OSU6162 as add-on in SSRI/SNRI-resistant Depression

Inclusion Criteria In order to be included in the study, subjects must meet the following criteria: 1. Signed informed consent. 2. Age: 25-75 on the day of screening. 3. Meeting DSM-5 criteria for major depressive disorder as confirmed by the Mini International Neuropsychiatric Interview (MINI). 4. A symptom-free period preceding the current episode within the past two years confirmed at interview. 5. Not significantly improved, as judged by both doctor and patient, after having been treated with one of the following SSRIs/SNRIs: citalopram, escitalopram, paroxetine, sertraline, fluoxetine, duloxetine, or venlafaxine for at least 6 weeks. 6. Displaying a sum score of MADRS ≥22. 7. In women of childbearing potential (WOCBP): negative result of a pregnancy test and a method of contraception with a failure rate of less than 1 %. Contraception must be used during the treatment and follow-up period. Acceptable forms of contraception are: 1. Use of combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation * oral * intravaginal * transdermal 2. progestogen-only hormonal contraception associated with inhibition of ovulation: * oral * injectable * implantable 3. Placement of intrauterine device (IUD) or intrauterine hormone releasing system (IUS) 4. Bilateral tubal occlusion or ligation 5. Vasectomised partner (with appropriate post-vasectomy documentation of the absence of sperm in the ejaculate and provided that male partner is the sole sexual partner of the WOCBP trial participant). 6. Sexual abstinence. 8. Male patients must agree to use condoms during the study and for 2 weeks after the end of the study/last dose of IMP, unless their partner is using a highly efficient method of contraception, as described above. Exclusion criteria Subjects must not be included in the study if any of the following criteria are met: 1. Meeting MINI criteria at interview for suicidality, manic episode, hypomanic episode, bipolar I, bipolar II, bipolar unspecified, bipolar I with psychotic symptoms, panic disorder (current), agoraphobia, posttraumatic stress disorder, alcohol dependency, alcohol abuse, substance dependency (non-alcoholic), substance abuse (non-alcoholic), psychotic disorders, mood disorders with psychotic features, anorexia nervosa, bulimia nervosa, anorexia nervosa binge eating / purging type, or antisocial personality disorder. Meeting MINI criteria at interview for generalised anxiety disorder, obsessive compulsive disorder or social anxiety (social phobia), unless the present symptoms can predominantly be attributed to a diagnosis of major depressive disorder. 2. A history of substance/alcohol abuse within 2 years prior to screening. 3. A previous diagnosis of a personality disorder, autism spectrum disorder, attention-deficit/hyperactivity disorder, or intellectual disability. 4. Any other previously diagnosed or suspected CNS disorder that according to the investigator renders the patient unsuitable for participation in the trial. 5. Any factor that according to the investigator renders it unlikely that the patient will comply with the instructions regarding treatment, visits, refraining from use of central stimulants etc. 6. Any somatic illness that according to the investigator renders the patient unsuitable for participation in the trial. 7. Any signs or symptoms of somatic illness resulting from assessment of vital signs, physical examination, clinical laboratory tests, and 12-lead ECG that according to the investigator renders the patient unsuitable for participation for safety reasons, including a QTc-time on ECG exceeding 450 ms in men and 460 ms in women. 8. Any change in dosage of said SSRI/SNRI within 4 weeks prior to screening or at any time during the course of the trial. 9. Treatment with any other psychoactive drug than said SSRI/SNRI with the exception of using mirtazapine up to 15 mg for sleep, occasional use of benzodiazepines and benzodiazepine-like anxiolytics or hypnotics and occasional use of antihistaminergic sedatives (without anti-dopaminergic effects) within 4 weeks prior to screening and at any time during the course of the trial. 10. Patients who are receiving concomitant therapy with potent cytochrome P450 enzyme inhibitors (e.g., bupropion, fluvoxamin, ketoconazole, itraconazole, telithromycin, clarithromycin, protease inhibitors, quinidine, and terbinafine). 11. Ongoing treatment with drugs with a narrow therapeutic window where either lower or higher serum levels are potentially harmful (including but not limited to warfarin along with other anticoagulants, digoxin along with other antiarrythmics, anticonvulsants prescribed for treatment of epilepsy, cyclosporine, immunosuppressants, and lithium). 12. Current treatment with any prescribed or OTC drug that according to the investigator renders the subject unsuitable for participation in the trial. 13. Previous intake of OSU6162. 14. Current participation in another clinical trial. 15. Nursing women. 16. Substudy only: Relative and/or absolute contraindications to lumbar puncture and functional Magnetic Resonance Imaging (fMRI), as per clinical practice. This includes ongoing treatment with anticoagulants such as NOAC or warfarin.