This study is to evaluates the safety and tolerability of Zerenex™ (ferric citrate) as a treatment for hyperphosphatemia in patients with End-Stage Renal Disease.

The purpose of this study is to evaluate the safety and tolerability of Zerenex™ (ferric citrate) as a treatment for hyperphosphatemia in patients with End-Stage Renal Disease. These patients will be switched to Zerenex™ from their current high dose of phosphate binder and, based on their serum phosphorus levels, will be titrated up from 3.4g/day of Zerenex™ to maximum tolerated and safe doses of Zerenex™. Doses will be adjusted weekly, based on serum phosphorus levels, with the maximum dose administered being approximately 12g/day.

Inclusion Criteria:

• Males and non-pregnant, nonlactating females
• Age > 18 years
• On thrice weekly hemodialysis for at least the previous 3 months prior to randomization
• Phosphorous levels ≥3.5mg/dL at Screening Visit
• On at least 12 tablets/capsules/day of calcium acetate (667mg), calcium carbonate (500mg), lanthanum carbonate (500mg), sevelamer hydrochloride (800mg or two 400mg tablets), or any combination of these agents
• Serum ferritin <1000micrograms/L and Transferrin Saturation (TSAT) <50%
• Willing to be discontinued from current phosphate binder(s) and initiated on Zerenex
• Willing and able to give informed consent

Exclusion Criteria:

• Parathyroidectomy within 6 months prior to Screening
• Actively symptomatic GI disease such as peptic ulcer disease, gastro esophageal reflux, diverticulosis, irritable bowel syndrome (treated asymptomatic is permitted)
• History of documented inflammatory bowel disease or erosive esophagitis
• Serum Phosphorus levels >10.0 mg/dL documented in the 3 monthly laboratories (done routinely in the dialysis unit) in the 3 months prior to the Screening Visit
• History of multiple drug allergies
• History of malignancy in the last 5 years (treated cervical or 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 [Neprocaps, 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 study (scheduled outpatient access surgery allowed)
• Any other medical condition that renders the patient unable to or unlikely to complete the study or that would interfere with optimal participation in the study or produce significant risk to the patient
• Receipt of any investigational drug within 30 days of randomization
• Inability to cooperate with study personnel or history of noncompliance

Intent-to-Treat