*2.1. Study Subjects*

Since MADRS score is regarded as the criterion for determining response to drug administration, plasma samples were collected from ten patients with MDD who participated in a clinical trial testing the efficacy and safety of escitalopram dose escalation at Seoul National University Hospital, Seoul, Republic of Korea, from February 2013 to February 2016 [23]. All the participants are Korean patients. The trial included two phases: open-label treatment for 4 weeks with a standard dose (10–20 mg/day) of escitalopram, followed by randomized, double-blinded treatment for 6 weeks with 20 mg/day or 30 mg/day escitalopram. Patients aged 18–65 years with a primary diagnosis of MDD, as defined by the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (text revision), were included. All patients had a total MADRS score ≥ 18 at initial screening and baseline visits. Subjects were excluded if they experienced hypersensitivity to escitalopram, had received any psychoactive medications such as antipsychotics, mood stabilizers, or selective monoamine oxidase inhibitors, had symptoms of depression and were deemed resistant to two or more antidepressant treatments, had psychiatric disorders other than MDD or a prior history of psychiatric disorders, such as manic or hypomanic episodes, schizophrenia, schizoaffective disorder, or substance abuse disorder, were at significant risk of suicide, as evaluated by an investigator or with score of ≥ 5 on item 10 of MADRS, or had a history of neurologic disorders or medically unstable conditions (e.g., renal or hepatic impairment, or cardiovascular, pulmonary, or gastrointestinal disorders). Of the patients who entered the clinical trial, five responders (1 male and 4 females) and five non-responders (1 male and 4 females) were selected, from each of whom plasma samples were obtained at four time points: baseline, week 1, week 4 (randomization), and week 10 (6 weeks after randomization) for proteomic analysis. An additional 24 patients were selected, 19 responders and five non-responders, from each of whom plasma samples were obtained at baseline and at week 1. The primary efficacy outcome was a change in total MADRS score. Response was defined as ≥50% reduction in baseline MADRS score after 4 and 10 weeks of treatment. None of these patients were taking medication that could alter the blood levels of relevant factors, such as nonsteroidal anti-inflammatory agents or steroids, and none had any acute or chronic diseases, such as cardiovascular disease, pulmonary disease, hypertension, endocrine abnormalities, rheumatic diseases, or cerebrovascular disease. The study protocol was approved by the Institutional Review Board of Seoul National University Hospital (Number: 1008-116-329, approved on 2 December 2010). The study was performed in accordance with the ethical principles stated in the Declaration of Helsinki and the International Conference on Harmonization Good Clinical Practice guidelines. All patients provided written informed consent and were free to discontinue the study at any time.
