3.5 μM and 8.2 μM, respectively), colorectal carcinoma (HTC116, IC₅₀ = 0.9 μM and 2.9 μM, respectively), MCF-7 (IC₅₀ = 8.4 μM and 15.0 μM, respectively), and PC3 (IC₅₀ = 7.5 μM and 26.8 μM, respectively).²¹⁴

The σ₁R/σ₂R-mixed ligands 101 (K_i(σ1) = 6.3 nM; K_i(σ2) = 9.23 nM; K_i(σ2)/K_i(σ1) = 1.5) and 102 (K_i(σ1) = 6.1 nM; K_i(σ2) = 152.76 nM; K_i(σ2)/K_i(σ1) = 25) were obtained by preserving the 4-fluorophenylbutanone moiety in the structure of haloperidol and substituting the piperidine ring with varying amines, such as spiro-piperidine and 4-(3,4-dimethoxybenzyl)piperidine (Figure 12).²¹⁵ In cytotoxicity assays, the compounds 101 and 102 demonstrated micromolar affinities for human neuroblastoma (SH-SY5Y, IC₅₀ = 57 μM and 58 μM, respectively) and hepatocarcinoma (HUH-7, IC₅₀ = 16 μM and 37 μM, respectively) cell lines.²¹⁵ In addition, compound 102, with certain σ₂R selectivity (K_i(σ2)/K_i(σ1) = 25), showed slightly increased potency against SH-SY5Y and HUH-7 cells, suggesting that subtype selectivity may modulate cytotoxic effects and influence ligand activity for tumors, which is similar to compound 97 described above.

Recently, progress has witnessed the evolution of a number of new radioligands for the sigma receptor, including PET and SPECT tracers.^{119,216−219} These radioligands have proven to be useful in investigating σ₁R and σ₂R functionality and developing therapies. The σ₁R and σ₂R radioligands, such as [¹¹C]SA4503 ([¹¹C]103),^220−222 [¹⁸F]FTC-146 ([¹⁸F]104),^223,224 [¹⁸F]Fluspidine ([¹⁸F]108),^225,226 [¹⁸F]ISO-1 ([¹⁸F]51),^19,174 and [¹⁸F]SYB4 ([¹⁸F]46),^26,168,169 have become increasingly important in the understanding of a variety of neuropsychiatric and tumor disorders.^119,217 These applications will be increasingly prominent and valuable in the future. Moreover, this review explores the discovery, development, and widespread application of σ₁R and σ₂R radioligands in recent years.

The σ₁R Radioligands

Currently, there are eight σ₁R radioligands ([¹⁸F]Haloperidol ([¹⁸F]1),²²⁷ (S)-(+)-[¹⁸F]FBFP ((S)-[¹⁸F]35) (NCT05335200)), [¹¹C]SA4503 ([¹¹C]103),^228,229 [¹⁸F]FPS ([¹⁸F]104),²³⁰ [¹¹C]Nemonapride ([¹¹C]105),²³¹ [¹²³I]TPCNE ([¹²³I]106),²³² [¹⁸F]FTC-146 ([¹⁸F]107),^224,233 and (S)-[¹⁸F]Fluspidine ((S)-[¹⁸F]108),^234,235 have been utilized in human trials (Figure 13). They are all employed for PET/SPET scans to investigate radiopharmaceuticals distribution in humans, σ₁R distribution in the brain, and CNS-related disease research, such as [¹¹C]SA4503 for AD and PD,²²⁸ [¹¹C]Nemonapride for PD,²³¹ (S)-[¹⁸F]Fluspidine for HD,^225,235 etc. In clinical trials, [¹⁸F]FTC-146 was administered to facilitate the detection and interpretation of σ₁R using PET imaging in conjunction with magnetic resonance imaging (MRI) in chronic pain patients and accurately identified the exact location of pain based on PET/MRI imaging.²³⁶ Furthermore, molecular imaging investigations of malignancies frequently employ σ₁R radioligands, such as [¹¹C]SA4503 in the VX-2 carcinoma²³⁷ and glioma cells (C6) tumor-bearing nude rats²³⁸ and [¹⁸F]Fluspidine (both isomers (S)-(−) and (R)-(+)) in the U87MG glioblastoma model.^226,239 These radioligands used in clinical trials or human exploratory studies have been described in detail in ref.^26,119

Figure 13.

