Elsevier

Nuclear Medicine and Biology

Volume 39, Issue 8, November 2012, Pages 1117-1121
Nuclear Medicine and Biology

Application of [11C]SA4503 to selection of novel σ1 selective agonists

https://doi.org/10.1016/j.nucmedbio.2012.06.004Get rights and content

Abstract

Introduction

The σ1 ligands are considered to be a new class of potential therapeutic agents for several types of central nervous system disorder. Carbon-11-labeled 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine ([11C]SA4503) was shown to be a promising PET ligand for mapping σ1 receptors, and was applied to measure receptor occupancy with several therapeutic drugs in the living human brain. In this study, we applied this technique for preclinical in vivo screening of novel σ1 selective agonists.

Methods

Six newly synthesized piperazine derivatives containing arylalkylamine groups and cyclohexylamine derivatives containing phenyl groups were selected and tested for their in vivo σ1 receptor binding with [11C]SA4503. The test compounds were administered by intravenous co-injection or oral administration. The in vivo receptor binding of [11C]SA4503 was evaluated by a tissue dissection method at a single time point.

Results

Our in vivo screen identified the most promising candidate of novel σ1 agonist in the piperazine derivatives. Some correlations between in vitro affinity and in vivo receptor blocking rate were observed when considering oral bioavailability. In vivo receptor blocking of piperazine derivatives after oral administration may be predictable by simple co-injection study.

Conclusion

Ligand selection with [11C]SA4503 by the in vivo receptor binding assay was performed successfully. This technique is a practical and high-throughput method that can directly evaluate blood–brain barrier permeability, receptor binding, and bioavailability of drug candidates at the same time.

Introduction

The σ1 receptors, which were initially regarded as one of the opioid receptor subtypes and later confused as the 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 [2]. They are involved in the modulation of various neurotransmitter systems, particularly the cholinergic [3], [4] and glutamatergic pathways [5]. Diverse classes of central nervous system (CNS) drugs show high to moderate affinities for σ1 receptors, i.e., acetylcholinesterase inhibitors (donepezil), antipsychotics (haloperidol), selective serotonin reuptake inhibitors (fluvoxamine), etc. [6], [7], [8]. These compounds can influence cognitive functions both via their primary targets and by activating σ1 receptors in the CNS [1]. Moreover, the discovery of selective ligands for σ1 receptors has suggested that these receptors may be useful potential targets for treatment of neuropsychiatric diseases (schizophrenia, depression, and cognition) and brain ischemia [9]. Taken together, σ1 receptor selective ligands are now recognized as a new class of potential therapeutic agents for several disorders of the CNS.

Drug discovery and development are time-consuming and costly procedures. The challenges for the pharmaceutical industry range from evaluation of potential new drug candidates, determination of drug pharmacokinetics/pharmacodynamics, measurement of receptor occupancy as a determinant of drug efficacy, and pharmacological characterization of the mechanisms of action of novel agents. Positron emission tomography (PET) is a powerful quantitative imaging technique for analyzing biochemical pathways, molecular interactions, drug pharmacokinetics and pharmacodynamics, and estimation of receptor occupancy by drugs in the human brain. We found carbon-11-labeled 1-[2-(3,4-dimethoxyphenyl)ethyl]-4-(3-phenylpropyl)piperazine ([11C]SA4503) to be a promising PET ligand for the mapping of 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 rates by several therapeutic drugs in the living human brain [10], [11], [12]. This technique is also applicable to preclinical in vivo screening of drug candidates, and directly evaluates blood–brain barrier (BBB) permeability, receptor occupancy, and bioavailability at the same time.

In this study, we screened a new series of piperidine and piperazine derivatives containing arylalkylamine groups and cyclohexylamine derivatives containing phenyl groups. Three piperazine derivatives and three cyclohexylamine derivatives were selected from ninety compounds by in vitro binding assay for σ1 and σ2 receptors and agonistic activity evaluated by neurite outgrowth. We measured the in vivo σ1 receptor binding of these compounds using σ1-selective [11C]SA4503. The high specific binding and stable brain kinetics of [11C]SA4503 enabled us to assess receptor blocking by a tissue dissection method at a single time point [13]. This is a much more practical and high-throughput method for in vivo screening of drug candidates than using 3H-labeled radioligands or dynamic small animal PET scanning. The relationships among in vivo receptor blocking, σ1 receptor affinity, and rat oral bioavailability are also discussed.

Section snippets

General

SA4503 was synthesized by the method reported previously [14]. [11C]SA4503 was prepared at the Tokyo Metropolitan Institute of Gerontology as described previously [15]. The chemical structures of six newly synthesized σ1 agonists are shown in Fig. 1. These compounds were N-ethyl-N-[2-(phenylthio)ethyl]cyclohexylamine (MC-46), N-ethyl-N-{2-[(4-fluorophenyl)thio]ethyl}cyclohexylamine (MC-54), N-ethyl-N-[3-(4-fluorophenyl)propyl]cyclohexylamine (MC-128),

Results

In vitro receptor binding affinity, rat oral bioavailability, ClogP, and in vivo blocking rates at the middle dose (333 nmol/kg) of co-injection studies for test compounds are summarized in Table 1. Dose-dependent effects of each compound on [11C]SA4503 uptake in the brain, blood, and muscle are shown in Fig. 2. All of the newly synthesized compounds showed high (Ki < 10 nM) and specific (σ21 > 28) binding to the σ1 receptor. These in vitro binding properties of MC-128 were comparable to

Discussion

We assessed the in vivo blocking effects of novel σ1 agonists on the brain σ1 receptors with [11C]SA4503 by a tissue dissection method at a single time point. The major findings of this study are: 1) the most promising candidate of a novel σ1 agonist was MC-101, 2) piperazine derivatives showed some correlation between in vitro affinities and in vivo receptor blocking rates when considering oral bioavailability, and 3) in vivo receptor blocking of piperazine derivatives after oral

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.

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