Major depressive disorder (MDD) is one of the most common psychiatric disorders, characterized by extremely high prevalence, relapse rate and therapeutic resistance. Treatment with ketamine and its enantiomer esketamine is the next step towards the successful treatment and understanding of depression that is resistant to treatment with standard antidepressants. The proposed study will include approximately 50 patients with MDD and 50 healthy control subjects of both sexes. Using untargeted metabolomic approach, we plan to detect changes in biochemical pathways related to the diagnosis of treatment-resistant MDD and changes related to the mechanism of action of esketamine. The proposed research will contribute to further understanding of the mechanism of action of esketamine in patients with MDD. Metabolic changes will be associated with improvements in specific domains of depressive symptoms and symptom severity in subjects with treatment-resistant major depressive disorder.