The purpose of the study is to evaluate the efficacy, safety, and tolerability of SLS-002 (intranasal racemic ketamine) in addition to standard of care on symptoms of Major Depressive Disorder (MDD) and suicidality, in participants who are assessed to be at imminent risk for suicide, as measured by the change from baseline on the Montgomery-Asberg Depression Rating Scale (MADRS) total score at 24 hours post first dose.

This is a 2-part study: Part 1 is open-label, while Part 2 is a randomized, double-blind, placebo-controlled, multicenter study. The study will enroll participants with MDD for whom a physician has assessed to be at risk for suicide. The study will consist of a screening evaluation, a treatment period, and a safety follow-up period. Participants' safety will be evaluated throughout the study. If you or a loved one are having thoughts of suicide, please seek immediate medical help. Go to the emergency room or call the National Suicide Prevention Lifeline at 1-800-273-8255.

Potential participants may visit http://frontiersstudy.com/

Inclusion Criteria:

• Participant with diagnosis of current MDD (unipolar without psychotic features) per Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), with symptoms present for at least 4 weeks, based on psychiatric intake and confirmed by the Mini International Psychiatric Interview Version 7.02 for Suicidality Disorders (MINI).
• Participant has a Montgomery Åsberg Depression Rating Scale (MADRS) total score of ≥28 predose on Day 1, and has a score of 5 or 6 on item 10.
• Participant has a Clinical Global Impression of Severity for Suicidal Ideation and Behavior (CGIS-SI/B) score of ≥4 predose on Day 1.
• Participant requires psychiatric hospitalization due to significant risk of suicide, has ≥15 on the S-STS CMCM total score, and a score of 6-9 (inclusive) on the S-STS CMCM Clinician judgment of subject's risk of a suicide attempt or death by suicide at this time.
• Participant has a history of previous suicide attempt(s), as confirmed on the Columbia Suicide Severity Rating Scale (C-SSRS) with a history of at least one actual attempt, or if the attempt was interrupted or aborted, is judged to have been serious in intent.
• Participant is willing and able to take prescribed non-investigational antidepressant therapy(ies) at investigator's discretion for at least the duration of the study.

Exclusion Criteria:

• Participant has seizures, intellectual disability, a neurocognitive disorder or a lifetime diagnosis of bipolar disorder, any mood disorder with psychotic features, schizophrenia or other psychotic disorder, obsessive compulsive disorder, or antisocial personality disorder.
• In the investigator's opinion, participant has chronic, refractory treatment-resistant depression from >4 adequate therapeutic trials of antidepressants (with or without adjuvants and/or ECT) as confirmed by Antidepressant Treatment Response Questionnaire (ATRQ).
• Participant has a body mass index (BMI) >40 or <18 at screening, uncontrolled hypertension, or any clinically significant medical condition that might confound the results.
• Participant meets the DSM-5 criteria for moderate or severe substance use disorder or a positive urine test for drugs of abuse.
• Participant does not meet or is not willing to comply with the requirements listed in prohibited and restricted medications and therapies in the protocol, as well as required washout periods prior to participation.

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

Part 1 is open-label. Part 2 is a double-blind placebo-controlled study.