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Hectorol Injection (Bone Care)
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Doxercalciferol, the active ingredient in Hectorol®, is a synthetic vitamin D 2 analog that undergoes metabolic activation in vivo to form 1(alpha),25-dihydroxyvitamin D 2 (1(alpha),25-(OH) 2 D 2 ), a naturally occurring, biologically active form of vitamin D 2 . Hectorol is available as a sterile, clear, essentially colorless to faint yellow, aqueous solution for intravenous injection. Each milliliter (mL) of solution contains doxercalciferol, 2 mcg; Polysorbate 20, 4 mg; sodium chloride, 1.5 mg; sodium ascorbate, 10 mg; sodium phosphate, dibasic, 7.6 mg; sodium phosphate, monobasic, 1.8 mg; and disodium edetate, 1.1 mg.
Doxercalciferol is a colorless crystalline compound with a calculated molecular weight of 412.66 and a molecular formula of C 28 H 44 O 2 . It is soluble in oils and organic solvents, but is relatively insoluble in water. Chemically, doxercalciferol is (1(alpha),3(beta),5Z,7E,22E)-9,10-secoergosta-5,7,10(19),22-tetraene-1,3-diol and has the following structural formula:
Other names frequently used for doxercalciferol are 1(alpha)-hydroxyvitamin D 2 , 1(alpha)-OH-D 2 , and 1(alpha)-hydroxyergocalciferol.
Vitamin D levels in humans depend on two sources: (1) exposure to the ultraviolet rays of the sun for conversion of 7-dehydrocholesterol in the skin to vitamin D 3 (cholecalciferol) and (2) dietary intake of either vitamin D 2 (ergocalciferol) or vitamin D 3 . Vitamin D 2 and vitamin D 3 must be metabolically activated in the liver and kidney before becoming fully active on target tissues. The initial step in the activation process is the introduction of a hydroxyl group in the side chain at C-25 by the hepatic enzyme, CYP 27 (a vitamin D-25-hydroxylase). The products of this reaction are 25-(OH)D 2 and 25-(OH)D 3 , respectively. Further hydroxylation of these metabolites occurs in the mitochondria of kidney tissue, catalyzed by renal 25-hydroxyvitamin D-1-(alpha)-hydroxylase to produce 1(alpha),25-(OH) 2 D 2 , the primary biologically active form of vitamin D 2 , and 1(alpha),25-(OH) 2 D 3 (calcitriol), the biologically active form of vitamin D 3 .
Mechanism of Action
Calcitriol (1(alpha),25-(OH) 2 D 3 ) and 1(alpha),25-(OH) 2 D 2 regulate blood calcium at levels required for essential body functions. Specifically, the biologically active vitamin D metabolites control the intestinal absorption of dietary calcium, the tubular reabsorption of calcium by the kidney and, in conjunction with parathyroid hormone (PTH), the mobilization of calcium from the skeleton. They act directly on bone cells (osteoblasts) to stimulate skeletal growth, and on the parathyroid glands to suppress PTH synthesis and secretion. These functions are mediated by the interaction of these biologically active metabolites with specific receptor proteins in the various target tissues. In uremic patients, deficient production of biologically active vitamin D metabolites (due to lack of or insufficient 25-hydroxyvitamin D-1-alpha-hydroxylase activity) leads to secondary hyperparathyroidism, which contributes to the development of metabolic bone disease in patients with renal failure.
Pharmacokinetics and Metabolism
After intravenous administration, doxercalciferol is activated by CYP 27 in the liver to form 1(alpha),25-(OH) 2 D 2 (major metabolite) and 1(alpha),24-dihydroxyvitamin D 2 (minor metabolite). Activation of doxercalciferol does not require the involvement of the kidneys.
Peak blood levels of 1(alpha),25-(OH) 2 D 2 are reached at 8 +/- 5.9 hours (mean +/- SD) after a single intravenous dose of 5 mcg of doxercalciferol. The mean elimination half-life of 1(alpha),25-(OH) 2 D 2 after an oral dose is approximately 32 to 37 hours with a range of up to 96 hours. The mean elimination half-life in patients with end stage renal disease (ESRD) and in healthy volunteers appears to be similar following an oral dose. Hemodialysis causes a temporary increase in 1(alpha),25-(OH) 2 D 2 mean concentrations presumably due to volume contraction. 1(alpha),25-(OH) 2 D 2 is not removed from blood during hemodialysis.
