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Methods

Eleven pediatric patients with end stage kidney disease underwent bone biopsy, were treated for 8 months with doxercalciferol, and then underwent a second bone biopsy. Bone expression of fibroblast growth factor 23 (FGF23), dentin matrix protein 1 (DMP1), and sclerostin were determined by immunohistochemistry and quantified by Ariol Scanning. Western blot analysis and qRT-PCR was performed on bone abstracts of a subset of study subjects to determine the nature (i.e. size) of FGF23 and DMP1 in bone before and after therapy.

Introduction

FGF23 processing and osteocyte biology appear to change with a progressive decline in kidney function. Although circulating FGF23 undergoes cleavage in patients with normal kidney function and in those with mild CKD,[] the majority of circulating FGF23 in dialysis patients is in its full-length form [] and changes in circulating mineral ion and hormone concentrations may play a significant role in osteocytic protein expression as CKD advances. Current data suggest that increasing PTH levels suppress osteocytic sclerostin expression;[] however, bone sclerostin is increased in CKD.[] Circulating phosphorus and PTH both increase FGF23 concentrations[, ] and therapies used to treat renal osteodystrophy, in addition to controlling secondary hyperparathyroidism and bone turnover, likely also alter osteocytic protein expression, potentially via changes in mineral metabolism. Indeed, vitamin D sterol therapy suppresses PTH while increasing circulating FGF23 levels dramatically in patients with end-stage kidney disease;[] however, the effect of this form of therapy on skeletal expression of FGF23, sclerostin, and DMP1 in the context of advanced kidney disease and secondary hyperparathyroidism remains unknown. Thus, in order to assess the interplay between these three skeletal proteins and parameters of mineral metabolism during active vitamin D sterol therapy in advanced CKD, the current study evaluated iliac crest bone tissue in pediatric dialysis patients before and after 8 months of therapy with 1α-(OH)vitamin D2 (doxercalciferol).

Study Subjects

Eleven pediatric patients with end-stage kidney disease (ESKD) treated with peritoneal dialysis who had bone biopsies before and 8 months after the initiation of doxercalciferol therapy were candidates for the study. Active vitamin D sterol therapy was withheld during the 4 weeks preceding the initial bone biopsy [] and high turnover renal osteodystrophy, as defined by an increase in bone formation rate and/or the presence of marrow fibrosis on the initial biopsy, was a pre-requisite for study participation. Exclusion criteria included: previous history of poor medication compliance; parathyroidectomy within the past 12 months; concurrent treatment immunosuppressive agents or growth hormone; or bone pathology not related to secondary hyperparathyroidism.

Patient Demographic Data, Biochemical Values, and Bone Histomorphometry at Baseline and After 8 Months of Therapy

Eleven pediatric dialysis patients (7 male and 4 female/ 9 Hispanic, 1 white, and 1 black) met entry criteria, agreed to participate, and were included in the study. The average age was 15.8 + 0.8 years and BMI was 21.7 + 2.2 kg/m2. Subjects had been on dialysis for an average of 1.1 + 0.4 years. All of the study participants had some residual urine output. During the course of therapy, subjects received an average of 19.3 + 3.8 mcg of doxercalciferol per week. Calcitriol levels increased by 10.9 + 5.0 pg/ml and doxercalciferol doses correlated with the change in circulating calcitriol concentration (r = 0.55, p = 0.08). Baseline and final biochemical parameters are shown in . At baseline, serum calcium levels were 8.6 + 0.2 mg/dl and serum phosphorus concentrations were 6.3 + 0.4 mg/dl; neither parameter changed with doxercalciferol therapy. Plasma PTH, alkaline phosphatase, and FGF23 levels were elevated at baseline. Overall, 6 patients achieved PTH levels within the target range while values remained elevated in the remainder due to the development of hypercalcemia and hyperphosphatemia. While PTH and alkaline phosphatase values did not change with therapy, FGF23 concentrations increased and final values were 1696 (565, 3973) RU/ml (p<0.05 from baseline).