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

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• 作者：Junko Noguchi ; Ming-Rong Zhang ; Kazuhiko Yanamoto ; Ryuji Nakao ; Kazutoshi Suzuki
• 关键词：Ultrahigh specific activity ; [¹¹C]Raclopride ; Homogenate assay ; Two-affinity binding sites ; Autoradiography
• 刊名：Nuclear Medicine and Biology
• 出版年：2008
• 出版时间：January 2008
• 年：2008
• 卷：35
• 期：1
• 页码：19-27
• 全文大小：679 K

文摘

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.