Paricalcitol, a new agent for the management of secondary hyperparathyroidism in patients undergoing chronic renal dialysis

Patients with end-stage renal disease commonly develop secondary hyperparathyroidism. Calcitriol may be administered to such patients to decrease the synthesis and secretion of parathyroid hormone (PTH) and to help maintain calcium and phosphorus homeostasis. However, the doses of calcitriol required to suppress serum PTH concentrations can lead to hypercalcemia or hyperphosphatemia in many patients undergoing hemodialysis. Paricalcitol is a new vitamin D analogue that is safe and effective in suppressing elevated concentrations of PTH in patients with established hyperparathyroidism who are maintained on chronic hemodialysis. As with vitamin D, the biologic action of paricalcitol is mediated through activation of the vitamin D receptor (VDR). The VDR functions as a ligand-induced transcription factor regulating the rare of expression of genes that are involved in controlling not only calcium homeostasis and bone remodeling but also hormone secretion, inhibition of cell growth, and induction of cell differentiation. In vitro studies have shown that paricalcitol inhibits PTH secretion from bovine parathyroid cells in a dose-dependent manner. Studies in renally insufficient rats demonstrated th-at paricalcitol caused approximately 10 times less elevation of serum calcium concentrations than calcitriol. In clinical studies, paricalcitol effectively decreased PTH by about 60% over a 12-week period. Mean serum concentrations of calcium were significantly increased but remained within the normal range. There were occasional (5/414 determinations) transient elevations in serum calcium above the upper limit of normal in some (5/401) patients. Serum phosphorus values did not change significantly compared with baseline, although they tended to be slightly higher in the paricalcitol-treated group than in the group receiving placebo. Elevations of the calcium-times-phosphorus product were relatively few but occurred more often in the paricalcitol than in the placebo group. The terminal half-life of paricalcitol was 5 to 7 hours in healthy subjects; in patients undergoing hemodialysis, it was 14 hours. Adverse events associated with paricalcitol use included, among others, chills, feeling unwell, fever, sepsis, palpitations, dry mouth, gastrointestinal bleeding, nausea, vomiting, edema, light-headedness, and pneumonia. Paricalcitol should be considered as an alternative to calcitriol in the treatment of patients who are undergoing maintenance hemodialysis for end-stage renal disease, as it has a decreased potential to induce hypercalcemia and hyperphosphatemia. Additional studies are required to determine the long-term effects of therapy.