This is a research study for people with high blood phosphorus levels who are on dialysis. This medical condition can cause weakening of the bones and damage other organs. This can lead to many health problems, and sometimes death. Phosphorus is in much of the food we eat, and is helpful to us in small amounts. Patients with kidney failure have trouble getting rid of the phosphorus eaten in food. Dialysis can help remove some of the phosphorus, but often patients must take a phosphate binder like PhosLo®, Renagel®, or Renvela® to bring the blood phosphorus levels back to normal. The purpose of this study is to see if KRX-0502 (ferric citrate) is safe and effective as a phosphate binder.

There will be a screening visit about 4 weeks receiving study drug. Upon qualifying for the study after the screening visit, patients will then be asked to stop taking their current phosphate binder for about 2 weeks. Then, if patients continue to qualify for the study, they will be entered in the study that lasts about 28 days. Study visits will happen every week during the patient's usual dialysis appointments. There will be a total of up to 9 visits for this study, and total participation time could last up to 8 weeks.

Inclusion Criteria:

• Males or non-pregnant, non-lactating females
• Age > 18 years
• On thrice weekly hemodialysis or peritoneal dialysis for at least the previous three months prior to Screening Visit (Visit 0)
• Serum phosphorus levels ≥ 3.5 mg/dL and < 8.0 mg/dL at Screening Visit (Visit 0)
• Serum phosphorus levels > 6.0 mg/dL during the washout period (Visits 2 or 3)
• Taking 3 to 15 tablets/capsules per day of 667mg calcium acetate or 800 mg sevelamer (hydrochloride or carbonate), or any combination of these agents as reported by the patient at Screening Visit (Visit 0)
• Serum ferritin <1000micrograms/L and Transferrin Saturation (TSAT) <50% at the Screening Visit (Visit 0)
• Willingness to be discontinued from current phosphate binder(s) and initiated on KRX-0502 (ferric citrate)
• Willing and able to give informed consent
• Willing and able to stay on a constant dose of Vitamin D (or its analogs) and Sensipar (cinacalcet) for the treatment period, if applicable.

Exclusion Criteria:

• Parathyroidectomy within six months prior to Screening Visit (Visit 0)
• Actively symptomatic gastrointestinal bleeding or inflammatory bowel disease
• Serum phosphorus levels >10.0 mg/dL documented in all of the three monthly laboratories (done routinely in the dialysis unit) in the three months prior to the Screening Visit (Visit 0)
• History of multiple drug allergies or intolerances
• History of malignancy in the last five years (treated cervical or non-melanomatous skin cancer may be permitted if approved by CCC)
• Previous intolerance to oral ferric citrate
• Absolute requirement for oral iron therapy
• Absolute requirement for Vitamin C (multivitamins [Nephrocaps, Renaphro, etc.] allowed)
• Absolute requirement for calcium-, magnesium-, or aluminum-containing drugs with meals
• Psychiatric disorder that interferes with the patient's ability to comply with the study protocol
• Inability to tolerate oral drug intake
• Planned surgery or hospitalization during the trial (scheduled outpatient access surgery allowed)
• Any other medical condition that renders the patient unable to or unlikely to complete the trial or that would interfere with optimal participation in the trial or produce significant risk to the patient
• Receipt of any investigational drug within 30 days of Screening Visit (Visit 0)
• Inability to cooperate with study personnel or history of noncompliance
• Unsuitable for this trial per Principal Investigator's clinical judgment