This study is testing a new medicine called ALTO-300 for people with Major Depressive Disorder (MDD). MDD is when a person feels very sad or hopeless for a long time. ALTO-300 will be taken with another antidepressant to see if it works better than a placebo (a fake medicine). People can join if they have moderate to severe MDD, are on a stable dose of SSRI (Selective Serotonin Reuptake Inhibitor), SNRI (Serotonin-Norepinephrine Reuptake Inhibitor), or bupropion for at least 6 weeks, and meet other study rules. They cannot join if they have certain medical problems, use sleep medicine every night, or have other mental health conditions like bipolar disorder.

• The study involves taking a new medicine with your current antidepressant.
• You must not be in another mental illness study at the same time.
• Participants should not have a history of allergic reactions to ALTO-300.