A Study of the Efficacy and Safety of SP-624 in the Treatment of Adults With Major Depressive Disorder

Key Inclusion Criteria: * Males and females, aged 18 to 65 years, inclusive. * Meet DSM-5 criteria for moderate to severe MDD, as confirmed by the Mini International Neuropsychiatric Interview (MINI). * In generally good physical health, in the opinion of the Investigator. * Body mass index (BMI) must be ≥ 18 and ≤ 45 kg/m2. Key Exclusion Criteria: * Female who is pregnant, breastfeeding, or less than 6 months postpartum at screen. * A history of or current DSM-5 diagnosis of MDD with psychotic features, any schizophrenia spectrum and other psychotic disorders, bipolar disorder, or personality disorder. * Presence or history of any known clinically significant cardiovascular disorders including, but not limited to: coronary artery disease, heart failure, valvular heart disease, cardiomyopathies, myocardial infarction, chamber enlargement or hypertrophy, or orthostatic hypotension. * Presence of uncontrolled hypertension, defined as consistent sitting systolic blood pressure (SBP) \>160 mmHg or consistent sitting diastolic blood pressure (DBP) \>95 mmHg despite present therapy. * Screening laboratory value(s) outside the laboratory reference range that are considered to be clinically significant by the Investigator (clinical chemistry, hematology, thyroid function, and urinalysis).