Evaluation of (+)-p-[11C]methylvesamicol for mapping sigma1 receptors: a comparison with [11C]SA4503

doi:10.1016/j.nucmedbio.2006.01.008

Nuclear Medicine and Biology

Volume 33, Issue 4, May 2006, Pages 543-548

https://doi.org/10.1016/j.nucmedbio.2006.01.008 Get rights and content

Abstract

Vesamicol is a leading compound for positron emission tomography (PET) and single photon emission computed tomography (SPECT) tracers for mapping the vesicular acetylcholine transporter (VAChT). Recently, we found that (+)-p-methylvesamicol ((+)-PMV) has low affinity for VAChT (K_i=199 nM), but has moderate to high affinity for sigma receptors: K_i=3.0 nM for sigma₁ and K_i=40.7 nM for sigma₂, and that sigma₁-selective SA4503 (K_i=4.4 nM for sigma₁ and K_i=242 nM for sigma₂) has moderate affinity for VAChT (K_i=50.2 nM). In the present study, we examined the potential of (+)-[¹¹C]PMV as a PET radioligand for mapping sigma₁ receptors as compared with [¹¹C]SA4503. In rat brain, similar regional distribution patterns of (+)-[¹¹C]PMV and [¹¹C]SA4503 were shown by tissue dissection and by ex vivo autoradiography. Blocking experiments using (±)-PMV, (−)-vesamicol, SA4503, haloperidol and (±)-pentazocine showed that the two tracers specifically bound to sigma₁ receptors, and that [¹¹C]SA4503 exhibited greater specific binding than (+)-[¹¹C]PMV. No sign of VAChT-specific binding by [¹¹C]SA4503 was observed in the striatum, which is rich in VAChT sites. In conclusion, (+)-[¹¹C]PMV specifically bound to sigma₁ receptors in the brain, but to a lesser extent than [¹¹C]SA4503, suggesting that (+)-[¹¹C]PMV is a less preferable PET ligand than [¹¹C]SA4503. On the other hand, the moderate affinity of [¹¹C]SA4503 for VAChT is negligible in vivo.

Introduction

The sigma receptors in the central nervous system are implicated in psychoses and movement disorders, with at least two subtypes, each having a different tissue distribution recognized to exist [1], [2]. Furthermore, both the sigma₁ and sigma₂ receptor subtypes are also expressed in a variety of tumors such as neuroblastoma, melanoma and breast cancer [3], [4]. Recent progress in the study of tumors has demonstrated that sigma₂ receptors are a potential biomarker of the proliferation of tumors [5], [6], [7]. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) can now be used as a diagnostic tool for understanding the pathophysiological functions of sigma receptors in the neurological and psychiatric diseases and for imaging malignant tumors. Therefore, many radioligands have been proposed for these in vivo imaging techniques [8].

2-(4-Phenylpiperidino)cyclohexanol (vesamicol) was found to block the release of acetylcholine from cholinergic nerve terminals [9] and has been used as a leading compound for PET and SPECT tracers for mapping vesicular acetylcholine transporter (VAChT) sites [10], [11]. Structural modifications and the optical isomerization of vesamicol analogues, however, lead to moderate to high affinity for sigma₁ and sigma₂ receptors, resulting in poor selectivity for VAChTs over sigma receptors [12], [13]. While searching for selective SPECT and PET ligands for VAChT, Shiba et al. [13] found that (+)-2-[4-(4-methyl)phenylpiperidino]cyclohexanol ((+)-p-methylvesamicol, (+)-PMV) has high affinity (K_i=3.0 nM) for the sigma₁ receptor and very low affinity for VAChT (K_i=199 nM) (Table 1), suggesting that the ¹¹C-labeled analog is a candidate PET tracer for mapping sigma₁ receptors. The in vitro data will be described elsewhere. For the mapping of sigma₁ receptors in the human brain by PET, we have clinically used [¹¹C]1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine ([¹¹C]SA4503) at the Tokyo Metropolitan Institute for Gerontology [14], [15], [16], [17]. SA4503 has little or no affinity for 36 other receptors, ion channels or second messenger systems [18], [19]; however, Shiba et al. found that SA4503 exhibited moderate affinity for VAChT (K_i=50.2 nM) (Table 1).

In the present study, we prepared [¹¹C](+)-PMV by substituting a tributylstannyl group with [¹¹C]methyl iodide in a palladium-promoted cross-coupling reaction (Fig. 1) [20] and evaluated its potential as a PET radioligand for mapping sigma₁ receptors as compared with [¹¹C]SA4503. We also investigated the possibility of the binding of [¹¹C]SA4503 to VAChT sites.

