This is a Phase 2 study randomized, quadruple masked, multi-center study , with a Open Label Extension, designed to investigate the efficacy and safety of BPL-003 combined with psychological support in patients with treatment resistant depression (TRD).

Approximately 200 eligible participants will receive a single dose of either low, medium, or high doses of BPL-003, given intranasally, with 8 weeks of follow up assessments. Following this participants may receive a second dose of either monophasic or biphasic dose of BPL-003, given intranasally, with another 8 weeks of follow up assessments.

Psychological support will be given before, during and after each dosing.

Inclusion Criteria:

1. At least moderate major depressive disorder.
2. Diagnosed with TRD defined as failure to respond to an adequate dose and duration of at least 2 pharmacological treatments based on the MGH ATRQ assessment.
3. Hamilton Depression Rating Scale score ≥19 at Screening and Baseline.
4. CGI-S ≥4 at Screening and Baseline.
5. If currently taking antidepressant medications, willing and able to discontinue current antidepressants.

Exclusion Criteria:

1. Current or past history of schizophrenia, psychotic disorder including psychotic depression, bipolar disorder, delusional disorder, schizoaffective disorder, or any other severe psychiatric disorder.
2. Current personality disorders.
3. First-degree family history of schizophrenia, bipolar disorder, delusional disorder, or schizoaffective disorder.
4. Current alcohol or substance use disorder (other than caffeine or nicotine).
5. A participant who at any time has been unresponsive to ketamine, esketamine, an adequate course of treatment with electroconvulsive therapy, or has received vagal nerve stimulation or deep brain stimulation.
6. Suicidal ideation or behavior within the 12 months prior to the start of Screening or on Day 1 prior to dosing.
7. Suicide attempt and/or self-injurious behavior within the last 12 months prior to Screening.
8. Uncontrolled medical conditions e.g. hypo/hyperthyroidism, diabetes, renal failure.
9. History or current uncontrolled hypertension.
10. Seizure disorder or any seizure in the 2 years prior to Screening.
11. Has clinically significant results on ECG during the Screening.
12. Any nasal obstruction, blockage, or symptoms of congestion at the time of dosing that in the investigator's opinion may interfere with the administration of the study medication.
13. Female participants who are pregnant, lactating, or of childbearing potential and not willing to use adequate forms of contraception during the study.
14. Male participants who are sexually active and not willing to use adequate forms of contraception during the study.

Due to the GDPR, individual participant data will not be shared publicly. Group data will be presented in publication after study completion

CORE: Quadruple masking: participant, Investigator, therapist, outcomes assessor and Sponsor.

OLE: Open Label Extension.