Longitudinal PET Imaging of Antipsychotic Binding to the Dopamine-3 Receptor in Schizophrenia

There is a great need for the development of new treatments for schizophrenia (SCZ). Aside from the recently approved muscarinic agonist xanomeline, all current treatments have been assumed to function by blocking dopamine-2 receptors (D2Rs). Interest in the dopamine-3 receptor (D3R) was encouraged by preclinical findings that D3R antagonists reverse cognitive impairment and improve negative symptoms. In vivo imaging of the D3R became possible with the development of [11C]-(+)-PHNO, a D3R-preferring radioligand. However, while numerous preclinical studies have demonstrated that the D3R is relevant to both the neurobiology and treatment of SCZ, in vivo studies initially and surprisingly reported that, after several weeks of administration, antipsychotic medications may not bind to the D3R and may paradoxically increase levels of the D3R. These findings were discrepant with our own findings in non-human primates and individuals with SCZ demonstrating that acute doses of antipsychotic medications bind to the D3R and D2R in ratios predicted by their in vitro binding profiles. In a later study conducted by our group, 10 days of chronic dosing of the D2R-preferring antipsychotic medication brexpiprazole (BREX) also led to increased levels of the D3R at 1mg and negligible binding at 4mg. Finally, in our study of the D3R-preferring antipsychotic medication cariprazine (CAR), there was robust binding to the D3R and D2R after both acute and subchronic dosing. These seemingly discrepant findings may be related to methodological differences, differences in the binding profiles of D2R- vs. D3R-preferring antipsychotic medications, or to homeostatic responses to chronic antipsychotic treatment (i.e., upregulation). Upregulation is a potentially critical, though underexamined, effect common to all D2R/D3R-binding antipsychotic medications, including partial agonists. It has been hypothesized to be largely responsible for differences in occupancy estimates from single and repeat dose studies and contribute to waning effects of antipsychotic medications and tardive dyskinesia. The goals of this proposal are to elucidate the contribution of D3R, compared to D2R, binding to antipsychotic action, both acutely (SA1) and subchronically (SA2), in the same patients. The relationship between occupancy, upregulation, and clinical effects (SA3, EA) will be investigated.

Inclusion criteria

• Individuals, any gender or sex, aged 18 to 55, inclusive at screen
• Capable of understanding the study procedures and able to provide informed consent
• Diagnosed with schizophrenia, schizoaffective, or schizophreniform disorder
• Negative urine toxicology
• Antipsychotic free (by choice and for reasons unrelated to the study), and for at least 3 weeks (4 for aripiprazole or LAIs) at the time of the baseline PET scan, inclusive of any antipsychotic-free time prior to consent. [Any patient who requires inpatient hospitalization or acute medication treatment for clinical stabilization during the medication free period will not be included in this study; any participant who in the clinical judgment of the PIs or any involved clinician is not stable or appropriate for a medication free period will not be included.]
• PANSS total score > 80 and < 120 (inclusive)

Exclusion criteria

• Diagnosis of substance use disorder within the previous month
• A history of poor or inadequate response or hypersensitivity to CAR or BREX for any reason
• EKG abnormality that is clinically significant including a QTc interval > 450 msec for men and > 470 msec for women,
• Pregnant or breast-feeding women. Women of child-bearing potential must have a negative serum β-hCG pregnancy test at Visit 1, must have been using an acceptable method of contraception for 30 days before the study (i.e., before the first PET or MRI scan, whichever comes first), and must agree to do so for the whole study and 30 days after (unless post-menopausal or surgically sterile)
• Any clinically significant or unstable medical/neurological illness, condition, or disorder that is anticipated to potentially compromise participant safety on study medication
• Any material in the body that is a contraindication for MRI procedures or participated in prior nuclear medicine procedures in the past year that exceed FDA-defined limits when combined with radiation dosimetry from PET scanning in this protocol to avoid exceeding annual dosimetry limits (metal screener repeated before MRI scan). Individuals exposed to radiation in the workplace in the previous year will be excluded.
• Acute risk for suicide (i.e., score of 4-5 within the previous month or 6 within the previous 3 months on the CSSRS) or violence/homicide (e.g., homicidal ideation), or history of severe violent behavior or behavioral dyscontrol while antipsychotic-free
• A history of treatment resistance to antipsychotics or who have a duration of illness of greater than 20 years
• Claustrophobia
• Use of nicotine products within the previous month (prior to first PET scan)