Name: Calcifediol/25-OH-cholecalciferol
Class: Vitamin
Mechanism: Increases intestinal absorption of calcium and phosphate. Stimulates bone resorption by facilitating effects of PTH. Stimulates renal reabsorption of calcium and phosphate. Net result = calcium, phosphate.
Absorption: Oral usu ® adequate absorption. Bile is essential for absorption.
Dist.: Stored in fat and muscle. Tightly bound to vitamin D-binding protein.
Metabolism.: Requires hydroxylation in kidney for full activity.
Excretion, t½: 1° = bile. Weeks.
Toxicity/S.E.s: Excess accumulation in fat/muscle, hypervitaminosis D, hypercalcemia. Drug interactions—phenytoin and phenobarbital reduce sensitivity to vit. D and/or increase rate of inactivation of calcitriol.
Utility: Treat osteomalacia 2° to liver disease. Treat renal osteodystrophy 2° to chronic renal disease—50-100 mg/d (larger dose necessary because kidney hydroxylation is deficient).
Special Features: Does not require liver activation. Fully activated in kidney. More effective than calcitriol for increasing renal absorption of calcium and phosphate. Much less effective than calcitriol for increasing bone resorption and increasing intestinal reabsorption of calcium and phosphate.