Chemical structures of σ₁R radioligands in humans investigating.

We summarized highlights the diagnostic and therapeutic use of σ₁R radioligands 109–114 (Figure 14) over the past 5 years in brain or CNS imaging ([¹⁸F]109, [^123/125I]OI5V ([^123/125I]110) and (S)-(+)-/(R)-(−)-[¹⁸F]FBFP ((S)- and (R)-[¹⁸F]35, Figure 13)) as well as tumor imaging ([¹²⁴I]IPAG ([¹²⁴I]111) and [¹²⁵I]112) and therapy ([²¹¹At]mAtC₂N5V ([²¹¹At]113) and [²¹¹At]mAtC₃N5V ([²¹¹At]114)). The affinities for σ₁R and σ₂R of corresponding stabilizing ligand are shown in Table 4.

Figure 14.

Recent σ₁R radioligands employed in diagnosis and therapy.

Table 4. Binding Affinity of σ₁R and σ₂R Nonradioligands^a.

compd.	σ1R	σ2R	selectivity	action	ref
(S)-35 ((S)-FBFP)	2.26	299	127	agonist	(26,151)
(R)-35 ((R)-FBFP)	1.61	246	152	antagonist	(26,151)
103 (SA4503)	3.3–4.6	51–242	14–55	agonist	(126,259−261)
104 (FPS)	4.3	143	33	ndb	(262)
	0.5c	nd	144263	nd	(264)
105 (Nemonapride)	5.6	38.6	6.9	nd	(265)
106 (TPCNE)	0.67	38.8	50	nd	(266)
107 (FTC-146)	2.5 × 10–3	364	145600	nd	(152)
(S)-108 ((S)-Fluspidine)	2.3	897	390	nd	(267)
(R)-108 ((R)-Fluspidine)	0.57	1650	2895	nd	(267)
109	6.63; 10.6	352; 403	53; 44	nd	(241)
110 (OI5V)	4.7	173.3	37	nd	(242)
111 (IPAG)	2.8–10.3c	nd	nd	antagonist	(243,268−270)
112	8.7	22.3	2.6	nd	(244)
113 (mAtC2N5V)d	7.9	73.6	9.3	nd	(246)
114 (mAtC3N5V)d	14.6	16.9	1.2	nd	(246)
115 (CT1812)	63	8.5	7.4	antagonist	(247)
(±)-116 ((±)-A03)	48.4	0.59	82	nd	(249)
(+)-116 ((+)-A03)	36.8	0.63	58	nd	(249)
(−)-116 ((−)-A03)	39.9	0.33	121	nd	(249)
(±)-117 ((±)-A04)	108	4.92	22	nd	(249)
(+)-117 ((+)-A04)	87.6	10.9	8	nd	(249)
(−)-117 ((−)-A04)	182	3.01	60	nd	(249)
118 (RM273)	691	1.6	432	nd	(250)
119	3610	2.30	1570	nd	(251)
120	6265	2.57	2438	nd	(252)
[Re]-121e	30.1	2.96	10.2	nd	(253)
122 (THQ-DTPA)	nd	16.32c	nd	nd	(254)
123	9.2	1.6	5.8	nd	(255)
124	6.3	10.2	1.6	nd	(256)
125 (mIC2N5V)	7.9	73.6	9.3	nd	(246,258)
126 (mIC3N5V)	14.6	16.9	1.2	nd	(246,258)

