This study will enroll participants with major depressive disorder (MDD) presenting with suicidal ideation who are assessed to be at imminent risk for suicide. The study will be conducted in 4 phases: a screening evaluation performed within 48 hours prior to Day 1 intranasal dose; a 25-day double-blind treatment phase (Days 1-25); an 8‑week initial post-treatment phase (Days 25-81); and a subsequent phase to complete a full 6‑month post-treatment follow-up (Days 81-200). Efficacy, safety, pharmacokinetic, biomarker, and pharmacogenomic evaluations will be performed in the study at defined schedule. The duration of the participant's participation will be approximately 29 weeks.