CALCITRIOL Drug Interactions: What You Need to Know

Drug Interactions Cholestyramine: Cholestyramine has been reported to reduce intestinal absorption of fat-soluble vitamins; as such it may impair intestinal absorption of calcitriol (see WARNINGS and PRECAUTIONS : General ). Phenytoin/Phenobarbital: The coadministration of phenytoin or phenobarbital will not affect plasma concentrations of calcitriol, but may reduce endogenous plasma levels of 25(OH)D 3 by accelerating metabolism. Since blood level of calcitriol will be reduced, higher doses of calcitriol may be necessary if these drugs are administered simultaneously. Thiazides: Thiazides are known to induce hypercalcemia by the reduction of calcium excretion in urine. Some reports have shown that the concomitant administration of thiazides with calcitriol causes hypercalcemia. Therefore, precaution should be taken when coadministration is necessary. Digitalis: Calcitriol dosage must be determined with care in patients undergoing treatment with digitalis, as hypercalcemia in such patients may precipitate cardiac arrhythmias (see PRECAUTIONS : General ). Ketoconazole: Ketoconazole may inhibit both synthetic and catabolic enzymes of calcitriol. Reductions in serum endogenous calcitriol concentrations have been observed following the administration of 300 mg/day to 1200 mg/day ketoconazole for a week to healthy men. However, in vivo drug interaction studies of ketoconazole with calcitriol have not been investigated. Corticosteroids: A relationship of functional antagonism exists between vitamin D analogues, which promote calcium absorption, and corticosteroids, which inhibit calcium absorption. Phosphate-Binding Agents: Since calcitriol also has an effect on phosphate transport in the intestine, kidneys and bones, the dosage of phosphate-binding agents must be adjusted in accordance with the serum phosphate concentration. Vitamin D: Since calcitriol is the most potent active metabolite of vitamin D 3 , pharmacological doses of vitamin D and its derivatives should be withheld during treatment with calcitriol to avoid possible additive effects and hypercalcemia (see WARNINGS ).

Calcium

Supplements: Uncontrolled intake of additional calcium-containing preparations should be avoided (see PRECAUTIONS : General ). Magnesium: Magnesium-containing preparations (e.g., antacids) may cause hypermagnesemia and should therefore not be taken during therapy with calcitriol by patients on chronic renal dialysis.

CONTRAINDICATIONS Calcitriol Oral Solution should not be given to patients with hypercalcemia or evidence of vitamin D toxicity. Use of Calcitriol Oral Solution in patients with known hypersensitivity to Calcitriol Oral Solution (or drugs of the same class) or any of the inactive ingredients is contraindicated.

WARNINGS Overdosage of any form of vitamin D is dangerous (see OVERDOSAGE ). Progressive hypercalcemia due to overdosage of vitamin D and its metabolites may be so severe as to require emergency attention. Chronic hypercalcemia can lead to generalized vascular calcification, nephrocalcinosis and other soft-tissue calcification. The serum calcium times phosphate (Ca x P) product should not be allowed to exceed 70 mg 2 /dL 2 . Radiographic evaluation of suspect anatomical regions may be useful in the early detection of this condition.

Calcitriol Oral

Solution is the most potent metabolite of vitamin D available. The administration of Calcitriol Oral Solution to patients in excess of their daily requirements can cause hypercalcemia, hypercalciuria, and hyperphosphatemia. Therefore, pharmacologic doses of vitamin D and its derivatives should be withheld during Calcitriol Oral Solution treatment to avoid possible additive effects and hypercalcemia. If treatment is switched from ergocalciferol (vitamin D 2 ) to calcitriol, it may take several months for the ergocalciferol level in the blood to return to the baseline value (see OVERDOSAGE ). Calcitriol increases inorganic phosphate levels in serum. While this is desirable in patients with hypophosphatemia, caution is called for in patients with renal failure because of the danger of ectopic calcification. A nonaluminum phosphate-binding compound and a low-phosphate diet should be used to control serum phosphorus levels in patients undergoing dialysis. Magnesium-containing preparations (eg, antacids) and Calcitriol Oral Solution should not be used concomitantly in patients on chronic renal dialysis because such use may lead to the development of hypermagnesemia. Studies in dogs and rats given calcitriol for up to 26 weeks have shown that small increases of calcitriol above endogenous levels can lead to abnormalities of calcium metabolism with the potential for calcification of many tissues in the body.