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

This is a randomized, double-blind, placebo-controlled study to evaluate the effect of tenapanor on change in s-P levels when tenapanor is administered orally, twice daily for 28 days as adjunctive therapy to ESRD subjects with hyperphosphatemia on stable phosphate binder therapy.

The study consists of a Screening visit; a Run-in Period of at least 2 weeks and up to 4 weeks, where existing phosphate binder treatment is maintained; and a 4-week Double-Blind Treatment Period.

At Screening, a subject must be on thrice daily phosphate binder therapy and have a serum phosphate (s-P) level ≥5.5 and ≤10.0 mg/dL to qualify for entering the study. s-P will be measured at each visit during the run-in period to enable the evaluation of the s-P randomization criteria.

Subjects who qualify to enroll in the study will be randomized in a 1:1 ratio to receive tenapanor or placebo while continuing their existing phosphate binder treatment.

During the Double-Blind Treatment Period, subjects will receive tenapanor or placebo starting at a dose of 30 mg bid

Eligibility Criteria:
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.