Sigma binding site ligands inhibit cell proliferation in mammary and colon carcinoma cell lines and melanoma cells in culture

Repurposing fluoxetine to treat lymphocytic leukemia: Apoptosis induction, sigma-1 receptor upregulation, inhibition of IL-2 cytokine production, and autophagy induction

BACKGROUND

Childhood cancer has a cure rate of as low as 15% in low-income countries, suggesting a need for cheaper treatment options. Fluoxetine is a thoroughly safety-tested drug that may target the sigma-1 receptor (σ1-R).

RESEARCH DESIGN AND METHODS

Using the human leukemic cell line, Jurkat, we investigated the effects of fluoxetine on cell survival using XTT and trypan blue staining. Apoptosis was measured using AnnexinV/PI staining and western blot analysis of caspase cleavage. IL-2 secretion of Jurkat cells in response to PHA/PMA was measured using ELISA, and the expression of AKT/pAKT and the σ1-R were measured using western blotting.

RESULTS

Fluoxetine-induced apoptosis and G-2 cell cycle arrest. Fluoxetine reduced IL-2 secretion dose-dependently and could be further potentiated by σ1-R antagonist BD1047 (P < 0.05). Fluoxetine inhibited pAKT six hours post-treatment (P < 0.05). The expression of the σ1-R showed a significant increase between 12 to 48 hours in Jurkat cells (P < 0.05). At the same time, there was a substantial increase in autophagy.

CONCLUSIONS

Fluoxetine may have the potential for acute leukemia treatment. Co-treatment with a σ1-R antagonist increases fluoxetine-induced apoptosis, possibly targeting AKT phosphorylation and autophagy activation.

Treatment of cancer with antipsychotic medications: Pushing the boundaries of schizophrenia and cancer

Over a century ago, the phenothiazine dye, methylene blue, was discovered to have both antipsychotic and anti-cancer effects. In the 20th-century, the first phenothiazine antipsychotic, chlorpromazine, was found to inhibit cancer. During the years of elucidating the pharmacology of the phenothiazines, reserpine, an antipsychotic with a long historical background, was likewise discovered to have anti-cancer properties. Research on the effects of antipsychotics on cancer continued slowly until the 21st century when efforts to repurpose antipsychotics for cancer treatment accelerated. This review examines the history of these developments, and identifies which antipsychotics might treat cancer, and which cancers might be treated by antipsychotics. The review also describes the molecular mechanisms through which antipsychotics may inhibit cancer. Although the overlap of molecular pathways between schizophrenia and cancer have been known or suspected for many years, no comprehensive review of the subject has appeared in the psychiatric literature to assess the significance of these similarities. This review fills that gap and discusses what, if any, significance the similarities have regarding the etiology of schizophrenia.

Effect of antipsychotics on breast tumors by analysis of the Japanese Adverse Drug Event Report database and cell-based experiments

Background

Since antipsychotics induce hyperprolactinemia via the dopamine D₂ receptor, long-term administration may be a risk factor for developing breast tumors, including breast cancer. On the other hand, some antipsychotic drugs have been reported to suppress the growth of breast cancer cells in vitro. Thus, it is not clear whether the use of antipsychotics actually increases the risk of developing or exacerbating breast tumors. The purpose of this study was to clarify the effects of antipsychotic drugs on the onset and progression of breast tumors by analyzing an adverse event spontaneous reporting database and evaluating the proliferation ability of breast cancer cells.

Methods

Japanese Adverse Drug Event Report database (JADER) reports from April 2004 to April 2019 were obtained from the Pharmaceuticals and Medical Devices Agency (PMDA) website. Reports of females only were analyzed. Adverse events included in the analysis were hyperprolactinemia and 60 breast tumor-related preferred terms. The reporting odds ratio (ROR), proportional reporting ratio (PRR), and information component (IC) were used to detect signals. Furthermore, MCF-7 cells were treated with haloperidol, risperidone, paliperidone, sulpiride, olanzapine and blonanserin, and cell proliferation was evaluated by WST-8 assay.

Results

In the JADER analysis, the IC signals of hyperprolactinemia were detected with sulpiride (IC, 3.73; 95% CI: 1.81–5.65), risperidone (IC, 3.69; 95% CI: 1.71–5.61), and paliperidone (IC, 4.54; 95% CI: 2.96–6.12). However, the IC signal of breast tumors was not observed with any antipsychotics. In cell-based experiments, MCF-7 cells were treated with six antipsychotics at concentrations of 2 and 32 μM, and none of the drugs showed any growth-promoting effects on MCF-7 cells. On the other hand, blonanserin markedly suppressed the growth of MCF-7 cells at a concentration of 32 μM, and the effect was concentration dependent.

Conclusions

Analysis of the JADER using the IC did not show breast tumor signals due to antipsychotic drugs. In in vitro experiments, antipsychotics did not promote MCF-7 cell proliferation whereas blonanserin suppressed MCF-7 cell growth. Further research on the effects of blonanserin on the onset and progression of breast tumor is expected.

Sigma-2 Receptor-A Potential Target for Cancer/Alzheimer's Disease Treatment via Its Regulation of Cholesterol Homeostasis

The sigma receptors were classified into sigma-1 and sigma-2 receptor based on their different pharmacological profiles. In the past two decades, our understanding of the biological and pharmacological properties of the sigma-1 receptor is increasing; however, little is known about the sigma-2 receptor. Recently, the molecular identity of the sigma-2 receptor has been identified as TMEM97. Although more and more evidence has showed that sigma-2 ligands have the ability to treat cancer and Alzheimer’s disease (AD), the mechanisms connecting these two diseases are unknown. Data obtained over the past few years from human and animal models indicate that cholesterol homeostasis is altered in AD and cancer, underscoring the importance of cholesterol homeostasis in AD and cancer. In this review, based on accumulated evidence, we proposed that the beneficial roles of sigma-2 ligands in cancer and AD might be mediated by their regulation of cholesterol homeostasis.

Sigma 1 Receptor is Overexpressed in Hepatocellular Adenoma: Involvement of ERα and HNF1α

Sigma receptor 1 (SigR1) is an endoplasmic reticulum resident integral membrane protein whose functions remain unclear. Although the liver shows the highest expression of SigR1, its role in this organ is unknown. SigR1 is overexpressed in many cancers and its expression is correlated to hormonal status in hormone-dependent cancers. To better understand the role of SigR1 in hepatocytes we focused our work on the regulation of its expression in tumoral liver. In this context, hepatocellular adenomas, benign hepatic tumors associated with estrogen intake are of particular interest. The expression of SigR1 mRNA was assessed in hepatocellular adenoma (HCA) patients using qPCR. The impact of estrogen on the expression of SigR1 was studied in vivo (mice) and in vitro (HepG2 and Huh7 cells). The effect of HNF1α on the expression of SigR1 was studied in vivo by comparing wild type mice to HNF1 knockout mice. Estrogen enhanced SigR1 expression through its nuclear receptor ERα. HNF1α mutated HCA (H-HCA) significantly overexpressed SigR1 compared to all other HCA subtypes. HNF1 knockout mice showed an increase in SigR1 expression. Overexpressing SigR1 in cellular models increases proliferation rate and storage of lipid droplets, which phenocopies the H-HCA phenotype. SigR1 is involved in hepatocyte proliferation and steatosis and may play an important role in the control of the H-HCA phenotype.

