CALCITRIOL

CALCITRIOL- calcitriol capsule
Carilion Materials Management

DESCRIPTION

Calcitriol is a synthetic vitamin D analog which is active in the regulation of the absorption of calcium from the gastrointestinal tract and its utilization in the body. Calcitriol is available as capsules containing 0.25 mcg or 0.5 mcg calcitriol and as an oral solution containing 1 mcg/mL of calcitriol. All dosage forms contain butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) as antioxidants. The capsules contain a fractionated triglyceride of coconut oil, and the oral solution contains a fractionated triglyceride of palm seed oil. Gelatin capsule shells contain glycerin, and sorbitol, with the following dye systems: 0.25 mcg — FD&C Yellow No. 6 and titanium dioxide; 0.5 mcg — FD&C Red No. 3, FD&C Yellow No. 6 and titanium dioxide. The oral solution contains no additional adjuvants or coloring principles.

Calcitriol is a white, crystalline compound which occurs naturally in humans. It has a calculated molecular weight of 416.65 and is soluble in organic solvents but relatively insoluble in water. Chemically, calcitriol is 9,10- seco(5Z,7E)-5,7,10(19)-cholestatriene-1α, 3β, 25-triol and has the following structural formula:

structural Formula

The other names frequently used for calcitriol are 1α,25-dihydroxycholecalciferol, 1,25-dihydroxyvitamin D , 1,25-DHCC, 1,25(OH) D and 1,25-diOHC. 3 2 3

CLINICAL PHARMACOLOGY

Man’s natural supply of vitamin D depends mainly on exposure to the ultraviolet rays of the sun for conversion of 7-dehydrocholesterol in the skin to vitamin D (cholecalciferol). Vitamin D must be metabolically activated in the liver and the kidney before it is fully active as a regulator of calcium and phosphorus metabolism at target tissues. The initial transformation of vitamin D is catalyzed by a vitamin D -25-hydroxylase enzyme (25-OHase) present in the liver, and the product of this reaction is 25-hydroxyvitamin D [25-(OH)D ]. Hydroxylation of 25-(OH)D occurs in the mitochondria of kidney tissue, activated by the renal 25-hydroxyvitamin D -1 alpha-hydroxylase (alpha-OHase), to produce 1,25-(OH) D (calcitriol), the active form of vitamin D . Endogenous synthesis and catabolism of calcitriol, as well as physiological control mechanisms affecting these processes, play a critical role regulating the serum level of calcitriol. Physiological daily production is normally 0.5 to 1.0 mcg and is somewhat higher during periods of increased bone synthesis (eg, growth or pregnancy). 3 3 3 3 3 3 3 3 2 3 3

Pharmacodynamics

The two known sites of action of calcitriol are intestine and bone. A calcitriol receptor-binding protein appears to exist in the mucosa of human intestine. Additional evidence suggests that calcitriol may also act on the kidney and the parathyroid glands. Calcitriol is the most active known form of vitamin D in stimulating intestinal calcium transport. In acutely uremic rats calcitriol has been shown to stimulate intestinal calcium absorption. 3

The kidneys of uremic patients cannot adequately synthesize calcitriol, the active hormone formed from precursor vitamin D. Resultant hypocalcemia and secondary hyperparathyroidism are a major cause of the metabolic bone disease of renal failure. However, other bone-toxic substances which accumulate in uremia (eg, aluminum) may also contribute.

The beneficial effect of calcitriol in renal osteodystrophy appears to result from correction of hypocalcemia and secondary hyperparathyroidism. It is uncertain whether calcitriol produces other independent beneficial effects. Calcitriol treatment is not associated with an accelerated rate of renal function deterioration. No radiographic evidence of extraskeletal calcification has been found in predialysis patients following treatment. The duration of pharmacologic activity of a single dose of calcitriol is about 3 to 5 days.

Pharmacokinetics

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) -26, 23S-lactone D . 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) -24-oxo-D ; 1α , 23,25(OH) -24-oxo-D ; 1α, 24R,25(OH) D ; 1α, 25S,26(OH) D ; 1α, 25(OH) -23-oxo-D ; 1α, 25R,26(OH) -23-oxo-D ; 1α, (OH)24,25,26,27-tetranor-COOH-D . 2 3 2 3 3 3 3 3 3 3 2 3 3 3 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.

Special Populations

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 was 116 pmol/L, mean serum half-life was 27.4 hours, and mean clearance was 15.3 mL/hr/kg. max 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.

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