| Summary: | Antipsychotic plasma levels have been extensively used in the assessment of poor treatment response, lack of adherence and adverse events in delusional disorder. It has not been used as an indicator of metabolizer status, to determine whether a delusional disorder patient is a poor, intermediate, or ultra-rapid metabolizer of antipsychotics. Pharmacogenetic probes are, of course, the right method for the latter task, but they are not readily available for clinical use. We report the case of a 46-year-old woman with delusional disorder who developed unexpected adverse effects to treatment with relatively low dose risperidone and poor symptomatic response. Blood level monitoring indicated high levels of risperidone and a high concentration-to-dose ratio, which suggested accumulation of unmetabolized risperidone. Paradoxically, extrapyramidal side effects increased when, after reducing the risperidone dose, 5 mg/day of aripiprazole was added. Consequently, the patient was switched to olanzapine 5 mg/day. Sertraline 150 mg/day was later added for comorbid depression. A complete symptomatic response was achieved. Although other factors may well have been at play, this sequence of events suggests that the patient was a slow metabolizer of CYP2D6, which metabolizes both risperidone and aripiprazole. With pharmacogenetic assessment not available, therapeutic drug monitoring helped clinicians decide on appropriate management.
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