This study is testing a new drug called ALTO-100 to see how well it helps people with bipolar disorder (BD) who are also feeling very sad or depressed. Bipolar disorder is a mental illness where people's moods can swing from very happy to very sad. The study compares ALTO-100 to a placebo (a fake pill with no medicine) while people also take their usual mood-stabilizing medicine. The study also checks if ALTO-100 is safe and how people handle it. Participants must have bipolar disorder I or II and be on a mood stabilizer like lithium, lamotrigine, or valproic acid. Pregnant or breastfeeding individuals cannot join. The study involves a double-blind period where neither the doctors nor the participants know who is taking ALTO-100 or the placebo, followed by an open-label period where everyone knows they are taking ALTO-100. Participants cannot have certain other medical conditions or be using other specific medications.

• The study lasts a few weeks and involves regular check-ins.
• Participants will take either ALTO-100 or a placebo along with their mood stabilizer.
• There might be side effects, so discussing participation with a doctor is important.