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Moustafa Moustafa,1 Lawrence Lehrner,2 Fahd Al-Saghir,3 Mark Smith,4 Sunita Goyal,5 Maureen Dillon,5 John Hunter,5 Randy Holmes-Farley5 1South Carolina Nephrology and Hypertension Center Inc., Orangeburg, SC, USA; 2Kidney Specialists of Southern Nevada, Las Vegas, NV, USA; 3Michigan Kidney Consultants, Pontiac, MI, USA; 4Kidney Care Associates, LLC, Augusta, GA, USA; 5Genzyme, a Sanofi company, Cambridge, MA, USA Background: Genz-644470 is a new, nonabsorbed phosphate binding polymer. In an in vitro competitive phosphate binding assay, Genz-644470 bound significantly more phosphate per gram than sevelamer. As a consequence, this clinical study evaluated the ability of Genz-644470 to lower serum phosphorus in patients on hemodialysis and compared serum phosphorus lowering of Genz-644470 with sevelamer carbonate and placebo. Because three different fixed doses of Genz-644470 and sevelamer carbonate were used, phosphate-lowering dose-responses of each agent were also analyzed. Methods: A randomized, double-blind, dose-ranging study was conducted. After a 2-week phosphate binder washout, 349 hyperphosphatemic (serum phosphorus >5.5 mg/dL) hemodialysis patients were randomized to one of seven fixed-dose groups: placebo, Genz-644470 2.4 g/day, Genz-644470 4.8 g/day, Genz-644470 7.2 g/day, sevelamer carbonate 2.4 g/day, sevelamer carbonate 4.8 g/day, or sevelamer carbonate 7.2 g/day. Indicated total daily doses were administered in fixed divided doses three times a day with meals for 3 weeks. The change in serum phosphorus during the treatment period and its dose-response patterns were assessed. Results: Dose-dependent reductions in serum phosphorus were observed with both Genz-644470 and sevelamer carbonate. Serum phosphorus-lowering responses to fixed doses of sevelamer carbonate and Genz-644470 were enhanced in a roughly linear fashion with increasing doses over a threefold range after 3 weeks of treatment. Genz-644470 did not show any advantage in phosphorus lowering per gram of binder compared with sevelamer carbonate. Overall tolerability was similar between active treatment groups. The tolerability of sevelamer carbonate was consistent with prior studies and with the established safety profile of sevelamer. Conclusion: Both Genz-644470 and sevelamer carbonate effectively lowered serum phosphate levels in a dose-dependent fashion in patients with chronic kidney disease on hemodialysis. However, Genz-644470 did not provide any advantage over sevelamer carbonate in phosphate lowering in vivo, as had been demonstrated in vitro. Keywords: sevelamer carbonate, clinical trial, serum phosphorus, hemodialysis