The objective of this study is to evaluate the safety and efficacy of LX1033 over a range of dose levels in subjects with diarrhea-predominant Irritable Bowel Syndrome (IBS).

Inclusion Criteria:

• Male and female subjects, aged 18 to 70 years, with diarrhea-predominant IBS (IBS-D) with symptom onset at least 6 months prior to diagnosis
• Two or more days per week with at least one stool with a consistency of Type 6 or 7 (Bristol Stool Form Scale)
• Weekly average of worst abdominal pain in past 24 hours score of greater than or equal to 3.0 using a 0-10 point scale
• Ability to provide written, informed consent

Exclusion Criteria:

• Inability to discontinue any current drug therapy for IBS, with the exception of bulking agents. Subjects will be allowed up to 2 doses of loperamide per week as rescue medication.
• Subjects who score severe abdominal pain (rated 7 or higher) 5 or more days per week
• Concomitant use of opioid analgesic drugs or drugs that affect bowel motility
• Any abnormalities or conditions deemed by the investigator as clinically significant