Substituted benzamides as ligands for visualization of dopamine receptor binding in the human brain by positron emission tomography.

Two substituted benzamides, FLB 524 and raclopride, were labeled with 11C and examined for their possible use as ligands for positron emission tomography (PET)-scan studies on dopamine-2 (D-2) receptors in the brains of monkeys and healthy human subjects. Both ligands allowed the in vivo visualization of D-2 receptor binding in the corpus striatum caudate nucleus/putamen complex in PET-scan images. [11C]Raclopride showed a high ratio of specific striatal to nonspecific cerebellar binding, and the kinetics of binding of this ligand made it optimal for PET studies. The in vivo binding of [11C]raclopride in the striatum of cynomolgus monkeys was markedly reduced by displacement with haloperidol. This and previous in vitro data indicate that [11C]raclopride binds selectively to striatal D-2 dopamine receptors. In healthy human subjects, [11C]raclopride binding in the caudate nucleus/putamen was 4- to 5-fold greater than nonspecific binding in the cerebellum. In comparison with previously available ligands for PET-scan studies on central dopamine receptors in man, [11C]raclopride appears to be advantageous with regard to (i) specificity of binding to D-2 receptors, (ii) the high ratio between binding in dopamine-rich (caudate, putamen) and dopamine-poor (cerebellum) human brain regions, and (iii) rapid association and reversibility of specific binding. [11C]Raclopride should be a valuable tool for characterizing D-2 receptors in the brains of patients with neuropsychiatric disorders.