^aK_i values (nM);

^bnd = no date or not determined;

^cK_d values (nM);

^dK_i values (nM) refer to iodine substituted derivatives mIC₂N5V (125) and mIC₃N5V (126) only;²⁴⁶

^eThe K_i value (nM) was obtained by measuring the corresponding rhenium (Re) complex.²⁵³

Racemate benzylpiperazine σ₁R radioligand rac-[¹⁸F]FBFP ([¹⁸F]35) is a high-affinity and selective brain PET imaging agent employed in receptor exploration in the senescence-accelerated prone (SAMP8) model of AD.¹⁴⁸ We formally isolated its isomers (S)-(+)- and (R)-(−)-[¹⁸F]FBFP ((S)- and (R)-[¹⁸F]35) in 2022¹⁵¹ and employed them in nonhuman primate research¹⁵⁰ as well as heart and brain assessment in the rat model of acute myocardial infarction²⁴⁰ with high affinities for σ₁R (K_i(σ1) = 2.26 and 1.61 nM, respectively) and subtype selectivity for σ₂R (K_i(σ2) = 299 and 246 nM; K_i(σ2)/K_i(σ1) = 127 and 152, respectively). The spirocyclic derivative radioligand [¹⁸F]109, with a high affinity for σ₁R (K_i(σ1) = 6.63 nM) and high subtype selectivity for σ₂R (K_i(σ2) = 352 nM; K_i(σ2)/K_i(σ1) = 44), was an effective brain PET tracer. It demonstrated a high initial brain uptake (9.84% ID/g) and high brain-to-blood ratios of 5.89 at 2 min radioligand postinjection.²⁴¹ The 4-phenylpiperidine derivative [^125/123I]OI5V ([^123/125I]110), a high affinity and selectivity σ₁R radioligand (K_i(σ1) = 4.7 nM; K_i(σ2) = 173.3 nM; K_i(σ2)/K_i(σ1) = 37), was among the few tracers considered for brain SPECT imaging, albeit with relatively low brain uptake of 1.49–2.10% ID/g at 2 min postinjection.²⁴²

As essential tumor imaging agents, two σ₁R radioligands, iodine-124 labeled [¹²⁴I]IPAG ([¹²⁴I]111) and iodine-124 labeled [¹²⁵I]112, are utilized in cancer investigation. The [¹²⁴I]IPAG ([¹²⁴I]111), a σ₁R antagonist with nanomolar affinity (K_d(σ1) = 2.8–10.3 nM), was employed for PET imaging in σ₁R-overexpressing tumors, such as mice bearing MCF-7 (1.53% ID/g uptake in tumor and tumor-to-muscle ratios of 2.04 at 4 h postinjection) and LNCaP (no biodistribution data available) tumor xenografts.²⁴³ Radioligand [¹²⁵I]112 displayed a high affinity for σ₁R (K_i(σ1) = 8.7 nM) as well as a moderate affinity for σ₂R (K_i(σ2) = 22.3 nM). Moreover, special accumulation of radioligand [¹²⁵I]112 was also revealed in DU-145 cells in vitro and tumor-bearing mice in vivo with 6.23% ID/g uptake in tumor and tumor-to-muscle ratios of 3.48 at 1 h postinjection.²⁴⁴

Furthermore, few studies have been conducted on radiotherapeutic ligands based on σ₁R. In 2009, Ogawa et al. first introduced the σ₁R radioligands (+)-[¹²⁵I]pIV and (+)-[¹³¹I]pIV, among which (+)-[¹²⁵I]pIV was regarded as an integrated diagnostic and therapeutic radioligand.²⁴⁵ In 2023, they reported the alpha (α) nuclide Astatine-211 (²¹¹At) labeled aza-vesamicol ((+)-pIV analog) σ₁R radioligands [²¹¹At]mAtC₂N5V ([²¹¹At]113) and [²¹¹At]mAtC₃N5V ([²¹¹At]114) for using radiotheranostics.²⁴⁶ Additionally, the tumor volume growth of DU-145 tumor-bearing mice treated with [²¹¹At]113 and [²¹¹At]114 (0.48 MBq) measurably decelerated by 2–3 fold.²⁴⁶ Inspired by that, the employment of therapeutic radionuclides, such as ¹³¹I, ²¹¹At and ¹⁷⁷Lu, to label high-affinity σ₁R ligands provides a new strategy for tumor treatment.

