Relationship between dopamine D(2) occupancy, clinical response, and side effects: a double-blind PET study of first-episode schizophrenia.
OBJECTIVE
Since all antipsychotics block dopamine D(2) receptors, the authors investigated how well D(2) receptor occupancy in vivo predicts clinical response, extrapyramidal side effects, and hyperprolactinemia.


METHODS
In a double-blind study, 22 patients with first-episode schizophrenia were randomly assigned to 1.0 or 2. 5 mg/day of haloperidol. After 2 weeks of treatment, D(2) receptor occupancy was determined with [(11)C]raclopride and positron emission tomography, and clinical response, extrapyramidal side effects, and prolactin levels were measured. Patients who showed adequate responses continued taking their initial doses, those who did not respond had their doses increased to 5.0 mg/day, and evaluations were repeated at 4 weeks for all patients.


RESULTS
The patients showed a wide range of D(2) occupancy (38%-87%). The degree of receptor occupancy predicted clinical improvement, hyperprolactinemia, and extrapyramidal side effects. The likelihood of clinical response, hyperprolactinemia, and extrapyramidal side effects increased significantly as D(2) occupancy exceeded 65%, 72%, and 78%, respectively.


CONCLUSIONS
The study confirms that D(2) occupancy is an important mediator of response and side effects in antipsychotic treatment. The data are consistent with a "target and trigger" hypothesis of antipsychotic action, i.e., that the D(2) receptor specificity of antipsychotics permits them to target discrete neurons and that their antagonist properties trigger within those neurons intracellular changes that ultimately beget antipsychotic response. While limited to haloperidol, the relationship between D(2) occupancy and side effects in this study helps explain many of the observed clinical differences between typical and atypical antipsychotics.
Kapur
S
S
Department of Psychiatry, University of Toronto, ON, Canada.
Zipursky
R
R
Jones
C
C
Remington
G
G
Houle
S
S
D000293:Adolescent; D000328:Adult; D014150:Antipsychotic Agents / Q000009:adverse effects / Q000494:pharmacology / Q000627:therapeutic use*; D001480:Basal Ganglia Diseases / Q000139:chemically induced; D002250:Carbon Radioisotopes; D003342:Corpus Striatum / Q000000981:diagnostic imaging / Q000187:drug effects* / Q000378:metabolism*; D065127:Dopamine D2 Receptor Antagonists; D004311:Double-Blind Method; D005260:Female; D006220:Haloperidol / Q000494:pharmacology / Q000627:therapeutic use; D006801:Humans; D006966:Hyperprolactinemia / Q000139:chemically induced; D008297:Male; D008875:Middle Aged; D020891:Raclopride; D017448:Receptors, Dopamine D2 / Q000378:metabolism; D012559:Schizophrenia / Q000000981:diagnostic imaging / Q000188:drug therapy* / Q000378:metabolism*; D014055:Tomography, Emission-Computed; D016896:Treatment Outcome
D014150:Antipsychotic Agents; D002250:Carbon Radioisotopes; D065127:Dopamine D2 Receptor Antagonists; D017448:Receptors, Dopamine D2; D020891:Raclopride; D006220:Haloperidol