Study to Evaluate the Efficacy of Tenapanor as Adjunctive Therapy to Phosphate Binder Therapy

Inclusion Criteria: * Signed and dated informed consent prior to any study specific procedures. * Males or females aged 18 to 80 years, inclusive, at Screening * Females must be non-pregnant, non-lactating and either be post-menopausal for at least -2 months, have documentation of irreversible surgical sterilization, use of acceptable -contraceptive method, or sexual abstinence, or a sterile sexual partner from Screening -until 30 days after the last subject visit. * Males must agree to avoid fathering a child (or donating sperm), and therefore be either sterile (documented) or agree to use approved methods of contraception, from the time of enrollment until 30 days after end of study. * Chronic maintenance hemodialysis (HD) 3x/week for at least 3 months or chronic maintenance peritoneal dialysis (PD) for a minimum of 6 months. * If receiving active vitamin D or calcimimetics, the dose should have been unchanged for the last 4 weeks prior to Screening. * Kt/V ≥1.2 at most recent measurement prior to Screening. * Prescribed and taking phosphate binder medication at least 3 times per day, and the prescribed dose should have been unchanged during the last 4 weeks prior to Screening. * Serum phosphorus levels must be ≥5.5 and ≤10.0 mg/dL at Screening and the end of the run-in period, analyzed at the central laboratory used in the study. Exclusion Criteria: * Severe hyperphosphatemia defined as having an s-P level \>10.0 mg/dL on phosphate-binders at any time point during routine clinical monitoring for the 3 preceding months before Screening. * Serum/plasma parathyroid hormone \>1200 pg/mL. * Clinical signs of hypovolemia at Screening as judged by the Investigator. * History of inflammatory bowel disease (IBD) or irritable bowel syndrome with diarrhea (IBS-D). * Scheduled for living donor kidney transplant or plans to relocate to another center during the study period. * Use of an investigational agent within 30 days prior to Screening. * Involvement in the planning and/or conduct of the study (applies to both Ardelyx/Contract Research Organization (CRO) staff and/or staff at the study site). * 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.