Haloperidol serum concentrations and D2 dopamine receptor occupancy during low-dose treatment with haloperidol decanoate.

The aim of this study was to examine the relationship between serum concentrations of haloperidol and central D2 receptor occupancy in eight schizophrenic patients treated with low doses of haloperidol decanoate. The accompanying psychopathology was assessed. During a 4-week interval after administration of haloperidol decanoate (dose range 30-70 mg), serum concentrations of haloperidol were determined once a week by using a sensitive high-performance liquid chromatography method. The patients' D2 receptor occupancy was determined with single-photon emission computed tomography on two separate occasions. One week after depot administration the mean haloperidol serum concentration was 7.3 nmol/l (range 3.9-22.7 nmol/l) and the mean D2 receptor occupancy was 75% (range 52-100%). After 4 weeks the mean haloperidol serum concentration had decreased to 1.8 nmol/l (range 0-5.7 nmol/l) and the mean D2 receptor occupancy to 53% (range 12-89%). Differences were seen in two subgroups, defined by their history of neuroleptic exposure before inclusion into the study. Patients treated with depots of haloperidol decanoate for months showed higher D2 receptor occupancy and corresponding higher serum haloperidol concentrations at week 4 than did patients who had a history of oral haloperidol treatment. Because the difference in the dynamics of D2 receptor occupancy could be reflected by corresponding serum concentrations of haloperidol, it seems useful to involve haloperidol drug monitoring as a possible surrogate marker for D2 receptor occupancy in optimizing low-dose treatment with haloperidol decanoate.