Is a nitrogen atom an important pharmacophoric element in sigma ligand binding?

Tanshinone IIA Liposomes Treat Doxorubicin-Induced Glomerulonephritis by Modulating the Microenvironment of Fibrotic Kidneys

Renal fibrosis plays a key role in the pathogenesis of chronic kidney disease (CKD), in which the persistent high expression of transforming growth factor β1 (TGF-β1) and α-smooth muscle actin (α-SMA) contributes to the progression of CKD to renal failure. In order to improve the solubility, bioavailability, and targeting of tanshinone IIA (Tan IIA), a novel targeting material, aminoethyl anisamide-polyethylene glycol-1,2-distearoyl-sn-glycero-3-phosphate ethanolamine (AEAA-PEG-DSPE, APD) modified Tan IIA liposomes (APD-Tan IIA-L) was constructed. An animal model of glomerulonephritis induced by doxorubicin in BALB/c mice was established. APD-Tan IIA-L significantly decreased blood urea nitrogen and serum creatinine (SCr), and the consequences of renal tissue oxidative stress indicators showed that APD-Tan IIA-L downregulated malondialdehyde, upregulated superoxide dismutase, catalase, and glutathione peroxidase. Masson's trichrome staining showed that the deposition of collagen in the APD-Tan IIA-L group decreased significantly. The pro-fibrotic factors (fibronectin, collagen I, TGF-β1, and α-SMA) and epithelial-mesenchymal transition marker (N-cadherin) were significantly inhibited by APD-Tan IIA-L. By improving the microenvironment of fibrotic kidneys, APD-Tan IIA-L attenuated TGF-β1-induced excessive proliferation of fibroblasts and alleviated oxidative stress damage to the kidney, providing a new strategy for the clinical treatment of renal fibrosis.

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

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

A Novel Bifunctional μOR Agonist and σ1R Antagonist with Potent Analgesic Responses and Reduced Adverse Effects

Bifunctional ligands possessing both μOR agonism and σ1R antagonism have shown promise in producing strong analgesic effects with reduced opioid-related side effects. However, the μOR agonism activity of most dual ligands diminishes compared with classical opioids, raising concern about their effectiveness in managing nociceptive pain. In this study, a new class of dual μOR agonist/σ1R antagonist was reported. Through structure-activity relationship analyses, we identified the optimal compound, 4x, which displayed picomolar μOR agonism activity (EC50: 0.6 ± 0.2 nM) and good σ1R inhibitory activity (Ki: 363.7 ± 5.6 nM) with excellent selectivity. Compound 4x exhibited robust analgesic effects in various pain models, with significantly reduced side effects. Importantly, compound 4x also possessed good safety profiles and no abnormalities were observed in biological parameters even under a high dosage. Our findings suggest that 4x may be a promising lead compound for developing safer opioids and warrants further in-depth studies.

Synthesis, Biophysical Properties, and Antitumor Activity of Antisense Oligonucleotides Conjugated with Anisamide

Antisense oligonucleotides (ASONs) have proven potential for the treatment of various diseases. However, their limited bioavailability restricts their clinical application. New structures with improved enzyme resistance stability and efficient drug delivery are needed. In this work, we propose a novel category of ASONs bearing anisamide conjugation at phosphorothioate sites for oncotherapy. ASONs can be conjugated with the ligand anisamide very efficiently and flexibly in a solution. The conjugation sites and the ligand amount both influence anti-enzymatic stability and cellular uptake, resulting in changes in antitumor activity that are detectable by cytotoxicity assay. The conjugate with double anisamide (T6) was identified as the optimal conjugate, and its antitumor activity and the underlying mechanism were examined further in vitro and in vivo. This paper presents a new strategy for the design of nucleic acid-based therapeutics with improved drug delivery and biophysical and biological efficacy.

Screening of σ2 Receptor Ligands and In Vivo Evaluation of 11C-Labeled 6,7-Dimethoxy-2-[4-(4-methoxyphenyl)butan-2-yl]-1,2,3,4-tetrahydroisoquinoline for Potential Use as a σ2 Receptor Brain PET Tracer

In this study, a panel of 46 compounds containing five different scaffolds known to have high σ₂ receptor affinity were screened. 6,7-Dimethoxy-2-[4-(4-methoxyphenyl)butan-2-yl]-1,2,3,4-tetrahydroisoquinoline [(±)-7] (K_i for σ₁ = 48.4 ± 7.7 nM, and K_i for σ₂ = 0.59 ± 0.02 nM) and its desmethyl analogue, (±)-8 (K_i for σ₁ = 108 ± 35 nM, and K_i for σ₂ = 4.92 ± 0.59 nM), showed excellent binding affinity and subtype selectivity for σ₂ receptors. In vitro cell binding indicated that σ₂ receptor binding of [¹¹C]-(±)-7 and [¹¹C]-(±)-8 was dependent on TMEM97 protein expression. In PET studies, the peak brain uptake of [¹¹C]-(±)-7 (8.28 ± 2.52%ID/cc) was higher than that of [¹¹C]-(±)-8 (4.25 ± 0.97%ID/cc) with specific distribution in the cortex and hypothalamus. Brain uptake or tissue binding was selectively inhibited by ligands with different σ₂ receptor binding affinities. The results suggest [¹¹C]-(±)-7 can be used as a PET radiotracer for imaging the function of σ₂ receptors in central nervous system disorders.

