Equivalent switching dose from oral risperidone to risperidone long-acting injection: a 48-week randomized, prospective, single-blind pharmacokinetic study.

Objective: Previous studies showed clinical benefit of risperidone long-acting injection in the treatment of schizophrenia. However, the equivalent switching dose from oral risperidone to risperidone long-acting injection was still in debate. This study, conducted among hospitalized patients, included a long-enough study period and optimal control of drug compliance to test the equivalent switching dose.

Method: Fifty symptomatic, stable hospitalized patients with DSM-IV schizophrenia were randomly assigned to receive either daily oral risperidone or risperi-done long-acting injection every 2 weeks. Those originally receiving an oral risperidone dose of 4 mg/day or less received 25 mg of risperidone long-acting injection, those taking an oral dose of more than 4 mg/day but of 6 mg/day or less received 37.5 mg of risperidone long-acting injection, and those taking more than 6 mg/day received 50 mg of risperidone long-acting injection. Assessments of clinical efficacy, side effects, metabolic safety, drug tolerance, and serum concentration of risperi-done metabolites were performed repeatedly. The study was conducted from March 2004 to May 2005.

Result: Forty-five patients (90%) completed the study. There were no significant differences in Positive and Negative Syndrome Scale (PANSS) scores between the 2 groups, but the risperidone long-acting injection group showed reduced UKU Side Effect Rating Scale total scores (p = .048), Simpson-Angus Scale scores (p = .028), prolactin levels (p = .046), and serum concentrations of risperidone metabolites (p = .028). Among the risperidone long-acting injection group, patients who received either 25 mg q 2 weeks or 37.5 mg q 2 weeks of risperidone long-acting injection showed increased PANSS scores (p = .058), decreased serum metabolite concentrations (p = .028), and an increased tendency to relapse.

Conclusions: The results support good tolerability of risperidone long-acting injection, but it is suggested that the equivalent switching dose be adjusted as follows: those originally on an oral risperidone dose of 3 mg/day or less should receive 25 mg of risperidone long-acting injection, those taking an oral dose of more than 3 mg/day but of 5 mg/day or less should receive 37.5 mg, and those taking an oral dose of more than 5 mg/day should receive 50 mg of risperidone long-acting injection.