The main goal of this clinical trial is to learn if the active study drug (code name: SP-624) has any effect on the electrical activity of the brain in adult healthy volunteers and in adults with major depression. Another goal of the study is to learn if SP-624 improves memory and learning in adults with major depression. The study will also provide additional information on the safety of SP-624 and how well it is tolerated in adult healthy volunteers and adults with major depression.

Researchers will compare SP-624 to a placebo (a look-alike substance that contains no drug) to see if SP-624 has any effect on study tests.

Study participants will:

Take capsules of study drug (SP-624 or a placebo) once daily for 2 weeks; visit the clinic at Screening, Day 1 (first dose of study drug), and Day 15 (last dose of study drug) for checkups and tests; and have phone call check-ups on Day 7 and about 1 week after the last dose of study drug.

Key Inclusion Criteria:

• In generally good health
• Body mass index (BMI) between 18.0 and 40.0 kg/m2
• Willing to comply with the requirements of the study
• For participants with depression: must meet study criteria for moderate to severe major depressive disorder

Key Exclusion Criteria:

• Female who is pregnant or breastfeeding.
• Clinically significant health condition or clinically significant abnormal results on screening health tests
• For participants with depression: presence of exclusionary study criteria for co-morbid psychiatric conditions or medication history.