Structural and Molecular Insight into Piperazine and Piperidine Derivatives as Histamine H3 and Sigma-1 Receptor Antagonists with Promising Antinociceptive Properties

In an attempt to extend recent studies showing that some clinically evaluated histamine H3 receptor (H3R) antagonists possess nanomolar affinity at sigma-1 receptors (σ1R), we selected 20 representative structures among our previously reported H3R ligands to investigate their affinity at σRs. Most of the tested compounds interact with both sigma receptors to different degrees. However, only six of them showed higher affinity toward σ1R than σ2R with the highest binding preference to σ1R for compounds 5, 11, and 12. Moreover, all these ligands share a common structural feature: the piperidine moiety as the fundamental part of the molecule. It is most likely a critical structural element for dual H3/σ1 receptor activity as can be seen by comparing the data for compounds 4 and 5 (hH3R Ki = 3.17 and 7.70 nM, σ1R Ki = 1531 and 3.64 nM, respectively), where piperidine is replaced by piperazine. We identified the putative protein-ligand interactions responsible for their high affinity using molecular modeling techniques and selected compounds 5 and 11 as lead structures for further evaluation. Interestingly, both ligands turned out to be high-affinity histamine H3 and σ1 receptor antagonists with negligible affinity at the other histamine receptor subtypes and promising antinociceptive activity in vivo. Considering that many literature data clearly indicate high preclinical efficacy of individual selective σ1 or H3R ligands in various pain models, our research might be a breakthrough in the search for novel, dual-acting compounds that can improve existing pain therapies. Determining whether such ligands are more effective than single-selective drugs will be the subject of our future studies.

Recent Advances in the Development of Sigma Receptor Ligands as Cytotoxic Agents: A Medicinal Chemistry Perspective

Since their discovery as distinct receptor proteins, the specific physiopathological role of sigma receptors (σRs) has been deeply investigated. It has been reported that these proteins, classified into two subtypes indicated as σ₁ and σ₂, might play a pivotal role in cancer growth, cell proliferation, and tumor aggressiveness. As a result, the development of selective σR ligands with potential antitumor properties attracted significant attention as an emerging theme in cancer research. This perspective deals with the recent advances of σR ligands as novel cytotoxic agents, covering articles published between 2010 and 2020. An up-to-date description of the medicinal chemistry of selective σ₁R and σ₂R ligands with antiproliferative and cytotoxic activities has been provided, including major pharmacophore models and comprehensive structure–activity relationships for each main class of σR ligands.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Using sigma-ligands as part of a multi-receptor approach to target diseases of the brain

INTRODUCTION

The sigma receptors are found abundantly in the central nervous system and are targets for the treatment of various diseases, including Alzheimer's (AD), Parkinson's (PD), Huntington's disease (HD), depression, amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). However, for many of these diseases, other receptors and targets have been the focus of the most, such as acetylcholine esterase inhibitors in Alzheimer's and dopamine replacement in Parkinson's. The currently available drugs for these diseases have limited success resulting in the requirement of an alternative approach to their treatment.

AREAS COVERED

In this review, we discuss the potential role of the sigma receptors and their ligands as part of a multi receptor approach in the treatment of the diseases mentioned above. The literature reviewed was obtained through searches in databases, including PubMed, Web of Science, Google Scholar, and Scopus.

EXPERT OPINION

Given sigma receptor agonists provide neuroprotection along with other benefits such as potentiating the effects of other receptors, further development of multi-receptor targeting ligands, and or the development of multi-drug combinations to target multiple receptors may prove beneficial in the future treatment of degenerative diseases of the CNS, especially when coupled with better diagnostic techniques.

Simple ammonium salts acting on sigma-1 receptors yield potential treatments for cancer and depression

Sigma-1 and sigma-2 receptors are emerging therapeutic targets. We have identified that simple ammonium salts bind to these receptors and are effective in vivo. Radioligand binding assays were used to obtain structure-activity relationships of these salts. MTS assays were performed to determine their effect on growth in MCF7 and MDA-MB-486 cells. Anticancer properties were tested in NMRI mice transplanted with a fragment of mouse adenocarcinoma (MAC13). Antidepressant activity was tested using the forced-swim test and tail suspension tests. Dipentylammonium (Ki 43 nM), tripentylammonium (Ki 15 nM) and trihexylammonium (Ki 9 nM) showed high affinity for the sigma-1 receptor. Dioctanoylammonium had the highest affinity (K50 0.05 nM); this also showed the highest affinity for sigma-2 receptors (Ki 13 nM). Dipentylammonium was found to have antidepressant activity in vivo. Branched-chain ammonium salts showed lower affinity. Bis(2-ethylhexyl)ammonium (K50 29 µM), triisopentylammonium (K50 196 µM) and dioctanoylammonium showed a low Hill slope, and fitted a 2-site binding model for the sigma-1 receptor. We propose this two-site binding can be used to biochemically define a sigma-1 receptor antagonist. Bis(2-ethylhexyl)ammonium and triisopentylammonium were able to inhibit the growth of tumours in vivo. Cheap, simple ammonium salts act as sigma-1 receptor agonists and antagonists in vivo and require further investigation.

Novel Dithiolane-Based Ligands Combining Sigma and NMDA Receptor Interactions as Potential Neuroprotective Agents

Sigma receptors (SRs) are recognized as valuable targets for the treatment of neurodegenerative disorders. A series of novel SRs ligands were designed by combining key pharmacophoric amines (i.e., benzylpiperidine or benzylpiperazine) with new 1,3-dithiolane-based heterocycles and their bioisosters. The new compounds exhibited a low nanomolar affinity for sigma-1 and sigma-2 receptors. Five selected compounds were evaluated for their neuroprotective capacity on SH-SY5Y neuroblastoma cell line. They were able to counteract the neurotoxicity induced by rotenone, oligomycin and NMDA. Competition studies with PB212, a S1R antagonist, confirmed the involvement of S1R in neuroprotection from the oxidative stress induced by rotenone. Electrophysiological experiments performed on cortical neurons in culture highlighted the compounds ability to reduce NMDA-evoked currents, suggesting a negative allosteric modulator activity toward the NMDA receptor. Altogether these results qualify our novel dithiolane derivatives as potential agents for fighting neurodegeneration.