Novel thymoquinone lipidic core nanocapsules with anisamide-polymethacrylate shell for colon cancer cells overexpressing sigma receptors

The biggest challenge in colorectal cancer therapy is to avoid intestinal drug absorption before reaching the colon, while focusing on tumor specific delivery with high local concentration and minimal toxicity. In our work, thymoquinone (TQ)-loaded polymeric nanocapsules were prepared using the nanoprecipitation technique using Eudragit S100 as polymeric shell. Conjugation of anisamide as a targeting ligand for sigma receptors overexpressed by colon cancer cells to Eudragit S100 was carried out via carbodiimide coupling reaction, and was confirmed by thin layer chromatography and ¹H-NMR. TQ nanocapsules were characterized for particle size, surface morphology, zeta potential, entrapment efficiency % (EE%), in vitro drug release and physical stability. A cytotoxicity study on three colon cancer cell lines (HT-29, HCT-116, Caco-2) was performed. Results revealed that the polymeric nanocapsules were successfully prepared, and the in vitro characterization showed a suitable size, zeta potential, EE% and physical stability. TQ exhibited a delayed release pattern from the nanocapsules in vitro. Anisamide-targeted TQ nanocapsules showed higher cytotoxicity against HT-29 cells overexpressing sigma receptors compared to their non-targeted counterparts and free TQ after incubation for 48 h, hence delineating anisamide as a promising ligand for active colon cancer targeting.

Sigma-1 receptor is involved in diminished ovarian reserve possibly by influencing endoplasmic reticulum stress-mediated granulosa cells apoptosis

Sigma non-opioid intracellular receptor 1 (sigma-1 receptor), a non-opioid transmembrane protein, is located on cellular mitochondrial membranes and endoplasmic reticulum. Current research has demonstrated that sigma-1 receptor is related to human degenerative diseases. This study is focused on the effects of sigma-1 receptor on the pathophysiological process of diminished ovarian reserve (DOR) and granulosa cells (GCs) apoptosis. Sigma-1 receptor concentration in follicular fluid (FF) and serum were negatively correlated with basal follicle-stimulating hormone (FSH) and positively correlated with anti-mullerian hormone (AMH), antral follicle count (AFC). Sigma-1 receptor reduction in GCs was accompanied by endoplasmic reticulum stress (ERS)-mediated apoptosis in women with DOR. Plasmid transfection was used to establish SIGMAR1-overexpressed and SIGMAR1-knockdown human granulosa-like tumor (KGN) cell and thapsigargin (TG) was used to induce ERS KGN cells. We found that KGN cells treated with endogenous sigma-1 receptor ligand dehydroepiandrosterone (DHEA) and sigma-1 receptor agonist PRE-084 showed similar biological effects to SIGMAR1-overexpressed KGN cells and opposite effects to SIGMAR1-knockdown KGN cells. DHEA may improve DOR patients' pregnancy outcomes by upregulating sigma-1 receptor and downregulating ERS-mediated apoptotic genes in GCs. Thus, sigma-1 receptor may be a potential ovarian reserve biomarker, and ligand-mediated sigma-1 receptor activation could be a future approach for DOR therapy.

Small-Molecule Modulators of Sigma1 and Sigma2/TMEM97 in the Context of Cancer: Foundational Concepts and Emerging Themes

There are two known subtypes of the so-called sigma receptors, Sigma1 and Sigma2. Sigma1 (encoded by the SIGMAR1 gene and also known as Sigma-1 receptor, S1R) is a unique pharmacologically regulated integral membrane chaperone or scaffolding protein that allosterically modulates the activity of its associated proteins. Sigma2, recently identified as transmembrane protein 97 (TMEM97), is an integral membrane protein implicated in cellular cholesterol homeostasis. A number of publications over the past two decades have suggested a role for both sigma proteins in tumor biology. Although there is currently no clinically used anti-cancer drug that targets Sigma1 or Sigma2/TMEM97, a growing body of evidence supports the potential of small-molecule compounds with affinity for these proteins, putative sigma ligands, as therapeutic agents to treat cancer. In preclinical models, these compounds have been reported to inhibit cancer cell proliferation, survival, adhesion, and migration; furthermore, they have been demonstrated to suppress tumor growth, to alleviate cancer-associated pain, and to exert immunomodulatory properties. Here, we will address the known knowns and the known unknowns of Sigma1 and Sigma2/TMEM97 ligand actions in the context of cancer. This review will highlight key discoveries and published evidence in support of a role for sigma proteins in cancer and will discuss several fundamental questions regarding the physiological roles of sigma proteins in cancer and sigma ligand mechanism of action.

Overexpression of sigma-1 receptor in MCF-7 cells enhances proliferation via the classic protein kinase C subtype signaling pathway

Sigma-1 receptor (sigma-1R), a 25-kDa integral membrane protein, is expressed at a high density in various tumor cell lines and its ligands mediate tumor cell proliferation. However, the effect of this receptor on proliferation and the associated intracellular molecules in tumors remains unclear. The present study aimed to investigate the effect of sigma-1R overexpression on MCF-7 cell proliferation and the associated intracellular molecules that serve a key role in this process. The sigma-1R proliferative function was examined by comparing the proliferation rates of a sigma-1R-overexpressing line, MCF-41 with a sigma-1R-defective line, MCF-7, in culture media with various serum concentrations. The results demonstrated that MCF-41 cells grew significantly faster compared with MCF-7 cells, indicating a proliferation-enhancing receptor function. This proliferation-enhancing effect was completely eliminated by adding a PKC inhibitor to the culture media for MCF-41 cells. To identify which PKC subtype affects the proliferative function of sigma-1R, five inhibitors of PKC subtypes or enzymes involved in the PKC signaling cascade were introduced to MCF-7 and MCF-41 cell culture media and their effects on cell proliferation were compared. It was revealed that only the classic PKC subtype inhibitor, GF109203×, significantly inhibited MCF-41 cell proliferation compared with the MCF-7 line. In conclusion, among PKC iso-enzymes only classic PKC subtype enzymes serve an important role in sigma-1R overexpression enhancing MCF-7 cell proliferation.

Synthesis and evaluation of haloperidol metabolite II prodrugs as anticancer agents

The use of haloperidol metabolite II (HP-metabolite II) prodrugs is an emerging strategy in the treatment of cancer. HP-metabolite II exhibits antiproliferative properties at micromolar concentrations inducing apoptosis in different types of cancer. Thus, the application of the prodrug approach appears as a useful method leading to much more desirable pharmacokinetic and pharmacodynamic properties. Some studies have shown that the esterification of the hydroxyl group of HP-metabolite II with 4-phenylbutiric acid (4-PBA) or valproic acid enhances the anticancer therapeutic potency. The current progresses in the design, synthesis and evaluation of anticancer activity of HP metabolite II prodrugs will be discussed in this review.

