The importance of presystemic extraction in clinical psychopharmacology

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.

Ketoconazole inhibition of triazolam and alprazolam clearance: differential kinetic and dynamic consequences

BACKGROUND

Kinetic and dynamic consequences of metabolic inhibition were evaluated in a study of the interaction of ketoconazole, a P4503A inhibitor, with alprazolam and triazolam, two 3A substrate drugs with different kinetic profiles.

METHODS

In a double-blind, 5-way crossover study, healthy volunteers received (A) ketoconazole placebo plus 1.0 mg alprazolam orally, (B) 200 mg ketoconazole twice a day plus 1.0 mg alprazolam, (C) ketoconazole placebo plus 0.25 mg triazolam orally, (D) 200 mg ketoconazole twice a day plus 0.25 mg triazolam, and (E) 200 mg ketoconazole twice a day plus benzodiazepine placebo. Plasma concentrations and pharmacodynamic parameters were measured after each dose.

RESULTS

For trial B versus trial A, alprazolam clearance was reduced (27 versus 86 mL/min; P < .002) and apparent elimination half-life (t1/2) prolonged (59 versus 15 hours; P < .03), whereas peak plasma concentration (Cmax) was only slightly increased (16.1 versus 14.7 ng/mL). The 8-hour pharmacodynamic effect areas for electroencephalographic (EEG) beta activity were increased by a factor of 1.35, and those for digit-symbol substitution test (DSST) decrement were increased by 2.29 for trial B versus trial A. For trial D versus trial C, triazolam clearance was reduced (40 versus 444 mL/min; P < .002), t1/2 was prolonged (18.3 versus 3.0 hours; P < .01), and Cmax was increased (2.6 versus 5.4 ng/mL; P < .001). The 8-hour effect area for EEG was increased by a factor of 2.51, and that for DSST decrement was increased by 4.33. Observed in vivo clearance decrements due to ketoconazole were consistent with those anticipated on the basis of an in vitro model, together with in vivo plasma concentrations of ketoconazole.

CONCLUSION

For triazolam, an intermediate-extraction compound, impaired clearance by ketoconazole has more profound clinical consequences than those for alprazolam, a low extraction compound.

Paroxetine potentiates the central nervous system side effects of perphenazine: contribution of cytochrome P4502D6 inhibition in vivo

BACKGROUND

Paroxetine is a frequently used antidepressant and a potent inhibitor of the CYP2D6 isozyme in vitro (inhibition constant [Ki] = 0.15 micromol/L). Most classic antipsychotic agents such as perphenazine are metabolized by the CYP2D6 isozyme and are often coadministered with antidepressant agents. This study assessed the extent of changes in CYP2D6 isozyme activity in vivo after pretreatment with paroxetine and its consequences on perphenazine kinetics and central nervous system effects.

METHODS

Eight extensive metabolizers for CYP2D6 were administered a single dose of perphenazine (0.11 mg/kg orally) or placebo following a randomized double-blind design. Perphenazine plasma concentrations and effects were assessed for a period of 8 hours. Subsequently, subjects were treated with a standard therapeutic dose of paroxetine (20 mg/day orally) for 10 days and test sessions with perphenazine and placebo were repeated.

RESULTS

Paroxetine treatment resulted in a twofold to 21-fold decrease in CYP2D6 activity (p < 0.001). After pretreatment with paroxetine, perphenazine peak plasma concentrations increased twofold to 13-fold (p < 0.01). This was associated with a significant increase in central nervous system side effects of perphenazine, including oversedation, extrapyramidal symptoms, and impairment of psychomotor performance and memory (p < 0.05).

CONCLUSION

Coadministration of perphenazine after pretreatment with a standard therapeutic dose of paroxetine increased the plasma concentration and central nervous system side effects of perphenazine, primarily as a result of inhibition of the CYP2D6 isozyme. In patients who are at steady state with paroxetine, a reduction of perphenazine dose may be required to prevent central nervous system side effects.