The goal of this project is to quantify the effectiveness and safety of escitalopram oxalate oral solution in the treatment of first-episode, drug-naïve patients with major depressive disorder, and to explore the mechanisms underlying its antidepressant effects using multi-omics approaches. By integrating clinical, cognitive, laboratory, imaging, genetic, and environmental data, the study aims to identify patient subgroups who are most likely to benefit from escitalopram, thereby promoting individualized and precision treatment for depression.

This multicenter, prospective, single-arm intervention study will enroll 200 adults aged 18-65 years with major depressive disorder, who will receive escitalopram oxalate oral solution for 8 weeks. Depressive symptoms, cognitive function, and adverse events will be assessed at baseline, during treatment, and after 8 weeks of treatment to evaluate efficacy and safety. Escitalopram blood concentrations will be measured at week 4 to monitor treatment adherence and support safety evaluation. Through comprehensive data collection and multimodal analysis, this project seeks to clarify the biological mechanisms of escitalopram and provide evidence to guide more precise clinical use of antidepressant therapy.

Inclusion Criteria:

1. Aged 18-65 years (including 18 and 65), no gender restriction, Han Chinese ethnicity.
2. Meets the diagnostic criteria for depressive disorders according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), confirmed by the Mini-International Neuropsychiatric Interview, version 5.0 (MINI).
3. Outpatients or inpatients; Hamilton Depression Rating Scale-17 items (HAMD-17) score ≥17; Hypomania Checklist-32 (HCL-32) score ≤13; and Clinical Global Impressions-Severity (CGI-S) score ≥4.
4. First depressive episode (duration ≤3 months), with no use of antidepressants or other psychotropic medications in the past 3 months.
5. Written informed consent obtained from the patient.

Exclusion Criteria:

1. Meet DSM-IV diagnostic criteria other mental disorders, including schizophrenia spectrum and other psychotic disorders, bipolar and related disorders, obsessive-compulsive disorder, etc..
2. Individuals with intellectual disabilities or who are unable to cooperate for other reasons, or those lacking or having incomplete civil capacity during the onset of illness;
3. Suffering from neurological or organic brain diseases (such as stroke, cerebral hemorrhage, brain tumors, Parkinson's disease, epilepsy, etc.) and a history of severe traumatic brain injury;
4. Have attempted suicide within the past 3 months, or currently present a high suicide risk, defined as a Montgomery-Åsberg Depression Rating Scale (MADRS) item 10 score ≥5.
5. Are pregnant or breastfeeding, cannot use reliable contraception during the study, or plan to conceive (or impregnate a partner) within 3 months after study initiation.
6. Have known allergies to escitalopram oxalate or its excipients.
7. Are currently taking medications that may interfere with the evaluation of escitalopram efficacy.
8. Have participated in another drug clinical trial within the past 3 months.
9. Have contraindications to MRI scanning, such as metal implants or claustrophobia.
10. Considered unsuitable for study participation by the investigators for any other reason.