Sigma1 Pharmacology in the Context of Cancer

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

(18)F-Labeled 1,4-Dioxa-8-azaspiro[4.5]decane Derivative: Synthesis and Biological Evaluation of a σ1 Receptor Radioligand with Low Lipophilicity as Potent Tumor Imaging Agent

We report the syntheses and evaluation of series of novel piperidine compounds with low lipophilicity as σ1 receptor ligands. 8-(4-(2-Fluoroethoxy)benzyl)-1,4-dioxa-8-azaspiro[4.5]decane (5a) possessed high affinity (K(i) = 5.4 ± 0.4 nM) for σ1 receptors and selectivity for σ2 receptors (30-fold) and the vesicular acetylcholine transporter (1404-fold). [(18)F]5a was prepared using a one-pot, two-step labeling procedure in an automated synthesis module, with a radiochemical purity of >95%, and a specific activity of 25-45 GBq/μmol. Cellular association, biodistribution, and autoradiography with blocking experiments indicated specific binding of [(18)F]5a to σ1 receptors in vitro and in vivo. Small animal positron emission tomography (PET) imaging using mouse tumor xenograft models demonstrated a high accumulation in human carcinoma and melanoma. Treatment with haloperidol significantly reduced the accumulation of the radiotracer in tumors. These findings suggest that radiotracer with suitable lipophilicity and appropriate affinity for σ1 receptors could be used for tumor imaging.

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

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

Sigma 1 Receptor antagonist potentiates the anti-cancer effect of p53 by regulating ER stress, ROS production, Bax levels, and caspase-3 activation

Over the last years, many improvements have been made in the treatment of breast cancer; however, novel and less toxic therapies are still needed, especially for relapsing and chemo-resistant patients. Here, we analyzed the therapeutic potential of p53 and Rimcazole, a Sigma 1 Receptor antagonist. Rimcazole and p53 are being evaluated in preclinical and clinical trials, respectively. While p53 is a promising antitumor therapeutic agent, antagonists of Sigma 1 Receptor also inhibit tumor cell survival and induce apoptosis. Our current study demonstrates for the first time the synergistic effect of p53 in combination with the Sigma 1 Receptor antagonist Rimcazole. Furthermore, we show that shRNA knockdown of Sigma 1 Receptor in combination with p53, lead to a similar synergistic effect, and that this synergistic effect, in breast cancer growth suppression occurs independent of p53 status. Furthermore, this combination treatment induced ER stress, p38 MAPK activation, ROS production, and proteins involved in apoptosis (caspases-3, Bax) in breast cancer cells. Combining these therapeutic anti-cancer molecules provides an innovative approach for potentially treating human breast cancer.

Altered expression level of Sigma1 receptor gene in human colorectal cancer

Nonopioid Sigma1 receptor (Sig1R) influences numerous metabolism functions including regulation of ion channels, reaction on stress and response to growth signals. Due to this influence, Sigma1 receptor ligands show anti-proliferative and cytotoxic action on tumor cells. Additionally its increased level is observed in some types of tumors. Colorectal cancer is one of the most common cancers worldwide and its clinical development is well described. The aim of the study was evaluation of Sigma1 receptor mRNA expression level in human colorectal cancer and colorectal cancer liver metastases at different stages of tumor development. The mRNA was isolated from 30 patients: 18 with colorectal cancer (CRC) and 12 with colorectal cancer liver metastases (CRCLM). The cDNA of Sig1R gene was amplified by polymerase chain reaction using specific primers. The level of Sig1R mRNA expression was determined by measurement of optical density. Sig1R expression level was increased in CRC and CRCLM. The highest level of Sig1R mRNA was observed in UICC stage III. We also showed significant interactions of UICC stage and tumor localization with Sig1R expression level. There were no interactions between UICC stage and age of patients, although we observed significantly decreased level of Sig1R mRNA in older patients. Clinical advancement stage, localization of tumor and age of patients seems to be an important factors influencing Sigma1 receptor expression level. It is probably due to double nature of Sig1R action - in certain conditions it could act pro- or antiapoptotic. This action might depend on Sig1R activity resulting from its expression level.

AGH is a new hemoglobin alpha-chain fragment with antinociceptive activity

Limited proteolysis of certain proteins leads to the release of endogenous bioactive peptides. Hemoglobin-derived peptides such as hemorphins and hemopressins are examples of intracellular protein-derived peptides that have antinociceptive effects by modulating G-protein coupled receptors activities. In the present study, a previously characterized substrate capture assay that uses a catalytically inactive form of the thimet oligopeptidase was combined with isotopic labeling and mass spectrometry in order to identify new bioactive peptides. Indeed, we have identified the peptide AGHLDDLPGALSAL (AGH), a fragment of the hemoglobin alpha-chain, which specifically bind to the inactive thimet oligopeptidase in the substrate capture assay. Previous peptidomics studies have identified the AGH as well as many other natural peptides derived from hemoglobin alpha-chain containing this sequence, further suggesting that AGH is a natural endogenous peptide. Pharmacological assays suggest that AGH inhibits peripheral inflammatory hyperalgesic responses through indirect activation of mu opioid receptors, without having a central nervous system activity. Therefore, we have successfully used the substrate capture assay to identify a new endogenous bioactive peptide derived from hemoglobin alpha-chain.

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

The sigma-2 receptor is highly expressed in various rapidly proliferating cancer cells and regarded as a cancer cell biomarker. Selective sigma-2 ligands have been shown to specifically label the tumor sites, induce cancer cells to undergo apoptosis, and inhibit tumor growth. Sigma-2 ligands are potentially useful as cancer diagnostics, anticancer therapeutics, or adjuvant anticancer treatment agents. However, both the cloning of this receptor and the identification of its endogenous ligand have not been successful, and the lack of structural information has severely hindered the understanding of its physiological roles, its signaling pathways, and the development of more selective sigma-2 ligands. Recent data have implicated that sigma-2 binding sites are within the lipid rafts and that PGRMC1 (progesterone receptor membrane component 1) complex and sigma-2 receptor may be coupled with EGFR (epidermal growth factor receptor), mTOR (mammalian target of rapamycin), caspases, and ion channels. Due to its promising applications in cancer management, there are rapidly increasing research efforts that are being directed into this field. This review article updates the current understanding of sigma-2 receptor and its potential physiological roles, applications, interaction with other effectors, with special focuses on the development of sigma-2 ligands, their chemical structures, pharmacological profiles, applications in imaging and anticancer therapy.

The sigma-1 receptor: a regulator of cancer cell electrical plasticity?

