Inhibition of parathyroid hormone: a dose equivalency study of paricalcitol and doxercalciferol.

Introduction: Paricalcitol and doxercalciferol are effective in reducing parathyroid hormone PTH concentrations in patients with secondary hyperparathyroidism. The purpose of this study was to determine the relative dose of doxercalciferol (compared to paricalcitol) required to maintain equivalent PTH concentrations in dialysis patients.

Methods: Chronic hemodialysis patients treated with a stable dose of paricalcitol for at least 3 months were randomized to receive doxercalciferol at either 35, 50, or 65% of the paricalcitol dose for 6 weeks. Serum iPTH, calcium, phosphorus, and albumin were determined at baseline and monitored every 2 weeks. A linear regression analysis of percent change in iPTH values by dose group was performed to determine the conversion factor.

Results: 27 patients were enrolled. Initial iPTH, adjusted serum calcium, serum phosphorus, and CaxP were similar among the treatment groups. Linear regression analysis demonstrated a conversion factor of 0.57 for the dose of doxercalciferol relative to paricalcitol resulting in equivalent suppression of iPTH. Corrected serum calcium, phosphorus, CaxP product, as well as incidence of hypercalcemia, hyperphosphatemia and CaxP >50 were similar for all groups.

Conclusion: In patients on a maintenance dose of paricalcitol, dosing doxercalciferol at 55-60% of the paricalcitol dose results in comparable inhibition of PTH.