The σ₂R radioligands

Compared to σ₁R, there is less structural information available about σ₂R, as the protein crystal structure was just recently characterized in less than 5 years.²⁴ Currently, only two σ₂R ligands have been employed in clinical trials, namely CT1812 (115, Figure 15 A, K_i(σ1) = 63 nM; K_i(σ2) = 8.5 nM; K_i(σ1)/K_i(σ2) = 7.4) for early/mild to moderate AD treatment^48,247,248 (phase II, NCT05531656) and geographic atrophy (GA) secondary to dry age-related macular degeneration (AMD) evaluation (phase II, NCT05893537)) and [¹⁸F]ISO-1 ([¹⁸F]51, Figure 15 A) for PET imaging in primary and metastatic breast cancer.¹⁷⁴ The compounds 51 and 115 were derived from isoindoline and isoquinoline pharmacophores σ₂R ligands, respectively, whose results have a particular guiding significance for σ₂R ligands development. We focused on recent developments of σ₂R-targeted radioligands 46–48 and 116–122, which were listed in Figure 15 (B), most notably those transpiring within the CNS and tumors.

Figure 15.

σ₂R ligands in clinical trials (A) and recent radioligands (B).

Indole-based derivatives 46–48, including the isoindoline pharmacophore, were thoroughly discussed (Section Selective Ligands for σ₂R Targeting), among which radioligand [¹⁸F]SYB4 ([¹⁸F]46) was highlighted as the first σ₂R specific brain imaging agent with high brain uptake (4.44% and 5.04% ID/g) and brain-to-blood ratio (1.87 and 10.57) at 2 and 30 min postinjection.¹⁶⁸ It can efficiently visualize σ₂R with high specificity in both in vitro autoradiograms and in vivo rats and nonhuman primates PET imaging.^168−170 Carbon-11-labeled isoquinoline-based derivatives (±)-[¹¹C]A03 ((±)-[¹¹C]116) and (±)-[¹¹C]A04 ((±)-[¹¹C]117) are also thought to potentially serve as PET imaging agents in the brain with high σ₂R affinity and subtype selectivity (K_i(σ1) = 48.4 and 108 nM; K_i(σ2) = 0.59 and 4.92 nM; K_i(σ1)/K_i(σ2) = 82 and 22, respectively). Radioligand (±)-[¹¹C]116, as a PET imaging agent for measuring function of σ₂R in CNS, displayed a certain uptake in the brain of C57BL/6J mice (8.28% ID/cc at 3 min), but it needs to improve the σ₂R specificity.²⁴⁹

