Label: CALCITRIOL capsule, liquid filled

Absorption

Calcitriol is rapidly absorbed from the intestine. Peak serum concentrations (above basal values) were reached within 3 to 6 hours following oral administration of single doses of 0.25 to 1.0 mcg of Calcitriol. Following a single oral dose of 0.5 mcg, mean serum concentrations of calcitriol rose from a baseline value of 40.0±4.4 (SD) pg/mL to 60.0±4.4 pg/mL at 2 hours, and declined to 53.0±6.9 at 4 hours, 50±7.0 at 8 hours, 44±4.6 at 12 hours, and 41.5±5.1 at 24 hours.

Following multiple-dose administration, serum calcitriol levels reached steady-state within 7 days.

Distribution

Calcitriol is approximately 99.9% bound in blood. Calcitriol and other vitamin D metabolites are transported in blood, by an alpha-globulin vitamin D binding protein. There is evidence that maternal calcitriol may enter the fetal circulation. Calcitriol is transferred into human breast milk at low levels (ie, 2.2±0.1 pg/mL).

Metabolism

In vivo and in vitro studies indicate the presence of two pathways of metabolism for calcitriol. The first pathway involves the 24-hydroxylase as the first step in catabolism of calcitriol. There is definite evidence of 24-hydroxylase activity in the kidney; this enzyme is also present in many target tissues which possess the vitamin D receptor such as the intestine. The end product of this pathway is a side chain shortened metabolite, calcitroic acid. The second pathway involves the conversion of calcitriol via the stepwise hydroxylation of carbon-26 and carbon-23, and cyclization to yield ultimately 1α, 25R(OH) 2-26, 23S-lactone D 3. The lactone appears to be the major metabolite circulating in humans, with mean serum concentrations of 131±17 pg/mL. In addition, several other metabolites of calcitriol have been identified: 1α, 25(OH) 2-24-oxo-D 3; 1α, 23,25(OH) 3-24-oxo-D 3; 1α, 24R,25(OH) 3D 3; 1α, 25S,26(OH) 3D 3; 1α, 25(OH) 2-23-oxo-D 3; 1α, 25R,26(OH) 3-23-oxo-D3; 1α, (OH)24,25,26,27-tetranor-COOH-D 3.

Excretion

Enterohepatic recycling and biliary excretion of calcitriol occur. The metabolites of calcitriol are excreted primarily in feces. Following intravenous administration of radiolabeled calcitriol in normal subjects, approximately 27% and 7% of the radioactivity appeared in the feces and urine, respectively, within 24 hours. When a 1-mcg oral dose of radiolabeled calcitriol was administered to normal subjects, approximately 10% of the total radioactivity appeared in urine within 24 hours. Cumulative excretion of radioactivity on the sixth day following intravenous administration of radiolabeled calcitriol averaged 16% in urine and 49% in feces. The elimination half-life of calcitriol in serum after single oral doses is about 5 to 8 hours in normal subjects.

Pediatric Pharmacokinetics

The steady-state pharmacokinetics of oral calcitriol were determined in a small group of pediatric patients (age range: 1.8 to 16 years) undergoing peritoneal dialysis. Calcitriol was administered for 2 months at an average dose of 10.2 ng/kg (SD 5.5 ng/kg). In this pediatric population, mean C maxwas 116 pmol/L, mean serum half-life was 27.4 hours, and mean clearance was 15.3 mL/hr/kg. 1

Geriatric

No studies have examined the pharmacokinetics of calcitriol in geriatric patients.

Gender

Controlled studies examining the influence of gender on calcitriol have not been conducted.

Hepatic Insufficiency

Controlled studies examining the influence of hepatic disease on calcitriol have not been conducted.

Renal Insufficiency

Lower predose and peak calcitriol levels in serum were observed in patients with nephrotic syndrome and in patients undergoing hemodialysis compared with healthy subjects. The elimination half-life of calcitriol increased by at least twofold in chronic renal failure and hemodialysis patients compared with healthy subjects. Peak serum levels in patients with nephrotic syndrome were reached in 4 hours. For patients requiring hemodialysis peak serum levels were reached in 8 to 12 hours; half-lives were estimated to be 16.2 and 21.9 hours, respectively.

