The Role of Metabolites of Antidepressants in the Treatment of Depression.

Recognition of the role of active metabolites in mediating therapeutic and/or adverse effects of many antidepressants is an important part of understanding the mechanisms of action of these medications. While virtually all antidepressants except lithium undergo extensive hepatic metabolism, the profile of activity of the resulting breakdown products varies considerably.The metabolites of some antidepressants share the primary biochemical actions of their parent compounds and appear to contribute to the therapeutic efficacy of those medications. Examples of this are the tricyclic antidepressant (TCA) nor-triptyline, the selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) fluoxetine and the serotonin-noradrenaline (norepinephrine) reuptake inhibitor venlafaxine. Less commonly, the activity of the primary metabolite may differ from that of the parent drug. An example is clomipramine. This drug is a potent serotonin reuptake blocking TCA, but its demethyl-metabolites are noradrenaline reuptake inhibitors. On the other hand, a number of effective anti-depressants, including most of the SSRIs other than fluoxetine, lack active metabolites.On the negative side, antidepressant metabolites may add to the adverse effect burden presented by their drugs of origin. At sufficiently high doses, the amphetamines resulting from the metabolism of some monoamine oxidase inhibitors, e.g. selegiline (deprenyl), may directly produce toxicity from the pharmacodynamic interaction with the parent antidepressant. While hydroxy-nortriptyline produces lesser anticholinergic effects than its parent compound, this metabolite may block the therapeutic action of nortriptyline when present in high concentrations. Excessive plasma concentrations of the major metabolite of amfebutamone (bupropion) have been associated with nonresponse and clinical worsening in some patients.Amfebutamone also illustrates the importance of pharmacokinetic factors in determining the magnitude of the influence of metabolites on antidepressant action. Active metabolites that have long elimination half-lives may predominate over the parent compound in plasma and CSF, exerting considerable clinical impact. With several of the newer drugs, notably amfebutamone, venlafaxine and nefazodone, the presence of active metabolites with half-lives approaching 1 day suggests that once-daily administration may be sufficient.The formation of most antidepressant metabolites is under strong genetic control and the metabolites themselves often exert effects on hepatic enzyme systems. This can lead to the possibility of drug-drug interactions. A key example is norfluoxetine, which is associated with potent inhibition of the cytochrome P450 isozyme 2D6 (and, consequently, reduced metabolism of drugs such as TCAs). This effect lasts for weeks even after fluoxetine discontinuation, due to the fact that norfluoxetine has a half-life of up to 2 weeks.The clearance of most antidepressant metabolites is ultimately dependent on elimination by the kidneys. Therefore, these substances tend to accumulate in states of reduced renal function, including normal aging. The relative increase in TCA hydroxy-metabolite concentrations in the elderly may contribute to the cardiovascular and other toxicities of these antidepressants in this vulnerable patient population.Attention to the existence and implications of active metabolites from the earliest stages of antidepressant drug development may help optimise the benefit: risk ratio of this valuable class of psychotropic medications.