Docking, Interaction Fingerprint, and Three-Dimensional Quantitative Structure-Activity Relationship (3D-QSAR) of Sigma1 Receptor Ligands, Analogs of the Neuroprotective Agent RC-33

The human Sigma1 receptor (S1R), which has been identified as a target with an important role in neuropsychological disorders, was first crystallized 3 years ago. Since S1R structure has no relation with another previous crystallized structures, the presence of the new crystal is an important hallmark for the design of agonists and antagonists against this important target. Some years ago, our group identified RC-33, a potent and selective S1R agonist, endowed with neuroprotective properties. In this work, drawing on new structural information, we studied the interactions of RC-33 and its analogs with the S1R binding site by using computational methods such as docking, interaction fingerprints, and receptor-guided alignment three dimensional quantitative structure–activity relationship (3D-QSAR). We found that RC-33 and its analogs adopted similar orientations within S1R binding site, with high similitude with orientations of the crystallized ligands; such information was used for identifying the residues involved in chemical interactions with ligands. Furthermore, the structure-activity relationship of the studied ligands was adequately described considering classical QSAR tests. All relevant aspects of the interactions between the studied compounds and S1R were covered here, through descriptions of orientations, binding interactions, and features that influence differential affinities. In this sense, the present results could be useful in the future design of novel S1R modulators.

Dyk S, Malan SF. Hexacyclododecylamines with Sigma-1 Receptor Affinity and Calcium Channel Modulating Ability

Introduction:

Recent research points to the Sigma Receptor (σR) as a possible neuromodulatory system with multi-functional action and σ1Rs have been suggested as a drug target for a number of CNS conditions. Hexacyclododecylamines have shown σ1R activity and provide an advantageous scaffold for drug design that can improve the blood-brain barrier permeability of privileged structures.

Methods and Materials:

A series of oxa- and aza- hexaxcyclododecylamines were synthesised and evaluated for sigma-1 receptor activity and voltage-gated calcium channel blocking ability to determine the effect of inclusion of amine containing heterocycles.

Results & Discussion:

The compounds had promising σ1R activities (Ki = 0.067 – 11.86 µM) with the aza-hexacyclododecylamines 12, 24 and 27 showing some of the highest affinities (Ki = 0.067 µM, 0.215 µM and 0.496 µM respectively). This confirms, as observed in previous studies, that the aza compounds are more favourable for σ1R binding than their oxa counterparts. The addition of the amine heterocycle showed affinities similar to that of related structures with only two lipophilic binding regions. This indicates that the inclusion of an amine heterocycle into these structures is a viable option in the design of new σ1R ligands. Significant voltage-gated calcium channel blocking ability was also observed for 12, 24 and 27, suggesting a link between σ1R activity and intracellular calcium levels.

Conclusion:

The σ1R activity and potential effect on other receptor classes and calcium channels could prove beneficial in pharmacological application.

In Pursuit of Fluorinated Sigma Receptor Ligand Candidates Related to [18F]-FPS

This paper describes the synthesis of N-arylmethyl(1-benzyl) and N-aroyl(1-benzoyl) 4-(4-fluoromethylphenoxymethyl)piperidines as potential sigma receptor ligands analogous to the potent and highly selective sigma-1 ligand [18F]-FPS, but with enhanced or alternative binding and transport profiles. The synthesis involves N-aroylation of 4-hydroxmethylpiperidine or ethyl nipecotate, functional group manipulation of the ester group or simple activation of the hydroxyl group to introduce the phenoxy component, and subsequent functional group manipulation to reduce the amide group and introduce the fluorine into the fluoromethyl substituent. In its development, the synthesis was found to require early N-aroylation of the piperidine precursor to avoid complications due to anchimeric assistance by its nitrogen in subsequent displacement reactions. New evidence is presented on the pathway followed in a literature report of direct displacement of a benzylic hydroxyl group by fluoride ion under Appel-like conditions. Relevant to the literature report, the halide ion in the fluoromethylphenoxy 1-benzylpiperidine derivatives was surprisingly labile to hydrolytic displacement on chromatography and this aspect is worthy of further study. Moreover, the NMR spectra of the amides were complicated by geometric isomerism about the amide C(O)–N bond, but detailed analysis of spectra from 2-anisoyl derivatives allowed the assignment of diastereomeric contributors to consistent, secondary atropisomerism about the aryl–C(O) bond.

Sigma Receptor (σR) Ligands with Antiproliferative and Anticancer Activity

Sigma receptor (σR) ligands have proven to be useful as cancer diagnostics and anticancer therapeutics and their ligands have been developed as molecular probes in oncology. Moreover, various σR ligands generate cancer cell death in vitro and in vivo. These σR ligands have exhibited promising results against numerous human and rodent cancers and are investigated under preclinical and clinical study trials, indicating a new category of drugs in cancer therapy.

4-aroylpiperidines and 4-(α-hydroxyphenyl)piperidines as selective sigma-1 receptor ligands: synthesis, preliminary pharmacological evaluation and computational studies

Background

Sigma (σ) receptors are membrane-bound proteins characterised by an unusual promiscuous ability to bind a wide variety of drugs and their high affinity for typical neuroleptic drugs, such as haloperidol, and their potential as alternative targets for antipsychotic agents. Sigma receptors display diverse biological activities and represent potential fruitful targets for therapeutic development in combating many human diseases. Therefore, they present an interesting avenue for further exploration. It was our goal to evaluate the potential of ring opened spipethiane (1) analogues as functional ligands (agonists) for σ receptors by chemical modification.

Results

Chemical modification of the core structure of the lead compound, (1), by replacement of the sulphur atom with a carbonyl group, hydroxyl group and 3-bromobenzylamine with the simultaneous presence of 4-fluorobenzoyl replacing the spirofusion afforded novel potent sigma-1 receptor ligands 7a–f, 8a–f and 9d–e. The sigma-1 receptor affinities of 7e, 8a and 8f were slightly lower than that of 1 and their selectivities for this receptor two to threefold greater than that of 1.

