Dopamine D2-receptor density in humans as assessed with SPET and the new high-affinity ligand 123I-NCQ298: a pilot study.

Astra (S)-3-[123I]iodo-5,6-dimethoxyl-N-[(1-ethyl-2-pyrrolidinyl)methyl] -salicylamide (123I-NCQ298) is a new high-affinity D2-receptor ligand for use in single photon emission tomography (SPET) studies. We have studied the biodistribution and absorbed radiation dose of 123I-NCQ298 in humans. The mean effective dose for adults was 0.055 mSv MBq(-1). Brain uptake and clearance was measured with a head dedicated SPET camera. 123I-NCQ298 showed specific uptake in the basal ganglia with a low clearance rate (time constant of 9-34 h). The extrastriatal binding was variable (average 30%, maximum 60% of that in the basal ganglia at 1 h), but with a clearance rate twice that of the basal ganglia. The fairly high level of extrastriatal binding precluded the use of a quotient between the basal ganglia and cerebellum-to-frontal cortex 123I-NCQ298 concentration as a measure for basal ganglia D2-receptor density. Chronic schizophrenic patients treated with conventional neuroleptics had a decreased affinity for 123I-NCQ298 in the basal ganglia in the range 10-60% of the median value for the control, untreated subjects.