Study to Evaluate the Efficacy, Safety, and Tolerability of BOS-589 in the Treatment of Patients With Diarrhea-predominant Irritable Bowel Syndrome (IBS-D)

Inclusion Criteria:

• Participant meets the diagnosis of diarrhea-predominant IBS (IBS-D) subtype based onRome IV diagnostic criteria within 3 months prior to randomization. On days when theparticipant experiences IBS symptoms
• At least 25% of stools are loose or watery; and
• Fewer than 25% of stools are hard.
• Recurrent abdominal pain occurring, on average, at least 1 day per week and associatedwith 2 or more of the following:
• Related to defecation;
• Associated with a change in frequency of bowel movements;
• Associated with a change in form (appearance) of stool.
• Over the week prior to randomization, the participant has
• An average of worst abdominal pain (WAP) scores in the prior 24 hours of 4.0 to 8.0 on a 0 to 10 numerical rating scale;
• An average daily Bristol Stool Form Scale (BSFS) score ≥ 5.0 (and at least 5 dayswith a BSFS score ≥ 5.0;
• An average daily IBS-Global Scale (IBS-GS) score of ≥ 2.0.
• Participant must undergo or previously have undergone (a) an appropriate evaluationfor their IBS symptoms, including an evaluation for organic/structural etiologies (ifin the presence of alarm symptoms); and (b) age-appropriate screening for colorectalcancer, if applicable.
• Participant is negative for serum tissue transglutaminase immunoglobulin A antibody (tTG-IgA) plus has evidence of detectable serum IgA within the normal reference range.

Exclusion Criteria:

• At the time of screening, participant has a diagnosis of an IBS subtype other thanIBS-D, based on Rome IV criteria.
• Participant has a history of inflammatory or immune-mediated gastrointestinal (GI)disorders including (but not limited to) inflammatory bowel disease (i.e., Crohn'sdisease, ulcerative colitis, microscopic colitis, and celiac disease).
• Participant has had an episode of diverticulitis within 3 months prior to Screening.
• Participant has a history of intestinal obstruction, stricture, toxic megacolon, GIperforation, fecal impaction, gastric banding, bariatric surgery, adhesions, ischemiccolitis, or impaired intestinal circulation (e.g., aortoiliac occlusive disease).
• Participant has any of the following surgical history:
• Cholecystectomy with any history of post-cholecystectomy biliary tract pain;
• Any abdominal surgery within the 3 months prior to Screening;
• Major gastric, esophageal, hepatic, pancreatic, or intestinal surgery (appendectomy, hemorrhoidectomy, or polypectomy greater than 3 monthspost-surgery are allowed).
• Confirmed alanine aminotransferase (ALT) > 2 upper limit of normal (ULN)
• Confirmed total bilirubin > ULN, unless the participant has a documented history ofGilbert's syndrome
• Evidence of active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection orHuman immunodeficiency virus (HIV)-1 or HIV-2 antibody positive
• Evidence of HCV infection based on a positive HCV antibody screen (Participants whohave been successfully treated for HCV are eligible if an undetectable HCV viral loadat least 6 months after completion of treatment can be demonstrated.)