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

Overview

This is a clinical trial in which 40 participants with schizophrenia will be randomized to 15 days of treatment with cariprazine (CAR) or brexpiprazole (BREX) in a single-blind manner. \[11C\]PHNO PET scans will be obtained before treatment and after 1 day and 15 days of treatment to examine the effects of antipsychotic medications on the dopamine-3 receptor (D3R). The overall objectives of the current study are to: 1) measure the acute binding of CAR/BREX to the D3R; 2) measure D3R availability for evidence of upregulation following subchronic administration of CAR/BREX in the same set of patients; 3) examine relationships between subchronic binding of CAR/BREX to, and upregulation of, the D3R vs. D2R and changes in positive symptoms, negative symptoms, and cognitive deficits. Relationships between subchronic binding of CAR/BREX and EPS will be explored.

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.