Conclusions

It was found that these compounds have higher selectivities for sigma-1 receptors compared to 1. Quantitatitive structure–activity relationship studies revealed that sigma-1 binding is driven by hydrophobic interactions.Graphical abstract

One-pot synthesis and sigma receptor binding studies of novel spirocyclic-2,6-diketopiperazine derivatives

New spirocyclic-2,6-diketopiperazine derivatives containing benzylpiperidine and cycloalkane moieties were synthesized by a one-pot two-step sequential Ugi/intramolecular N-amidation process in moderate to good yields. The in vitro ligand-binding profile studies performed on the sigma-1 and sigma-2 receptors revealed that the σ1 affinities and subtype selectivities of three spirocyclic piperidine derivatives are generally comparable to those of spirocycloalkane analogues. Compared to the low σ1 affinities obtained for cycloalkyl-substituted spirocyclic-2,6-diketopiperazines with n=2, those with n=1 proved to have optimal fitting with σ2 subtype by exhibiting higher affinities. Moreover, the best binding affinity and subtype selectivity was identified for compound 3c with Kiσ1=5.9±0.5nM and Kiσ2=563±21nM as well as 95-fold σ1/σ2 selectivity ratio, respectively.

Sigma Receptor Binding Assays

Sigma receptors, both Sigma-1(S1R) and Sigma-2 (S2R), are small molecule-regulated, primarily endoplasmic reticulum (ER) membrane-associated sites. A number of drugs bind to sigma receptors, including the antipsychotic haloperidol and (+)-pentazocine, an opioid analgesic. Sigma receptors are implicated in many central nervous system disorders, in particular Alzheimer's disease and conditions associated with motor control, such as Amyotrophic Lateral Sclerosis (ALS). Described in this unit are radioligand binding assays used for the pharmacological characterization of S1R and S2R. Methods detailed include a radioligand saturation binding assay for defining receptor densities and a competitive inhibition binding assay employing [³H]-(+)-pentazocine for identifying and characterizing novel ligands that interact with S1R. Procedures using [³H]-1,3-di(2-tolyl)guanidine ([³H]-DTG), a nonselective sigma receptor ligand, are described for conducting a saturation binding and competitive inhibition assays for the S2R site. These protocols are of value in drug discovery in identifying new sigma ligands and in the characterization of these receptors.

Recent Advances in the Development of Sigma-1 Receptor Ligands

The existence of two distinct sigma (σ) receptor subtypes was established in the early 1990s. Sigma-1 and sigma-2 receptors (S1Rs and S2Rs, respectively) were shown to possess distinct molecular size, anatomical distribution, and ligand discrimination. S2R is overexpressed in numerous human cancers, and has therapeutic potential for the imaging and treatment of certain tumours. In contrast, S1R is more broadly involved in a wide variety of central nervous system (CNS) diseases including motor disorders, memory deficits, depression, schizophrenia, anxiety, pain, drug addiction, and many more. Since the human S1R was cloned in 1996, numerous high affinity ligands with excellent selectivity for S1R have been developed. This review focuses on recent developments in the generation of structurally diverse S1R-selective ligands and novel therapeutic candidates targeting S1Rs.

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.

Sigma-1 receptor regulates early steps of viral RNA replication at the onset of hepatitis C virus infection

Hepatitis C virus (HCV) genome replication is thought to occur in a membranous cellular compartment derived from the endoplasmic reticulum (ER). The molecular mechanisms by which these membrane-associated replication complexes are formed during HCV infection are only starting to be unraveled, and both viral and cellular factors contribute to their formation. In this study, we describe the discovery of nonopioid sigma-1 receptor (S1R) as a cellular factor that mediates the early steps of viral RNA replication. S1R is a cholesterol-binding protein that resides in lipid-rich areas of the ER and in mitochondrion-associated ER membranes (MAMs). Several functions have been ascribed to this ER-resident chaperone, many of which are related to Ca(2+) signaling at the MAMs and lipid storage and trafficking. Downregulation of S1R expression by RNA interference (RNAi) in Huh-7 cells leads to a proportional decrease in susceptibility to HCV infection, as shown by reduced HCV RNA accumulation and intra- and extracellular infectivity in single-cycle infection experiments. Similar RNAi studies in persistently infected cells indicate that S1R expression is not rate limiting for persistent HCV RNA replication, as marked reduction in S1R in these cells does not lead to any decrease in HCV RNA or viral protein expression. However, subgenomic replicon transfection experiments indicate that S1R expression is rate limiting for HCV RNA replication without impairing primary translation. Overall, our data indicate that the initial steps of HCV infection are regulated by S1R, a key component of MAMs, suggesting that these structures could serve as platforms for initial RNA replication during HCV infection.

3D-QSAR and docking studies of pentacycloundecylamines at the sigma-1 (σ1) receptor

Pentacycloundecylamine (PCU) derived compounds have been shown to be promising lead structures for the development of novel drug candidates aimed at a variety of neurodegenerative and psychiatric diseases. Here we show for the first time a 3D quantitative structure-activity relationship (3D-QSAR) for a series of aza-PCU-derived compounds with activity at the sigma-1 (σ1) receptor. A comparative molecular field analysis (CoMFA) model was developed with a partial least squares cross validated (q(2)) regression value of 0.6, and a non-cross validated r(2) of 0.9. The CoMFA model was effective at predicting the sigma-1 activities of a test set with an r(2) >0.7. We also describe here the docking of the PCU-derived compounds into a homology model of the sigma-1 (σ1) receptor, which was developed to gain insight into binding of these cage compounds to the receptor. Based on docking studies we evaluated in a [(3)H]pentazocine binding assay an oxa-PCU, NGP1-01 (IC50=1.78μM) and its phenethyl derivative (IC50=1.54μM). Results from these studies can be used to develop new compounds with specific affinity for the sigma-1(σ1) receptor.

