Metabolism of tricyclic antidepressants

1. Despite the considerable advances in the treatments available for mood disorders over the past generation, tricyclic antidepressants (TCAs) remain an important option for the pharmacotherapy of depression. 2. The pharmacokinetics of TCAs are characterized by substantial presystemic first-pass metabolism, a large volume of distribution, extensive protein binding, and an elimination half-life averaging about 1 day (up to 3 days for protriptyline). 3. Clearance of tricyclics is dependent primarily on hepatic cytochrome P450 (CYP) oxidative enzymes. Although the activities of some P450 isoenzymes are largely under genetic control, they may be influenced by external factors, such as the concomitant use of other medications or substances. Patient variables, such as ethnicity and age, also affect TCA metabolism. The impact of gender and related reproductive issues is coming under increased scrutiny. 4. Metabolism of TCAs, especially their hydroxylation, results in the formation of active metabolites, which contribute to both the therapeutic and the adverse effects of these compounds. 5. Renal clearance of the polar metabolites of TCAs is reduced by normal aging, accounting for much of the increased risk of toxicity in older patients. 6. Knowledge of factors affecting the metabolism of TCAs can further the development and understanding of newer antidepressant medications.