A Study of CLE-100 (Oral Esketamine) in Addition to Standard Antidepressant Drug for Major Depressive Disorder - CLEO Study

CLEO study is performed in two parts (part A and Part B). Part A will be an inpatient study to assess the safety, tolerability, and pharmacokinetics of CLE-100 (oral esketamine) in MDD participants currently treated with an antidepressant drug. It will include a screening phase (up to 35 days), a 1 week inpatient double-blind treatment phase and an outpatient post treatment safety follow-up phase of 1 week after last study drug administration. Part B will be a study to assess the safety and efficacy of CLE-100 (oral esketamine) in MDD participants currently treated with an antidepressant drug with inadequate response to standard antidepressant therapy. The participants will remain on their current antidepressant therapy with no dose change during the study.