In addition, the radioligand [¹⁸F]RM273 ([¹⁸F]118), an azaindole derivative, is based on a tetrahydroisoquinoline pharmacophore with high affinity for σ₂R and subtype selectivity (K_i(σ1) = 691 nM; K_i(σ2) = 1.6 nM; K_i(σ1)/K_i(σ2) = 432). As a brain imaging agent, radioligand [¹⁸F]118 demonstrated acceptable brain uptake (SUV = 1.3 at 2.25 min) and low nonspecific binding in CD1 mice PET imaging.²⁵⁰ The radioligand [¹⁸F]118, when employed as a tumor imaging agent, was able to orthotopically image F98 rat glioblastoma in vitro through autoradiography. However, in vivo investigation data is lacking.²⁵⁰ Tumor imaging agent [¹⁸F]119 and [¹⁸F]120 are CM398 (50) analogs with high σ₂R affinity and subtype selectivity (K_i(σ1) = 3610 and 6265 nM; K_i(σ2) = 2.30 and 2.57 nM; K_i(σ1)/K_i(σ2) = 1570 and 2438, respectively), with a benzimidazolone scaffold based on isoindoline and tetrahydroisoquinoline pharmacophores, respectively.^251,252 These agents demonstrate great imaging capabilities in specific σ₂R for subcutaneous A549 lung cancer ([¹⁸F]119, SUV = 1.71 and tumor-to-muscle ratios of 3.43 at 30 min), U87MG glioma xenograft tumor ([¹⁸F]119, SUV = 1.04 and tumor-to-muscle ratios of 3.21, and [¹⁸F]120, SUV = 1.14 and tumor-to-muscle ratios of 3.58, at 30 min) and intracranial orthotopic U87MG glioma model ([¹⁸F]119, tumor-to-background of 3.03 at 30 min) imaged.^251,252 Furthermore, [¹⁸F]119 and [¹⁸F]120 were determined to be the most specific σ₂R tumor imaging agents currently available.

Clinically, SPECT imaging offers significant advantages in terms of affordability and versatility. Developing ^99mTc-labeled σ₂R radioligands with high affinity and specificity could provide a valuable tool for early cancer diagnosis and proliferation state evaluation. A limited number of ^99mTc-labeled σ₂R ligands have been developed to date,²¹⁷ among which radioligand [^99mTc]121 was the most representative ^99mTc-labeled small molecule σ₂R tumor SPECT imaging agent with high affinity (K_i(σ2) = 2.96 nM) and moderate subtype selectivity (K_i(σ1) = 30.1 nM; K_i(σ1)/K_i(σ2) = 10.2).²⁵³ Tumor cellular association of [^99mTc]121 showed 25% (incubation for 120 min), and it achieved high tumor uptake (5.92% ID/g), tumor-to-blood (20.6), and tumor-to-muscle (16.1) ratios at 240 min in C6 glioma xenografts biodistribution. SPECT imaging also showed clear visualization of solid tumors in C6 glioma xenografted mice.²⁵³ Recently, Mishra et al. reported a ^99mTc-labeled tetrahydroisoquinoline-based (THQ) diethylenetriaminepentaacetic acid (DTPA) derivative [^99mTc]122 with moderate σ₂R affinity (K_d(σ2) = 16.32 nM). The biodistribution study in nude mice bearing MDA-MB-231 cells has highlighted the notable tumor uptake of [^99mTc]122 with 1.56% ID/g and tumor-to-blood (1.98) and tumor-to-muscle (3.58) ratios at 30 min. In SPECT imaging of triple-negative breast tumor model mice, tumor radioactivity uptake was observed at 30 to 60 min.²⁵⁴ At the same time, its specificity is still a matter of deliberation and requires further improvement.

Dual/Multitargeted σRs Radioligands

Various receptor dual/multitargeting ligands were reviewed (Section Dual/Multitargeted σRs Radioligands) that have been investigated recently for CNS treatment or antitumor proliferation. There are relatively few σR dual/multitargeted imaging agents, which is most likely due to challenges with specificity. Although this scientific issue is challenging, several imaging agents, such as [¹¹C]Cimbi-701 ([¹¹C]123), [¹⁸F]124, [¹²⁵I]mIC₂N5V ([¹²⁵I]125), and [¹²⁵I]mIC₃N5V ([¹²⁵I]126), have been developed and employed for CNS and tumor imaging (Figure 16 and Table 4).

Figure 16.

σR dual/multitargeted radioligands.

