This study tests how well a new drug, SP-624, works for adults with Major Depressive Disorder (MDD) compared to a dummy pill called a placebo. MDD is a condition that makes people feel very sad, empty, or irritable for a long time. The study is in Phase 2B, meaning it's checking both how safe and effective the drug is. Participants must be between 18 and 65 years old, have moderate to severe MDD, and be generally healthy. Some people cannot join, like those with certain heart problems or women who are pregnant. The study uses the DSM-5, a book that doctors use to identify mental disorders, to confirm MDD. A special interview called the MINI helps with this diagnosis. Your body mass index (BMI), a measure of body fat using height and weight, must be between 18 and 45 kg/m².

• The study lasts several weeks, with multiple visits to the clinic.
• Participants will receive compensation for their time and travel.
• Potential risks include side effects from the drug.