Originally mistaken as an opioid receptor, the sigma-1 receptor (Sig1R) is a ubiquitous membrane protein that has been involved in many cellular processes. While the precise function of Sig1R has long remained mysterious, recent studies have shed light on its role and the molecular mechanisms triggered. Sig1R is in fact a stress-activated chaperone mainly associated with the ER-mitochondria interface that can regulate cell survival through the control of calcium homeostasis. Sig1R functionally regulates ion channels belonging to various molecular families and it has thus been involved in neuronal plasticity and central nervous system diseases. Interestingly, Sig1R is frequently expressed in tumors but its function in cancer has not been yet clarified. In this review, we discuss the current understanding of Sig1R. We suggest herein that Sig1R shapes cancer cell electrical signature upon environmental conditions. Thus, Sig1R may be used as a novel therapeutic target to specifically abrogate pro-invasive functions of ion channels in cancer tissue.

Overexpression of sigma1 receptor and its positive associations with pathologic TNM classification in esophageal squamous cell carcinoma

Sigma1 receptor (sigma1R), a significant protein, has been found to be frequently upregulated in human tumor cells and tissues. It has been demonstrated that sigma1R is involved in proliferation and adhesion of cancer cells. However, the significance of sigma1R expression in esophageal squamous cell carcinoma (ESCC) remains unclear. In this article, by a series of methods, the authors examined the expression of sigma1R protein in ESCC cell lines and tissues. Flow cytometry indicated intense staining of sigma1R in ESCC cells. Immunocytochemistry staining demonstrated that sigma1R was mainly distributed in cytoplasm and nucleus in ESCC cell lines. Western blotting was performed to characterize the relative expression of sigma1R in different ESCC cell lines. Moreover, different levels of sigma1R were presented from normal epithelium to carcinoma by immunohistochemistry analysis, which demonstrated that sigma1R was highly expressed in tumors. Association analysis showed significant correlations between total sigma1R protein levels and pathologic TNM (pTNM) classification of tumors (r=0.216, p=0.011). Furthermore, the sigma1R in the nucleus was significantly correlated with pTNM classification and lymph node metastasis (r=0.263, p=0.002, and r=0.269, p=0.002, respectively). These data indicated that sigma1R may serve as a potential predictive factor for pTNM classification and tumor development in ESCC.

Antagonists show GTP-sensitive high-affinity binding to the sigma-1 receptor

BACKGROUND AND PURPOSE

Sigma-1 receptors are atypical receptors with potentially two transmembrane domains. Antagonists require doses significantly higher than their published affinities to have biological effects. We have reassessed the binding characteristics of these ligands and found antagonists bind to high- and low-affinity states not distinguished by agonists.

EXPERIMENTAL APPROACH

The affinities of sigma-1 receptor ligands was assessed using radioligand saturation and competition binding of [³H]-(+)-pentazocine to permeabilized MDA-MB-468 cells. This was compared with the effect of ligands on metabolic activity using an MTS-based assay and calcium signalling using cells loaded with the calcium dye, Fura-2.

KEY RESULTS

Sigma-1 receptor antagonists, but not agonists, show GTP- and suramin-sensitive high-affinity binding. Functional responses (calcium signalling and metabolic activity), while associated with sigma-1 receptor binding, required binding to an unidentified, low-affinity target.

CONCLUSIONS AND IMPLICATIONS

Sigma-1 receptors are coupled to G proteins. This interaction is only observed when analysing antagonist binding. The identity of the G protein remains to be resolved. The concept of agonist and antagonist at the sigma-1 receptor needs to be revisited.

Synthesis and in vitro biological evaluation of carbonyl group-containing analogues for σ1 receptors

To identify the ligands for σ(1) receptors that are potent and selective, analogues of prezamicol and trozamicol scaffolds of carbonyl-containing vesicular acetylcholine transporter (VAChT) inhibitors were explored. Of the 23 analogues synthesized and tested, 5 displayed very high affinity for σ(1) (K(i) = 0.48-4.05 nM) and high selectivity for σ(1) relative to σ(2) receptors (σ(1)/σ(2) selectivity of >749-fold). Four of the five compounds (14a, 14b, 14c, and 14e) showed very low affinity for VAChT (K(i) > 290 nM), and the fifth compound (14g) showed moderate affinity for VAChT (K(i) = 44.2 nM). The compound [1'-(4-fluorobenzyl)-3'-hydroxy[1,4']bipiperidinyl-4-yl]-(4-fluorophenyl)methanone (14a) displayed very high affinity and selectivity for σ(1) receptor (K(i) = 0.48 nM, σ(1)/σ(2) > 3600). All four of these most promising compounds (14a, 14b, 14c, and 14e) can be radiosynthesized with fluorine-18 or carbon-11, which will allow further evaluation of their properties as PET probes for imaging σ(1) receptor in vivo.

Structure-activity study to develop cationic lipid-conjugated haloperidol derivatives as a new class of anticancer therapeutics

Haloperidol (HP), a neuroleptic drug, shows high affinity toward σ receptors (SR). HP and reduced-HP at higher concentration were known to induce apoptosis in SR-overexpressing carcinomas and melanomas. Herein, we report the development of cationic lipid-conjugated haloperidol as a new class of anticancer therapeutics. In comparison to HP, the C-8 carbon chain analogue (HP-C8) showed significantly high, SR-assisted antiproliferative activity against cancer cells via caspase-3-mediated apoptosis and down-regulation of pAkt. Moreover, melanoma tumor aggressiveness in HP-C8-treated mice was significantly lower than that in HP-treated mice. HP-C8 simultaneously reduced Akt-protein level and increased Bax/Bcl-2 ratio in vascular endothelial cells, thereby indicating a possible protein kinase down-regulatory and apoptosis inducing role in tumor-associated vascular cells. In conclusion, we developed σ receptor-targeting cationic lipid-modified HP derivatives as a promising class of anticancer therapeutic that concurrently affects cancer and tumor environment associated angiogenic vascular cells through induction of apoptosis and Akt protein down-regulation.

Early development of sigma-receptor ligands

Sigma receptors (σ-1 and σ-2) are non-opioid proteins implicated in the pathophysiology of various neurological disorders and cancer. The σ-1 subtype is a chaperon protein widely distributed in the CNS and peripheral tissues. These receptors are involved in the modulation of K+- and Ca2+-dependent signaling cascades at the endoplasmic reticulum and modulation of neurotransmitter release. σ-1 receptors are emerging targets for the treatment of neurophychiatric diseases (schizophrenia and depression) and cocaine addiction. σ-2 receptors are lipid raft proteins. They are highly expressed on many tumor cells and hence considered potential targets for anticancer drugs. σ receptors bind to a diverse class of pharmacological compounds like cocaine, methamphetamine, benzomorphans like (±)-pentazocine, (±)-SKF-10,047 and endogenous neurosteroids and sphingolipids. In this review we focus on the early development of σ receptor-specific ligands and radiolabeling agents.

