Key Inclusion Criteria:
1. Age: 18 to 45 years old (inclusive), female.
2. Participant had a major depressive episode that began no earlier than the third trimester and no later than the first 4 weeks following delivery, as diagnosed by Structured Clinical Interview for Diagnostic and Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) .
3. Participant was \<=12 months postpartum.
4. Participant has a Hamilton Rating Scale for Depression (HAM-D) total score ≥26 at screening and Day 1 (prior to dosing).
5. Participant either must have ceased lactating at screening or, if still lactating or actively breastfeeding at screening, must agree to temporarily cease giving breast milk to her infant(s).
6. Fully understand the procedures and sigh the informed consent.
Key Exclusion Criteria:
1. Participant is currently at significant risk of suicide, or has attempted suicide associated with the current episode of MDD.
2. Participant has a history of treatment-resistant depression, defined as persistent depressive symptoms despite treatment with adequate doses of antidepressants from two different classes for an adequate amount of time.
3. Participant has active psychosis.
4. Participant had used modified electroconvulsive therapy (MECT), transcranial magnetic stimulation (TMS), vagus nerve stimulation (VNS), deep brain stimulation (DBS), light therapy, etc. within 1 month prior to Day 1.
5. Subject has a history of sleep apnea.
6. Participant has a medical history of bipolar disorder, schizophrenia, and/or schizoaffective disorder (such as olanzapine, risperidone, quetiapine, aripiprazole, eptifamol, ziprasidone, caliparazine, sodium valproate, lithium carbonate, etc.) within the current major depressive episode.
7. Participant has a clinically significant abnormal 12-lead ECG at the screening or baseline visits. NOTE: mean QT interval calculated using the Fridericia method (QTcF) of \>450 msec in males or \>470 msec in females will be the basis for exclusion from the study.
