This study is testing a new treatment called ALTO-100 for people with Bipolar Disorder (BD) who are currently experiencing depression (BD-D). Bipolar Disorder is a mental condition where people have extreme mood swings, including emotional highs (mania) and lows (depression). ALTO-100 is being compared to a placebo, which is a harmless pill that looks like the real medicine, to see if it can help improve depression when taken with a mood stabilizer. Mood stabilizers are medicines like lithium, lamotrigine, or valproic acid that help manage mood swings.

To join the study, participants must have Bipolar I or II disorder and be on a stable dose of mood stabilizers for at least 6 weeks. They must not have a severe medical condition or be pregnant. The study will check how safe ALTO-100 is and how well it works over a period of time, and requires participants to follow all study-related activities.

• The study includes a double-blind period and an open-label treatment period.
• Participants must be willing to comply with all study procedures.
• Participants should not be in another clinical trial for mental illness.