The safety and effectiveness of Hectorol® Injection were evaluated in two open-label, single-arm, multi-centered clinical studies (Study C and Study D) in a total of 70 patients with chronic kidney disease on hemodialysis (Stage 5 CKD). Patients in Study C were an average age of 54 years (range: 23-73), were 50% male, and were 61% African-American, 25% Caucasian, and 14% Hispanic, and had been on hemodialysis for an average of 65 months. Patients in Study D were an average age of 51 years (range: 28-76), were 48% male, and 100% African-American and had been on hemodialysis for an average of 61 months. This group of 70 of the 138 patients who had been treated with Hectorol® Capsules in prior clinical studies (Study A and Study B) received Hectorol® Injection in an open-label fashion for 12 weeks following an 8-week washout (control) period. Dosing of Hectorol® Injection was initiated at the rate of 4 mcg administered at the end of each dialysis session (3 times weekly) for a total of 12 mcg per week. The dosage of Hectorol® was adjusted in an attempt to achieve iPTH levels within a targeted range of 150 to 300 pg/mL. The dosage was increased by 2 mcg per dialysis session after 8 weeks of treatment if the iPTH levels remained above 300 pg/mL and were greater than 50% of baseline levels. The maximum dosage was limited to 18 mcg per week. If at any time during the trial iPTH fell below 150 pg/mL, Hectorol® Injection was immediately suspended and restarted at a lower dosage the following week.
Fifty-two of the 70 patients who were treated with Hectorol® Injection achieved iPTH levels </= 300 pg/mL. Forty-one of these patients exhibited plasma iPTH levels </= 300 pg/mL on at least 3 occasions. Thirty-six patients had plasma iPTH levels < 150 pg/mL on at least one occasion during study participation.
Mean weekly doses in Study C ranged from 8.9 mcg to 12.5 mcg. In Study D, the mean weekly doses ranged from 9.1 mcg to 11.6 mcg.
Decreases in plasma iPTH from baseline values were calculated using as baseline the average of the last 3 values obtained during the 8-week washout period and are displayed in the table below. Plasma iPTH levels were measured weekly during the 12-week study.
In both studies, iPTH levels increased progressively and significantly in 62.9% of patients during the 8-week washout (control) period during which no vitamin D derivatives were administered. In contrast, Hectorol® Injection treatment resulted in a clinically significant reduction (at least 30%) from baseline in mean iPTH levels during the 12-week open-label treatment period in more than 92% of the 70 treated patients.
The following table shows the numbers of patients who achieved iPTH levels below 300 pg/mL on one, two, or three or more non-consecutive occasions during the 12-week treatment period. Thirty-seven of 70 patients (53%) had plasma iPTH levels within the targeted range (150-300 pg/mL) during Weeks 10-12.
INDICATIONS AND USAGE
Hectorol® is indicated for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease on dialysis.
Hectorol® should not be given to patients with a tendency towards hypercalcemia or current evidence of vitamin D toxicity.
Overdosage of any form of vitamin D, including Hectorol®, is dangerous (see OVERDOSAGE ). Progressive hypercalcemia due to overdosage of vitamin D and its metabolites may be so severe as to require emergency attention. Acute hypercalcemia may exacerbate tendencies for cardiac arrhythmias and seizures and may potentiate the action of digitalis drugs. Chronic hypercalcemia can lead to generalized vascular calcification and other soft-tissue calcification. The serum calcium times serum phosphorus (Ca X P) product should be maintained at <55 mg 2 /dL 2 in patients with chronic kidney disease. Radiographic evaluation of suspect anatomical regions may be useful in the early detection of this condition.
Since doxercalciferol is a precursor for 1(alpha),25-(OH) 2 D 2 , a potent metabolite of vitamin D 2 , pharmacologic doses of vitamin D and its derivatives should be withheld during Hectorol® treatment to avoid possible additive effects and hypercalcemia.
Oral calcium-based or other non-aluminum-containing phosphate binders and a low phosphate diet should be used to control serum phosphorus levels in patients undergoing dialysis. Uncontrolled serum phosphorus exacerbates secondary hyperparathyroidism and can lessen the effectiveness of Hectorol® in reducing blood PTH levels. If hypercalcemia occurs after initiating Hectorol® therapy, the dose of Hectorol® and/or calcium-containing phosphate binders should be decreased. If hyperphosphatemia occurs after initiating Hectorol®, the dose of Hectorol® should be decreased and/or the dose of phosphate binders increased. (See dosing recommendations for Hectorol® under DOSAGE AND ADMINISTRATION section.)
