Σ1 Ligands Research Articles

Exploring the Importance of Piperazine N-Atoms for σ2 Receptor Affinity and Activity in a Series of Analogs of 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28)

sigma(2)-Agonist 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (7, PB 28), which proved to revert doxorubicin resistance in breast cancer cells, was taken as a template to prepare new analogs. One of the two basic N-atoms was alternatively replaced by a methine or converted into an amide or ammonium function, with the aim of finding out which of them was essential for sigma(2) receptor affinity and activity. Some simply 4-substituted 1-cyclohexylpiperazines were also investigated. None of the compounds was as high-affinity as 7 (sigma(2)K(i) = 0.68, sigma(1)K(i) = 0.38 nM), proving that both basic N-atoms ensure better sigma(2) receptor binding. Amide 36 emerged as high-affinity (K(i) = 0.11 nM) and noteworthy() selective (1627-fold) sigma(1) ligand. Small N-cyclohexylpiperazine 59 displayed the highest sigma(2) affinity (K(i) = 4.70 nM). The sigma(2)/sigma(1) selectivities were generally low. Antiproliferative assay in SK-N-SH cells revealed piperidines 24 and 15 as putative sigma(2) agonists (EC(50)s 1.40 and 3.64 muM respectively) more potent than 7.

Selective Sigma-1 (σ1) Receptor Antagonists: Emerging Target for the Treatment of Neuropathic Pain

A large number of therapeutic roles have been proposed for sigma(1) receptors but the involvement of sigma(1) receptor in non-acute pain had not been well explored up to now. sigma(1) receptor knock-out mice became available offering us the possibility to study the role of sigma(1) receptor in nociception, particularly in models where central sensitization processes play a significant role. Given the attractive therapeutic potential, we have developed a chemical program aimed at the discovery of novel and selective sigma(1) ligands. Herein we discuss the rational basis of this approach and report preliminary pharmacological results of several chemical series and aspects of their structure-activity relationship on sigma(1) receptor. Functional data in pain models are presented mainly on one series that provide evidence to consider selective sigma(1) receptor antagonists an innovative and alternative approach for treating neuropathic pain.

A Structure-Affinity and Comparative Molecular Field Analysis of Sigma-2 (σ2) Receptor Ligands

Several sigma(1) receptor ligands with sub-nanomolar affinity and excellent selectivity have been reported, but relatively few sigma(2)-selective ligands are known. 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl] piperazine (PB28; 1) has been reported by us as a high-affinity sigma(2) receptor ligand with significant sigma(2) selectivity, and several analogs of (1) now have been developed. Among these are the class of cyclohexylpiperazines that display a good compromise between affinity/activity and selectivity for sigma(2) receptors. Very little is currently known about the nature of sigma(2) receptors. In the absence of structure-based receptor information, we applied a comparative molecular field analysis (CoMFA) - a three-dimensional structure-activity relationship (3D-QSAR) method - to a set of cyclohexylpiperazine sigma(2) ligands to develop a predictive model that might provide information about the stereoelectronic nature of the receptor binding site. Two CoMFA models were generated from two different alignments: the first used an automated FlexS algorithm, and the second used a rationally-driven manual alignment. Significantly better predictivity was obtained with the manual alignment (TSET: q(2) = 0.73, r(2) = 0.95; PSET: r(2) = 0.55/0.73) than from the automated alignment (TSET: q(2) = 0.69, r(2) = 0.98; PSET: r(2) = 0.13/0.16). The resulting CoMFA maps account for observed structure-affinity relationships and suggest a possible anatomy for the sigma(2) receptor/cyclohexylpiperazine binding site.

Synthesis, Biological Evaluation, and Three-Dimensional in Silico Pharmacophore Model for σ1 Receptor Ligands Based on a Series of Substituted Benzo[d]oxazol-2(3H)-one Derivatives

Novel benzo[d]oxazol-2(3H)-one derivatives were designed and synthesized, and their affinities against sigma receptors were evaluated. On the basis of 31 compounds, a three-dimensional pharmacophore model for the sigma(1) receptor binding site was developed using the Catalyst 4.9 software package. The best 3D pharmacophore hypothesis, consisting of one positive ionizable, one hydrogen bond acceptor, two hydrophobic aromatic, and one hydrophobic features provided a 3D-QSAR model with a correlation coefficient of 0.89. The best hypothesis was also validated by three independent methods, i.e., the Fisher randomization test included in the CatScramble functionality of Catalyst, the leave-one-out test, and activity prediction of an additional test set. The achieved results will allow researchers to use this 3D pharmacophore model for the design and synthesis of a second generation of high affinity sigma(1) ligands, as well as to discover other lead compounds for this class of receptors.

