Re-evaluation of in vivo selectivity of [11C]SA4503 to σ1 receptors in the brain: Contributions of emopamil binding protein
Introduction
The σ1 receptors, first regarded as one of the opioid receptor subtypes and later confused as phencyclidine binding sites of the N-methyl-d-aspartate receptor, have now been confirmed to be independent receptors [1]. Recently, σ1 receptors have been shown to act as ligand-regulated molecular chaperones in the endoplasmic reticulum (ER) [2]. They are involved in the modulation of various neurotransmitter systems, particularly the cholinergic [3], [4] and glutamatergic pathways [5]. The σ1 receptor was successfully cloned and characterized as a protein of 233 amino acids with two transmembrane domains [6]. Although the σ1 receptor is not homologous to any mammalian proteins, it shows 30% identity with yeast Δ8-Δ7 sterol isomerase [6]. The structural homology of mammalian cloned σ1 receptor with yeast Δ8-Δ7 sterol isomerase suggested that σ1 receptor belongs to the family of sterol isomerases [7]. Although endogenous ligands for σ1 receptor were proposed to be neurosteroids, no enzymatic activity has been demonstrated for σ1 receptor [8].
In vertebrates, emopamil binding protein (EBP) catalyzes the same Δ8-Δ7 sterol isomerization and shows a strikingly similar pharmacological profile and molecular mass to yeast Δ8-Δ7 sterol isomerase, but their structures are unrelated [9], [10]. Interestingly, several structural classes of σ receptor ligand bind to EBP [11], [12], [13]. Therefore, EBP is also thought to belong to the σ receptor family on the basis of its pharmacological, but not structural, homology with the mammalian σ1 receptor.
We have found carbon-11-labeled 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine ([11C]SA4503) to be a promising PET ligand for mapping σ1 receptors, and applied it to human subjects. Using PET with [11C]SA4503, we demonstrated that [11C]SA4503 is useful for evaluation of σ1 receptor occupancy by several therapeutic drugs in the living human brain [14], [15], [16]. In addition, a widespread decrease in [11C]SA4503 binding in patients with Alzheimer's disease [17] and a significant decrease in binding on the more affected side of the anterior putamen in patients with Parkinson's disease [18] have been reported. Hence, [11C]SA4503-PET is useful for studying the pathophysiology of neurological and psychiatric disorders and for evaluation of the pharmacodynamics of psychiatric drugs. However, [11C]SA4503 has high affinity not only to σ1 receptors (Ki between 4 and 14 nM; [19], [20], [21]) but also to the above-mentioned EBP with Ki of 1.7 nM [11] and to the vesicular acetylcholine transporter (VAChT, Ki = 50 nM; [21]), which may affect the interpretation of neuroimaging findings. A rodent study confirmed that [11C]SA4503 shows negligible binding to VAChT sites in vivo [22]. To our knowledge, no information is available about the possibility of [11C]SA4503 binding to EBP in the brain in vivo. In the present study, to further confirm the selectivity of [11C]SA4503, we performed an in vivo blocking experiment using high-affinity EBP and σ1 blockers. The affinities of blockers used for EBP and σ1 receptor are summarized in Table 1.
Section snippets
General
SA4503 was synthesized by the method reported previously [25]. Carbon-11-labeled SA4503 ([11C]SA4503) was prepared at the Tokyo Metropolitan Institute of Gerontology as described previously [26]. The chemical structures of blockers tested are shown in Fig. 1. These compounds were: 4-(2-(4-benzylpiperidin-1-yl)-1-hydroxypropyl)phenol (ifenprodil), 4-(4-(4-chlorophenyl)-4-hydroxy-1-piperidyl)-1-(4-fluorophenyl)butan-1-one (haloperidol),
Results
In vitro σ1 receptor and EBP binding affinities and in vivo IC50 values of test compounds are summarized in Table 1. Dose-dependent effects of each blocker on [11C]SA4503 uptake in the brain and blood and the brain-to-blood ratio are shown in Fig. 2. The brain uptake of [11C]SA4503 was dose-dependently decreased by SA4503 and the high-affinity σ1 blockers haloperidol, ifenprodil, and trifluperidol (Fig. 2A). The SA4503 and σ1-selective blockers haloperidol (σ1/EBP = 0.006) and trifluperidol (σ1
Discussion
In this study, we assessed the in vivo blocking effects of selective σ1 receptor and EBP blockers on the brain uptake of [11C]SA4503 by the tissue dissection method at a single time point. The results indicated that the contribution of in vivo EBP binding of [11C]SA4503 was negligible in the mouse brain. The in vivo IC50 values of σ1 receptor-selective blockers tended to correlate with the in vitro Ki values toward σ1 receptors. From these results, we confirmed the σ1 receptor-selective binding
Acknowledgments
This work was supported by Grants-in Aid for Scientific Research (B) No. 22390241 and Challenging Exploratory Research No. 23659605 from the Japan Society for the Promotion of Science (to Jun Toyohara). The authors thank Mr. Kunpei Hayshi for technical assistance.
References (34)
- et al.
Sigma-1 receptor chaperons at the ER-mitochondrion interface regulate Ca2+ signaling and cell survival
Cell
(2007) - et al.
Neurosteroids enhance spontaneous glutamate release in hippocampal neurons. Possible role of metabotropic σ1-like receptors
J Biol Chem
(2002) - et al.
The mysteries of sigma receptors: new family members reveal a role in cholesterol synthesis
Trends Pharmacol Sci
(1997) - et al.