Magnesium-containing antacids and Hectorol® should not be used concomitantly in patients on chronic renal dialysis because such use may lead to the development of hypermagnesemia.
The principal adverse effects of treatment with Hectorol® Injection are hypercalcemia, hyperphosphatemia, and oversuppression of iPTH (less than 150 pg/mL). Prolonged hypercalcemia can lead to calcification of soft tissues, including the heart and arteries, and hyperphosphatemia can exacerbate hyperparathyroidism. Oversuppression of iPTH may lead to adynamic bone syndrome. All of these potential adverse effects should be managed by regular patient monitoring and appropriate dosage adjustments. During treatment with Hectorol®, patients usually require dose titration, as well as adjustment in co-therapy (i.e., dietary phosphate binders) in order to maximize iPTH suppression while maintaining serum calcium and phosphorus levels within prescribed ranges.
In two open-label, single-arm, multi-centered studies, the incidence of hypercalcemia and hyperphosphatemia increased during therapy with Hectorol® Injection (see Adverse Reactions section). The observed increases during Hectorol® treatment underscore the importance of regular safety monitoring of serum calcium and phosphorus levels throughout treatment. Patients with higher pre-treatment serum levels of calcium (> 10.5 mg/dL) or phosphorus (> 6.9 mg/dL) were more likely to experience hypercalcemia or hyperphosphatemia. Therefore, Hectorol® should not be given to patients with a recent history of hypercalcemia or hyperphosphatemia, or evidence of vitamin D toxicity.
Information for the Patient
The patient, spouse, or guardian should be informed about adherence to instructions about diet, calcium supplementation, and avoidance of the use of nonprescription drugs without prior approval from the patient's physician. Patients should also be carefully informed about the symptoms of hypercalcemia (see ADVERSE REACTIONS section).
Serum levels of iPTH, calcium, and phosphorus should be determined prior to initiation of Hectorol® treatment. During the early phase of treatment (i.e., first 12 weeks), serum iPTH, calcium, and phosphorus levels should be determined weekly. For dialysis patients in general, serum or plasma iPTH and serum calcium, phosphorus, and alkaline phosphatase should be determined periodically.
Specific drug interaction studies have not been conducted. Magnesium-containing antacids and Hectorol® should not be used concomitantly because such use may lead to the development of hypermagnesemia (see WARNINGS ). Although not examined specifically, enzyme inducers (such as glutethimide and phenobarbitol) may affect the 25-hydroxylation of Hectorol® and may necessitate dosage adjustments. Cytochrome P450 inhibitors (such as ketoconazole and erythromycin) may inhibit the 25-hydroxylation of Hectorol®. Hence, formation of the active Hectorol® moiety may be hindered.
Carcinogenesis, Mutagenesis, Impairment of Fertility
Long-term studies in animals to evaluate the carcinogenic potential of doxercalciferol have not been conducted. No evidence of genetic toxicity was observed in an in vitro bacterial mutagenicity assay (Ames test) or a mouse lymphoma gene mutation assay. Doxercalciferol caused structural chromatid and chromosome aberrations in an in vitro human lymphocyte clastogenicity assay with metabolic activation. However, doxercalciferol was negative in an in vivo mouse micronucleus clastogenicity assay. Doxercalciferol had no effect on male or female fertility in rats at oral doses up to 2.5 mcg/kg/day (approximately 3 times the maximum recommended human oral dose of 60 mcg/wk based on mcg/m 2 body surface area).
Use in Pregnancy
Pregnancy Category B
Reproduction studies in rats and rabbits, at doses up to 20 mcg/kg/day and 0.1 mcg/kg/day (approximately 25 times and less than the maximum recommended human oral dose of 60 mcg/week based on mcg/m 2 body surface area, respectively) have revealed no teratogenic or fetotoxic effects due to doxercalciferol. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.
It is not known whether doxercalciferol is excreted in human milk. Because other vitamin D derivatives are excreted in human milk and because of the potential for serious adverse reactions in nursing infants from doxercalciferol, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.
Safety and efficacy of Hectorol® in pediatric patients have not been established.
Of the 70 patients treated with Hectorol® Injection in the two Phase 3 clinical studies, 12 patients were 65 years or over. In these studies, no overall differences in efficacy or safety were observed between patients 65 years or older and younger patients.
Studies examining the influence of hepatic insufficiency on the metabolism of Hectorol® were inconclusive. Since patients with hepatic insufficiency may not metabolize doxercalciferol appropriately, the drug should be used with caution in patients with impaired hepatic function. More frequent monitoring of iPTH, calcium, and phosphorus levels should be done in such individuals.