Melanoma-associated Chondroitin Sulfate Proteoglycan (MCSP)-targeted delivery of soluble TRAIL potently inhibits melanoma outgrowth in vitro and in vivo

BACKGROUND

Advanced melanoma is characterized by a pronounced resistance to therapy leading to a limited patient survival of ~6 - 9 months. Here, we report on a novel bifunctional therapeutic fusion protein, designated anti-MCSP:TRAIL, that is comprised of a melanoma-associated chondroitin sulfate proteoglycan (MCSP)-specific antibody fragment (scFv) fused to soluble human TRAIL. MCSP is a well-established target for melanoma immunotherapy and has recently been shown to provide important tumorigenic signals to melanoma cells. TRAIL is a highly promising tumoricidal cytokine with no or minimal toxicity towards normal cells. Anti-MCSP:TRAIL was designed to 1. selectively accrete at the cell surface of MCSP-positive melanoma cells and inhibit MCSP tumorigenic signaling and 2. activate apoptotic TRAIL-signaling.

RESULTS

Treatment of a panel of MCSP-positive melanoma cell lines with anti-MCSP:TRAIL induced TRAIL-mediated apoptotic cell death within 16 h. Of note, treatment with anti-MCSP:sTRAIL was also characterized by a rapid dephosphorylation of key proteins, such as FAK, implicated in MCSP-mediated malignant behavior. Importantly, anti-MCSP:TRAIL treatment already inhibited anchorage-independent growth by 50% at low picomolar concentrations, whereas > 100 fold higher concentrations of non-targeted TRAIL failed to reduce colony formation. Daily i.v. treatment with a low dose of anti-MCSP:TRAIL (0.14 mg/kg) resulted in a significant growth retardation of established A375 M xenografts. Anti-MCSP:TRAIL activity was further synergized by co-treatment with rimcazole, a σ-ligand currently in clinical trials for the treatment of various cancers.

CONCLUSIONS

Anti-MCSP:TRAIL has promising pre-clinical anti-melanoma activity that appears to result from combined inhibition of tumorigenic MCSP-signaling and concordant activation of TRAIL-apoptotic signaling. Anti-MCSP:TRAIL alone, or in combination with rimcazole, may be of potential value for the treatment of malignant melanoma.

Synthesis and characterization of N,N-dialkyl and N-alkyl-N-aralkyl fenpropimorph-derived compounds as high affinity ligands for sigma receptors

The sigma-1 receptor is a unique non-opioid, non-PCP binding site that has been implicated in many different pathophysiological conditions including psychosis, drug addiction, retinal degeneration and cancer. Based on the structure of fenpropimorph, a high affinity (K(i)=0.005 nM)(1) sigma-1 receptor ligand and strong inhibitor of the yeast sterol isomerase (ERG2), we previously deduced a basic sigma-1 receptor pharmacophore or chemical backbone composed of a phenyl ring attached to a di-substituted nitrogen atom via an alkyl chain.(2) Here, we report the design and synthesis of various N,N-dialkyl or N-alkyl-N-aralkyl derivatives based on this pharmacophore as well as their binding affinities to the sigma-1 receptor. We introduce three high affinity sigma-1 receptor compounds, N,N-dibutyl-3-(4-fluorophenyl)propylamine (9), N,N-dibutyl-3-(4-nitrophenyl)propylamine (3), and N-propyl-N'-4-aminophenylethyl-3-(4-nitrophenyl)propylamine (20) with K(i) values of 17.7 nM, 0.36 nM, and 6 nM, respectively. In addition to sigma receptor affinity, we show through cytotoxicity assays that growth inhibition of various tumor cell lines occurs with our high affinity N,N-dialkyl or N-alkyl-N-aralkyl derivatives.

Synthesis and evaluation of novel radioiodinated nicotinamides for malignant melanoma

INTRODUCTION

A series of iodonicotinamides based on the melanin-binding iodobenzamide compound N-2-diethylaminoethyl-4-iodobenzamide was prepared and evaluated for the potential imaging and staging of disseminated metastatic melanoma.

METHODS

[(123)I]Iodonicotinamides were prepared by iododestannylation reactions using no-carrier-added iodine-123 and evaluated in vivo by biodistribution and competition studies and by single photon emission computed tomography (SPECT) imaging in black and albino nude mice bearing B16F0 murine melanotic and A375 human amelanotic melanoma tumours, respectively.

RESULTS

The iodonicotinamides displayed low-affinity binding for sigma(1)-sigma(2) receptors (K(i)>300 nM). In biodistribution studies in mice, N-(2-(diethylamino)ethyl)-5-[(123)I]iodonicotinamide ([(123)I]1) exhibited the fastest and highest uptake of the nicotinamide series in the B16F0 tumour at 1 h ( approximately 8% ID/g), decreasing slowly over time. No uptake was observed in the A375 tumour. Clearance from the animals by urinary excretion was more rapid for N-alkyl-nicotinamides than for piperazinyl derivatives. At 1 h postinjection, the urinary excretion was 66% ID for [(123)I]1, while the gastrointestinal tract amounted to 17% ID. Haloperidol was unable to reduce the uptake of [(123)I]1 in pigmented mice, indicating that this uptake was likely due to an interaction with melanin. SPECT imaging of [(123)I]1 in black mice bearing the B16F0 melanoma indicated that the radioactivity was predominately located in the tumour and eyes. No specific localisation was observed in nude mice bearing A375 amelanotic tumours.

CONCLUSION

These findings suggest that [(123)I]1, which displays high tumour uptake with rapid clearance from the body, could be a promising imaging agent for the detection of melanotic tumours.

Selective sigma-2 ligands preferentially bind to pancreatic adenocarcinomas: applications in diagnostic imaging and therapy

BACKGROUND

Resistance to modern adjuvant treatment is in part due to the failure of programmed cell death. Therefore the molecules that execute the apoptotic program are potential targets for the development of anti-cancer therapeutics. The sigma-2 receptor has been found to be over-expressed in some types of malignant tumors, and, recently, small molecule ligands to the sigma-2 receptor were found to induce cancer cell apoptosis.

RESULTS

The sigma-2 receptor was expressed at high levels in both human and murine pancreas cancer cell lines, with minimal or limited expression in normal tissues, including: brain, kidney, liver, lung, pancreas and spleen. Micro-PET imaging was used to demonstrate that the sigma-2 receptor was preferentially expressed in tumor as opposed to normal tissues in pancreas tumor allograft-bearing mice. Two structurally distinct sigma-2 receptor ligands, SV119 and WC26, were found to induce apoptosis to mice and human pancreatic cancer cells in vitro and in vivo. Sigma-2 receptor ligands induced apoptosis in a dose dependent fashion in all pancreatic cell lines tested. At the highest dose tested (10 muM), all sigma-2 receptor ligands induced 10-20% apoptosis in all pancreatic cancer cell lines tested (p < 0.05). In pancreas tumor allograft-bearing mice, a single bolus dose of WC26 caused approximately 50% apoptosis in the tumor compared to no appreciable apoptosis in tumor-bearing, vehicle-injected control animals (p < 0.0001). WC26 significantly slowed tumor growth after a 5 day treatment compared to vehicle-injected control animals (p < 0.0001) and blood chemistry panels suggested that there is minimal peripheral toxicity.

