Calcitriol capsules are also indicated in the management of hypocalcemia and its clinical manifestations in patients with postsurgical hypoparathyroidism, idiopathic hypoparathyroidism, and pseudohypoparathyroidism.
Calcitriol should not be given to patients with hypercalcemia or evidence of vitamin D toxicity. Use of calcitriol in patients with known hypersensitivity to calcitriol (or drugs of the same class) or any of the inactive ingredients is contraindicated.
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 × P) product should not be allowed to exceed 70 mg2 /dL2 . Radiographic evaluation of suspect anatomical regions may be useful in the early detection of this condition.
Calcitriol is the most potent metabolite of vitamin D available. The administration of calcitriol 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 treatment to avoid possible additive effects and hypercalcemia. If treatment is switched from ergocalciferol (vitamin D2 ) 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 non-aluminum 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 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.
Excessive dosage of calcitriol induces hypercalcemia and in some instances hypercalciuria; therefore, early in treatment during dosage adjustment, serum calcium should be determined twice weekly. In dialysis patients, a fall in serum alkaline phosphatase levels usually antedates the appearance of hypercalcemia and may be an indication of impending hypercalcemia. An abrupt increase in calcium intake as a result of changes in diet (eg, increased consumption of dairy products) or uncontrolled intake of calcium preparations may trigger hypercalcemia.
Should hypercalcemia develop, treatment with calcitriol should be stopped immediately. During periods of hypercalcemia, serum calcium and phosphate levels must be determined daily. When normal levels have been attained, treatment with calcitriol can be continued, at a daily dose 0.25 mcg lower than that previously used. An estimate of daily dietary calcium intake should be made and the intake adjusted when indicated. Calcitriol should be given cautiously to patients on digitalis, because hypercalcemia in such patients may precipitate cardiac arrhythmias.
Immobilized patients, eg, those who have undergone surgery, are particularly exposed to the risk of hypercalcemia.
In patients with normal renal function, chronic hypercalcemia may be associated with an increase in serum creatinine. While this is usually reversible, it is important in such patients to pay careful attention to those factors which may lead to hypercalcemia. Calcitriol therapy should always be started at the lowest possible dose and should not be increased without careful monitoring of the serum calcium. An estimate of daily dietary calcium intake should be made and the intake adjusted when indicated.
Patients with normal renal function taking calcitriol should avoid dehydration. Adequate fluid intake should be maintained.
The patient and his or her caregivers should be informed about compliance with dosage instructions, adherence to instructions about diet and calcium supplementation, and avoidance of the use of unapproved nonprescription drugs. Patients and their caregivers should also be carefully informed about the symptoms of hypercalcemia (see ADVERSE REACTIONS).
The effectiveness of calcitriol therapy is predicated on the assumption that each patient is receiving an adequate daily intake of calcium. Patients are advised to have a dietary intake of calcium at a minimum of 600 mg daily. The U.S. RDA for calcium in adults is 800 mg to 1200 mg.
For dialysis patients, serum calcium, phosphorus, magnesium, and alkaline phosphatase should be determined periodically. For hypoparathyroid patients, serum calcium, phosphorus, and 24-hour urinary calcium should be determined periodically. For predialysis patients, serum calcium, phosphorus, alkaline phosphatase, creatinine, and intact PTH (iPTH) should be determined initially. Thereafter, serum calcium, phosphorus, alkaline phosphatase, and creatine should be determined monthly for a 6-month period and then determined periodically. Intact PTH (iPTH) should be determined periodically every 3 to 4 months at the time of visits. During the titration period of treatment with calcitriol, serum calcium levels should be checked at least twice weekly (see DOSAGE AND ADMINISTRATION).
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).
The coadministration of phenytoin or phenobarbital will not affect plasma concentrations of calcitriol, but may reduce endogenous plasma levels of 25(OH)D3 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 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.
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 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.
A relationship of functional antagonism exists between vitamin D analogues, which promote calcium absorption, and corticosteroids, which inhibit calcium absorption.
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.
Since calcitriol is the most potent active metabolite of vitamin D3 , pharmacological doses of vitamin D and its derivatives should be withheld during treatment with calcitriol to avoid possible additive effects and hypercalcemia (see WARNINGS).
Uncontrolled intake of additional calcium-containing preparations should be avoided (see PRECAUTIONS: General).
Magnesium-containing preparations (eg, antacids) may cause hypermagnesemia and should therefore not be taken during therapy with calcitriol by patients on chronic renal dialysis.
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