Study to Assess Safety and Efficacy of Tenapanor for Treatment of IBS-C in Pediatric Patients 12 to Less Than 18 Years

This study consists of 2-week screening period followed by 12 week randomized treatment period (RTP) and a 2-week treatment-free Follow-Up period (only for patients who will not enter the 40-week Long Term Safety Extension Study \[TEN-01-306\]). At the beginning of the 2-week Screening period, patients who provide written assent will be fully assessed for eligibility into the study and will be asked to self-report daily information about the status of their IBS symptoms via an electronic diary (eDiary) device. Patient compliance with the eDiary will be monitored actively by the site staff and will be reviewed to determine eligibility at the end of screening. Eligible patients will be randomized to receive one of the study medications: tenapanor 25 mg BID, tenapanor 50 mg BID, or placebo. During the 12-week double-blind RTP, patients will continue recording daily assessments via the eDiary system as instructed and compliance with eDiary entries will be monitored on an ongoing basis. Patients will return for study visit every two or four weeks (Visits 3-6) and will undergo safety assessments at these visits. Patients who do not enter the 40-week Long Term Safety Extension Study \[TEN-01-306\] including those who complete the RTP but do not enter study TEN-01-306 and those who prematurely discontinue from the RTP, a Follow-Up Visit will be scheduled approximately 2 weeks after the completion of the RTP (Visit 6) or the Early Termination Visit at which safety assessments will be performed