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 risperidone 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 risperidone 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.