Σ1 Receptor Affinity Research Articles

Synthesis and biological evaluation of a radioiodinated spiropiperidine ligand as a potential σ1 receptor imaging agent

We report the synthesis and evaluation of 1′-(4-[125I]iodobenzyl)-3H-spiro[isobenzofuran-1,4′-piperidine] ([125I]Spiro-I) as a potential SPECT tracer for imaging of σ1 receptors. [125I]Spiro-I was prepared in 55–65% isolated radiochemical yield, with radiochemical purity of >99%, via iododestannylation of the corresponding tributyltin precursor. In receptor binding studies, Spiro-I displayed low nanomolar affinity for σ1 receptors (σ1: Ki=2.75±0.12 nM; σ2: Ki=340 nM) and high subtype selectivity (σ2/σ1=124). Biodistribution in mice demonstrated relatively high concentration of radioactivity in organs known to contain σ1 receptors, including the lung, kidney, heart, spleen, and brain. Administration of haloperidol 5 min prior to injection of [125I]Spiro-I significantly reduced the concentration of radioactivity in the above-mentioned organs. These findings suggest that the binding of [125I]Spiro-I to σ1 receptors in vivo is specific. Copyright © 2010 John Wiley & Sons, Ltd.

Distribution of sigma receptors in EMT-6 cells: preliminary biological evaluation of PB167 and potential for in-vivo PET

Sigma(1) and sigma(2) receptors have been detected in many tissues and are highly expressed in several tumour cell lines from various tissues. The high level of expression observed for sigma receptors and their involvement in cell proliferation and apoptosis has led to the development of several sigma ligands in order to obtain a molecular probe for in-vivo diagnostic imaging techniques such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT). The EMT-6 cells implanted in mice were a good model for evaluating the proliferation of solid tumours by in-vivo PET. Moreover, we developed the sigma ligand PB167, a cyclohexylpiperazine derivative, previously evaluated for sigma(2) receptor affinity and activity in standard protocols. The related results encouraged us to verify if this compound could be developed as a radiotracer for in-vivo PET in order to visualize sigma(2) receptors expressed in EMT-6 cells when implanted in mice. This perspective was thought to be favourable because PB167 bears a methoxy substituent on the tetraline nucleus, an easy point for (11)C labelling. The aims of this preliminary study were both to assess the relative distribution of sigma(1) and sigma(2) receptors in EMT-6 cells and to verify if PB167 could be developed as a sigma(2) radiotracer for in-vivo PET. The results showed that both sigma(1) and sigma(2) receptors were overexpressed in EMT-6 cells and that the ligand PB167 can be positively considered for radiosynthesis preparation in order to suitably visualize sigma(2) receptors by the in-vivo PET technique and correlate their presence to tumour proliferation.

Synthesis and biological evaluation of conformationally restricted σ1 receptor ligands with 7,9-diazabicyclo[4.2.2]decane scaffold

The key step in the synthesis of the 7,9-diazabicyclo[4.2.2]decane system was a modified Dieckmann condensation of piperazinebutyrate 11, which makes use of trapping the first cyclized intermediate with TMS-Cl. Reduction of the bicyclic ketone 14 with LiBH(4) at -90 °C provided diastereoselectively (>99 : 1) the syn-configured alcohol 15a, which was converted into the final alcohol and ethers 16a-g. The configuration at the 2-position was established by X-ray structure analysis of methyl and ethyl ethers 15b and 15c. In contrast to bicyclic systems with a three-carbon bridge, inversion of the configuration at the 2-position of the alcohol 15a failed to give the inverted alcohol 19a. However, an unselective reduction of the ketone 24 with L-Selectride led to the diastereomeric alcohols 16a and 25a in the ratio 36 : 64. LiAlH(4) reduction of the tosylate 20 and the alkene 18 yielded the diazabicyclo-decane 26 and -decene 27 without further substituents at the four-carbon bridge. The σ(1) and σ(2) receptor affinities were investigated in receptor binding studies with radioligands. All test compounds showed a lower σ(1) affinity than the corresponding bicyclic derivatives with a three-membered bridge. The reduced σ(1) receptor affinity is attributed to the larger four-membered bridge. This hypothesis is supported by the alkene 27, which represents the most potent σ(1) ligand of this series (K(i) = 7.5 nM). In the alkene 27 the size and flexibility of the bridge is considerably reduced by the double bond. The methyl ether 25b and the unsubstituted derivatives 26 and 27 revealed moderate inhibition of the growth of the human tumor cell lines A-427, 5637 and MCF-7. Again, these compounds are less potent than the analogues with a three-membered bridge. The IC(50)-value of the most potent σ(1) ligand 27 against the small cell lung cancer cell line A-427 (IC(50) = 10 μM) should be emphasized, since this cell line is particularly sensitive to homologues with a three-carbon bridge.

