This phase 3, open label study will evaluate the safety of tenapanor 50 mg BID in subjects with constipation-predominant irritable bowel syndrome (IBS-C) defined by the ROME III criteria. Subjects who have completed either TEN-01-301 (16 weeks) or TEN-01-302 (26 weeks) studies may be enrolled. Subjects will take tenapanor for approximately 52-55 weeks total based on previous protocol and this study.

During the treatment period of up to 39-weeks, subjects will return for study visits approximately every 13 weeks. Subjects will undergo safety assessments at these visits, which may include a physical exam, ECG, vital signs, and clinical labs. Adverse events and concomitant medications will be recorded. Medication compliance will be monitored and the subjects will be given additional study drug as appropriate.

Inclusion Criteria:

• Subjects completed all 16 weeks of TEN-01-301 or all 26 weeks of TEN-01-302
• Subject demonstrated adequate compliance with the study procedures during either the TEN-01-301 or TEN-01-302 studies
• Females must be of non-childbearing potential; If of child-bearing potential, must have negative pregnancy test and confirm the use of one of the appropriate means of contraception
• Males must agree to use appropriate methods of barrier contraception or have documented surgical sterilization

Exclusion Criteria:

• Subject has been withdrawn or discontinued prematurely from either TEN-01-301 or TEN-01-302
• The subject reports using any prohibited medication and is not willing to abide by the restrictions for intake
• Pregnant or lactating women

This was an extension study that ran for up to 1 year