No evidence of increased adverse drug reactions in cytochrome P450 CYP2D6 poor metabolizers treated with fluoxetine or nortriptyline.

The polymorphic enzyme cytochrome P450 CYP2D6 is involved in the metabolism of many antidepressants, including nortriptyline and fluoxetine. Some 7%-10% Caucasians have inactivating mutations in both alleles of the CYP2D6 gene, and are referred to as poor metabolizers (PMs). Several case reports and clinical studies suggest that CYP2D6 PMs are at a greater risk of developing adverse drug reactions (ADRs) on antidepressant medication than extensive metabolizers (EMs). However, few clinical trials have investigated whether CYP2D6 PM genotype is predictive of ADRs during antidepressant treatment. This paper explores the link between CYP2D6 genotype and antidepressant-associated ADRs in outpatients being treated for major depression with either nortriptyline or fluoxetine. Patients were randomized to fluoxetine (n=65) or nortriptyline (n=60) for the 6 week trial. CYP2D6 genotypes predicted that of these patients 115 were EM and the remaining 10 were PMs. ADRs attributed to antidepressant usage were recorded over the 6-week trial. Although the type of ADR was, as expected, different between drugs, the frequency of ADRs experienced did not differ significantly between the two antidepressants or between CYP2D6 PMs and EMs. In addition, the frequency at which PMs discontinued antidepressant medication was not noticeably different from EMs, although with only 10 PMs the study is under powered to detect moderate or small differences. These findings suggest that inability to efficiently metabolize antidepressants that are CYP2D6 substrates does not necessarily lead to increased occurrence of antidepressant-associated ADRs. Thus, for clinicians prescribing antidepressant monotherapy, CYP2D6 polymorphisms are probably not of relevance to antidepressant side effects and therapy.