This phase 3, 8-week, randomized, double-blind, parallel group, multi-center study with a 4-week, placebo-controlled, randomized withdrawal period will evaluate the efficacy, safety and tolerability of Tenapanor to treat hyperphosphatemia in end-stage renal disease patients on hemodialysis (ESRD-HD). Subjects who qualify are randomized into the study will either receive 3 mg BID, 10 mg BID, or a titration regimen of tenapanor.

The study consists of a screening visit, a wash out period of up to 3 weeks, when existing phosphate lowering medication is withheld, an 8-week treatment period, in which all groups receive tenapanor, and a 4-week placebo-controlled, randomized withdrawal period, during which patients are re-randomized 1:1 to either remain on their current tenapanor treatment or placebo.

Depending on the increase in serum phosphate levels, subjects can be randomized 1,2, or 3 weeks after being taken off their phosphate lowering medication.

Inclusion Criteria:

• 18 to 80 years old
• Females must be non-pregnant, non-lactating, and either be post-menopausal for at least 12 months, have documentation of irreversible surgical sterilization, or confirm the use of one of the acceptable contraceptive methods.
• Males must agree to avoid fathering a child and agree to use an appropriate method of contraception
• Chronic maintenance hemodialysis 3x/week for at least 3 months
• Kt/V ≥ 1.3 at most recent measurement prior to screening
• Prescribed and taking at least 3 doses of phosphate binder per day
• Serum phosphate levels should be between 4.0 and 7.0 mg/dL (inclusive) at screening
• For randomization in the study, after 1 week wash-out of phosphate binders, subjects must have serum phosphate level of at least 9 mg/dL but below 10 mg/dL and have had an increase of at least 1.5 mg/dL versus pre-wash out value
• For randomization in the study, after 2 or 3 weeks wash-out of phosphate binders, subjects must have serum phosphate level of at least 6 mg/dL but below 10 mg/dL and have had an increase of at least 1.5 mg/dL versus pre-wash out value

Exclusion Criteria:

• Severe hyperphosphatemia defined as >10 mg/dL on Phosphate-binders at any time point during clinical routine monitoring for the 3 preceding months before screening
• Serum parathyroid hormone >1200 pg/mL
• Persistent metabolic acidosis defined as serum carbon dioxide <18 mmol/L from two consecutive measurements during screening and washout periods
• Clinical signs of hypovolemia at randomization
• History of inflammatory bowel disease (IBD) or diarrhea predominant irritable bowel syndrome (IBS-D)
• Scheduled for living donor kidney transplant, change to peritoneal dialysis, home HD or plans to relocate to another center during the study period
• Diarrhea or loose stools during the week before randomization defined as BSFS ≥ 6 and frequency ≥ 3 for 2 or more days
• Any evidence of or treatment of malignancy within one year, excluding non-melanomatous malignancies of the skin
• Positive serology with evidence of significant hepatic impairment or WBC elevation according to the Investigator
• Life expectancy < 6 months