This study tests a new treatment called SP-624 for adults with Major Depressive Disorder (MDD). MDD is a type of depression that makes people feel very sad and lose interest in daily activities. The study compares SP-624 to a placebo, which is a harmless pill with no medicine. This helps doctors see if SP-624 really works and is safe.

Eligibility: To join, you must be between 18 and 65 years old, have moderate to severe MDD, and be in good health. You cannot join if you are pregnant, have certain mental health conditions like bipolar disorder, or have serious heart problems.

• This is a Phase 2B study, which means SP-624 has been tested before but needs more research.
• Participants must attend several visits for tests and evaluations.
• There may be risks, like side effects, and benefits, like improving depression symptoms.