This study aimed to evaluate the efficacy and safety of fecal microbiota transplantation (FMT) in patients with major depressive disorder (MDD) who exhibit suboptimal early response to antidepressant treatment. Additionally, it sought to investigate the impact of FMT on biological indicators, including intestinal microbiota and metabolites, in individuals with MDD

This multicenter, randomized, double-blind, placebo-controlled study aims to compare the efficacy and safety of adjunctive FMT in patients with major depressive disorder (MDD) who show limited response to initial drug therapy.

A total of 600 patients experiencing depressive episodes will be screened, with all receiving escitalopram oxalate for an initial 2-week period. Of these, 214 participants who exhibit suboptimal therapeutic response to early antidepressant treatment will be enrolled and randomized in a 1:1 ratio to either the experimental group or control group.

During the intervention, participants will continue their existing antidepressant regimen and receive a 4-week treatment with either microbiota capsules or placebo. An additional 20-week follow-up assessment will then be conducted to evaluate outcomes.

Inclusion Criteria (All 8 criterion are met):

• Outpatient or inpatient, aged 18 to 65 years (inclusive), regardless of gender;
• At the start of the screening phase, participants must meet the Diagnostic and Statistical Manual of Mental Disorders-fifth edition (DSM-5) diagnostic criteria for recurrent major depressive disorder (MDD) or single-episode MDD;
• At the start of the screening phase, participants' score on the 17-item Hamilton Depression Rating Scale (HAMD-17) must be ≥ 17;
• At the start of the screening phase, participants have not been treated with medication for their current depressive episode;
• At the start of the screening phase, participants are intended to be treated with a single antidepressant medication, Escitalopram;
• The HAMD-17 score after two weeks of treatment with the maximum tolerated dose of escitalopram was reduced by less than 20% compared with the HAMD-17 score at screening;
• Participants must have an education level above primary school and be able to understand the content of the scale;
• Participants sign the informed consent form.

Exclusion Criteria (Exclude if 1 criterion is met):

• According to DSM-5 criteria, currently or previously diagnosed as bipolar disorder, neurodevelopmental disorder, neurocognitive disorder, schizophrenia spectrum and other psychotic disorders, substance-related and addiction disorders;
• Accompanied by significant psychotic symptoms (delusions, hallucinations, etc.);
• The patient currently has severe or unstable central nervous system, cardiovascular, respiratory, liver, kidney, endocrine, blood system or other system diseases, and the researcher believes that the patient is not suitable for inclusion in this study;
• The patient currently has a serious suicide risk, and the HAMD-17 suicide risk item is ≥3 points;
• Suffering from inflammation-related diseases;
• Suffering from gastrointestinal infections, tumors and other structural abnormalities of the digestive system, including but not limited to irritable bowel syndrome, Crohn's disease, ulcerative colitis, celiac disease, etc.;
• Previous history of gastrointestinal surgery;
• Continuous use of antibiotics, probiotics, prebiotics or traditional Chinese medicine products for medical purposes for more than 2 weeks within 3 months before enrollment in the study;
• Those who are allergic to capsule ingredients and contents;
• Pregnant or lactating patients;
• Patients who are unable (such as difficulty swallowing) or unwilling to swallow capsules;
• Patients who received MECT treatment in the past 6 months.