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.

Evaluation of Spirocyclic 3-(3-Fluoropropyl)-2-benzofurans as σ1 Receptor Ligands for Neuroimaging with Positron Emission Tomography

A series of various N-substituted 3-(3-fluoropropyl)-3H-spiro[[2]benzofuran-1,4'-piperidines] (7) has been synthesized. In receptor binding studies, the N-benzyl derivative 7a (WMS-1813) revealed extraordinarily high sigma(1) receptor affinity (K(i) = 1.4 nM) and excellent sigma(1)/sigma(2) selectivity (>600 fold). In vitro biotransformation of 7a with rat liver microsomes led to three main metabolites. N-Debenzylation was inhibited by introduction of an N-phenylethyl residue (7 g). The PET tracer [(18)F]7a was synthesized by nucleophilic substitution of the tosylate 13 with K[(18)F]F-K222-carbonate complex. The decay corrected radiochemical yield of [(18)F]7a was 35-48% with a radiochemical purity of >99.5% and a specific activity of 150-238 GBq/micromol. The radiotracer properties were evaluated in female CD-1 mice by organ distribution and ex vivo brain autoradiography. The radiotracer uptake in the brain was fast and sufficient, with values of approximately 4% injected dose per gram. Target specificity of [(18)F]7a was validated in blocking studies by preapplication of haloperidol, and significant reduction in the uptake of radioactivity was observed in the brain and peripheral organs expressing sigma(1) receptors.

Synthesis and Evaluation of Novel Radioiodinated Benzamides for Malignant Melanoma

The imaging potential of a series of [123I]benzamides was studied in mice bearing B16F0 melanoma tumors. Compound [123I]25 exhibited tumor uptake >8 %ID/g at 1 h, while that of [123I]14d and [123I]25 reached a maximum of 9-12 %ID/g at 6 h. Standardized uptake values of [123I]14d were higher than 100 between 24 and 72 h after injection. In haloperidol treated animals, the tumor uptake of [123I]14d was not significantly different to controls, while significant reduction of [123I]25 uptake was observed, supporting that [123I]14d uptake relates to melanin interaction, whereas part of the mechanism of [123I]25 uptake is related to its sigma 1-receptor affinity. Benzamides 14d and 25, which display rapid and high tumor uptake, appear to be promising imaging agents for melanoma detection, while 14d, which displays a long lasting and high melanoma/nontarget ratio, is more suitable for evaluation as a potential radiotherapeutic.

Fluorine-18-Labeled Benzamide Analogues for Imaging the σ2 Receptor Status of Solid Tumors with Positron Emission Tomography

A series of fluorine-containing benzamide analogs was synthesized and evaluated as candidate ligands for positron emission tomography (PET) imaging of the sigma-2 (sigma2) receptor status of solid tumors. Four compounds having a moderate to high affinity for sigma2 receptors and a moderate to low affinity for sigma-1 (sigma1) receptors were radiolabeled with fluorine-18 via displacement of the corresponding mesylate precursor with [18F]fluoride. Biodistribution studies in female Balb/c mice bearing EMT-6 tumor allografts demonstrated that all four F-18-labeled compounds had a high tumor uptake (2.5-3.7% ID/g) and acceptable tumor/normal tissue ratios at 1 and 2 h post-i.v. injection. An analysis of the chemistry and biodistribution data suggested that N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2-(2-[18F]-fluoroethoxy)-5-methylbenzamide ([18F]3c) and N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2-(2-[18F]-fluoroethoxy)-5-iodo-3-methoxybenzamide ([18F]3f) are acceptable compounds for imaging the sigma2 receptor status of solid tumors.

