The purpose of this study is to determine if treatment with a single dose of RE104 for Injection reduces depressive symptoms in participants with moderate-to-severe postpartum depression (PPD) as compared to active-control.

Inclusion Criteria:

• Is ≤15 months postpartum at Screening.
• Meet DSM-5 criteria for postpartum depression (PPD): experiencing a major depressive episode that began at any time starting at the beginning of the second trimester (≥14 weeks) of pregnancy through 4 weeks post delivery.
• Has a Hamilton Depression Scale (HAM-D) total score meeting severity threshold at Screening and Baseline.
• Is not using any psychotropic medications or psychotherapy for 30 days prior to Screening, OR are on an already stable/established regimen of SSRIs or psychotherapy for 30 days prior to Screening.
• Has ceased breastfeeding at Screening.
• Has a negative pregnancy test at Screening and Day 0 prior to study drug administration.

Exclusion Criteria:

• History or active postpartum psychosis per Investigator assessment.
• History of treatment-resistant depression within the current postpartum depressive episode.
• Has a significant risk of suicide.
• Active or medical history of bipolar disorder, schizophrenia, schizoaffective disorder, psychotic disorder and/or borderline personality disorder, or first-degree family history of psychosis or bipolar disorder.
• Medically significant condition rendering unsuitability for the study .
• Has received electroconvulsive therapy (ECT) or transcranial magnetic stimulation within 90 days prior to Screening.
• Has used psychedelics such as psilocybin, ayahuasca, mescaline, or LSD (with the exception of cannabis) within 12 months prior to Screening.
• Has used or will need to use prohibited medications.

Data sharing will be consistent with the results submission policy of ClinicalTrials.gov.