A novel radioligand for glycine transporter 1: characterization and use in autoradiographic and in vivo brain occupancy studies

Zeng, Zhizhen; O'Brien, Julie A.; Lemaire, Wei; O'Malley, Stacey S.; Miller, Patricia J.; Zhao, Zhijian; Wallace, Michael A.; Raab, Conrad; Lindsley, Craig W.; Sur, Cyrille; Williams, David L.

doi:10.1016/j.nucmedbio.2007.12.002

« Previous Next »Nuclear Medicine and Biology
Volume 35, Issue 3 , Pages 315-325, April 2008

A novel radioligand for glycine transporter 1: characterization and use in autoradiographic and in vivo brain occupancy studies

Zhizhen Zeng
- Imaging, Merck Research Laboratories, West Point, PA 19486, USA
- Corresponding author. Imaging, WP44C-2, Merck Research Laboratories, West Point, PA 19486, USA. Tel.: +1 215 652 5627; fax: +1 215 993 6803.
Julie A. O'Brien
- Sleep and Psychiatric Disorders, Merck Research Laboratories, West Point, PA 19486, USA
Wei Lemaire
- Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
Stacey S. O'Malley
- Imaging, Merck Research Laboratories, West Point, PA 19486, USA
Patricia J. Miller
- Imaging, Merck Research Laboratories, West Point, PA 19486, USA
Zhijian Zhao
- Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
Michael A. Wallace
- Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA
Conrad Raab
- Drug Metabolism, Merck Research Laboratories, West Point, PA 19486, USA
Craig W. Lindsley
- Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
- Departments of Pharmacology and Chemistry, Vanderbilt University, Nashville, TN 37232, USA
Cyrille Sur
- Imaging, Merck Research Laboratories, West Point, PA 19486, USA
David L. Williams Jr.
- Imaging, Merck Research Laboratories, West Point, PA 19486, USA

Received 10 September 2007; received in revised form 24 October 2007; accepted 5 December 2007.

Abstract

Introduction

In an effort to develop agents to test the NMDA hypofunction hypothesis of schizophrenia, benchmark compounds from a program to discover potent, selective, competitive glycine transporter 1 (GlyT1) inhibitors were radiolabeled in order to further study the detailed pharmacology of these inhibitors and the distribution of GlyT1 in brain. We here report the in vitro characterization of [³⁵S](S)-2-amino-4-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide ([³⁵S]ACPPB), a radiotracer developed from a potent and selective non-sarcosine-derived GlyT1 inhibitor, its use in autoradiographic studies to localize (S)-2-amino-6-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide (ACPPB) binding sites in rat and rhesus brain and for in vivo occupancy assays of competitive GlyT1 inhibitors.

Methods

Functional potencies of unlabeled compounds were characterized by [¹⁴C]glycine uptake into JAR (human placental choriocarcinoma) cells and synaptosomes. Radioligand binding studies were performed with tissue homogenates. Autoradiographic studies were performed on tissue slices.

Results

ACPPB is a potent (K_d=1.9 nM), selective, GlyT1 inhibitor that, when radiolabeled with [³⁵S], is a well-behaved radioligand with low nondisplaceable binding. Autoradiographic studies of rat and rhesus brain slices with this ligand showed that specific binding sites were plentiful and nonhomogeneously distributed, with high levels of binding in the brainstem, cerebellar white matter, thalamus, cortical white matter and spinal cord gray matter. In vivo studies demonstrate displaceable binding of [³⁵S]ACPPB in rat brain tissues following iv administration of this radioligand.

Conclusions

This is the first report of detailed anatomical localization of GlyT1 using direct radioligand binding, and the first demonstration that an in vivo occupancy assay is feasible, suggesting that it may also be feasible to develop positron emission tomography tracers for GlyT1.

Keywords: Glycine, GlyT1, Schizophrenia, Autoradiography