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 on Rome IV diagnostic criteria within 3 months prior to randomization. On days when the participant 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 associated with 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 days with 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 evaluation for their IBS symptoms, including an evaluation for organic/structural etiologies (if in the presence of alarm symptoms); and (b) age-appropriate screening for colorectal cancer, 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 than IBS-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's disease, 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, GI perforation, fecal impaction, gastric banding, bariatric surgery, adhesions, ischemic colitis, 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 months post-surgery are allowed). * Confirmed alanine aminotransferase (ALT) \> 2 upper limit of normal (ULN) * Confirmed total bilirubin \> ULN, unless the participant has a documented history of Gilbert's syndrome * Evidence of active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection or Human immunodeficiency virus (HIV)-1 or HIV-2 antibody positive * Evidence of HCV infection based on a positive HCV antibody screen (Participants who have been successfully treated for HCV are eligible if an undetectable HCV viral load at least 6 months after completion of treatment can be demonstrated.)