Hectorol® Injection has been evaluated for safety in 70 patients with chronic renal disease on hemodialysis (who had been previously treated with oral Hectorol®) from two 12-week, open-label, single-arm, multi-centered studies. (Dosage titrated to achieve target plasma iPTH levels, see CLINICAL PHARMACOLOGY / Clinical Studies .)
Because there was no placebo group included in the studies of Hectorol® Injection, the table below provides the adverse event incidence rates from placebo-controlled studies of oral Hectorol®.
Potential adverse effects of Hectorol® are, in general, similar to those encountered with excessive vitamin D intake. The early and late signs and symptoms of vitamin D intoxication associated with hypercalcemia include:
Weakness, headache, somnolence, nausea, vomiting, dry mouth, constipation, muscle pain, bone pain, metallic taste, and anorexia.
Polyuria, polydipsia, anorexia, weight loss, nocturia, conjunctivitis (calcific), pancreatitis, photophobia, rhinorrhea, pruritus, hyperthermia, decreased libido, elevated blood urea nitrogen (BUN), albuminuria, hypercholesterolemia, elevated serum aspartate transaminase (AST) and alanine transaminase (ALT), ectopic calcification, hypertension, cardiac arrhythmias, sensory disturbances, dehydration, apathy, arrested growth, urinary tract infections, and, rarely, overt psychosis.
Administration of Hectorol® to patients in excess doses can cause hypercalcemia, hypercalciuria, hyperphosphatemia, and over-suppression of PTH secretion leading in certain cases to adynamic bone disease. High intake of calcium and phosphate concomitant with Hectorol® may lead to similar abnormalities. High levels of calcium in the dialysate bath may contribute to hypercalcemia.
Treatment of Hypercalcemia and Overdosage
General treatment of hypercalcemia (greater than 1 mg/dL above the upper limit of the normal range) consists of immediate suspension of Hectorol® therapy, institution of a low calcium diet, and withdrawal of calcium supplements. Serum calcium levels should be determined at least weekly until normocalcemia ensues. Hypercalcemia usually resolves in 2 to 7 days. When serum calcium levels have returned to within normal limits, Hectorol® therapy may be reinstituted at a dose that is at least 1 mcg lower than prior therapy. Serum calcium levels should be obtained weekly after all dosage changes and during subsequent dosage titration. Persistent or markedly elevated serum calcium levels may be corrected by dialysis against a reduced calcium or calcium-free dialysate.
Treatment of Accidental Overdosage of Hectorol®
The treatment of acute accidental overdosage of Hectorol® should consist of general supportive measures. 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 institution of a low calcium diet are also indicated in accidental overdosage. If persistent and markedly elevated serum calcium levels occur, there are a variety of therapeutic alternatives that may be considered. These include the use of drugs such as phosphates and corticosteroids as well as measures to induce diuresis. Also, one may consider dialysis against a calcium-free dialysate.
DOSAGE AND ADMINISTRATION
The optimal dose of Hectorol® must be carefully determined for each patient.
The recommended initial dose of Hectorol® is 4 mcg administered as a bolus dose three times weekly at the end of dialysis (approximately every other day). The initial dose should be adjusted, as needed, in order to lower blood iPTH into the range of 150 to 300 pg/mL. The dose may be increased at 8-week intervals by 1 to 2 mcg if iPTH is not lowered by 50% and fails to reach the target range. Dosages higher than 18 mcg weekly have not been studied. Drug administration should be suspended if iPTH falls below 100 pg/mL and restarted one week later at a dose that is at least 1 mcg lower than the last administered dose. During titration, iPTH, serum calcium, and serum phosphorus levels should be obtained weekly. If hypercalcemia, hyperphosphatemia, or a serum calcium times phosphorus product greater than 55 mg 2 /dL 2 is noted, the dose of Hectorol® should be decreased or suspended and/or the dose of phosphate binders should be appropriately adjusted. If suspended, the drug should be restarted at a dose that is 1 mcg lower.
Dosing must be individualized and based on iPTH levels with monitoring of serum calcium and serum phosphorus levels. The following is a suggested approach in dose titration:
Hectorol® (doxercalciferol) Injection is supplied in pre-scored amber glass ampules.
Store at 15° to 25°C (59° to 77°F). Protect from light.
Manufactured by Draxis Pharma, Inc. for
Bone Care International, Inc., Middleton, WI 53652
©2000, Bone Care International, Inc.