Dose and dosing frequency of long-acting injectable antipsychotics: a systematic review of PET and SPECT data and clinical implications

Brain imaging data of antipsychotics have mainly been derived from oral antipsychotic drugs, which hampers our understanding of the requirement of dose/dosing frequency of long-acting injectable (LAI) antipsychotics for the maintenance treatment of schizophrenia. A systematic literature search was performed to identify positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies that assessed dopamine D2 receptor occupancy levels with LAI antipsychotic drugs in humans, using PubMed, EMBASE, and PsycINFO (last search, February 2013). Twenty studies (15 PET and 5 SPECT studies) were identified. The most investigated drug in these PET and SPECT studies was haloperidol decanoate (44 subjects; 11 studies), followed by risperidone LAI (24 subjects; 3 studies), olanzapine pamoate (14 subject; 1 study), and fluphenazine decanoate (12 subjects; 3 studies). The data have demonstrated high and continuous D2 receptor blockade with LAIs; the effects of LAI first-generation antipsychotics on the central nervous system may persist for several months. The prospective and cross-sectional studies showed that continuous dopamine D2 receptor blockade above 65% (ie, the lower end of the established therapeutic window for the acute phase treatment) was not always necessary for maintenance treatment for at least some of the patients. In conclusion, because of the limited brain imaging data on LAI antipsychotics, we still do not know the best way to dose them. Still, the currently available brain imaging data raises a possibility that the dosing interval of LAI antipsychotics may be extended beyond the currently indicated range in some patients.