CONCLUSION

We demonstrate a novel therapeutic strategy that induces a significant increase in pancreas cancer cell death. This strategy highlights a new potential target for the treatment of pancreas cancer, which has little in the way of effective treatments.

Synthesis and evaluation of fluorine-18-labeled SA4503 as a selective sigma1 receptor ligand for positron emission tomography

The [(18)F]fluoromethyl analog of the sigma(1) selective ligand 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) ([(18)F]FM-SA4503) was prepared and its potential evaluated for the in vivo measurement of sigma(1) receptors with positron emission tomography (PET). FM-SA4503 had selective affinity for the sigma(1) receptor (K(i) for sigma(1) receptor, 6.4 nM; K(i) for sigma(2) receptor, 250 nM) that was compatible with the affinity of SA4503 (K(i) for sigma(1) receptor, 4.4 nM; K(i) for sigma(2) receptor, 242 nM). [(18)F]FM-SA4503 was synthesized by (18)F-fluoromethylation of O-demethyl SA4503 in the radiochemical yield of 2.9-16.6% at the end of bombardment with a specific activity of 37.8-283 TBq/mmol at the end of synthesis. In mice, the uptake of [(18)F]FM-SA4503 in the brain was gradually increased for 30 min after injection, and then decreased. In the blocking study, brain uptake was significantly decreased by co-injection of haloperidol to 32% of control, and FM-SA4503 to 52% of control. In PET study of the monkey brain, high uptake was found in the cerebral cortex, thalamus and striatum. The radioactivity level of [(18)F]FM-SA4503 in the brain regions gradually increased over a period of 120 min after injection, followed by a stable plateau phase until 180 min after injection. In pretreatment with haloperidol measurement of the monkey brain, the radioactivity level was 22-32% and 11-25% of the baseline at 60 and 180 min, respectively, after injection, suggesting high receptor-specific binding. [(18)F]FM-SA4503 showed specific binding to sigma(1) receptors in mice and monkeys; therefore, [(18)F]FM-SA4503 has the potential for mapping sigma(1) receptors in the brain.

4-IBP, a sigma1 receptor agonist, decreases the migration of human cancer cells, including glioblastoma cells, in vitro and sensitizes them in vitro and in vivo to cytotoxic insults of proapoptotic and proautophagic drugs

Although the molecular function of sigma receptors has not been fully defined and the natural ligand(s) is still not known, there is increasing evidence that these receptors and their ligands might play a significant role in cancer biology. 4-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4-IBP), a selective sigma1 agonist, has been used to investigate whether this compound is able to modify: 1) in vitro the migration and proliferation of human cancer cells; 2) in vitro the sensitivity of human glioblastoma cells to cytotoxic drugs; and 3) in vivo in orthotopic glioblastoma and non-small cell lung carcinoma (NSCLC) models the survival of mice co-administered cytotoxic agents. 4-IBP has revealed weak antiproliferative effects on human U373-MG glioblastoma and C32 melanoma cells but induced marked concentration-dependent decreases in the growth of human A549 NSCLC and PC3 prostate cancer cells. The compound was also significantly antimigratory in all four cancer cell lines. This may result, at least in U373-MG cells, from modifications to the actin cytoskeleton. 4-IBP modified the sensitivity of U373-MG cells in vitro to proapoptotic lomustin and proautophagic temozolomide, and markedly decreased the expression of two proteins involved in drug resistance: glucosylceramide synthase and Rho guanine nucleotide dissociation inhibitor. In vivo, 4-IBP increased the antitumor effects of temozolomide and irinotecan in immunodeficient mice that were orthotopically grafted with invasive cancer cells.

Cyclohexylpiperazine derivative PB28, a sigma2 agonist and sigma1 antagonist receptor, inhibits cell growth, modulates P-glycoprotein, and synergizes with anthracyclines in breast cancer

sigma Ligands have recently been shown to have cytotoxic activity, to induce ceramide-dependent/caspase-independent apoptosis, and to down-regulate P-glycoprotein (P-gp) mRNA levels in some mouse and human models. In this study, we verified whether a mixed sigma(2) agonist/sigma(1) antagonist, PB28, was able to have antitumor activity and to enhance anthracycline efficacy in two human breast cancer cell lines, MCF7 and MCF7 ADR, both characterized by significant sigma(2) receptor expression, by high and low sigma(1) receptor expression, and low and high P-gp expression, respectively. In both cell lines, PB28 showed high sigma(2) receptor affinity and low sigma(1) receptor affinity; furthermore, it inhibited cell growth with a clear effect at 48 hours (IC(50) in nanomolar range), a consistent time exposure-independent increase of G(0)-G(1)-phase fraction (of approximately 20% of both cell lines) and caspase-independent apoptosis (15% increased after 1-day drug exposure). PB28 also reduced P-gp expression in a concentration- and time-dependent manner ( approximately 60% in MCF7 and 90% in MCF7 ADR). We showed also a strong synergism between PB28 and doxorubicin by adopting either simultaneous or sequential schedules of the two drugs. We suggest that this synergism could depend on PB28-induced increase of intracellular accumulation of doxorubicin ( approximately 50% in MCF7 and 75% in MCF7 ADR by flow cytometry analysis). In conclusion, we suggest that the sigma(2) agonist PB28 could be an interesting antitumor agent either in monotherapy or in combination with conventional drugs.

Haloperidol induces apoptosis via the sigma2 receptor system and Bcl-XS

Toxicity of the typical antipsychotic haloperidol (HAL) comprises an apoptotic component that we link to pro-apoptotic Bcl-XS in PC12 preneuronal and N2a neuroblastoma cells. The mitochondrial translocation of Bcl-XS and its interaction with the pore-forming voltage-dependent anion channel (VDAC) correlates with the redistribution of cytochrome c and the cleavage of Poly(ADP-ribose) polymerase. Haloperidol-induced apoptosis is mediated by the sigma2 (sigma2) receptor system and does not involve the expected antagonism of the dopamine D(2) receptor, nor is it influenced by Vitamin E- or p53/Bax-mediated events. Pathological relevance is demonstrated by the cytotoxic synergism between HAL and the Alzheimer disease-related peptide beta-amyloid(1-40), which correlates with Bcl-XS expression and its interaction with VDAC, and with cytosolic cytochrome c translocation. These data provide for a unique apoptotic mechanism that could underscore the clinical risks associated with HAL, particularly following chronic regimens or in the elderly.

