This is a single-center, prospective, open-label, controlled, randomized, cross-over study in 34 prevalent end-stage renal disease patients on chronic hemodialysis treatment with hyperphosphatemia.

Patients will be randomized in a 1:1 ratio to either receive low-dose (250mg/d) PA-21 followed by high-dose (2000mg/d) PA-21 (sequence A-B) or to receive high-dose PA21 followed by low-dose PA21 (sequence B-A) with washout-phases in between. An open label design with a sub-therapeutic dose of PA21 as control treatment was chosen because creating a convincing placebo for PA21 is not feasible.

The study will start with two study visits on the 2nd and 3rd dialysis session of the week , where dialysis patients are still treated with standard of care to establish baseline values (duration: 0.5 weeks). Next will be an initial run-in wash-out phase, where all phosphate binders taken by the patient as standard of care treatment will be discontinued and no phosphate binders will be introduced. Consequently, there will be a PA21 treatment (high dose/low dose) phase, followed by a wash-out phase, followed by another PA21 treatment (high dose/low dose) phase, followed by a final wash-out phase. Each study phase, including wash-out phases as well as low-dose and high-dose treatment phases will be 14 days of duration. Patients will be followed up to 10.5 weeks after randomization until the last study visit of the final wash-out phase.

Inclusion Criteria:

• Prevalent patients (≥ 3 months on dialysis) treated with thrice weekly hemodialysis (HD) or hemodiafiltration (HDF)
• Hyperphosphatemia (serum phosphate > upper limit of normal within the last 3 months) or current phosphate binder use
• No use or constant dose of vitamin D and/or calcimimetics for ≥2 weeks

Exclusion Criteria:

• Allergy to sucroferric oxyhydroxide (PA21), to other ingredients of Velphoro or to any iron-medication
• Current or history of calciphylaxis (calcific uremic arteriolopathy - CUA)
• parathormone >800 pg/ml
• Parathyreoidektomie planned or expected
• Significant GI or hepatic disorders
• Hypercalcemia (total serum calcium >2.6 mmol/l) at screening
• Antacids containing aluminum, calcium, magnesium or bicarbonate
• Oral iron treatments/supplements
• Pregnant and nursing (lactating) women