The multitargeted radioligand [¹¹C]Cimbi-701 ([¹¹C]123), which has been investigated as a PET imaging agent for CNS assessment, simultaneously displays high affinity for the serotonin 7 receptor (5-HT₇, K_i(5-HT7) = 18 nM), σ₁R (K_i(σ1) = 9.2 nM), and σ₂R (K_i(σ2) = 1.6 nM). Although brain uptake was lower in rats, where it was determined to be a P-gp substrate, in PET imaging, this limitation was lifted in pig and baboon, with peak SUV values of 2.1 and 1.25, respectively.²⁵⁵ The [¹⁸F]124 is a small molecule radioligand that targets both σ₁R and σ₂R with high affinity (K_i(σ1) = 6.3 nM; K_i(σ2) = 10.2 nM).²⁵⁶ In PC-3 prostate tumor cells, which have elevated overexpression of both σ₁R and σ₂R,²⁵⁷ the majority of the radioactivity from [¹⁸F]124 was localized on the surface of these σRs. In PET imaging, the radioligand [¹⁸F]124 successfully imaged solid tumors in PC-3 tumor-bearing mice, with radioactivity buildup rising over time and peaking at 4.4% ID/g at 1.5 h, improving tumor visibility.²⁵⁶ Substituting radionuclide ¹²⁵I for ²¹¹At in radioligands [²¹¹At]113 and [²¹¹At]114 transforms therapeutic radiopharmaceuticals into diagnostic tumor imaging agents [¹²⁵I]mIC₂N5V ([¹²⁵I]125) and [¹²⁵I]mIC₃N5V ([¹²⁵I]126). They demonstrated high or moderate affinity for both σ₁R (7.9 nM for 125 and 14.6 nM for 126) and σ₂R (73.6 nM for 125 and 16.9 nM for 126), particularly [¹²⁵I]126, which exhibited equal affinity for both receptors. Both radioligands [¹²⁵I]125 and [¹²⁵I]126 successfully conducted SPECT imaging of tumors in DU-145 (overexpression of both σ₁R and σ₂R²⁵⁷) tumor-bearing mice.²⁵⁸ Few studies have focused on dual or multitargeted radioligands for σR imaging or disease treatment, and the quality of imaging requires enhancements. Consequently, pertinent research still has arduous progress yet to achieve.

Conclusions and Perspectives

Since initially being identified in 1976,²⁰ σ₁R and σ₂R have been shown to play critical roles in a variety of biological processes associated with numerous human diseases, including cancer²⁷¹ and CNS disorders,²⁷² etc., such as overexpression occurring in many tumors. To date, many highly effective and selective σR ligands have been developed. These primarily consist of conventional ligands, fluorescence probes, and radioligands. Among them, SA4503, CT1812, [¹⁸F]FTC-146, and [¹⁸F]ISO-1 have progressed to clinical evaluation for CNS disease treatment and tumor imaging.^{26,100,118,119,217} In cancer research, σ₁R was engaged in modulating cell proliferation, differentiation, and survival, serving as a chaperone at the mitochondrial-associated ER membrane. Its expression has been connected to cancer development, progression, and metastasis, with implications for therapy responses and patient outcomes.^10,51,273 Simultaneously, the σ₂R, although less characterized, has been identified as a potential prognostic marker and therapeutic target due to its high expression in proliferating tumor cells and capacity to influence cell cycle, apoptosis, and lipid synthesis.^{67,75,274−276}

Understanding the complicated involvement of σ₁R and σ₂R in tumor biology is crucial for the development of cancer targeted therapies, offering a promising avenue for improving the management of cancer patients. Prioritizing the development of ligands or probes that specifically target σ₁R and σ₂R has become essential and a priority. More noteworthy, differences in ligand chirality, agonist and antagonist properties, broad receptor scope, and biodistribution in vivo warrant careful consideration.

