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

ObjectiveThe 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).ResultsIn 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, Kd,high and Bmax,high values were 0.005±0.002 nM and 0.19±0.04 fmol/mg tissue, respectively, while Kd,low and Bmax,low values were 2.2±1.0 nM and 35.8±16.4 fmol/mg tissue, respectively. In the cerebral cortex, Kd,high and Bmax,high values were 0.061±0.087 nM and 0.2±0.2 fmol/mg tissue, respectively, while Kd,low and Bmax,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).ConclusionUsing 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.