A Study of ONO-1110 in Patients With Major Depressive Disorder

Inclusion Criteria: 1. Japanese (sex not specified) 2. Participants who, in the opinion of the principal (or sub-investigator), are capable of understanding the content of the clinical trial and complying with its requirements 3. Participants diagnosed with major depressive disorder based on DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) criteria, as determined through an interview using the M.I.N.I. (Mini-international neuropsychiatric interview) 4. Outpatients 5. Participants whose current depressive episode has lasted for at least 2 months but no more than 12 months 6. Participants with a HAM-D17 (Hamilton depression rating scale 17 items) total score of 18 or higher and a CGI-S (Clinical global impression-Improvement) score of 4 or higher Exclusion Criteria: 1. Participants with a current or past history of psychiatric or neurological disorders that meet any of the following criteria: * Participants with a comorbid psychiatric disorder other than major depressive disorder as defined by DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) (assessed using the M.I.N.I.(Mini-international neuropsychiatric interview)) * Participants with major depressive disorder with mixed features, psychotic features, or catatonia as defined by DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) * Participants with a current or past history of schizophrenia or other psychotic disorders as defined by DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) * Participants with neurodevelopmental disorders or personality disorders as defined by DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) * Participants with a current or past history of clinically significant neurological disorders (including epilepsy) * Participants with neurodegenerative diseases (such as Alzheimer's disease, Parkinson's disease, multiple sclerosis, etc.) 2. Participants who, in the opinion of the principal (or sub-investigator), have not responded to at least two different antidepressants, each administered at an adequate dose for at least 6 weeks, during the current or past depressive episode. 3. Participants who have used adjunctive treatments such as lithium, triiodothyronine/thyroxine, lamotrigine, valproate, carbamazepine, or atypical antipsychotics, or who have used combination therapy with antidepressants for the current depressive episode.