Following our description above, it is required to perform further property research on these σR ligands containing chiral isomers. For instance, from the perspective of ligands 3, 6, 7, 27, 38, and 61–63, distinct isomers (stereo-, cis-, and trans-isomers) demonstrated significant changes in affinity for the σ₁R and σ₂R (Tables 1 and 2), e.g., isomer (R,Z)-3 possesses a 23-fold higher affinity for the σ₁R than (R,E)-3, while isomer (S,Z)-3 shows a 15-fold higher affinity than (S,E)-3. Additionally, various pharmacological characteristics may emerge, e.g., the S-configuration of FBFP ((S)-35) acts as an agonist, while the R-configuration ((R)-35) functions as an antagonist for the σ₁R.¹⁵¹ Although there are limited reports on σ₂R ligands with isomers, our overview shown that chiral ligands 61–63 and 66 existed just one identified isomer^{177−179,185} (Figure 8 and Table 2), implying that the affinity of alternate enantiomer may be quite negligible.

Those variations in stereochemistry and pharmacology can result in significant disparities in pharmacokinetic characteristics in vivo, particularly those observed in radiopharmaceutical procedures. Chiral radioligands [¹⁸F]FBFP ([¹⁸F]35) and [¹⁸F]Fluspidine ([¹⁸F]108) serve as σ₁R neuroimaging agents in CNS. Notably, the R-configuration isomers ((R)-[¹⁸F]FBFP and (R)-[¹⁸F]Fluspidine) demonstrate a higher affinity for σ₁R compared to the (S)-[¹⁸F]FBFP and (S)-[¹⁸F]Fluspidine counterparts, while also exhibiting lower brain clearance rates.^{150,151,267,277} Furthermore, in recent years, we discovered a highly noteworthy phenomenon through in vivo pharmacokinetic investigations of σ₁R and σ₂R radio-probes. Radioactive absorption in the pancreas is substantially higher than in other tissues and organs, and it can be blocked by highly selective σ₁R and σ₂R ligands SA4503 or CM398.^{151,168,241,251,252} The high and specific expression of σ₁R and σ₂R in the pancreas suggests potential strategies for early diagnosis, treatment, and prognosis of pancreatic related diseases, notably pancreatic cancer and diabetes.

Consequently, as novel biomarkers, exploring the involvement of σ₁R and σ₂R in tumor physiology and their contribution to proliferation and metastasis mechanisms could provide valuable insights into cancer diagnosis and treatments.

Glossary

Abbreviations

AD

Alzheimer’s disease

ALS

Amyotrophic lateral sclerosis

CNS

Central nervous system

EBP

Emopamil binding protein

ER

Endoplasmic reticulum

ESCC

Esophageal squamous cell carcinoma

HD

Huntington’s disease

HER2

Human epidermal growth factor receptor 2

IARC

International Agency for Research on Cancer

ISR

Integrated stress response

IAP

Inhibitor of apoptosis protein

LDLR

Low-density lipoprotein receptor

MAM

Mitochondria-associated endoplasmic reticulum membranes

MAC30

Meningioma-related protein 30

MDD

Major depressive disorder

MDR

Multidrug resistance

MAPK

Mitogen-activated protein kinase

MTDLs

Multitarget directed ligands

NCC

National Cancer Center

NMDA

N-methyl-D-aspartate

PET

Positron emission tomography

PD

Parkinson’s disease

PDX

Patient-derived xenograft

ROS

Reactive oxygen species

SPECT

Single photon emission computed tomography

SREBP2

Sterol regulatory element-binding protein 2

TMEM97

Transmembrane protein 97

TNBC

Triple-negative breast cancer

VRCC

Volume-regulated chloride channels

VEGF

Vascular endothelial growth factor

Author Contributions

T. W. conceived the project, processed the data and prepared the original draft. T. W., N. S., and Y. M., created the pictures. N. S., Y. M., and S. Z., revised the manuscript. All authors have read and approved this version of manuscript.

This work was supported by the Young Doctoral Incubation Project of Xinqiao Hospital of Army Medical University (No. 2022YQB026).

The authors declare no competing financial interest.