Section snippets

General

(−)-Vesamicol and haloperidol were purchased from Sigma-RBI (Natick, MA, USA). (±)-Pentazocine injection (30 mg/ml) and ascorbic acid injection (250 mg/ml) for human use were purchased from Sankyo (Tokyo, Japan) and Nipro Pharma (Osaka, Japan), respectively. SA4503 was prepared by M's Science (Kobe, Japan). (±)-PMV and (+)-2-[4-(4-tributylstannyl)phenylpiperidino]cyclohexanol [(+)-p-tributylstannylvesamicol] were synthesized by the methods to be described elsewhere. All other chemicals and

Tissue distribution of (+)-[¹¹C]PMV and [¹¹C]SA4503

The tissue distribution of the radioactivity after injection of (+)-[¹¹C]PMV into rats is summarized in Table 2. The clearance of the radioactivity from blood was very rapid. The lung showed a high initial uptake followed by a rapid decrease. The mean levels of radioactivity in the heart, kidney and muscle gradually decreased. The levels in the liver, pancreas, spleen and small intestine gradually increased.

As shown in Fig. 2, the two tracers exhibited a similar pattern of distribution in the

Discussion

(+)-[¹¹C]PMV and [¹¹C]SA4503 showed a similar regional distribution pattern in the rat brain (Fig. 2, Fig. 4). Blocking experiments supported that the uptake of each tracer reflects specific binding to sigma₁ receptors, and [¹¹C]SA4503 bound more specifically than (+)-[¹¹C]PMV. The small specific binding ratio of (+)-[¹¹C]PMV was not due to the labeled metabolites. The blocking effects of four compounds were slightly different. SA4503 is more effective in the blocking study, because it

Acknowledgments

This work was partially supported by Grant-in-aid for Scientific Research (B) No. 13557077 from the Japan Society for the Promotion of Science.

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Fluvoxamine reduced [11C]SA4503 binding in a dose-dependent manner and produced >50% receptor occupancy at the therapeutic doses (50–200 mg), suggesting that σ1 receptors may play an important role in the antidepressant activity of fluvoxamine. Although it has been used in humans, [11C]SA4503 is not an ideal radiotracer, as it was later found to display significant affinity for the σ2 receptor (Ki = 51 nM) [135] and vesicular acetylcholine transporters (Ki = 50 nM) [136]. Further, the use of [11C]SA4503 needs an on-site cyclotron due to the short half-life of 11C (t1/2 = 20 min), and its kinetics is relatively slow in the brain of non-human primates [132] and humans [126] for a 11C-labeled radiotracer.
The sigma-1 (σ₁) receptor is a chaperone protein located on the mitochondria-associated membrane of the endoplasmic reticulum, while the sigma-2 receptor (σ₂) is an endoplasmic reticulum-resident membrane protein. Recent evidence indicates that both of these receptors figure prominently in the pathophysiology of Alzheimer’s disease (AD) and thus are targets for the development of novel, disease-modifying therapeutic strategies. Radioligand-based molecular imaging technique such as positron emission tomography (PET) imaging is a powerful tool for the investigation of protein target expression and function in living subjects. In this review, we survey the development of PET radioligands for the σ₁ or σ₂ receptors and assess their potential for human imaging applications. The availability of PET imaging with σ₁ or σ₂ receptor-specific radioligands in humans will allow the investigation of these receptors in vivo and lead to further understanding of their respective roles in AD pathogenesis and progression. Moreover, PET imaging can be used in target occupancy studies to assess target engagement and correlate receptor occupancy and therapeutic response of σ₁ receptor agonists and σ₂ receptor antagonists currently in clinical trials. It is expected that neuroimaging of σ₁ and σ₂ receptors in the brain will shed new light on AD pathophysiology and may provide us with new biomarkers for diagnosis of AD and efficacy monitoring of emerging AD therapeutic strategies.
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Citation Excerpt :
We synthesized compound 1 possessing a hydroxyl group at the para position in the benzene ring of the vesamicol structure and introduced a bromine molecule at the next position of the hydroxyl group in the benzene ring (Fig. 2). In competitive binding assays of sigma receptors, the compounds with halogen molecule or methyl group at the para position of the benzene ring in (+)-vesamicol, (+)-pIV, (+)-pBrV, and (+)-para-methylvesamicol had a higher affinity for sigma-1 receptor than (+)-vesamicol as a starting compound [19,23,38]. However, the introduction of a hydroxyl group [(+)-Ves-OH, (+)-(1)] reduced the affinity for sigma-1 receptor against (+)-vesamicol, whereas the additional introduction of bromine at the next position of the hydroxyl group at the benzene ring [(+)-BrV-OH, (+)-(2)] recovered the affinity (Table 1).
Sigma-1 receptor is a target for tumor imaging. In a previous study, we synthesized a vesamicol analog, (+)-2-[4-(4-bromophenyl)piperidino]cyclohexanol [(+)-pBrV], with a high affinity for sigma-1 receptor, and synthesized radiobrominated (+)-pBrV. This radiobrominated (+)-pBrV showed high tumor uptake in tumor-bearing mice; however, radioactivity accumulation in normal tissues, such as the liver, was high. We assumed that the accumulation of (+)-pBrV in the non-target tissues was partially derived from its high lipophilicity; therefore, we synthesized and evaluated (+)-4-[1-(2-hydroxycyclohexyl)piperidine-4-yl]-2-bromophenol [(+)-BrV-OH], which is a more hydrophilic compound. Although we aimed to develop a PET tracer using ⁷⁶Br, in these initial studies, we used ⁷⁷Br because of its longer half-life.
(+)-[⁷⁷Br]BrV-OH was synthesized using the chloramine-T method with a radiochemical purity of 95%. Lipophilicity and affinity for sigma-1 receptor of (+)-[⁷⁷Br]BrV-OH were determined, and biodistribution experiments were performed. We also performed an in vivo blocking study by co-injecting excess amounts of the sigma-1 receptor ligand, SA4503, into mice.
The lipophilicity and affinity for sigma-1 receptor of (+)-[⁷⁷Br]BrV-OH were lower than those of (+)-[⁷⁷Br]pBrV. (+)-[⁷⁷Br]BrV-OH also showed high tumor uptake in biodistribution experiments in DU-145 tumor-bearing mice,. Although (+)-[⁷⁷Br]pBrV was retained in most tissues, (+)-[⁷⁷Br]BrV-OH was cleared from these tissues. In blocking studies, the co-injection of SA4503 significantly decreased the tumor uptake of (+)-[⁷⁷Br]BrV-OH.
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¹: Present address: Brain Research Institute, Niigata University.

