Serum concentrations of risperidone and 9-hydroxyrisperidone after administration of the long-acting injectable form of risperidone: evidence from a routine therapeutic drug monitoring service.

There is limited documentation on the pharmacokinetics of long-acting intramuscular risperidone in a naturalistic setting. The objective of this study was to investigate the concentrations of risperidone and its active metabolite 9-hydroxyrisperidone as well as the concentration/dose (C/D) ratios achieved after intramuscular depot administration in a routine therapeutic drug monitoring setting. Thirty samples were collected from 10 female and 20 male patients receiving depot injections of risperidone. For 6 of the patients, the results could be compared with corresponding data available after previous oral administration of risperidone. In addition, data from a group of 278 patients using oral risperidone were retrieved. The median serum concentrations of risperidone plus 9-hydroxyrisperidone were 38 nmol/L 12-14 days after a intramuscular dose of 25 mg/14 days, 67 nmol/L after a dose of 37.5 mg/14 days, 99 nmol/L after a dose of 50 mg/14 days, and 148 nmol/L after a dose of 75 mg/14 days. The median C/D ratio for risperidone plus 9-hydroxy-risperidone was 22.2 (nmol/L)/(mg/d). In the group on oral medication, the median C/D ratio was 18.6 (nmol/L)/(mg/d). In the 6 patients previously using oral risperidone, switching to depot injections gave an average increase of 33% (range 12%-68%) in the C/D ratio. In conclusion, the authors' data indicate that there are great interindividual differences in the extent to which the daily dose has to be reduced when switching from oral to intramuscular depot administration to achieve the same serum concentrations of risperidone plus 9-hydroxyrisperidone. Because the degree of dose adjustment required for the individual patient so far cannot be predicted, guidance by therapeutic drug monitoring might be helpful.