Phosphate and FGF-23

Fibroblast growth factor (FGF)-23 is probably the most important regulator of serum phosphate and calcitriol (1,25(OH)2D3) levels. It is secreted by osteocytes and osteoblasts in response to oral phosphate loading or increased serum 1,25(OH)2D3 levels. In human chronic kidney disease (CKD), plasma FGF-23 appears to be a sensitive biomarker of abnormal renal phosphate handling, as FGF-23 levels increase during early stages of kidney malfunction. In humans and animals with CKD, elevated FGF-23 levels increase fractional phosphate excretion, reduce serum phosphate levels, and reduce 1α-hydroxylase activity, which reduces 1,25(OH)2D3 formation thereby increasing parathyroid hormone (PTH) secretion. FGF-23 thus has a key adaptive role in maintaining normophosphatemia. Plasma FGF-23 continues to increase as CKD progresses, increasing by orders of magnitude in end-stage renal disease. At the same time, responsiveness to FGF-23 declines as the number of intact nephrons declines, which is associated with reduced expression of Klotho, the co-receptor required for FGF-23 signaling. In late CKD, FGF-23 cannot reduce serum phosphate levels, and abnormally high plasma FGF-23 concentrations appear to exert unwarranted off-target effects, including left ventricular hypertrophy, faster CKD progression, and premature mortality. Lowering serum phosphate levels through the use of oral phosphate binders and/or long-acting PTH agents may reduce FGF-23 levels in early CKD stages, thereby limiting off-target effects, which may improve patient outcomes.