Treatment-Resistant Depression: A Literature Review of Innovative and Emerging Treatment Strategies

Abstract

Introduction and objective: Major depressive disorder (MDD) is a heterogeneous psychiatric condition characterized by affective, cognitive and somatic symptoms that considerably impair daily functioning. It is defined clinically by prolonged low mood, diminished interest or pleasure, and a range of associated disturbances, including changes in cognitive skills, appetite, and sleep patterns. Although many treatment strategies are well-established and available, not all patients achieve satisfactory improvement. Treatment-resistant depression (TRD), often defined as insufficient response to at least two antidepressant trials of proper dose and duration, represents a major therapeutic challenge. This review aims to present and assess emerging treatment strategies for TRD, focusing on their mechanisms, effectiveness and safety. Brief description of the state of knowledge: Recent advances in the understanding of depression pathophysiology have initiated the development of new therapeutic approaches targeting various systems thought to be involved in mood regulation. Glutamatergic modulators, such as ketamine and esketamine, have demonstrated prompt antidepressant effects, particularly in patients with severe or refractory symptoms. Neuromodulatory techniques, including repetitive transcranial magnetic stimulation (rTMS) and electroconvulsive therapy (ECT), remain important treatment options with growing research interest. Simultaneously, heightened attention has been directed toward psychedelic-assisted therapies, especially psilocybin, which may produce sustained antidepressant effects through modulation of various neural networks. Additionally, invasive interventions such as vagus nerve stimulation (VNS) and deep brain stimulation (DBS) are being studied for resistant cases, although their use remains limited. Summary: Emerging therapies for TRD offer promising alternatives to conventional treatment strategies, particularly for patients not responding to routine interventions. Further research is crucial for the optimization of therapeutic outcomes and integration of those novel approaches into clinical practice.