Phenylalkylamine Ca2+ antagonist binding protein
J Biol Chem
(1995) - et al.
Emopamil-binding protein, a mammalian protein that binds a series of structurally diverse neuroprotective agents, exhibits Δ8-Δ7 sterol isomerase activity in yeast
J Biol Chem
(1996) - et al.
A multireceptorial binding reinvestigation on an extended class of σ ligands: N-[ω-(indan-1-yl and tetralin-1-yl)alkyl] derivatives of 3,3-dimethylpiperidine reveal high affinities towards σ1 and EBP sites
Bioorg Med Chem
(2001) - et al.
High occupancy of sigma-1 receptors in the human brain after single oral administration of fluvoxamine: a positron emission tomography study using [11C]SA4503
Biol Psychiatry
(2007) - et al.
Binding properties of SA4503, a novel and selective σ1 receptor agonist
Eur J Pharmacol
(1996) - et al.
Evaluation of (+)-p-[11C]methylvesamicol for mapping sigma1 receptors: a comparison with [11C]SA4503
Nucl Med Biol
(2006) - et al.
Synthesis, structure and quantitative structure-activity relationships of σ receptor ligands, 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazines
Bioorg Med Chem
(1997)
The application of receptor theory to receptor-binding and enzyme-binding oncologic radiopharmaceuticals
Nucl Med Biol
In vivo evaluation of [11C]SA4503 as a PET ligand for mapping CNS sigam1 receptors
Nucl Med Biol
Mapping of human cerebral sigam1 receptors using positron emission tomography and [11C]SA4503
Neuroimage
The role of sigma-1 receptors in the pathophysiology of neuropsychiatric diseases
J Recept Ligand Channel Res
Effect of SA4503 on the electrically evoked release of 3H-acetylcholine from striatal and hippocampal rat brain slices
Synapse
Enhancement of acetylcholine release by SA4503, a novel sigma 1 receptor agonist, in the brain
J Pharmacol Exp Ther
Purification, molecular cloning, and expression of the mammalian sigma1-binding site
Proc Natl Acad Sci USA
Cited by (13)
Chiral resolution of serial potent and selective σ<inf>1</inf>ligands and biological evaluation of (−)-[<sup>18</sup>F]TZ3108 in rodent and the nonhuman primate brain
2017, Bioorganic and Medicinal ChemistryCitation Excerpt :Although SA4503 was reported to have high binding affinity for emopamil binding protein (EBP) (Ki = 1.7 nM)17 and moderate binding affinity for vesicular acetylcholine transporter (VAChT) (Ki = 50 nM),18 subsequent in vivo study in mice suggested the contribution of EBP binding of [11C]SA4503 was negligible in the brain, probably due to a low EBP density. Blocking experiments using four σ1 specific agents and two EBP specific agents confirmed the σ1 selective binding of [11C]SA4503 in the brain.19 [ 11C]SA4503 displayed less than 100-fold selectivity over both σ2 receptor and VAChT (55-fold for σ2 and 11-fold for VAChT, respectively).20
(R)-[<sup>11</sup>C]Emopamil as a novel tracer for imaging enhanced P-glycoprotein function
2016, Nuclear Medicine and BiologyCitation Excerpt :The densities of L-type calcium channels and 5-HT2 receptors in the rodent brain were reported as 3.8 nM [31] and 1.1 nM [32], respectively. The estimated density of EBP in the rodent brain was 4.7 nM [33]. According to these analyses, Bmax/KD values of each molecule were estimated to be < 1.
Potential applications for sigma receptor ligands in cancer diagnosis and therapy
2015, Biochimica et Biophysica Acta - BiomembranesCitation Excerpt :The brain uptake of [11C]SA4503 was dose-dependently decreased by sigma-1 receptor ligands, but not by inhibitors of the emopamil binding protein. Thus, [11C]SA4503 shows sigma-1 selective binding in the living brain [49]. In an attempt to develop novel probes for sigma-2 receptors, Abate et al. prepared benzamides which combined moieties of 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28) N-[4-(3,4-Dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl]-2-methoxy-5-methylbenzamide (RHM-1) in a single molecule.
Sigma (σ) receptors as potential therapeutic targets to mitigate psychostimulant effects
2014, Advances in PharmacologyCitation Excerpt :Thus far, the only one that has been utilized in humans in both normal and CNS disease states is [11C]SA4503 (Toyohara, Sakata, & Ishiwata, 2009). [ 11C]SA4503 has high affinity and selectivity for σ1 receptors and is suitable for brain imaging (Toyohara, Sakata, & Ishiwata, 2012). When used in humans, it has shown decreased levels of σ1 receptors in the putamen of patients with Parkinson's disease (Mishina et al., 2005).
Synthesis and biological evaluation of <sup>18</sup>F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging
2013, Bioorganic and Medicinal ChemistryCitation Excerpt :But [11C]SA4503 did not seem to bind to VAChT in the rat brain in vivo.19 Recently, Toyohara et al. confirmed that the brain uptake of [11C]SA4503 in mice was not blocked by high affinity EBP blockers tamoxifen (EBP Ki = 2.8 nM, Kiσ1/KiEBP = 12) and trifluoperazine (EBP Ki = 3.9 nM, Kiσ1/KiEBP = 52).20 The newly developed compound [18F]fluspidine showed weak affinity for VAChT (Ki = 1400 nM) and EBP (Ki = 211 nM).21
The emerging role of the sigma-1 receptor in autophagy: hand-in-hand targets for the treatment of Alzheimer’s
2021, Expert Opinion on Therapeutic Targets