σ-1 receptor at the mitochondrial-associated endoplasmic reticulum membrane is responsible for mitochondrial metabolic regulation

The mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) is a small section of the outer mitochondrial membrane tethered to the ER by lipid and protein filaments. One such MAM protein is the σ-1 receptor, which contributes to multiple signaling pathways. We found that short interfering RNA-mediated knockdown of σ-1 reduced pregnenolone synthesis by 95% without affecting expression of the inner mitochondrial membrane resident enzyme, 3-β-hydroxysteroid dehydrogenase 2. To explore the underlying mechanism of this effect, we generated a series of σ-receptor ligands: 5,6-dimethoxy-3-methyl-N-phenyl-N-(3-(piperidin-1-yl)propyl)benzofuran-2-carboxamide (KSCM-1), 3-methyl-N-phenyl-N-(3-(piperidin-1-yl)propyl)benzofuran-2-carboxamide (KSCM-5), and 6-methoxy-3-methyl-N-phenyl-N-(3-(piperidin-1-yl) propyl)benzofuran-2-carboxamide (KSCM-11) specifically bound to σ-1 in the nanomolar range, whereas KSCM-5 and KSCM-11 also bound to σ-2. Treatment of cells with the KSCM ligands led to decreased cell viability, with KSCM-5 having the most potent effect followed by KSCM-11. KSCM-1 increased σ-1 expression by 4-fold and progesterone synthesis, whereas the other compounds decreased progesterone synthesis. These differences probably are caused by ligand molecular structure. For example, KSCM-1 has two methoxy substituents at C-5 and C-6 of the benzofuran ring, whereas KSCM-11 has one at C-6. KSCM ligands or σ-1 knockdown did not alter the expression of ER resident enzymes that synthesize steroids. However, coimmunoprecipitation of the σ-1 receptor pulled down voltage-dependent anion channel 2 (VDAC2), whose expression was enhanced by KSCM-1. VDAC2 plays a key role in cholesterol transport into the mitochondria, suggesting that the σ-1 receptor at the MAM coordinates with steroidogenic acute regulatory protein for cholesterol trafficking into the mitochondria for metabolic regulation.

Synthesis of N-phenyl-N-(3-(piperidin-1-yl)propyl)benzofuran-2-carboxamides as new selective ligands for sigma receptors

Novel benzofuran-2-carboxamide ligands, which are selective for sigma receptors, have been synthesized via a microwave-assisted Perkin rearrangement reaction and a modified Finkelstein halogen-exchange used to facilitate N-alkylation. The ligands synthesized are the 3-methyl-N-phenyl-N-(3-(piperidin-1-yl)propyl)benzofuran-2-carboxamides (KSCM-1, KSCM-5 and KSCM-11). The benzofuran-2-carboxamide structure was N-arylated and N-alkylated to include both N-phenyl and N-(3-(piperidin-1-yl)propyl substituents, respectively. These new carboxamides exhibit high affinity at the sigma-1 receptor with K(i) values ranging from 7.8 to 34nM. Ligand KSCM-1 with two methoxy substituents at C-5 and C-6 of the benzofuran ring, and K(i)=27.5nM at sigma-1 was found to be more selective for sigma-1 over sigma-2.

Characterization of interactions of 4-nitrophenylpropyl-N-alkylamine with ς receptors

Sigma receptors are small membrane proteins implicated in a number of pathophysiological conditions, including drug addiction, psychosis, and cancer; thus, small molecule inhibitors of sigma receptors have been proposed as potential pharmacotherapeutics for these diseases. We previously discovered that endogenous monochain N-alkyl sphingolipids, including d-erythro-sphingosine, sphinganine, and N,N-dimethylsphingosine, bind to the sigma-1 receptor at physiologically relevant concentrations [Ramachandran, S., et al. (2009) Eur. J. Pharmacol. 609, 19-26]. Here, we investigated several N-alkylamines of varying chain lengths as sigma receptor ligands. Although the K(I) values for N-alkylamines were found to be in the micromolar range, when N-3-phenylpropyl and N-3-(4-nitrophenyl)propyl derivatives of butylamine (1a and 1b, respectively), heptylamine (2a and 2b, respectively), dodecylamine (3a and 3b, respectively), and octadecylamine (4a and 4b, respectively) were evaluated as sigma receptor ligands, we found that these compounds exhibited nanomolar affinities with both sigma-1 and sigma-2 receptors. A screen of high-affinity ligands 2a, 2b, 3a, and 3b against a variety of other receptors and/or transporters confirmed these four compounds to be highly selective mixed sigma-1 and sigma-2 ligands. Additionally, in HEK-293 cells reconstituted with K(v)1.4 potassium channel and the sigma-1 receptor, these derivatives were able to inhibit the outward current from the channel, consistent with sigma receptor modulation. Finally, cytotoxicity assays showed that 2a, 2b, 3a, and 3b were highly potent against a number of cancer cell lines, demonstrating their potential utility as mixed sigma-1 and sigma-2 receptor anticancer agents.

Synthesis and characterization of [³H]-SN56, a novel radioligand for the σ₁ receptor

The study of the binding characteristics of σ ligands in vivo and in vitro requires radiolabeled probes with high affinity and selectivity. The radioligand presently used for in vitro studies of the σ₁ receptor, [³H](+)-pentazocine, has significant limitations; it is difficult to synthesize, has limited chemical stability, and can be problematic to obtain. Evaluation of a series of novel 2(3H)-benzothiazolone compounds revealed SN56 to have sub-nanomolar and preferential affinity for the σ₁ subtype, relative to σ₂ and non-sigma, binding sites. The goal of this study was to characterize the binding of [³H]-SN56 to σ₁ receptors isolated from rat brain. Standard in vitro binding techniques were utilized to 1) determine the specificity and affinity of binding to σ₁ receptors, 2) confirm that[³H]-SN56 labels sites previously identified as σ₁ by comparing binding to sites labeled by [³H](+)-pentazocine, and 3) characterize the kinetics of binding. The results indicate that [³H]-SN56 exhibits 1) specific, saturable, and reversible binding to the σ₁ receptor, with B(max)=340±10 fmol/mg and K(d)=0.069±0.0074 nM, 2) competitive displacement by classical sigma compounds, yielding σ₁ K(i) values consistent with those reported in the literature, and 3) binding kinetics compatible with a 90 min incubation, and filtration for separation of free and bound radioligand. The results of these studies suggest that [(3)H]-SN56 may serve as a viable alternative to [³H](+)-pentazocine in radioligand binding assays.

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.