Synthesis and Pharmacological Characterization of Sila-panamesine, a Sila-Analogue of the σ Receptor Ligand Panamesine (EMD 57445)

Sila-substitution (C/Si exchange) at the 4-position of the piperidine skeleton of the σ receptor ligand panamesine (1a, EMD 57445) leads to sila-panamesine (1b). Compounds 1a and 1b were synthesized in multistep syntheses, starting from 1-benzyl-4-piperidone and tetramethoxysilane, respectively, and were isolated as the hydrochlorides 1a·HCl and 1b·HCl. ESI-MS studies of aqueous solutions of the silanol 1b and the corresponding disiloxane 14 at different pH values revealed a remarkable stability of 1b. The identities of 1a·HCl and 1b·HCl were established by elemental analyses and multinuclear NMR studies. The σ1 and σ2 receptor affinities of the C/Si analogues panamesine (1a) and sila-panamesine (1b) were determined in competitive radioligand receptor binding studies. It was shown that the C/Si exchange results in a 3-fold enhancement of σ1 receptor affinity.

Cyclohexylpiperazine derivative PB28, a σ2 agonist and σ1 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.

Clinical effects of pharmacological variations in selective serotonin reuptake inhibitors: an overview

Although the selective serotonin reuptake inhibitor (SSRI) class of antidepressants shares a common primary pharmacology, namely the inhibition of serotonin reuptake, their secondary pharmacology is remarkably heterogeneous. Inhibition of serotonin reuptake and the consequent increase in serotonin availability are responsible for the relief of depressive symptoms and for some of the adverse effects of this class of drugs. Transsynaptic effects such as modulation of signalling cascades, gene expression processes and neuroplasticity are also important in the mechanism of action of antidepressants. However, this review shows that secondary properties of the SSRIs may contribute to the differences in efficacy and tolerability between members of the class. For example, fluvoxamine has affinity for sigma(1)-receptors -- a property likely to be responsible for its particular efficacy in delusional depression. By understanding the properties of SSRIs and employing careful selection of agents for individual patients, physicians are more able to tailor antidepressant treatments to their patients' particular circumstances.

Chiral, nonracemic (piperazin-2-yl)methanol derivatives with $sigma;-receptor affinity

Starting with the proteinogenic amino acid (S)-serine a series of chiral nonracemic (piperazin-2-yl)methanols 3 with various N-4 substituents is described. The key step in the synthesis of 3 is the reaction of the chloroacetamide 5 with various primary amines to yield the diastereomeric bicyclic piperazinediones cis-6 and trans-6. The scope and limitation of this transformation is thoroughly investigated. The σ1- and σ2-receptor affinities of the piperazines 3 are determined in receptor binding studies with guinea pig brain and rat liver membrane preparations using [3H]-labeled (+)-pentazocine and ditolylguanidine, respectively. It was found, that an additional phenyl residue in the N-4 substituent is favorable to high σ1-receptor affinity. In this series the p-methoxybenzyl substituted piperazine 3d reveals the highest σ1-receptor affinity (Ki=12.4 nM) with selectivity toward σ2-, NMDA-, κ-opioid, and μ-opioid receptors.

1-Phenyl-3-azabicyclo[3.1.0]hexane derivatives as new ligands for sigma receptors

A series of 1-phenyl-3-azabicyclo(3.1.0)hexanes were synthesized as more conformationally restricted prototypical σ ligands 3-phenylpiperidines with the aim to developing new σ ligands. Compared with 3-phenylpiperidines reported by Largent et al., binding data showed that conformational restriction was not detrimental for σ receptor affinity. Specifically, except for secondary amine 4, all racemic 1-phenyl-3-azabicyclo(3.1.0)hexane derivatives (12-19) showed moderate to high affinity for both σ1 and σ2 receptors. Dextrorotatory isomers with the same configuration of 3-phenylpiperidines to C-1 carbon linked to the phenyl ring showed a better affinity and selectivity for σ1 receptors compared to the respective levorotatory isomers. Compounds (+)-14 and (+)-15 displayed very high affinity for σ1 (Ki = 0.9 and 2.3 nM respectively) but low selectivity for receptor subtypes. Compound (+)-18 with N-phenethyl substituent embodies the highest selectivity for σ1 receptors.

