Study of Asimadoline to Treat Diarrhea-Predominant Irritable Bowel Syndrome (D-IBS)

Inclusion Criteria: 1. Signs and dates a written informed consent form. 2. Male and female subjects aged 18-79 who are fluent in English 3. All subjects must use protocol specified contraceptive measures 4. The subject is or has been diagnosed with IBS with symptom onset at least 6 months prior to diagnosis. IBS is defined as the subject having recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months associated with at least two of the following symptoms: Improvement with defecation Onset associated with a change in frequency of stool Onset associated with a change in form (appearance) of stool 5. The subject has been diagnosed with diarrhea-predominant IBS 6. Within 2 years of the randomization visit, the subject has normal results from a flexible sigmoidoscopy, a colonoscopy, or a barium enema plus flexible sigmoidoscopy, according to the subject's age by a specified algorithm. Exclusion Criteria: 1. The subject exhibits evidence of a biochemical or structural abnormality of the digestive tract. 2. Subject has a concurrent illness or disability (excluding IBS) that may affect the interpretation of clinical efficacy and/or safety data or otherwise contraindicates participation in this clinical study (e.g., an unstable cardiovascular, renal, hepatic, pulmonary, endocrine, metabolic, GI, hematological, or neurological condition). 3. The subject has a family history of prolonged QT syndrome. 4. The subject has been diagnosed with a major psychiatric disorder. 5. The subject has a history of alcohol or substance abuse within the past 2 years. 6. The subject has a history or current evidence of laxative abuse 7. The subject has a positive stool sample for ova or parasite. 8. The subject has used an investigational drug or participated in an investigational study within 30 days of screening. 9. The subject refuses to discontinue one (or more) prohibited medications at least 7 days prior to the screening visit. 10. The subject refuses to maintain a stable dose of one (or more) allowable concurrent medications for at least 30 days prior to the screening visit. 11. The subject is a pregnant woman or a woman who is breast feeding. 12. The subject is unable or unwilling to follow directions or use the electronic diary system.