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/dL. 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 D) 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.
This product contains FD&C Yellow No. 5 (tartrazine) which may cause allergic-type reactions (including bronchial asthma) in certain susceptible persons. Although the overall incidence of FD&C Yellow No. 5 (tartrazine) sensitivity in the general population is low, it is frequently seen in patients who also have aspirin hypersensitivity.
Information for Patients
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).
Pregnancy Category C
Calcitriol has been found to be teratogenic in rabbits when given at doses of 0.08 and 0.3 mcg/kg (approximately 2 and 6 times the maximum recommended dose based on mg/m2). All 15 fetuses in 3 litters at these doses showed external and skeletal abnormalities. However, none of the other 23 litters (156 fetuses) showed external and skeletal abnormalities compared with controls.
Teratogenicity studies in rats at doses up to 0.45 mcg/kg (approximately 5 times maximum recommended dose based on mg/m2) showed no evidence of teratogenic potential. There are no adequate and well-controlled studies in pregnant women. Calcitriol should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.
In the rabbit, dosages of 0.3 mcg/kg/day (approximately 6 times maximum recommended dose based on surface area) administered on days 7 to 18 of ges-tation resulted in 19% maternal mortality, a decrease in mean fetal body weight and a reduced number of newborn surviving to 24 hours. A study of perinatal and postnatal development in rats resulted in hypercalcemia in the offspring of dams given calcitriol at doses of 0.08 or 0.3 mcg/kg/day (approximately 1 and 3 times the maximum recommended dose based on mg/m2), hypercalcemia and hypophosphatemia in dams given calcitriol at a dose of 0.08 or 0.3 mcg/kg/day, and increased serum urea nitrogen in dams given calcitriol at a dose of 0.3 mcg/kg/day. In another study in rats, maternal weight gain was slightly reduced at a dose of 0.3 mcg/kg/day (approximately 3 times the maximum recommended dose based on mg/m2) administered on days 7 to 15 of gestation. The offspring of a woman administered 17 mcg/day to 36 mcg/day of calcitriol (approximately 17 to 36 times the maximum recommended dose), during pregnancy manifested mild hypercalcemia in the first 2 days of life which returned to normal at day 3.
Calcitriol from ingested calcitriol capsules may be excreted in human milk. Because many drugs are excreted in human milk and because of the potential for serious adverse reactions from calcitriol in nursing infants, a mother should not nurse while taking calcitriol capsules.
Safety and effectiveness of calcitriol in pediatric patients undergoing dialysis have not been established. The safety and effectiveness of calcitriol in pediatric predialysis patients is based on evidence from adequate and well-controlled studies of calcitriol in adults with predialysis chronic renal failure and additional supportive data from non-placebo controlled studies in pediatric patients. Dosing guidelines have not been established for pediatric patients under 1 year of age with hypoparathyroidism or for pediatric patients less than 6 years of age with pseudohypoparathyroidism (see DOSAGE AND ADMINISTRATION: Hypoparathyroidism).
Oral doses of calcitriol ranging from 10 to 55 ng/kg/day have been shown to improve calcium homeostasis and bone disease in pediatric patients with chronic renal failure for whom hemodialysis is not yet required (predialysis). Long-term calcitriol therapy is well tolerated by pediatric patients. The most common safety issues are mild, transient episodes of hypercalcemia, hyperphosphatemia, and increases in the serum calcium times phosphate (Ca x P) product which are managed effectively by dosage adjustment or temporary discontinuation of the vitamin D derivative.
Clinical studies of calcitriol did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.