The sigma-1 receptor chaperone as an inter-organelle signaling modulator

Inter-organelle signaling plays important roles in many physiological functions. Endoplasmic reticulum (ER)-mitochondrion signaling affects intramitochondrial calcium (Ca(2+)) homeostasis and cellular bioenergetics. ER-nucleus signaling attenuates ER stress. ER-plasma membrane signaling regulates cytosolic Ca(2+) homeostasis and ER-mitochondrion-plasma membrane signaling regulates hippocampal dendritic spine formation. Here, we propose that the sigma-1 receptor (Sig-1R), an ER chaperone protein, acts as an inter-organelle signaling modulator. Sig-1Rs normally reside at the ER-mitochondrion contact called the MAM (mitochondrion-associated ER membrane), where Sig-1Rs regulate ER-mitochondrion signaling and ER-nucleus crosstalk. When cells are stimulated by ligands or undergo prolonged stress, Sig-1Rs translocate from the MAM to the ER reticular network and plasmalemma/plasma membrane to regulate a variety of functional proteins, including ion channels, receptors and kinases. Thus, the Sig-1R serves as an inter-organelle signaling modulator locally at the MAM and remotely at the plasmalemma/plasma membrane. Many pharmacological/physiological effects of Sig-1Rs might relate to this unique action of Sig-1Rs.

Targeted intracellular delivery of antisense oligonucleotides via conjugation with small-molecule ligands

Selective delivery of antisense or siRNA oligonucleotides to cells and tissues via receptor-mediated endocytosis is becoming an important approach for oligonucleotide-based pharmacology. In most cases receptor targeting has been attained using antibodies or peptide-type ligands. Thus, there are few examples of delivering oligonucleotides using the plethora of small-molecule receptor-specific ligands that currently exist. In this report we describe a facile approach to the generation of mono- and multivalent conjugates of oligonucleotides with small-molecule ligands. Using the sigma-receptor ligand anisamide as an example, we describe conversion of the ligand to a phosphoramidite and direct incorporation of this moiety into the oligonucleotide by solid-phase DNA synthesis. We generated mono- and trivalent conjugates of anisamide with a splice switching antisense oligonucleotide (SSO) and tested their ability to modify splicing of a reporter gene (luciferase) in tumor cells in culture. The trivalent anisamide-SSO conjugate displayed enhanced cellular uptake and was markedly more effective than an unconjugated SSO or the monovalent conjugate in modifying splicing of the reporter. Significant biological effects were attained in the sub-100 nM concentration range.

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.

The sigma1 receptor interacts with N-alkyl amines and endogenous sphingolipids

The sigma1 receptor is distinguished for its ability to bind various pharmacological agents including drugs of abuse such as cocaine and methamphetamine. Some endogenous ligands have been identified as putative sigma1 receptor regulators. High affinity ligands for the sigma1 receptor contain a nitrogen atom connected to long alkyl chains. We found that long alkyl chain primary amines including endogenous amines belonging to the sphingolipid family such as D-erythro-sphingosine and sphinganine bind with considerable affinity to the sigma1 receptor but not to the sigma2 receptor. The binding of D-erythro-sphingosine to the sigma1 receptor appears to be competitive in nature as assessed against the radioligand [3H]-(+)-pentazocine. Interestingly, the well studied sphingolipid mediator sphingosine-1 phosphate did not bind to the sigma1 or the sigma2 receptor. Sphingosine is converted to sphingosine-1 phosphate by a family of sphingosine kinases that regulate the relative levels of these two bioactive lipids in the cell. The selective binding of sphingosine but not sphingosine-1 phosphate to the sigma1 receptor suggests a mechanism for regulation of sigma1 receptor activity by the sphingosine kinase. We have successfully reconstituted this hypothetical model in HEK-293 cells overexpressing both the sigma1 receptor and sphingosine kinase-1. The data presented here strongly supports sphingosine as an endogenous modulator of the sigma1 receptor.

Probing the steroid binding domain-like I (SBDLI) of the sigma-1 receptor binding site using N-substituted photoaffinity labels

Radioiodinated photoactivatable photoprobes can provide valuable insights regarding protein structure. Previous work in our laboratory showed that the cocaine derivative and photoprobe 3-[ (125)I]iodo-4-azidococaine ([ (125)I]IACoc) binds to the sigma-1 receptor with 2-3 orders of magnitude higher affinity than cocaine [Kahoun, J. R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1393-1397]. Using this photoprobe, we demonstrated the insertion site for [ (125)I]IACoc to be Asp188 [Chen, Y. (2007) Biochemistry 46, 3532-3542], which resides in the proposed steroid binding domain-like II (SBDLII) region (amino acids 176-194) [Pal, A. (2007) Mol. Pharmacol. 72, 921-933]. An additional photoprobe based on the sigma-1 receptor ligand fenpropimorph, 1- N-(2-3-[ (125)I]iodophenyl)propane ([ (125)I]IAF), was found to label a peptide in both the SBDLII and steroid binding domain-like I (SBDLI) (amino acids 91-109) [Pal, A. (2007) Mol. Pharmacol. 72, 921-933]. In this report, we describe two novel strategically positioned carrier-free, radioiodinated photoaffinity labels specifically designed to probe the putative "nitrogen interacting region" of sigma-1 receptor ligands. These two novel photoprobes are (-)-methyl 3-(benzoyloxy)-8-2-(4-azido-3-[ (125)I]iodobenzene)-1-ethyl-8-azabicyclo[3.2.1]octane-2-carboxylate ([ (125)I]-N-IACoc) and N-propyl- N-(4-azido-3-iodophenylethyl)-3-(4-fluorophenyl)propylamine ([ (125)I]IAC44). In addition to reporting their binding affinities to the sigma-1 and sigma-2 receptors, we show that both photoaffinity labels specifically and covalently derivatized the pure guinea pig sigma-1 receptor (26.1 kDa) [Ramachandran, S. (2007) Protein Expression Purif. 51, 283-292]. Cleavage of the photolabeled sigma-1 receptor using Endo Lys C and cyanogen bromide (CNBr) revealed that the [ (125)I]-N-IACoc label was located primarily in the N-terminus and SBDLI-containing peptides of the sigma-1 receptor, while [ (125)I]IAC44 was found in peptide fragments consistent with labeling of both SBDLI and SBDLII.

