A pharmacokinetic interaction between the selective serotonin reuptake inhibitor citalopram and a tricyclic antidepressant, clomipramine, was noted in a patient treated for major depression and obsessive-compulsive disorder. After the addition of citalopram, a desmethylclomipramine plasma level increase and an 8-hydroacy-desmethylclomipramine plasma level decrease were observed. The CYP2D6 phenotype, determined when the patient received the antidepressant comedication, characterized a poor metabolizer status (dextromethorphan metabolic ratio > 0.3), despite a heterozygous genotype containing a wild-type allele with extensive metabolic capacity and a mutant non-functional allele (CYP2D6*1A/CYP2D6*4A). This case seems to be one of the first descriptions of the clinical relevance of a CYP2D6 heterozygous genotype in a patient treated with antidepressant.
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