Targeted delivery to tumors in vivo with sigma‐2 ligands

Radiolabeled sigma‐2 (σ2) receptor‐specific ligands image pancreas adenocarcinoma allografts in vivo and fluorescently labeled σ2 ligands are rapidly internalized and distributed throughout membrane‐encapsulated organelles. Based on these data, we hypothesized that σ2 ligands could target the delivery of therapeutic compounds to tumors in vivo. To test this hypothesis we synthesized a novel amino acid with a σ2 ligand side chain. This amino acid was incorporated as the N‐terminal residue in a series of pro‐apoptotic peptides without appreciable alteration of the Kd for the σ2 receptor (~10–20 nM). One of these peptides is the BH3‐domain of the pro‐apoptotic Bcl‐2 family member Bim. ~2‐BH3 causes dose‐dependent apoptosis to pancreas adenocarcinoma cells in vitro and is approximately the sum of the apoptotic activity of the σ2 ligand and the TAT‐BH3 peptide (Figure). When administered to tumor bearing mice this chimeric molecule is capable of halting tumor growth without appreciable effects on organ function or blood chemistry. Seven day treatment with this peptide increased median time to death in these mice from 33 days to 45 days (P < 0.0001). These data support the hypothesis that σ2 ligands target the delivery of cancer therapy in vivo and demonstrate a platform approach to delivering peptides selectively to cancer cells in the immunocompetent host.

Design, synthesis, and SAR analysis of novel selective σ1 ligands

A new series of arylalkyl- and alkenylamines was designed, synthesized, and evaluated for binding to σ 1 and σ 2 receptors. Many compounds exhibited nanomolar affinity for σ 1 subtype receptor with good selectivity over σ 2. A molecular modeling study was conducted in order to rationalize the experimental data, and the structure-receptor affinities are discussed.

Is the σ2 Receptor a Histone Binding Protein?

Starting from the high affinity sigma(2) receptor ligand 2, (PB28), we synthesized amino derivative 4 and coupled it to an NHS-ester activated sepharose stationary phase column to elute a crude protein prepared by lysed human SK-N-SH neuroblastoma cells. We characterized the SDS-PAGE gel electrophoresis stained bands by MALDI-MS and LC-MS-MS analysis. The MASCOT MS-MS ion search program led to the identification of the protein components. The six eluted proteins had a molecular weight ranging from 13 kDa to 26 kDa and were human histone proteins. A human 40S ribosomal protein S3 (SwissProt accession number: P23396) was also identified as a comigrated band. The human histone proteins that were characterized were H3.3A histone (NCBI accession number: 51859376), H2B histone (NCBI accession number: 1568557), H2A.5 histone (NCBI accession number: 70686), H1 (NCBI accession number: 22770677), and H2.1 histone (SwissProt accession number: P16403). These results disclosed a dual hypothesis about the sigma(2) receptor, that is, that it is formed by histones or that the sigma(2) ligands also bind histone proteins.

Molecular modeling of σ1 receptor ligands: a model of binding conformational and electrostatic considerations

We have performed molecular modeling studies on several σ1 specific ligands, including PD144418, spipethiane, haloperidol, pentazocine, and others to develop a pharmacophore for σ1 receptor–ligand binding, under the assumption that all the compounds interact at the same receptor binding site. The modeling studies have investigated the conformational and electrostatic properties of the ligands. Superposition of active molecules gave the coordinates of the hypothetical 5-point σ1 pharmacophore, as follows: R1 (0.85, 7.26, 0.30); R2 (5.47, 2.40, −1.51); R3 (−2.57, 4.82, −7.10); N (−0.71, 3.29, −6.40); carbon centroid (3.16, 4.83, −0.60), where R1, R2 were constructed onto the aromatic ring of each compound to represent hydrophobic interactions with the receptor; and R3 represents a hydrogen bond between the nitrogen atom and the receptor. Additional analyses were used to describe secondary binding sites to electronegative groups such as oxygen or sulfur atom. Those coordinates are (2.34, 5.08, −4.18). The model was verified by fitting other σ1 receptor ligands. This model may be used to search conformational databases for other possibly active ligands. In conjunction with rational drug design techniques the model may be useful in design and synthesis of novel σ1 ligands of high selectivity and potency. Calculations were performed using Sybyl 6.5.

Spiro[1,2,4-benzotriazine-3(4H),4′-(1′-substituted)piperidines] and related compounds as ligands for sigma receptors

As analogues of some conformationally restricted spiropiperidine derivatives which are endowed with high affinity for σ1 receptor, a set of 16 spiro[1,2,4-benzotriazine-3(4H),4′-(1′-substituted)piperidines] and congeneric compounds was prepared and tested for affinity to σ1 receptor subtype. All N-arylalkyl substituted derivatives exhibited high affinity for the relevant receptor, with Ki in the low nanomolar range. Affinity for σ2 subtype (assayed only for a few representative compounds) was from one to three order of magnitude lower. Spiro[1,2,4-benzotriazine-3(4H),4′-(1′-benzyl)piperidine] (2), with a ratio Kiσ2/Kiσ1=7000 should represent the most selective σ1 ligand so far described.

