Early observations and pilot data that first suggested a new direction
The STAR*D trial (Sequenced Treatment Alternatives to Relieve Depression) remains the most important study ever conducted in depression therapeutics. This NIMH-funded, multi-step, pragmatic trial of 4,041 real-world outpatients demonstrated that only 37% achieved remission with first-line citalopram, and cumulative remission after four sequential treatment steps was just 67% — meaning one-third of depressed patients failed all available evidence-based treatments. Critically, relapse rates increased with each treatment step, and patients who required third- or fourth-line treatments had sustained remission rates below 15%. STAR*D definitively quantified treatment-resistant depression (TRD) as a massive unmet clinical need affecting millions of patients worldwide.
Landmark RCTs and pivotal trials that established the evidence base
The observation by Berman et al. (2000) and Zarate et al. (2006) that a single subanesthetic dose of intravenous ketamine produced rapid antidepressant effects within hours — compared to weeks for conventional antidepressants — represented a paradigm shift in depression neurobiology. The Zarate NIMH crossover trial showed that 71% of TRD patients responded to IV ketamine versus 0% placebo within 24 hours, with effects lasting up to one week. This finding challenged the monoamine hypothesis that had dominated depression research for 50 years and identified the glutamate/NMDA system as a novel therapeutic target. The speed of response was unprecedented and suggested entirely different mechanisms of action, later linked to BDNF release and rapid synaptogenesis.
Follow-up studies, subgroup analyses, and real-world validation
Esketamine (Spravato), the S-enantiomer of ketamine delivered as a nasal spray, gained FDA approval in 2019 for TRD based on the SUSTAIN and TRANSFORM trial programs. TRANSFORM-1 and TRANSFORM-2 showed significant improvement in MADRS scores at 28 days when esketamine was added to a new oral antidepressant, though effect sizes were modest. The SUSTAIN-1 trial demonstrated that continued esketamine maintenance reduced relapse risk by 51-70% compared to discontinuation. Separately, psilocybin-assisted therapy emerged as a promising investigational approach, with the COMPASS Pathways phase IIb trial showing a 25mg dose produced rapid, significant MADRS improvement in TRD patients at 3 weeks. The convergence of ketamine, esketamine, and psychedelic research fundamentally expanded the mechanistic understanding and therapeutic options for TRD beyond traditional monoamine-based pharmacology.
Integration into clinical practice guidelines and recommendations
The APA 2023 depression guideline update incorporated esketamine as a recommended option for TRD, defined as failure of two adequate antidepressant trials. IV ketamine remains off-label but is acknowledged in guidelines as a rapid-acting option with emerging evidence. The CANMAT 2023 update similarly endorsed esketamine while noting concerns about long-term safety data and abuse potential. All major guidelines continue to recommend augmentation strategies (lithium, atypical antipsychotics, thyroid hormone) as first-line approaches to TRD before novel agents. Psilocybin and MDMA remain investigational and are not yet included in mainstream treatment guidelines, though their evidence base is growing rapidly.
APA Clinical Practice Guideline for Depression
Esketamine nasal spray recommended for TRD (≥2 failed adequate antidepressant trials) in combination with oral antidepressant; REMS monitoring required
CANMAT Guidelines for Management of MDD
Esketamine as second-line for TRD; IV ketamine as third-line; augmentation with atypical antipsychotics or lithium remains first-line for TRD
NICE Depression Guideline (CG90 update)
Esketamine recommended for TRD in combination with SSRI/SNRI; specialist initiation only; limited to patients with documented failure of 2 adequate trials
Now
Current standard of care and ongoing research directions
TRD management in 2025-2026 is being reshaped by rapid-acting therapies and novel mechanisms. Esketamine has established a commercial foothold with expanded indications including major depressive disorder with suicidal ideation. IV ketamine clinics have proliferated, though standardization of dosing, frequency, and monitoring remains variable. Psilocybin-assisted therapy continues through phase III trials (COMPASS Pathways), with potential FDA approval anticipated. Augmentation with atypical antipsychotics (aripiprazole, brexpiprazole, cariprazine) remains the most common first step for TRD in practice. Research frontiers include psychedelic-assisted therapy integration into healthcare systems, biomarker-guided treatment selection, neuromodulation approaches (TMS, ECT optimization), and combination strategies targeting glutamate alongside traditional monoamine pathways. The definition of 'adequate treatment' itself is being scrutinized, with measurement-based care emphasizing that many apparent TRD cases reflect inadequate dosing or duration.
TRD is most commonly defined as failure to achieve adequate response after two adequate trials of antidepressants from different classes, each at therapeutic doses for adequate duration (typically 6-8 weeks). STAR*D showed approximately 33% of patients meet this criterion. Some experts advocate a staging system (Thase-Rush or Maudsley) that grades resistance severity based on number and type of failed treatments.
How does esketamine differ from IV ketamine?+
Esketamine (Spravato) is the S-enantiomer nasal spray with FDA approval, standardized dosing (56mg or 84mg), REMS monitoring requirements, and demonstrated efficacy in phase III trials. IV racemic ketamine (off-label) uses the full racemic mixture, allows dose titration, has a larger evidence base of smaller studies, and is significantly less expensive. Both require supervised administration and monitoring for dissociation and blood pressure changes.
What is the evidence for psilocybin in depression?+
The COMPASS Pathways phase IIb trial (NEJM 2022) showed 25mg psilocybin with psychological support significantly improved MADRS scores versus 1mg control at 3 weeks in TRD. Earlier open-label studies from Johns Hopkins and Imperial College showed promising results. Phase III trials are underway. Key considerations include the requirement for trained therapist support, 6-8 hour supervised sessions, and unknown long-term safety with repeated dosing.
Should augmentation or switching be tried first in TRD?+
STAR*D and subsequent meta-analyses suggest augmentation (adding lithium, atypical antipsychotic, or thyroid hormone to the failing antidepressant) is generally preferred over switching, as it avoids losing any partial benefit from the current agent. Aripiprazole and brexpiprazole augmentation have the strongest evidence base. Switching may be preferred when the current antidepressant is poorly tolerated or there has been zero response.
