title: Inhibition of risperidone metabolism by fluoxetine in patients with schizophrenia: a clinically relevant pharmacokinetic drug interaction.
abstract: The effect of fluoxetine on the steady-state plasma concentrations of risperidone and its active metabolite 9-hydroxyrisperidone (9-OH-risperidone) was evaluated in 10 patients with schizophrenia or schizoaffective disorder. Patients stabilized on risperidone (4-6 mg/day) received additional fluoxetine (20 mg/day) to treat concomitant depression. One patient dropped out after 1 week due to the occurrence of akathisia associated with markedly increased plasma risperidone concentrations. In the other subjects, mean plasma concentrations of risperidone increased during fluoxetine administration from 12 +/- 9 ng/mL at baseline to 56 +/- 31 at week 4 (p < 0.001), while the levels of 9-OH-risperidone were not significantly affected. After 4 weeks of combined treatment, the levels of the active moiety (sum of the concentrations of risperidone and 9-OH-risperidone) increased by 75% (range, 9-204%, p < 0.01) compared with baseline. The mean plasma risperidone/9-OH-risperidone ratio also increased significantly. During the second week of adjunctive therapy, two patients developed Parkinsonian symptoms, which were controlled with anticholinergic medication. These findings indicate that fluoxetine, a potent inhibitor of the cytochrome P450 enzyme CYP2D6 and a less potent inhibitor of CYP3A4, reduces the clearance of risperidone by inhibiting its 9-hydroxylation or alternative metabolic pathways. This interaction may lead to toxic plasma risperidone concentrations. In addition to careful clinical observation, monitoring plasma risperidone levels may be of value in patients given adjunctive therapy with fluoxetine.
authors.affiliation: Department of Clinical and Experimental Medicine and Pharmacology, Section of Pharmacology, University of Messina, Messina, Italy. espina@unime.it
authors.forename: Edoardo
authors.initials: E
authors.lastname: Spina
authors.forename: Angela
authors.initials: A
authors.lastname: Avenoso
authors.forename: Maria Gabriella
authors.initials: MG
authors.lastname: Scordo
authors.forename: Maria
authors.initials: M
authors.lastname: Ancione
authors.forename: Aldo
authors.initials: A
authors.lastname: Madia
authors.forename: Giuliana
authors.initials: G
authors.lastname: Gatti
authors.forename: Emilio
authors.initials: E
authors.lastname: Perucca
mesh_terms: D000328:Adult; D014150:Antipsychotic Agents / Q000037:antagonists & inhibitors* / Q000097:blood / Q000493:pharmacokinetics*; D003863:Depression / Q000097:blood / Q000188:drug therapy / Q000209:etiology; D004347:Drug Interactions; D004359:Drug Therapy, Combination; D005260:Female; D005473:Fluoxetine / Q000097:blood / Q000493:pharmacokinetics* / Q000627:therapeutic use; D006801:Humans; D008297:Male; D008875:Middle Aged; D018967:Risperidone / Q000037:antagonists & inhibitors* / Q000097:blood / Q000493:pharmacokinetics*; D012559:Schizophrenia / Q000097:blood* / Q000150:complications / Q000188:drug therapy; D017367:Selective Serotonin Reuptake Inhibitors / Q000097:blood / Q000493:pharmacokinetics* / Q000627:therapeutic use; D018709:Statistics, Nonparametric
chemical_list: D014150:Antipsychotic Agents; D005473:Fluoxetine; D018967:Risperidone; D017367:Selective Serotonin Reuptake Inhibitors