Selective serotonin reuptake inhibitors in affective disorders — I. Basic pharmacology

The selective serotonin reuptake inhibitors (SSRIs), citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline, are the result of rational research to find drugs that were as effective as the tricyclic antidepressants but with fewer safety and tolerability problems. The SSRIs selectively and powerfully inhibit serotonin reuptake and result in a potentiation of serotonergic neurotransmission. The property of potent serotonin reuptake appears to give a broad spectrum of therapeutic activity in depression, anxiety, obsessional and impulse control disorders. However, despite the sharing of the same principal mechanism of action, SSRIs are structurally diverse with clear variations in their pharmacodynamic and pharmacokinetic profiles. The potency for serotonin reuptake inhibition varies amongst this group, as does the selectivity for serotonin relative to noradrenaline and dopamine reuptake inhibition. The relative potency of sertraline for dopamine reuptake inhibition differentiates it pharmacologically from other SSRIs. Affinity for neuroreceptors, such as sigma1, muscarinic and 5-HT2c, also differs widely. Furthermore, the inhibition of nitric oxide synthetase by paroxetine, and possibly other SSRIs, may have significant pharmacodynamic effects. Citalopram and fluoxetine are racemic mixtures of different chiral forms that possess varying pharmacokinetic and pharmacological profiles. Fluoxetine has a long acting and pharmacologically active metabolite. There are important clinical differences among the SSRIs in their pharmacokinetic characteristics. These include differences in their half-lives, linear versus non-linear pharmacokinetics, effect of age on their clearance and their potential to inhibit drug metabolising cytochrome P450 (CYP) isoenzymes. These pharmacological and pharmacokinetic differences underly the increasingly apparent important clinical differences amongst the SSRIs.