Synthesis of 99mTc-nitrido heterocomplex of piperidine and in vitro and in vivo evaluation of its affinity for sigma receptors

Sigma receptors are overexpressed in various types of cancer cells, making ligands that bind to these receptors attractive vectors for targeting radiation to specific sites for the imaging and therapy of oncologic disorders. In this paper, we report the synthesis of a dithiocarbamate derivative of 4-amino-N-benzylpiperidine and its radiolabeling with the [(99m)TcN(PNP)](2+) metal synthon. The radiolabeled tracer has been evaluated for sigma-receptor specificity. The radiochemical purity of the (99m)Tc-complex was >98%. The in vitro cell-binding and competition studies of the complex showed affinity and specificity toward fibrosarcoma and melanoma cells. In vivo studies carried out in mice bearing melanoma and fibrosarcoma tumors showed tumor uptakes of 1% and 1.9%, respectively, at 3 hours postinjection. In vivo blocking studies were carried out, using (+)-pentazocine, a sigma-receptor-specific agent where approximately 40% decrease in the tumor uptake was observed. The affinity of [(99m)TcN(PNP)Pip-DTC](+) complex for sigma-receptor sites ascertained through in vitro and in vivo studies makes it a potential agent for further investigation.

Identification of regions of the sigma-1 receptor ligand binding site using a novel photoprobe

sigma Receptors, once considered a class of opioid receptors, are now regarded as a unique class of receptors that contain binding sites for a wide range of ligands, including the drug 1-N(2',6'-dimethylmorpholino)3-(4-t-butylpropylamine) (fenpropimorph), a yeast sterol isomerase inhibitor. Because fenpropimorph has high-binding affinity to the sigma-1 receptor, we have synthesized a series of fenpropimorph-like derivatives with varying phenyl ring substituents and have characterized their binding affinities to the sigma-1 receptor. In addition, we have synthesized a carrier-free, radioiodinated fenpropimorph-like photoaffinity label, 1-N-(2',6'-dimethyl-morpholino)-3-(4-azido-3-[(125)I]iodo-phenyl)propane ([(125)I]IAF), which covalently derivatized the sigma-1 receptor (25.3 kDa) in both the rat liver and guinea pig liver membranes and the sigma-2 receptor (18 kDa) in rat liver membranes with high specificity. Furthermore, after cleaving the specific [(125)I]IAF-photolabeled sigma-1 receptor in guinea pig and rat liver membranes and the pure guinea pig sigma-1 receptor with EndoLys-C and cyanogen bromide, the [(125)I]IAF label was identified both in a peptide containing steroid binding domain-like I (SBDLI) (amino acids 91-109) and in a peptide containing steroid binding domain-like II (SBDLII) (amino acids 176-194). Because a single population of binding sites (R(2) = 0.992) for [(125)I]IAF interaction with the sigma-1 receptor was identified by (+)-[(3)H]pentazocine competitive binding with nonradioactive [(127)I]IAF, it was concluded that SBDLI (amino acids 91-109) and SBDLII (amino acids 176-194) comprises, at least in part, regions of the sigma-1 receptor ligand binding site(s).

Design and Evaluation of Naphthol- and Carbazole-Containing Fluorescent σ Ligands as Potential Probes for Receptor Binding Studies

Some 3,3-dimethyl-1-(3-naphthylpropyl)piperidine and 1-cyclohexyl-4-(3-naphthylpropyl)piperazine derivatives, structurally containing naphthol as a fluorescent moiety, were prepared for being potentially used as fluorescent sigma ligands. Structurally related analogs were also prepared, where the naphthalene nucleus was replaced by the fluorescent carbazole moiety and chain length was varied. For all compounds the in vitro affinities toward sigma receptors and Delta8-Delta7 sterol isomerase site were measured, and the fluorescent properties were determined. Compound 19 gave the best results both for sigma receptor affinities (sigma1, Ki = 6.78 nM and sigma2, Ki = 26.4 nM) and fluorescence features; thus, it was chosen for in vitro saturation binding analysis at sigma receptors. The good results obtained in such assay suggested that the fluorescent compound 19 could be used instead of a radioligand in "green" binding assays.

Synthesis and pharmacological evaluation of Tic-hydantoin derivatives as selective sigma1 ligands. Part 2

Herein is described a new class of selective sigma1 ligands consisting of tetrahydroisoquinoline-hydantoin (Tic-hydantoin) derivatives. Compound 1a has high affinity (IC50 = 16 nM) for sigma1 receptor and is selective in a large panel of therapeutic targets. This study presents structural changes on the side chain of the Tic-hydantoin core. Analogs of higher affinity could be identified (IC50 approximately 2-3 nM).

Synthesis and binding affinity of novel 3-aminoethyl-1-tetralones, potential atypical antipsychotics

A series of 3-aminoethyl-1-tetralones, conformationally constrained higher homologues of haloperidol (standard for typical antipsychotic profile), have been obtained by a four-step route from valerolactone. Their binding affinities at dopamine D(2) and serotonin 5-HT2A and 5-HT2C receptors were determined, showing in some cases an atypical antipsychotic profile.

Synthesis and evaluation of 2-[18F]fluoro-CP-118,954 for the in vivo mapping of acetylcholinesterase

5,7-Dihydro-3-[2-[1-(2-fluorobenzyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2,f]-1,2-benzisoxazol-6-one (2-flouro-CP-118,954; 1), a potent acetylcholinesterase (AChE) inhibitor, was prepared as a radioligand by reductive alkylation of CP-144,885 the debenzylated form of CP 118,954, with 2-[18F]fluorobenzaldehyde. The decay-corrected radiochemical yield was 25-30% and the effective specific activity was 41-53 GBq/micromol. Tissue distribution studies of 2-[18F]fluoro-CP-118,954 ([18F]1) in mice showed that the regional brain distribution correlated well with the known density of AChE in the mouse brain. A high level of uptake in the striatum was also shown at all time points in the olfactory tubercle, which is known to have dopaminergic neurons. Blocking studies showed that radioligand uptake in all brain regions was not altered by either the dopamine receptor antagonists or the sigma receptor agonist. On the other hand, radioligand uptake in both the striatum and the olfactory tubercle was significantly blocked (80%) by ligand 1. The low level of bone uptake over time suggested that [18F]1 underwent little in vivo metabolic defluorination. The lack of metabolite formation in the mouse brain indicated that the regional distribution was attributed to [18F]1. These results demonstrated that [18F]1 binds specifically and selectively to AChE in mice and appears to be a suitable radioligand for the in vivo mapping of AChE.

