Clinically Relevant Pharmacology of Selective Serotonin Reuptake Inhibitors: An Overview with Emphasis on Pharmacokinetics and Effects on Oxidative Drug Metabolism

Abstract

This paper presents an overview of the clinically relevant pharmacology of selective serotonin reuptake inhibitors (SSRIs) with an emphasis on their pharmacokinetics and effects on cytochrome P450 (CYP) enzymes. The SSRIs are potent inhibitors of the neuronal reuptake pump for serotonin (5-hydroxytryptamine; 5-HT) and have minimal effects on a number of other sites of actions (e.g. neuroreceptors and fast sodium channels). For this reason, drugs in this class have remarkable similarity as regards acute and maintenance antidepressant efficacy and tolerability profile. However, individual members of this class differ substantially in their pharmacokinetics and effects on CYP enzymes. Most SSRIs have a half-life (t1/2) of approximately 1 day. Fluoxetine, however, has a longer t1/2 of 2 to 4 days, and its active metabolite, norfluoxetine, has an extended t1/2 of 7 to 15 days. Fluoxetine, paroxetine and, to a lesser extent, fluvoxamine inhibit their own metabolism. That is not the case for citalopram or sertraline. There are nonlinear increases in paroxetine plasma concentrations with dosage increases, but proportional changes with citalopram and sertraline. Indirect data suggest that fluoxetine and fluvoxamine also have nonlinear pharmacokinetics over their usual dosage range. Age-related increases in plasma drug concentrations for citalopram (approximately 130%) and paroxetine (approximately 50 to 100%) have been observed in healthy elderly (65 to 75 years) persons versus those who are younger. There is an age-gender interaction for sertraline, with its plasma concentrations being 35 to 40% lower in young men than in elderly or young females or elderly males. While there is no apparent change in fluvoxamine plasma levels as a function of age, plasma drug concentrations are 40 to 50% lower in males than in females. Limited data from clinical trials suggest that age-related differences with fluoxetine may be comparable to those of citalopram and paroxetine. Marked differences exist between the SSRIs with regard to effects on specific CYP enzymes and, thus, the likelihood of clinically important pharmacokinetic drug-drug interactions. The most extensive in vitro and in vivo research has been done with fluoxetine, fluvoxamine and sertraline; there has been less with paroxetine and citalopram. The available in vivo data at each drug's usually effective antidepressant dose are summarised below. Citalopram produces mild inhibition of CYP2D6. Fluvoxamine produces inhibition (which would be expected to be clinically meaningful) of two CYP enzymes. CYP1A2 and CYP2C19, and probably a third, CYP3A3/4. Fluoxetine substantially inhibits CYP2D6 and probably CYP2C9/10, moderately inhibits CYP2C19 and mildly inhibits CYP3A3/4. Paroxetine substantially inhibits CYP2D6 but doses not appear to inhibit any other CYP enzyme. Sertraline produces mild inhibition of CYP2D6 but has little, if any, effect on CYP1A2, CYP2C9/10, CYP2C19 or CYP3A3/4. Understanding the similarities and differences in the pharmacology of SSRIs can aid the clinician in optimal use of this important class of antidepressants.