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.

Conformationally constrained ethylenediamines: synthesis and receptor binding of 6,8-Diazabicyclo[3.2.2]nonanes

The synthesis and receptor affinity of 6,8-diazabicyclo[3.2.2]nonanes representing conformationally constrained ethylenediamines are described. The Dieckmann analogous cyclization of the (piperazin-2-yl)propionate 9 provided the bicyclononane 10 only, when the first cyclization product was trapped with chlorotrimethylsilane. 10 was stereoselectively transformed into the bicyclic amines 19a,b and amides 22a,b, which were investigated in competition experiments with radioligands for their σ1-, σ2-, κ-, and μ-receptor affinities. The (2R)-configured dimethylamine 19a showed promising σ1-receptor affinity (Ki=23.8nM) and selectivity, whereas the (2S)-configured (dichlorophenyl)acetamide 22b displayed a σ-receptor binding profile (σ1: Ki=184nM; σ2: Ki=263nM) very similar to the binding profile of the atypical antipsychotic BMY-14802 (26).

The analgesic tropane analogue (±)-SM 21 has a high affinity for σ2 receptors

The analgesic properties of the tropane analogue (±)-SM 21 have been attributed to the antagonism of presynaptic m2 receptors resulting in a potentiation of acetylcholine release. However, drugs targeting a number of other neurotransmitter receptors have been shown to enhance acetylcholine release. In the current study, in vitro studies were conducted in order to determine the affinity of (±)-SM 21 for serotonin 5-HT 3, 5-HT 4, and sigma receptors. Our results indicate that (±)-SM 21, and its structural congeners, have a relatively high affinity for σ 2 receptors relative to their reported affinity for muscarinic receptors. The higher affinity for σ 2 versus σ 1 receptors indicates that (±)-SM 21 may be a suitable lead compound for developing σ 2-selective ligands.

Ibogaine possesses a selective affinity for σ2 receptors

The alkaloid ibogaine is potentially useful to reduce craving for several drugs of abuse, but its mechanism of action is not known. In the current study, in vitro studies were conducted in order to determine the affinity of ibogaine for sigma receptors. Our results indicate that ibogaine has a relatively high affinity for σ 2 receptors (K i = 90.4 and 250 nM) and a significantly lower affinity for σ 1 receptors (K i = 9310 nM). These data suggest that ibogaine may have a higher affinity at σ 2 receptors than any other known CNS receptor. Its low affinity for σ 1 receptors also suggests that ibogaine may be a suitable lead compound for structure-activity relationship studies aimed at developing σ 2-selective ligands.

CB-64D and CB-184: ligands with high σ2 receptor affinity and subtype selectivity

Four members of a novel class of sigma (σ) ligands were investigated for σ subtype selectivity. (−)-1 S,5 S- and (+)-1 R,5 R-( E)-8-Benzylidene-5-(3-hydroxyphenyl)-2-methylmorphan-7-one (CB-64L and CB-64D, respectively) exhibited σ 1 K 1 = 10.5 nM and 3063 nM; σ 2 K i = 154 nM and 16.5 nM, respectively. The corresponding 3,4-dichloro derivatives, (−)-1 S,5 S- and (+)-1 R,5 R-( E)-8-(3,4-dichlorobenzylidene)-5-(3-hydroxyphenyl)-2-methylmorphan-7-one (CB-182 and CB-184, respectively) were also examined. CB-182 ((−)-isomer) showed σ 1 and σ 2 K i = 27.3 nM and 35.5 nM, respectively, whereas CB-184 ((+)-isomer) exhibited σ 1 and σ 2 K i = 7436 nM and 13.4 nM, respectively. Thus, the two σ subtypes showed opposite enantioselectivity for these compounds, with (−) > (+) at σ 1 and (+) > (−) at σ 2. Importantly, CB-64D and CB-184 showed high σ 2 affinity and, respectively, 185-fold and 554-fold selectivity for σ 2 receptors over σ 1. While high σ 2 selectivity relative to σ 1 was achieved with these compounds, they both exhibited high affinity at mu (μ) opioid receptors ( K i = 37.6 nM and 4.5 nM, respectively). Despite this, CB-64D and CB-184 will be useful tools for further characterization of σ 2 receptors.