Chapter 178: Atypical and Serotonergic Antidepressants

INTRODUCTION

Atypical and serotonergic antidepressants are commonly referred to as newer or second-generation antidepressants, to distinguish them from the first-generation monoamine oxidase inhibitors and cyclic antidepressants. As a group, these antidepressants are the most popular form of psychopharmacologic therapy for the treatment of major depression, obsessive-compulsive disorder, panic disorders, and eating disorders.¹ These antidepressants produce less severe toxicity in overdose and are associated with fewer fatalities than either cyclic antidepressants or monoamine oxidase inhibitors.^2,3 This favorable overdose profile is tempered by the U.S. Food and Drug Administration black box warning regarding the use of some agents by patients <24 years old due to increased suicidal ideation and behavior.

PHARMACOLOGY

This group of antidepressants is a heterogeneous collection of drugs that differ significantly in chemical structure, mechanism of action, pharmacokinetic characteristics, and adverse effect profile.¹ Nonetheless, they also share many important similarities.

Most possess serotonergic activity and, especially in combination with other serotonergic agents, have the potential to produce serotonin syndrome. Thus, they all carry specific warnings about this syndrome, particularly against their combination with monoamine oxidase inhibitors and other high-potency agents.

Most of these antidepressants do not significantly inhibit cardiac sodium, calcium, or potassium ion channels, with a lower risk for cardiac toxicity, which in large part explains their greater safety in overdose compared with cyclic antidepressants. These agents do not inhibit monoamine oxidase activity and are not associated with tyramine-like reactions. Unlike monoamine oxidase inhibitor toxicity, indirect sympathomimetics can be used for hypotension induced by these atypical antidepressants.

These agents have negligible affinity for acetylcholine, dopamine, γ-aminobutyric acid, glutamate, and β-adrenergic receptors. Although their exact mechanism of action remains poorly understood, it is traditionally attributed to inhibition of neurotransmitter reuptake or interruption of negative feedback loops.

These antidepressants are metabolized primarily by hepatic enzyme systems (cytochrome P-450 pathways). If two drugs are given that interact with a common metabolic pathway, drug levels can increase or decrease depending on the interaction. In addition, hepatic dysfunction can lead to elevated drug levels and subsequent drug toxicity. These antidepressants are not detected by routine hospital serum and urine drug screens. Specialty laboratories can measure parent drug and metabolite plasma levels, but this information is useful only for the confirmation of suspected drug overdose. Specific levels are not immediately available, nor do they affect patient management. Postmortem drug redistribution is likely to occur with these agents, which affects forensic investigation.

ATYPICAL ANTIDEPRESSANTS

The atypical antidepressants have unique chemical structures different from each other and from the other antidepressant classes (Table 178-1). They possess some unique clinical features at therapeutic doses and with overdoses.