Regulation of Bone Mineralization in Renal Osteodystrophy

The study outlined is designed to measure and to determine whether the combined use of vitamin D2 (ergocalciferoI) and 1-alpha-hydroxyvitamin D2 (doxercalciferol)) or doxercalciferol alone will correct the mineralization defect in pediatric patients with established secondary hyperparathyroidism (2°HPT) undergoing regular peritoneal dialysis. Serum phosphorus levels will be controlled with a calcium¬-free-metal free phosphate binder; (obtained at baseline and after 8 months of treatment) sevelamer. Indices of bone mineralization obtained at baseline and after 8 months of treatment will be measured by quantitative histomorphometry in iliac crest bone biopsies after double tetracycline labeling. Immunohistochemistry will be done in specimens of bone biopsies from iliac crest to examine the expression for selected markers of bone turnover and mineralization such as FGF-23, DMP1, MEPE and OPG. Serum PTH levels will be measured with the 1st and 2nd generation immunometric assay (PTH-IMAs) and fibroblast growth factor-23 (FGF-23) will be determined by one assay with specific detection antibodies that are against epitopes within the C-terminus of FGF-23 and another assay that uses antibodies against epitopes within the N- and C-terminal portions of the molecule respectively. The value of non-invasive assessment of bone mass by quantitative computed tomography (QCT) and its relationship with vascular disease determined by ultrasound (US) of intimal carotid thickness (CIMT) will be correlated with bone histomorphometry and the different biochemical determinations.