Synthesis and structure-activity relationships of 1-aralkyl-4-benzylpiperidine and 1-aralkyl-4-benzylpiperazine derivatives as potent sigma ligands

In the attempt to define more accurately structure-affinity relationships for sigma(1) and sigma(2) ligands, we synthesized and tested on sigma subtype receptors a series of aralkyl derivatives of 4-benzylpiperidine, in which the effect of modifications on the aralkyl moiety was studied in a systematic way. The affinity of the compounds here described varied to a great extent, with a sigma(2)/sigma(1) selectivity ranging from 0.1 to 9. Thus, to confirm the ability of the piperazine derivative to bind to sigma(1) receptors in a different way than piperidines, we synthesized and tested a series of piperazine compounds; the comparison of their affinity with that of the corresponding piperidines strongly supports the possibility of a different binding mode. While the compounds here described are on the whole selective for sigma vs serotonin 5-HT(1A) and dopamine D(2) receptors, 9aa, 9ba and 9ab possess a remarkable affinity for both sigma and 5-HT(1A) receptors, with K(i) in the nanomolar range, and are selective with respect to D(2) receptors. They displayed also a partial agonist profile in a human 5-HT(1A) [(35)S]GTP gamma S binding assay, suggesting their potential use as atypical antipsychotic agents.

Thioxanthene-derived analogs as sigma(1) receptor ligands

An investigation of the structure-affinity relationships for the binding of thioxanthene-related structures indicates that an intact thioxanthene ring is not required for binding at sigma(1) receptors, and that with the appropriate structural modifications, affinity can be enhanced to the subnanomolar level. Certain of the analogs displayed sigma(1)-fold selectivity for sigma(1) versus sigma(2) receptors.

Synthesis of potent sigma-1 receptor ligands via fragmentation of dextromethorphan

Treatment of dextromethorphan 1 with various alkylating agents followed by base treatment led to Hoffman-type elimination reactions to produce a series of tricyclic derivatives, 6. These derivatives were characterized in vitro as sigma-1 receptor ligands.

4-(Tetralin-1-yl)- and 4-(Naphthalen-1-yl)alkyl Derivatives of 1-Cyclohexylpiperazine as σ Receptor Ligands with Agonist σ2 Activity

Several 1-cyclohexylpiperazine derivatives related to sigma(2) receptor ligand 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (33, K(i) = 0.34 nM) were synthesized and tested in radioligand binding assays, to attempt a structure-affinity relationship study. Intermediate alkyl chain length and methoxyl group position on the tetralin nucleus were varied. A few naphthalene analogues were also prepared. High affinities were found in sigma(2) receptor binding for almost all compounds, some of which displayed K(i) values in subnanomolar range, but low sigma(2)/sigma(1) selectivities were found. The highest sigma(2) affinities were displayed by compounds with an intermediate alkyl chain of three (32 and 43) or five methylenes (39 and 46). Quite high sigma(1) receptor affinity was found for compounds with a four-methylene chain; 36 (K(i) = 0.036 nM) and 45 (K(i) = 0.22 nM) displaying good sigma(1)/sigma(2) selectivity (406- and 139-fold, respectively). Moreover, homologues of compound 33 displayed also satisfactory selectivities over dopamine D(2)-like, serotonin 5-HT(3), and adrenergic alpha(1) receptors. These compounds and a few others were tested in the inhibition of the electrically evoked contractions in guinea pig bladder and were demonstrated to be full sigma(2) agonists. The activity values correlated well to the affinity scale (EC(50) in microM range). 33 and related compounds are proposed as a class of potential antineoplastic and PET diagnosis agents.

Dual probes for the dopamine transporter and sigma1 receptors: novel piperazinyl alkyl-bis(4'-fluorophenyl)amine analogues as potential cocaine-abuse therapeutic agents

Both dopamine uptake inhibitors and sigma(1) receptor antagonists have been implicated as potential pharmacotherapeutics for the treatment of cocaine abuse. While the dopamine uptake inhibitors may share with cocaine neurochemical mechanisms underlying reinforcing properties, sigma(1) antagonists have been shown to attenuate some behavioral actions and toxic side effects associated with cocaine overdose. Rimcazole, a sigma(1) receptor antagonist that binds to the DAT (K(i) = 224 nM), is not behaviorally cocaine-like and attenuates some of the behavioral actions of cocaine. To determine the roles of both DAT and sigma(1) receptors in the behavioral actions of rimcazole, a series of analogues was synthesized. Initial studies identified two analogues (1 and 4) that showed high to moderate affinities for both DAT and sigma(1) receptors and failed to show cocaine-like discriminative stimulus (DS) effects. A second series of bis(4'-fluorophenyl)amine analogues have now been prepared in which the most potent DAT compound, 19 (K(i) = 8.5 nM), was selective over serotonin transporter (SERT/DAT = 94), norepinephrine transporter (NET/DAT = 63), and sigma(1) receptor binding (sigma(1)/DAT = 44). In addition, two other analogues 10 and 17 showed superior selectivity for DAT over SERT (170- and 140-fold, respectively) and DAT over NET (219- and 190-fold, respectively) but were essentially equipotent at DAT and sigma(1) receptors (10; K(i) = 77 and 124 nM, respectively, 17; K(i) = 28 and 13 nM, respectively). CoMFA studies at both DAT and sigma(1) receptors were performed to examine structural requirements for optimal binding at these two targets as well as to assess differences between them. Behavioral evaluation of analogues with varying affinities for both DAT and sigma(1) receptors may provide a novel approach toward designing medications for cocaine abuse.