This phase 2, randomized, double blind, placebo-controlled, multi-center study will evaluate the safety and efficacy of three dose levels of AZD1722 in subjects with constipation predominant IBS (IBS-C) as defined by the ROME III criteria and who have active disease as determined during a two-week screening period. Subjects who qualify and are randomized into the study will receive 5, 20, or 50 mg of AZD1722 BID or placebo BID for 12 consecutive weeks. At the end of this treatment period, subjects will be followed for an additional 4 weeks.

Inclusion Criteria:

• Females and males must agree to use appropriate methods of contraception or be sterile (with appropriate documentation)
• Subject is ambulatory
• Subject meets definition of IBS-C using Rome III Criteria for the Diagnosis of IBS with a history of <5 SBMs per week
• Subject meets Screening eligibility criteria (see below)
• A colonoscopy based on AGA guidelines; every 10 years at ≥ 50 years old, or the occurrence of any warning signs (i.e., unexplained weight loss, non-hemorrhoid blood in stools)
• Ability to communicate well with the Investigator and to comply with the requirements of the entire study, including an understanding of how to use the touch-tone telephone electronic diary.
• Written informed consent and a willingness to participate in the study as it is described.
• Daily access to a touch tone telephone.

Exclusion Criteria:

• Functional diarrhea as defined by Rome III criteria
• IBS with diarrhea (IBS-D), mixed IBS (IBS-M), or unsubtyped IBS as defined by Rome III criteria
• Diagnosis or treatment of any clinically symptomatic biochemical or structural abnormality of the GI tract within 6 months prior to screening, or active disease within 6 months prior to screening. Including but not limited to cancer, inflammatory bowel disease, diverticulitis, duodenal ulcer, erosive esophagitis, gastric ulcer, pancreatitis (within 12 months of screening), cholelithiasis, amyloidosis, ileus, non-controlled GERD, gastrointestinal obstruction or carcinoid syndrome.
• Use of medications that are known to affect stool consistency (Prohibited Medications), including fiber supplements, anti-diarrheals, cathartics, antacids, opiates, prokinetic drugs, laxatives, enemas, antibiotics within 4 weeks of enrollment, probiotics (including probiotic yogurt); or salt or electrolyte supplements containing sodium, potassium, chloride, or bicarbonate formulations during the seven days prior to treatment; unless specified as rescue medication, and used accordingly.
• Clinical evidence of significant cardiovascular, respiratory, renal, hepatic, gastrointestinal, hematologic, neurologic, psychiatric or any disease that may interfere with the subject successfully completing the trial.
• The subject has a history or current evidence of laxative abuse (in the clinical judgment of the physician).
• Hepatic dysfunction (ALT [SGPT] or AST [SGOT] >2.5 times the upper limit of normal) or renal impairment (serum creatinine > 2mg/dL).
• Any evidence of or treatment of malignancy (other than localized basal cell, squamous cell skin cancer or cancer in situ that has been resected) within the previous year.
• Any surgery on the stomach, small intestine or colon, excluding appendectomy.
• Pregnant or lactating women.
• A major psychiatric disorder (DSM-III-R or DSM-IV) including major depression or other psychoses that has required hospitalization in the last 3 years. History of attempted suicide or uncontrolled bipolar disorder. Alcohol or substance abuse in the last year.
• Participation in other clinical trials within 1 month prior to Day -14 (beginning of screening period).
• If, in the opinion of the Investigator the subject is unable or unwilling to fulfill the requirements of the protocol or has a condition, which would render the results uninterpretable.

Safety Analysis Set