Clinical Significance of a CYP2D6 Poor Metabolizer -A Patient With Schizophrenia on Risperidone TreatmentBožina, Nada MD, PhD*; Jovanović, Nikolina MD, Pre-PhD†; Lovrić, Mila BSc, MSc*; Medved, Vesna MD, PhD†Therapeutic Drug Monitoring: December 2008 - Volume 30 - Issue 6 - p 748-751 doi: 10.1097/FTD.0b013e3181896afc Case Report Buy Abstract Author InformationAuthors Article MetricsMetrics A case of a 46-year-old woman with schizophrenia who was treated with risperidone and followed up for 1 year is reported. She was genotyped as a CYP2D6 poor metabolizer (PM): CYP2D6-4*/*6, which was confirmed by a dextromethorphan (DM) test (metabolic ratio = 5.8). Genotypes of ABCB1 (MDR1) were 2677TT and 3435TT. Because risperidone is CYP2D6 and P-glycoprotein substrate, the patient might have been expected to accumulate risperidone and suffer from significant side effects. However, the patient tolerated the drug extremely well. Plasma concentration of risperidone was 73.2 nmol/L and of 9-OH-risperidone was below the limit of quantitation (6.1 nmol/L). Target range of risperidone plus 9-hydroxyrisperidone is 50-150 nmol/L. During the follow-up, patient was continuously taking 3 mg/day of risperidone. Plasma levels of risperidone and 9-OH-risperidone were 70.2 and 18.1 nmol/L, respectively. We repeated a DM test, metabolic ratio was 3.6, thus confirming that the patient remained a PM. Psychopathology was assessed with Positive and Negative Syndrome Scale, and stable remission of illness was achieved over the stated period. No adverse effects were observed or reported by the patient. We conclude that PM phenotype for CYP2D6 does not necessarily have clinical significance in regard to risperidone treatment. DM and risperidone are both CYP2D6 and P-glycoprotein substrates and significant interactions might occur with both drugs, in parallel with the possible impact of ABCB1 and CYP2D6 polymorphic gene variants. From the *Clinical Institute of Laboratory Diagnosis, Zagreb University School of Medicine; and †Department of Psychiatry, Zagreb University Hospital, Zagreb, Croatia. Received for publication March 18, 2008; accepted August 5, 2008. Correspondence: Nada Božina, MD, PhD, Clinical Institute of Laboratory Diagnosis, Zagreb University School of Medicine and Zagreb University Hospital, Kišpatićeva 12, 10000 Zagreb, Croatia (e-mail: email@example.com). © 2008 Lippincott Williams & Wilkins, Inc.