A Randomized, Double-Blind, Placebo-Controlled, Exploratory Study to Evaluate the Efficacy and Safety of Bifidobacterium PB-18 in Adults With Mild to Moderate Depressive Disorder and to Explore Its Gut-Brain Axis Mechanisms

This clinical trial is designed to evaluate whether Bifidobacterium PB-18 can improve symptoms in adults with mild to moderate depressive disorder who are not taking antidepressants or other psychotropic medications during the study. The study will also assess the safety of Bifidobacterium PB-18 and explore whether treatment-related changes in gut microbiota, metabolites, and other biological markers may help explain its potential effects through the gut-brain axis.

Depressive disorder is associated with a substantial disease burden, and many people with mild to moderate symptoms are reluctant to start antidepressant treatment because of concerns about adverse effects, delayed onset of action, or long-term medication use. Increasing evidence suggests that the gut-brain axis may play an important role in mood regulation. Bifidobacteria are probiotics that may influence emotional symptoms through effects on neurotransmitter-related pathways, inflammation, and stress responses. Based on preclinical findings and the study rationale described in the protocol, this trial will evaluate PB-18 as a potential non-drug intervention for this patient population.

This is a single-center, randomized, double-blind, placebo-controlled, parallel-group exploratory study. A total of 88 eligible participants will be enrolled and randomly assigned in a 1:1 ratio to receive either Bifidobacterium PB-18 or placebo for 8 weeks. Participants in the active treatment group will take a daily powder containing 1 × 10^10 colony-forming units of PB-18. Participants in the placebo group will take a matching maltodextrin powder with the same appearance, color, smell, and taste but without PB-18. Study product allocation and coding will be managed independently, and participants, investigators, and outcome assessors will remain blinded during the study.

Eligible participants are adults 18 to 65 years of age who meet DSM-5 diagnostic criteria for depressive disorder and have a baseline HAMD-17 score from 7 to 24. Participants must be willing to complete study visits and must not use antidepressants or other psychotropic medications during the trial. The study includes three visits: baseline, Week 4, and Week 8. At these visits, participants will undergo clinical efficacy and safety assessments, and information on concomitant medications, prohibited medications, and adverse events will be recorded throughout the study.

The main efficacy endpoint is the difference between groups in response rate at Week 8, defined as the proportion of participants with at least a 50% reduction from baseline in the 17-item Hamilton Depression Rating Scale (HAMD-17). Secondary outcomes include remission rate at Week 8, response and remission at Week 4, and changes from baseline in HAMD-17, HAMA, PSQI, and CBCT scores across follow-up visits. Safety will be evaluated by monitoring adverse events and serious adverse events during the study.