Randomized, Double-Blind, Placebo-Controlled Trial of REL-1017 as Adjunctive Treatment for MDD

Inclusion Criteria: * Body mass index (BMI) between 18.0 and 35.0 kg/m2. * Diagnosed with Major Depressive Disorder (MDD) based on Structured Clinical Interview for DSM-5 (SCID-5) for MDD. * Inadequate response to 1 to 3 adequate antidepressant treatments in the current MDE. Exclusion Criteria: * Psychiatric hospitalization during the current major depressive episode. * History or presence of clinically significant health conditions which in the opinion of the Investigator would negatively impact the safety of the participant. * Participants who, in the Investigator's judgment, are at significant risk for suicide. * Pregnant or planning to become pregnant.