Synthesis and in Vitro Evaluation of Iodinated Derivatives of Piperazine as a New Ligand for Sigma Receptor Imaging by Single Photon Emission Computed Tomography

A new series of radioiodinated analogues of 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine (SA4503) was synthesized and evaluated as a potential brain sigma-1 receptor imaging ligands by single photon emission computed tomography (SPECT). Iodinated analogues of SA4503 (4a-c) were prepared from piperazine in a high yield. The in vitro competition binding studies using [3H] DTG (sigma-1, 2), [3H] (+)-pentazocine (sigma-1), and [3H] DTG in the presence of carbetapentane (sigma-2) as sigma receptor selective radioligands were revealed that iodinated analogues 4a-c possess high affinities to sigma receptors (IC50: 4a=7.1, 4b=31.0, and 4c=77.3 nM). In particular, the affinity of 4a, bearing iodine at ortho position on the phenyl ring, was 4.4 times greater than SA4503, and 3 times greater than that of haloperidol. The meta-iodo analogue 4b was the same to SA4503, the lead compound. The radioiodinated derivatives, [125I] 4a, 4b were synthesized no-carrier-added from the corresponding tributyltin precursors by the iododestannylation reaction with high yields. The binding of [125I] 4a, 4b have been characterized in the rat brain membranes. These compounds were indicated single population binding to sigma receptor with high affinity (4a: Kd=1.86+/-0.34 nM, Bmax=205+/-28.9 fmol/mg protein, 4b: Kd=3.30+/-0.51 nM, Bmax=231.5+/-13.8 fmol/mg protein). In vitro blocking studies were confirmed that the high specificity of 4a, 4b. These results suggest that radioiodinated 4a and 4b are promising sigma receptors imaging ligand for pursuing further in vivo studies.

In Vivo Evaluation of Radioiodinated 1-[2-(3,4-Dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)-piperazine Derivatives as New Ligands for Sigma Receptor Imaging Using Single Photon Emission Computed Tomography

New series of radioiodinated analogues of 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-[3-(2-iodophenyl)propyl]piperazine (o-BON) and 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-[3-(3-iodophenyl)propyl]piperazine (m-BON) were evaluated as single photon emission computed tomography (SPECT) radiopharmaceuticals for mapping sigma receptors in the central nervous system (CNS) and peripheral organs. In vivo biodistribution studies of [125I] o- and m-BON in mice demonstrated high initial uptakes and prolonged retention in the brain. In contrast to high brain uptake and retention, the blood accumulations were low, resulting in good brain-blood ratios (7.9-9.2). In the other tissues, high uptake of [125I] o- and m-BON were observed in the liver, kidney, heart, lung, and pancreas. Moreover, selective interactions of [125I] o- and m-BON with sigma receptors were confirmed by pretreatment experiments with various sigma and other receptor ligands. Haloperidol posttreatment induced decreases in the accumulation of [125I] o- and m-BON. These data suggest that [125I] o- and m-BON binding to sigma receptors is reversible and competitive. Furthermore, ex vivo autoradiograms of [125I] o- and m-BON in rats showed high uptake in the parietal cortex, vestibular nucleus, and pons nucleus and moderate uptake in the thalamus, inferior colliculus, hippocampus, hypothalamus, and temporal cortex. These ex vivo autoradiograms were comparable with the histochemical distribution of sigma receptors. Furthermore, the uptake of [125I] o- and m-BON reflected quantitative amounts of sigma receptor in the brain. These results demonstrated that radiolabeled o- and m-BON have good characteristics for mapping sigma receptors in the CNS and the peripheral organs with SPECT.

Effective tumor cell death by sigma-2 receptor ligand siramesine involves lysosomal leakage and oxidative stress

Acquired resistance to classic caspase-mediated apoptosis is a common problem for the treatment of human cancer. Here, we show that siramesine, a novel sigma-2 receptor ligand, effectively induces caspase-independent programmed cell death in immortalized and transformed cells of various origins. Siramesine-treated tumor cells displayed increased levels of reactive oxygen species, lysosomal membrane permeabilization, chromatin condensation, and shrinkage and detachment of cells. Lipid antioxidants (alpha-tocopherol and gamma-tocopherol), but not other tested antioxidants (butylated hydroxyanisol or N-acetyl cysteine), effectively inhibited siramesine-induced morphologic changes and cell death. Cathepsin B inhibitors (CA-074-Me and R-2525) conferred similar, but less pronounced protection, whereas ectopic expression of antiapoptotic protein Bcl-2, lack of wild-type p53 as well as pharmacologic inhibitors of caspases (zVAD-fmk, DEVD-CHO, and LEHD-CHO), calpains (PD150606), and serine proteases (N-tosyl-L-phenylalanine chloromethyl ketone and pefabloc) failed to protect cells against siramesine-induced death. Importantly, transformation of murine embryonic fibroblasts with activated c-src or v-Ha-ras oncogenes greatly sensitized them to siramesine-induced cytotoxicity. Furthermore, p.o. administration of well-tolerated doses of siramesine had a significant antitumorigenic effect in orthotopic breast cancer and s.c. fibrosarcoma models in mice. These results present siramesine as a promising new drug for the treatment of tumors resistant to traditional therapies.

Evaluation of [11C]SA5845 and [11C]SA4503 for imaging of sigma receptors in tumors by animal PET

Sigma receptors are expressed in a wide variety of tumor cell lines, and are expressed in proliferating cells. A radioligand for the visualization of sigma receptors could be useful for selective detection of primary tumors and their metastases, and for non-invasive assessment of tumor proliferative status. To this end we evaluated two sigma receptor ligands, [11C]SA5845 and [11C]SA4503. In an in vitro study, AH109A hepatoma showed moderate densities of sigma1 and sigma2 receptors, and VX-2 carcinoma showed a high density of sigma2 receptors: Bmax (fmol/mg protein) for sigma1 vs. sigma2, 1,700 vs. 1,200 for AH109A hepatoma and 800 vs. 10,000 for VX-2 carcinoma. In a cell growth assay in vitro, neither SA5845 nor SA4503 (<10 microM) showed any inhibitory effect on proliferation of the AH109A hepatoma cells. In rats, the uptake of [11C]SA5845 and [11C]SA4503 in AH109A tissues was accumulated over the first 60 minutes; however, the uptake of both tracers increased by co-injection with haloperidol as a sigma receptor ligand. On the other hand, in the PET studies of rabbits, the uptake of [11C]SA5845 in the VX-2 carcinoma was relatively higher than that of [11C]SA4503, because of a much higher density of sigma2 receptors compared to sigma1 receptors in the VX-2 tissue, and the uptake of both tracers in the VX-2 tissue was decreased by carrier-loading and pre-treatment with haloperidol ([11C]SA5845, 53% and 26%, respectively; [11C]SA4503, 41% and 22%, respectively at 30 minutes after injection). Therefore, [11C]SA5845 and [11C]SA4503 may be potential ligands for PET imaging of sigma receptor-rich tumors.

