A Study of SDI-118 in Participants in Remission From Depression

Inclusion Criteria: * Male and female participants between 25 and 55 years of age (inclusive) at screening. * Are remitted from depression. * Have received prescribed treatment with an antidepressant or a recognised psychotherapy for depression (e.g. cognitive behaviour therapy) for a previous MDE * Report present subjective cognitive impairment (such as difficulty concentrating, slow thinking, and difficulty in learning new things or remembering things). * Have not been treated with antidepressants or received other psychotherapy for depression for at least six weeks prior to Screening Visit 1. * Otherwise healthy with no clinically significant abnormalities as determined by medical history, physical examination, blood chemistry assessments, haematologic assessments, and urinalysis, measurement of vital signs, and Electrocardiogram (ECG). * Negative serology test for human immunodeficiency virus (HIV) antibodies, hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) antibodies at screening. * Have a body mass index (BMI) of 18 to 36 inclusive. * Agree not to use herbal medications (including herbal tea, St. John's Wort), within 14 days prior to study agent administration through to the final follow-up visit. * Participants must be able and willing to give written, informed consent, indicating that they understand the purpose of and procedures required for the study and are willing to participate in the study. * The participant, in the opinion of the investigator, is willing and able to adhere to the study visit schedule and other requirements, prohibitions and restrictions of the study. Exclusion Criteria: * They are left-handed. * Have immediate recall of greater than 22 words from the International Shopping List Test (ISLT) and have delayed recall of greater than 8 words from the ISLT 15 mins after the presentation of the word list. * Positive urine drug screen or alcohol breath test at screening or assessment visits. * History or presence of significant neurological or psychiatric conditions except those related to MDD. * Any suicidal ideation of type 4 or 5 in the Columbia Suicide Severity Rating Scale (C-SSRS) in the past 3 months prior to screening or at screening or baseline visit. * Has a known clinically relevant structural brain abnormality as determined by e.g. previous MRI, or, persistent MRI imaging artefact which is judged to produce extensive imaging distortions. * Has a disease or takes medication that could, in the investigator's and/or sponsor's opinion, interfere with the assessments of safety, tolerability, or efficacy, or interfere with the conduct or interpretation of the study.