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Nuclear Medicine and Biology

Abstract

Introduction

Section snippets

General

Tissue distribution of (+)-[¹¹C]PMV and [¹¹C]SA4503

Discussion

Acknowledgments

References (21)

The vesicular acetylcholine transport system

Trends Neurosci

Vesamicol analogues as sigma ligands. Molecular determinants of selectivity at the vesamicol receptor

Biochem Pharmacol

Characterization of radioiodinated (−)-ortho-iodovesamicol binding in rat brain preparations

Life Sci

Binding properties of SA4503 a novel and selective σ₁ receptor agonist

Eur J Pharmacol

Delineating biochemical and functional properties of sigma receptors: emerging concepts

Crit Rev Neurobiol

Drugs acting on sigma and phencyclidine receptors: a review of their nature, function and possible therapeutic importance

Clin Neuropharmacol

Overexpression of σ receptors in nonneural human tumors

Cancer Res

Sigma-1 and sigma-2 receptors are expressed in a wide variety of human and rodent tumor cell lines

Cancer Res

Sigma 2 receptors as potential biomarkers of proliferation in breast cancer

Cancer Res

Effect of ploidy, recruitment, environmental factors, and tamoxifen treatment on the expression of sigma-2 receptors in proliferating and quiescent tumour cells

Br J Cancer

Cited by (34)

Sigma-1 receptor: A potential target for the development of antidepressants

Imaging sigma receptors in the brain: New opportunities for diagnosis of Alzheimer's disease and therapeutic development

Development of a novel radiobromine-labeled sigma-1 receptor imaging probe

Tactics for preclinical validation of receptor-binding radiotracers

Synthesis and evaluation of a <sup>18</sup>F-labeled spirocyclic piperidine derivative as promising σ<inf>1</inf>receptor imaging agent

Brain Imaging of Sigma Receptors

Research articleEvaluation of (+)-p-[11C]methylvesamicol for mapping sigma1 receptors: a comparison with [11C]SA4503

Abstract

Introduction

Section snippets

General

Tissue distribution of (+)-[11C]PMV and [11C]SA4503

Discussion

Acknowledgments

Trends Neurosci

Biochem Pharmacol

Life Sci

Eur J Pharmacol

Delineating biochemical and functional properties of sigma receptors: emerging concepts

Crit Rev Neurobiol

Drugs acting on sigma and phencyclidine receptors: a review of their nature, function and possible therapeutic importance

Clin Neuropharmacol

Overexpression of σ receptors in nonneural human tumors

Cancer Res

Sigma-1 and sigma-2 receptors are expressed in a wide variety of human and rodent tumor cell lines

Cancer Res

Sigma 2 receptors as potential biomarkers of proliferation in breast cancer

Cancer Res

Effect of ploidy, recruitment, environmental factors, and tamoxifen treatment on the expression of sigma-2 receptors in proliferating and quiescent tumour cells

Br J Cancer

Research article
Evaluation of (+)-p-[¹¹C]methylvesamicol for mapping sigma₁ receptors: a comparison with [¹¹C]SA4503

Tissue distribution of (+)-[¹¹C]PMV and [¹¹C]SA4503