Expression of sigma 1 receptor in human breast cancer

The sigma 1 receptor (S1R) represents a unique drug-binding site that is distinct from any other receptors. We examined S1R expression in human breast cancer and assessed the activity of S1R ligands in breast cancer cell lines. One-hundred nine breast specimens from normal breast, benign breast disease and cancer were examined with immunohistochemistry or RT-PCR and six different cell lines were also evaluated. S1R mRNA overexpression was detected in 64% of breast cancers compared to normal breast tissue. Immunohistochemistry showed positive epithelial cell staining in 60% of invasive and 41% of in situ cancers, 75% of ductal hyperplasia and in 33% of normal breast. The pattern of expression was more diffuse in invasive breast carcinoma compared to other conditions (p = 0.02). S1R expression was neither a prognostic nor a predictive factor for efficacy of adjuvant chemotherapy but the study only included 58 cancer patients and therefore the statistical power is limited. MDA-MB-361, MDA-MB-435, BT20 and MCF7 cells all expressed S1R protein by Western blot. The non-specific S1R ligands haloperidol, reduced haloperidol and progesterone produced a dose-dependent inhibition of the growth at high (>10 microM) concentrations. Reduced haloperidol also showed additive cytotoxic effects when combined with doxorubicin, vinorelbine , paclitaxel and docetaxel in vitro. The S1R-specific ligand, SKF 10047 demonstrated the least growth inhibitory activity and showed no interaction with chemotherapy. These results demonstrate that some normal and most neoplastic breast epithelial cells and cell lines commonly express S1R. High concentrations of haloperidol inhibit the growth of these cells and potentiate the effect of chemotherapy in vitro.

Sigma Receptors and Cancer

The sigma (sigma) receptor and its agonists have been implicated in a myriad of cellular functions, biological processes and diseases. Whereas the precise molecular mechanism(s) of sigma receptors and their involvement in cancer cell biology have not been elucidated, recent work has started to shed some light on these issues. A molecular model has been proposed for the cloned sigma1 receptor; the precise molecular nature of the sigma2 receptor remains unknown. sigma receptors have been found to be frequently up-regulated in human cancer cells and tissues. sigma2 receptor drugs particularly have been shown to have antiproliferative effects. An interesting possibility is that sigma and/or sigma1 drugs could produce anticancerous effects by modulating ion channels. As well as proliferation, a variety of other metastatic cellular behaviors such as adhesion, motility, and secretion may also be affected. Other mechanisms of sigma receptor action may involve interaction with ankyrin and modulation of intracellular Ca(2+) and sphingolipid levels. Although more research is needed to further define the molecular physiology of sigma receptors, their involvement in the cellular pathophysiology of cancer raises the possibility that sigma drugs could be useful as novel therapeutic agents.

Small molecule antagonists of the sigma-1 receptor cause selective release of the death program in tumor and self-reliant cells and inhibit tumor growth in vitro and in vivo

The acquisition of resistance to apoptosis, the cell's intrinsic suicide program, is essential for cancers to arise and progress and is a major reason behind treatment failures. We show in this article that small molecule antagonists of the sigma-1 receptor inhibit tumor cell survival to reveal caspase-dependent apoptosis. sigma antagonist-mediated caspase activation and cell death are substantially attenuated by the prototypic sigma-1 agonists (+)-SKF10,047 and (+)-pentazocine. Although several normal cell types such as fibroblasts, epithelial cells, and even sigma receptor-rich neurons are resistant to the apoptotic effects of sigma antagonists, cells that can promote autocrine survival such as lens epithelial and microvascular endothelial cells are as susceptible as tumor cells. Cellular susceptibility appears to correlate with differences in sigma receptor coupling rather than levels of expression. In susceptible cells only, sigma antagonists evoke a rapid rise in cytosolic calcium that is inhibited by sigma-1 agonists. In at least some tumor cells, sigma antagonists cause calcium-dependent activation of phospholipase C and concomitant calcium-independent inhibition of phosphatidylinositol 3'-kinase pathway signaling. Systemic administration of sigma antagonists significantly inhibits the growth of evolving and established hormone-sensitive and hormone-insensitive mammary carcinoma xenografts, orthotopic prostate tumors, and p53-null lung carcinoma xenografts in immunocompromised mice in the absence of side effects. Release of a sigma receptor-mediated brake on apoptosis may offer a new approach to cancer treatment.

Review of the pharmacological and clinical profile of rimcazole

Rimcazole is a carbazole derivative that acts in part as a sigma receptor antagonist. Wellcome Research Laboratories introduced this compound during the 1980s when it was hypothesized to be a novel antipsychotic with an improved side effect profile. However, subsequent clinical trials demonstrated that rimcazole lacked efficacy in schizophrenic patients and it is now primarily used as an experimental tool. In addition to its actions as a sigma receptor antagonist, rimcazole also has high affinity for dopamine transporters, and in recent years it has served as a lead compound for the development of novel dopamine transporter ligands. Although rimcazole cannot be considered a selective ligand for sigma receptors, the recent development of other selective agonists and antagonists for sigma receptors have aided in clarifying the involvement of these receptors in the actions of rimcazole. Many of the physiological and behavioral effects of rimcazole can in fact be ascribed to its action as a sigma receptor antagonist, although there are exceptions. Rimcazole is likely to have a continued role in elucidating sigma receptor function in either in vitro or in vivo systems where sigma receptor-mediated effects can be studied independently of the influence of dopamine and serotonin transporters.

Preclinical acute toxicity studies and rodent-based dosimetry estimates of the novel sigma-1 receptor radiotracer [(18)F]FPS

[(18)F]1-(Fluoropropyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]FPS) is a novel high affinity (KD = 0.5 nM) sigma receptor radioligand that exhibits saturable and selective in vivo binding to sigma receptors in rats, mice and non-human primates. In order to support an IND application for the characterization of [(18)F]FPS through PET imaging studies in humans, single organ and whole body radiation adsorbed doses associated with [(18)F]FPS injection were estimated from distribution data obtained in rats. In addition, acute toxicity studies were conducted in rats and rabbits and limited toxicity analyses were performed in dogs. Radiation dosimetry estimates obtained using rat biodistribution analysis of [(18)F]FPS suggest that most organs would receive around 0.012-0.015 mGy/MBq. The adrenal glands, brain, kidneys, lungs, and spleen would receive slightly higher doses (0.02-0.03 mGy/MBq). The adrenal glands were identified as the organs receiving the greatest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [(18)F]FPS is well below the FDA defined limits for yearly cumulative and per study exposures to research participants. Extended acute toxicity studies in rats and rabbits, and limited acute toxicity studies in beagle dogs suggest at least a 175-fold safety margin in humans at a mass dose limit of 2.8 microg per intravenous injection. This estimate is based on the measured no observable effect doses (in mg/m(2)) in these species. These data support the expectation that [(18)F]FPS will be safe for use in human PET imaging studies at a maximum administration of 5 mCi and a mass dose equal to or less than 2.8 microg FPS per injection.