Cytochrome P450 2D6 deficiency and its clinical relevance in a patient treated with risperidone.

In contrast to several authors who found hepatic cytochrome P 450 2D6 (CYP2D6) metabolising status to be clinically unimportant in treatment with the CYP2D6 substrate, risperidone, we report on a 17-year-old schizophrenic patient who suffered from severe extrapyramidal side effects (EPS) while being treated with risperidone at 4 mg per day. He was genotyped as a CYP2D6 poor metaboliser (PM). The active moiety of risperidone (sum of risperidone and 9-hydroxyrisperidone) was elevated and increased even further under co-medication with haloperidol and biperiden. We conclude that the PM phenotype for CYP2D6 of this patient had major clinical importance in treatment with risperidone. Most likely metabolic pathways other than CYP2D6 were also involved that are probably inhibited by haloperidol.