Search

Secondary hyperparathyroidism (2HPT), a common disorder in patients with chronic renal failure, develops in response to phosphate retention and low serum 1,25-dihydroxyvitamin D(3) [1,25 (OH) (2)D(3), calcitriol] . Replacement therapy with calcitriol or its precursor 1alpha-hydroxyvitamin D(3) [1alpha (OH) D(3), alfacalcidol] often produces hypercalcemia and hyperphosphatemia in these patients. Several vitamin D analogues have been developed that retain the direct suppressive action of 1,25 (OH) (2)D(3) on the parathyroid glands but have less calcemic activity, therapy offering a safer and more effective means of controlling 2HPT. 1,25-D dihydroxy-19-norvitamin D(2) (19-nor D(2)) and 1alpha-hydroxyvitamin D(2) (1alphaOHD(2)) are available in the United States and 1,25-dihydrox-22-oxavitamin D(3) (22-oxacalcitriol, OCT) and 1,25-dihydroxy-26,26,26,27,27,27-hexafluorovitamin D(3) [1,25 (OH)(2)26,27F(6)D(3), falecalcitriol] have been approved for use in Japan. Animal studies have demonstrated that OCT and 19-nor D(2) have a wider therapeutic window for suppression of parathyroid hormone (PTH) because of their lower calcemic activities of OCT has been attributed to its rapid clearance which prevents sustained effects on intestinal calcium absorption and bone resorption, but still allows a prolonged suppression of PTH gene expression and parathyroid cell growth. The calcemic activity of 19-norD(2) diminishes with the duration of treatment by as yet unknown mechanisms. The lower toxicity of 1alphaOHD(2), compared 1alphaOHD(3). has also been noted with chronic, but not acute administration, perhaps due to differential metabolism. The unique actions of falecalcitriol may also result from altered metabolism. A clear understanding of the molecular basis for the selectivity of vitamin D analogues on parathyroid function may allow the design of even more effective analogues.