A Study to Assess Adverse Events and Change in Disease Activity of Oral Icalcaprant in Adult Participants With Bipolar I or II Disorder

Inclusion Criteria: * Participants with a diagnosis of bipolar I or II according to the DSM-5-TR without psychotic features, confirmed by the MINI 7.0.2, and currently experiencing an MDE beginning at least 4 weeks prior to consent and not exceeding 12 months prior to Screening. * Body Mass Index (BMI) is \>= 18.0 to ≤\<= 35.0 kg/m\^2. * A condition of general good health, based upon the results of a medical history, physical examination, vital signs, laboratory profile and a 12-lead ECG. * CGI-S-BP score of ≥ 4 for depression and overall bipolar illness at screening (Visit 1) and baseline (Visit 2). * YMRS total score \<= 12 at screening (Visit 1) and baseline (Visit 2). * Participants on treatment with a single mood stabilizer (lithium, valproate, or lamotrigine), maintained at a stable dose for \>= 28 days prior to screening. Current mood stabilizer dose must remain unchanged for the duration of the study. * If taking lithium or valproate, participant must have a therapeutic blood level at screening of lithium (0.4 - 1.2 mg/dL) or valproate (50 - 125 mg/dL). * If taking lamotrigine, participant should be taking a dose within the FDA accepted range (50-400 mg/day, target dose is 200 mg/day) and investigator clinical judgment determines that the dose the participant is taking is likely to prevent onset of mood episodes. Exclusion Criteria: * History of an allergic reaction or significant sensitivity to constituents of the study drug (and its excipients) and/or other products in the same class. * History of or active medical conditions(s) that might interfere with the conduct of the study, confound the interpretation of the study results, or endanger the subject's well-being. This includes any unstable condition, history or evidence of malignancy (other than treated basal or squamous cell carcinoma), or any significant hematologic, endocrine, cardiovascular, respiratory, renal, hepatic, gastrointestinal, or neurological disorder (if there is a history of such disease but the condition has been stable for more than 1 year, does not require treatment with prohibited medications, and is judged by the investigator not to interfere with the participant's participation in the study, the participant may be included in the study).