Deep Phenotyping for Clinical Inferring Response in Treatment Resistant Depression

Inclusion Criteria: * Obligated Participation in "Biobanking" of Max Planck Institute of psychiatry * Age: ≥18 years * DSM-V diagnosis of major depressive disorder (MDD; confirmed by Mini-Interview) * Treatment resistant depression (TRD): TRD stage I or TRD stage II * Indication for antidepressant pharmacotherapy * Indication for lithium augmentation * Ability to provide informed consent Exclusion Criteria: * Age: \< 18 years * Medical conditions incompatible with lithium therapy * History of hypomanic or manic episode * Two or more antidepressant substances simultaneously to lithium, except the combination with sleep-promoting antidepressants like Mirtazapine, Mianserin, or Trazodone * An alternative pharmacological augmentation strategy simultaneously to lithium disorder * Quetiapine immediate- and extended release more than 100 mg in total * Current substance use disorder except for moderate alcohol or benzodiazepine use bound to the current episode, or smoking * Patients who are not suitable for the study in the opinion of the investigator * Patients with a relevant comorbidity of the central nervous system such as focal neurological disease (stroke, tumour), current or past epilepsy, CNS inflammation including autoimmune disease, traumatic brain injury, past brain surgery * Patients that are not able to provide informed consent * Electroconvulsive therapy (ECT) in the current depressive episode * Repetitive transcranial magnetic stimulation (rTMS) in the current depressive episode