To evaluate the effect and safety of a switch from phosphate binders to KHK7791 to treat Hyperphosphatemia in patients on HD.

Inclusion Criteria:

• Stable chronic renal failure patients who have undergone hemodialysis 3 times per week for at least 12 weeks until screening examination.
• Met certain conditions for dialysis excluding Dry Weight (Dialysate, Dialyzer, Frequency of Dialysis per Week, Dialysis Time, Blood Flow Rate, and Dialysate and Substitution Fluid Flow Rate) 2 weeks before screening examination.
• Taking at least 2 tablets of phosphate binder 3 times per day. The prescribed dosage regimen should have been unchanged for the period from 2 weeks before screening examination through pre-enrollment.
• Serum phosphorus levels should be between 3.5 and 7.0 mg/dL (inclusive) at screening examination.
• If on any vitamin D or calcimimetics regimen, the dosage regimen should have been unchanged for the last 2 weeks before screening examination.

Exclusion Criteria:

• iPTH > 600 pg/mL (should be based on the most recent value from patient's medical records before pre-enrollment)
• History of inflammatory bowel disease (IBD) or diarrhea predominant irritable bowel syndrome
• History of gastrectomy or enterectomy (excluding endoscopic resection and cecectomy) or having undergone gastrointestinal tract surgery within 3 months before screening examination.
• Severe heart disease (including congestive heart failure, defined as New York Heart Association [NYHA] cardiac functional classification of Class III or IV, and vascular lesions requiring hospitalization such as myocardial infarction), hepatic impairment (including AST/ALT ≥ 100 U/L before the start of observation period), or concurrent cirrhosis.
• Developed cerebrovascular disease (such as cerebral infarction and cerebral hemorrhage) or cardiovascular disease (such as acute myocardial infarction and unstable angina) requiring hospitalization within 6 months before screening examination.
• Uncontrollable hypertension or diabetes
• Scheduled for living donor kidney transplant, change in the mode of dialysis, home hemodialysis or plans to change the dialysis center (relocate to another hospital/clinic) during the study period.
• Any diagnosis of or treatment of malignancy within 5 years before screening examination (excluding basal cell carcinoma or surgically resected intraepithelial carcinoma of uterine cervix).