Newer Antidepressants

Newer Antidepressants

Scope note: "Newer antidepressants" = the post-TCA/post-MAOI second- and later-generation agents: SSRIs, SNRIs, the receptor-modulator/atypical class (SARIs, NaSSA, NDRI), the multimodal/partial-agonist agents (vortioxetine, vilazodone/SPARI), the melatonergic agonist (agomelatine), and the mechanistically distinct rapid-acting agents (esketamine/ketamine, the neurosteroid brexanolone, the bupropion–dextromethorphan combination). TCAs and MAOIs are covered only as the comparator "first-generation" backdrop the newer agents were designed to improve upon. Lithium/mood-stabilisers and bipolar pharmacotherapy appear only as antidepressant context.

1. Definition, scope & the "generation" framing

• Antidepressants are drugs that elevate mood in depressive illness; practically all clinically used agents act on monoaminergic (5HT, NE, ± DA) transmission in the brain in one way or another (KDT 8e Ch.33, p.481).
• First-generation antidepressants = the tricyclic antidepressants (TCAs) and the monoamine oxidase inhibitors (MAOIs); they have established efficacy but serious toxicity (anticholinergic, cardiac, hypotensive, lethal in overdose) and major food/drug interactions, so are no longer first-line (G&G 14e Ch.18, pp.343–4; KDT 8e Ch.33, pp.486–7).
• Second-generation ("newer") antidepressants were developed from the 1980s to overcome three TCA limitations: frequent anticholinergic/cardiovascular/neurological side effects, low safety margin (overdose fatalities), and incomplete/absent response in a subset of patients (KDT 8e Ch.33, p.487).
• The two most significant newer classes are the SSRIs and the SNRIs, which selectively inhibit membrane-associated SERT (SSRIs) or both SERT and NET (SNRIs) with no direct action on cholinergic/adrenergic/histaminergic receptors — giving improved tolerability at therapeutic doses and safety in overdose (KDT 8e Ch.33, pp.487–8; G&G 14e Ch.18, p.344).
• A further wave of mechanistically distinctive agents has emerged: the NMDA-receptor antagonists (ketamine/esketamine) and the neurosteroid GABA_A modulator brexanolone — described by G&G as "the first mechanistically distinct antidepressants approved in decades" (G&G 14e Ch.18, p.348).
• R&D reframes the whole field by rate of onset: "conventional" (delayed-onset, 4–6-week) monoamine-targeting drugs versus rapid-acting antidepressants (RAADs) such as ketamine that act within hours and last ~7–14 days (R&D 10e Ch.48 OVERVIEW, pp.4–5).
• Therapeutic empiricism has led the field — mechanistic understanding lags, and no biological marker/biomarker of depression has been reliably identified to date (R&D 10e Ch.48, pp.1, 7).