The study is a Phase 2, double-blind, randomized, placebo-controlled study in Major Depressive Disorder (MDD) participants with an inadequate response to standard antidepressants The objective of the study is to assess CLE-100 (oral esketamine) for the treatment of MDD in participants currently treated with an oral antidepressant medication and who have an inadequate response to at least 2 antidepressants.

This is a Phase 2 study in subjects with MDD currently treated with an oral antidepressant medication with an inadequate response to at least 2 antidepressants. SOLEO will be an outpatient study to assess the safety, efficacy, and tolerability of CLE-100 as compared to placebo.

Eligible subjects will be randomized to receive either placebo or CLE-100 (oral esketamine) once daily in addition to their current oral antidepressant monotherapy for 4 weeks. All subjects who adequately completed the 4-week Double-Blind Treatment Period and who meet the eligibility criteria will be offered the option to roll-over to a 6-month Open-label Extension (OLE) Treatment Period with CLE-100.

Inclusion Criteria:

1. Male or female between 18 to 65 years of age at Screening
2. Diagnosis of MDD, single or recurrent, without psychotic features, in the current or previous episode(s), according to the Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5). The diagnosis of MDD must be supported by the Mini International Neuropsychiatric Interview (MINI) Screen 7.0.2 for DSM-5.
3. Currently experiencing a Major Depressive Episode (MDE) that began at least 12 weeks but no more than 5 years prior to Screening. The current MDE must be confirmed by the independent SAFER assessor.
4. MADRS score of 24 or higher at Screening as confirmed by an independent SAFER assessor.
5. At Screening, a history of inadequate response to at least 2 antidepressant medications in the current MDE. Inadequate response is defined as less than 50% improvement of depression symptoms following at least 6 weeks of treatment with a therapeutic dose and is assessed by the site using the Massachusetts General Hospital Antidepressant Treatment Response Questionnaire (MGH-ATRQ). Inadequate response to at least 2 antidepressant medications must be verified by documented medical or pharmacy records and confirmed by an independent SAFER assessor.
6. Able and competent to read and sign the informed consent form (ICF).

Exclusion Criteria:

1. At Screening, a history of inadequate response (as defined in inclusion criterion #5) to more than 5 antidepressant medications in the current MDE using MGH-ATRQ and confirmed by the independent SAFER assessor.

2. A high risk of suicide based on any of the following:

   1. Item 10 of MADRS score (suicidal thoughts) is 5 or higher at Screening or Baseline.
   2. Suicide attempt in the previous 6 months.
   3. Significant risk, as judged by the Investigator, based on the psychiatric interview or information collected with the C SSRS at Screening or Baseline.

3. Current or lifetime history of substance use disorder with ketamine, phencyclidine (PCP), lysergic acid diethylamide (LSD), or 3.4-methylenedioxymethamphetamine (MDMA) hallucinogen-related use disorders or has any other current substance use disorder or history within 12 months prior to Screening (Substance Use Disorder is diagnosed per DSM-5 criteria and does not include tobacco use disorder).

4. Use of ketamine, esketamine, PCP, or dextromethorphan recreationally in the past 6 months.

5. History or current diagnosis of bipolar disorder, schizophrenia, or schizoaffective disorder or other schizophrenia spectrum disorders.

6. Dementia, delirium, amnesia, or any other significant cognitive disorder.

7. Any medical condition for which an increase in blood pressure or intracranial pressure or intraocular pressure or tachycardia poses a serious risk (e.g., aneurysmal vascular disease, arteriovenous malformation, or history of intracerebral hemorrhage).