Cholestyramine

Cholestyramine has been reported to reduce intestinal absorption of fat-soluble vitamins; as such it may impair intestinal absorption of calcitriol (see WARNINGSand 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 3by 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 (eg, antacids) may cause hypermagnesemia and should therefore not be taken during therapy with calcitriol by patients on chronic renal dialysis.

Teratogenic Effects

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/m 2). 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/m 2) 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.

Nonteratogenic Effects

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 gestation 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/m 2), 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/m 2) 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.

Treatment of Hypercalcemia and Overdosage in Predialysis Patients

If hypercalcemia ensues (greater than 1 mg/dL above the upper limit of the normal range), adjust dosage to achieve normocalcemia by reducing calcitriol therapy from 0.5 mcg to 0.25 mcg daily. If the patient is receiving a therapy of 0.25 mcg daily, discontinue calcitriol until patient becomes normocalcemic. Calcium supplements should also be reduced or discontinued. Serum calcium levels should be determined 1 week after withdrawal of calcium supplements. If serum calcium levels have returned to normal, calcitriol therapy may be reinstituted at a dosage of 0.25 mcg/day if previous therapy was at a dosage of 0.5 mcg/day. If calcitriol therapy was previously administered at a dosage of 0.25 mcg/day, calcitriol therapy may be reinstituted at a dosage of 0.25 mcg every other day. If hypercalcemia is persistent at the reduced dosage, serum PTH should be measured. If serum PTH is normal, discontinue calcitriol therapy and monitor patient in 3 months' time.

Treatment of Hyperphosphatemia in Predialysis Patients

If serum phosphorus levels exceed 5.0 mg/dL to 5.5 mg/dL, a calcium-containing phosphate-binding agent (ie, calcium carbonate or calcium acetate) should be taken with meals. Serum phosphorus levels should be determined as described earlier (see PRECAUTIONS: Laboratory Tests). Aluminum-containing gels should be used with caution as phosphate-binding agents because of the risk of slow aluminum accumulation.

Treatment of Accidental Overdosage of Calcitriol

The treatment of acute accidental overdosage of calcitriol should consist of general supportive measures. If drug ingestion is discovered within a relatively short time, induction of emesis or gastric lavage may be of benefit in preventing further absorption. If the drug has passed through the stomach, the administration of mineral oil may promote its fecal elimination. Serial serum electrolyte determinations (especially calcium), rate of urinary calcium excretion, and assessment of electrocardiographic abnormalities due to hypercalcemia should be obtained. Such monitoring is critical in patients receiving digitalis. Discontinuation of supplemental calcium and a low-calcium diet are also indicated in accidental overdosage. Due to the relatively short duration of the pharmacological action of calcitriol, further measures are probably unnecessary. Should, however, persistent and markedly elevated serum calcium levels occur, there are a variety of therapeutic alternatives which may be considered, depending on the patient's underlying condition. These include the use of drugs such as phosphates and corticosteroids as well as measures to induce an appropriate forced diuresis. The use of peritoneal dialysis against a calcium-free dialysate has also been reported.

Hypoparathyroidism

The recommended initial dosage of calcitriol is 0.25 mcg/day given in the morning. If a satisfactory response in the biochemical parameters and clinical manifestations of the disease is not observed, the dose may be increased at 2- to 4-week intervals. During the dosage titration period, serum calcium levels should be obtained at least twice weekly and, if hypercalcemia is noted, calcitriol should be immediately discontinued until normocalcemia ensues (see PRECAUTIONS: General). Careful consideration should also be given to lowering the dietary calcium intake. Serum calcium, phosphorus, and 24-hour urinary calcium should be determined periodically.

Most adult patients and pediatric patients age 6 years and older have responded to dosages in the range of 0.5 mcg to 2 mcg daily. Pediatric patients in the 1 to 5 year age group with hypoparathyroidism have usually been given 0.25 mcg to 0.75 mcg daily. The number of treated patients with pseudohypoparathyroidism less than 6 years of age is too small to make dosage recommendations.

Malabsorption is occasionally noted in patients with hypoparathyroidism; hence, larger doses of calcitriol may be needed.

Predialysis Patients

The recommended initial dosage of calcitriol is 0.25 mcg/day in adults and pediatric patients 3 years of age and older. This dosage may be increased if necessary to 0.5 mcg/day.

For pediatric patients less than 3 years of age, the recommended initial dosage of calcitriol is 10 to 15 ng/kg/day.