In vitro binding of [11C]raclopride with ultrahigh specific activity in rat brain determined by homogenate assay and autoradiography

Noguchi, Junko; Zhang, Ming-Rong; Yanamoto, Kazuhiko; Nakao, Ryuji; Suzuki, Kazutoshi

doi:10.1016/j.nucmedbio.2007.09.009

« Previous Next »Nuclear Medicine and Biology
Volume 35, Issue 1 , Pages 19-27, January 2008

In vitro binding of [¹¹C]raclopride with ultrahigh specific activity in rat brain determined by homogenate assay and autoradiography

Junko Noguchi
- Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
- SHI Accelerator Service Co. Ltd., 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan
Ming-Rong Zhang
- Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
Kazuhiko Yanamoto
- Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
Ryuji Nakao
- Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
Kazutoshi Suzuki
- Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
- Corresponding author. Tel.: +81 43 206 4039; fax: +81 43 206 3261.

Received 20 July 2007; received in revised form 11 September 2007; accepted 29 September 2007.

Abstract

Objective

The aim of this study was to characterize the in vitro binding of [¹¹C]raclopride with ultrahigh specific activity (SA) in the striatum and cerebral cortex of rat brain.

Methods

[¹¹C]Raclopride, a dopamine D₂ receptor ligand, with an ultrahigh SA of 4880±2360 GBq/μmol (132±64 Ci/μmol, n=25) was synthesized. In vitro binding experiment was performed using brain homogenate assay and autoradiography (ARG).

Results

In vitro homogenate assay demonstrated that high SA [¹¹C]raclopride (2520–6340 GBq/μmol; 68–171 Ci/μmol) had two-affinity (high and low) binding sites in the striatum and cerebral cortex of rat brain. In the striatum, K_d,high and B_max,high values were 0.005±0.002 nM and 0.19±0.04 fmol/mg tissue, respectively, while K_d,low and B_max,low values were 2.2±1.0 nM and 35.8±16.4 fmol/mg tissue, respectively. In the cerebral cortex, K_d,high and B_max,high values were 0.061±0.087 nM and 0.2±0.2 fmol/mg tissue, respectively, while K_d,low and B_max,low values were 2.5±3.2 nM and 5.5±4.8 fmol/mg tissue, respectively. On the other hand, only one binding site was found in the striatum and no binding site was identified in the cerebral cortex using low SA [¹¹C]raclopride (44 GBq/μmol; 1.2 Ci/μmol). In vitro ARG for the rat brain using high SA [¹¹C]raclopride (6212 GBq/μmol; 168 Ci/μmol) gave a coronal image of the striatum and cerebral cortex with a higher signal/noise ratio than using low SA [¹¹C]raclopride (40 GBq/μmol; 1.1 Ci/μmol).

Conclusion

Using ultrahigh SA [¹¹C]raclopride for the in vitro homogenate assay, we succeeded in detecting two-affinity binding sites of [¹¹C]raclopride, not only in the striatum but also in the cerebral cortex of rat brain.

Keywords: Ultrahigh specific activity, [¹¹C]Raclopride, Homogenate assay, Two-affinity binding sites, Autoradiography