Therapeutic window for striatal dopamine D_2/3 receptor occupancy in older patients with schizophrenia: A pilot PET study

Objective In younger patients with schizophrenia, positron emission tomography (PET) studies have identified a therapeutic window of striatal dopamine D_2/3 receptor occupancy of 65%-80%. This type of empirical information is not available in late life. Our primary aim was to assess the effect of changes in D_2/3 relative receptor occupancy (RRO) on clinical outcomes in this population. Design Open-label intervention. Setting Centre for Addiction and Mental Health, Toronto. Participants Subjects with schizophrenia age 50 years or more who were clinically stable and previously maintained on oral risperidone for more than 6 months. Intervention A dose reduction of risperidone of up to 40%, followed by a 3-month follow-up. Measurements Dopamine D_2/3 RRO in dorsal putamen was assessed, using the region of interest analysis of [¹¹C]raclopride PET scans, before and after the dose reduction. Clinical assessments included the Positive and Negative Syndrome Scale and the Simpson-Angus Scale. Results Nine subjects (mean ± SD age: 58 ± 7 years; mean ± SD baseline risperidone dose: 3.4 ± 1.6 mg/day) participated in the study. Extrapyramidal symptoms (EPS) were present in six subjects and were associated with 70% or more D_2/3 RRO in the putamen (range: 70%-87%). Following the dose reduction, EPS resolved in five subjects. Two subjects experienced a clinical worsening at 52% and at less than 50% D_2/3 RRO. Conclusion EPS diminished less than 70% D_2/3 RRO, which suggests a lower therapeutic window for older patients with schizophrenia than that for younger patients. Although these findings have to be replicated in a larger sample, they have important implications for future drug development and clinical guidelines in late-life schizophrenia.