PET has shown that dose-dependent in vivo occupancy of dopamine receptors by antipsychotic drugs is associated with clinical response to antipsychotic agents and the production of extrapyramidal side effects. We studied the feasibility of administering [123I]IBZM as a bolus plus continuous infusion over 8 hr to achieve unchanging regional brain activity levels, and the application of [123I]IBZM continuous infusion to examine the effects of the antipsychotic agent RWJ-37796, on striatal activity in humans.
Methods: Five healthy male subjects received a bolus of [123I]IBZM followed by a continuous infusion at a bolus (mCi):infusion (mCi/hr) ratio of 6:1. Serial SPECT images were obtained every 2-3 min for a total of 8 hr with a 1-2 hr break in the scanning session. Serial venous blood samples were obtained every 30 min for the duration of the study. All five subjects achieved unchanging plasma [123I]IBZM and striatal brain-activity levels over the 300-420 min postinitiation of tracer infusion. Two subjects achieved flat brain time-activity curves later than the others, suggesting the bolus-to-infusion ratio was slightly high. An additional six healthy male subjects received a similar bolus plus constant infusion of [123I]IBZM. RWJ-37796 (0.04 mg/kg) was administered intravenously 157 +/- 13.7 min after the initiation of [123I]IBZM infusion. Serial SPECT brain images, serum prolactin and extrapyramidal side effect ratings were obtained for an additional 330 min.
Results: All six subjects demonstrated rapid and marked reduction of striatal activity following RWJ-37796 without return of striatal activity to baseline levels over the 5.5 hr of continued [123I]IBZM administration. Estimated receptor occupancy by RWJ-37796 was 57% +/- 5% (range 47%-67%). Prolactin was only transiently increased in all subjects by 1054% +/- 1084% over baseline. One subject experienced moderate extrapyramidal symptoms (akasthisia) during RWJ-37796 injection.
Conclusion: SPECT imaging during continuous [123I]IBZM infusion provides a powerful within-scan method for determining both temporal binding characteristics and receptor occupancy of striatal dopamine receptors by antipsychotic agents.