Some of the effects of the selective sigma ligand (+)pentazocine are mediated via a naloxone‐sensitive receptor

Recently, in an attempt to isolate the nonopioid σ receptor, Su and colleagues purified a protein from rat liver and brain which appeared to resemble the original σ opioid receptor as proposed by Martin in 1976, and for which the nonopiate σ1 ligand (+)pentazocine presents a high affinity. Previous in vivo electrophysiological studies from our laboratory have demonstrated that several selective σ1 ligands potentiate the neuronal response to NMDA. The goal of the present series of experiments was to assess the effects of some selective σ1 ligands on the potentiation of the NMDA response and to determine if this potentiation was mediated by the naloxone-sensitive σ receptor. Extracellular unitary recordings from pyramidal neurons of the CA3 region of the rat dorsal hippocampus were obtained. The σ1 ligands BD 737, L 687-384, and JO-1784 (igmesine), administered intravenously at low doses, potentiated the NMDA response but the opiate antagonist naloxone failed to reverse this potentiation. However, the potentiation of the NMDA response induced by the σ1 ligand (+)pentazocine was suppressed by naloxone but not by the μ antagonist cyprodime hydrobomide, the κ antagonist DIPPA nor by the δ antagonist naltrindole. (±) Cyclazocine, which presents a high affinity for the above-mentioned σ-opiate receptor acted as an antagonist by suppressing the potentiation of the NMDA response induced by both JO-1784 and (+)pentazocine. These results suggest that the effects induced by some σ1 ligands may, in fact, be sensitive to naloxone while others may not. The original classification of σ receptors as opiates might have been partly accurate. Synapse 39:323–331, 2001. © 2001 Wiley-Liss, Inc.

Modulation of the neuronal response to N‐Methyl‐D‐Aspartate by selective sigma2 ligands

It has now been accepted for several years that sigma (σ) receptors exist in, at least, two distinct entities denoted σ1 and σ2. Previous electrophysiological studies from our laboratory have demonstrated that several selective σ1 ligands potentiate the neuronal response to NMDA. The nonselective σ1/σ2 ligand DTG also potentiates the NMDA response. However, when DTG is administered at doses between 3 and 40 μg/kg, the increase of NMDA-induced activation turns to an epileptoid activity. Until recently, the physiological role of σ2 receptors had been less studied due to the lack of selective σ2 ligands. The goal of the present electrophysiological studies was to assess the effect of the intravenous administration of new selective σ2 ligands on the neuronal response to NMDA in the CA3 region of the rat dorsal hippocampus. Lu 28-179 and BD 1008 potentiated dose-dependently the NMDA response and generated bell-shaped dose-response curves. These ligands failed to generate any epileptoid activity on their own but the subsequent administration of a low dose of a σ1 agonist (JO-1784) induced an epileptoid activity. Interestingly, the potentiations of the NMDA response induced by Lu 28-179 or BD 1008 were not reversed by haloperidol, by the neurosteroid progesterone, nor by the selective σ1 antagonist NE-100. Ibogaine, a high affinity σ2 ligand, slightly increases the NMDA response, which was reversed by progesterone. These data suggest that, similarly to σ1 ligands, σ2 agonists potentiate the NMDA response and that the coactivation of σ1 and σ2 receptors could be necessary to induce an epileptoid activity. They also suggest that haloperidol may not act as a σ2 antagonist and that several subtypes of σ2 receptors could exist. Synapse 29:62–71, 1998. © 1998 Wiley-Liss, Inc.

Dehydroepiandrosterone sulfate attenuates dizocilpine-induced learning impairment in mice via σ1-receptors

We previously reported that high-affinity sigma type 1 (σ1) ligands attenuate the learning impairment induced in mice by dizocilpine, a non-competitive N-methyl-d-aspartate (NMDA) antagonist. Neurosteroids, such as pregnenolone sulfate, progesterone and dehydroepiandrosterone sulfate (DHEAS), modulate NMDA-evoked responses in the central nervous system. Furthermore, some of them were reported to interact with a-receptors. This study was carried out to investigate whether DHEAS, a neurosteroid with memory-enhancing effects, attenuates the dizocilpine-induced learning impairment in mice, and, if so, by a mechanism involving α1-receptors. Learning was evaluated using spontaneous alternation in the Y-maze for spatial working memory and step-down type of passive avoidance for long-term memory. At doses about 10–20 mg/kg s.c., DHEAS significantly attenuated dizocilpine (0.15 mg/kg i.p.)-induced impairment of learning on both tests. The enhancing effect of DHEAS (20 mg/kg s.c.) was antagonized by co-administration of the σ-antagonist BMY-14802 (5 mg/kg i.p.) and suppressed by a subchronic treatment with haloperidol (4 mg/kg/day s.c. for 7 days). These results indicate that DHEAS attenuates dizocilpine-induced learning impairment via an interaction with a,-receptors.

ChemInform Abstract: Novel 1‐Phenylcycloalkanecarboxylic Acid Derivatives are Potent and Selective σ1 Ligands.

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