Background: Different phosphate binders exert differing effects on bone mineral metabolism and levels of regulating hormones. The objective of this post hoc evaluation of the CALcium acetate MAGnesium carbonate (CALMAG) study was to compare the effects of calcium acetate/magnesium carbonate (CaMg) and a calcium-free phosphate binder, sevelamer-hydrochloride (HCl), on serum levels of fibroblast growth factor-23 (FGF-23) and markers of bone turnover.
Methods: This secondary analysis of the controlled, randomized CALMAG study, comparing the effect of CaMg and sevelamer-HCl on serum phosphorus (P), aimed to investigate the parameters described above. The analysis included 204 patients who completed the initial study per protocol (CaMg, n = 105; sevelamer-HCl, n = 99).
Results: The study showed that serum levels of FGF-23 were significantly reduced with CaMg and sevelamer-HCl, with no difference between groups at Week 25 [analysis of covariance (ANCOVA); log-intact FGF-23 (iFGF-23), P = 0.1573]. FGF-23 levels strongly correlated with serum P levels at all time points in both groups. The bone turnover parameters alkaline phosphatase (AP), bone AP (BAP), procollagen type 1 amino-terminal propeptide 1 (P1NP), osteoprotegerin (OPG), beta-crosslaps (β-CTX) and tartrate-resistant acid phosphatase 5b (TRAP 5b) increased significantly in the sevelamer-HCl group; they remained almost unchanged in the CaMg group, after the initial phase of P lowering (ANCOVA, P < 0.0001 for all except OPG, P = 0.1718).
Conclusions: CaMg and sevelamer-HCl comparably lower serum levels of iFGF-23. Changes in bone parameters were dependent on characteristics of the phosphate binder; in contrast with sevelamer-HCl, CaMg had no influence on bone turnover markers.
Keywords: bone markers; calcium acetate; fibroblast growth factor-23; haemodialysis; magnesium carbonate; phosphate binder.