The study was withdrawn based on business-related matters and there are no new safety concerns related to the trial. No patients were enrolled.

Study Overview This clinical trial is a Phase 3 research study designed to test the safety and effectiveness of an investigational drug called REL-1017 (esmethadone HCl). The goal is to determine if REL-1017 can help people with major depressive disorder (MDD) who are already taking other antidepressant medications but have not experienced enough improvement.

Why This Study Is Important MDD is a serious condition that affects mood, cognition, motivation, energy, and daily life. Some people do not respond fully to standard antidepressants. REL-1017 is being tested as an additional treatment to see if it can improve symptoms more effectively.

Who Can Participate The study is enrolling adults diagnosed with major depressive disorder who do not have enough relief from their current antidepressant medications. Specific criteria must be met to join the study.

What Happens in the Study Participants will be randomly assigned to receive either REL-1017 or a placebo (an inactive substance) along with their current antidepressant. This process helps researchers understand the effects of the drug. Neither the participants nor the researchers will know who receives REL-1017 or the placebo during the study.

Study Details Duration: The study involves several weeks of treatment and follow-up visits. Assessments: Participants will complete health evaluations, including physical exams and questionnaires, to monitor progress and side effects.

Costs: Study-related treatments and assessments are provided at no cost to participants.

Potential Benefits and Risks Benefits: Participants may experience improvements in depressive symptoms. Risks: As with any investigational medication, there may be side effects. All participants will be closely monitored by the study team.

Inclusion Criteria:

1. Written informed consent.
2. Male or female participant, aged 18 to 65 years, inclusive, at Screening.
3. Body mass index (BMI) between 18.5 and 30.0 kg/m2, inclusive, at Screening.
4. Participant understands the study requirements, is willing and able to commit to meet all study requirements, adhere to both approved ADT and study drug regimen, and complete all assessments and all scheduled visits, per Investigator judgment.

Exclusion Criteria:

1. History or presence of clinically significant abnormality as assessed by physical examination, medical history, 12-lead ECG, vital signs, or laboratory values, which in the opinion of the Investigator would jeopardize the safety of the participant or the validity of the study results, including established QT prolongation, long QT syndrome, torsades de pointes, bradyarrhythmia, ventricular tachycardia, uncompensated heart failure (greater than NYHA Class 1 CHF), uncontrolled hypokalemia, or uncontrolled hypomagnesemia.
2. Any medical, psychiatric condition, or social context that, in the opinion of the Investigator, is likely to unfavourably alter the risk-benefit of participant, to interfere with protocol compliance, or to confound safety or efficacy assessments.
3. Triplicate 12-lead ECG with average QTcF ≥450 msec, and/or a QRS interval ≥120 msec at Screening.
4. Poorly controlled diabetes as defined by a glycosylated hemoglobin (HbA1c) >8.5% (69 mmol/mol), despite standard care. (Note: re-screening of patients who has failed the screening process due to not meeting this exclusion criterion is allowed after diabetes treatment adjustment and level of glycosylated hemoglobin (HbA1c) back to <8.5%)