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Hectorol Injection

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Hectorol Injection

CLINICAL PHARMACOLOGY

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 D3 (cholecalciferol) and (2) dietary intake of either vitamin D2 (ergocalciferol) or vitamin D3. Vitamin D2 and vitamin D3 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)D2 and 25-(0H)D3, respectively. Further hydroxylation of these metabolites occurs in the mitochondria of kidney tissue, catalyzed by renal 25-hydroxyvitamin D-1-a-hydroxylase to produce 1a,25-(0H)2D2, the primary biologically active form of vitamin D2, and 1 a,25-(0H)2D3 (calcitriol), the biologically active form of vitamin D3.

Mechanism of Action

Calcitriol (1α,25-(OH)2D3) and 1α,25-(0H)2D2 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 1α,25-(OH)2D2 (major metabolite) and 1α,24-dihydroxyvitamin D2 (minor metabolite). Activation of doxercalciferol does not require the involvement of the kidneys.

Peak blood levels of 1α,25-(0H)2D2 are reached at 8 +/-5.9 hours (mean +/-SD) after a single intravenous dose of 5 meg of doxercalciferol. The mean elimination half-life of 1α,25-(0H)2D2 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 1α,25-(OH)2D2 mean concentrations presumably due to volume contraction. 1α,25-(OH)2D2 is not removed from blood during hemodialysis.

Clinical Studies

The safety and effectiveness of Hectorol (doxercalciferol injection) 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 (doxercalciferol injection) Capsules in prior clinical studies (Study A and Study B) received Hectorol (doxercalciferol injection) Injection in an open-label fashion for 12 weeks following an 8-week washout (control) period. Dosing of Hectorol (doxercalciferol injection) Injection was initiated at the rate of 4 meg administered at the end of each dialysis session (3 times weekly) for a total of 12 meg per week. The dosage of Hectorol (doxercalciferol injection) 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 meg 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 meg per week. If at any time during the trial iPTH fell below 150 pg/mL, Hectorol (doxercalciferol injection) Injection was immediately suspended and restarted at a lower dosage the following week.

Results

Fifty-two of the 70 patients who were treated with Hectorol (doxercalciferol injection) 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 meg to 12.5 meg. In Study D, the mean weekly doses ranged from 9.1 meg to 11.6 meg.

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.

Table 1: iPTH Summary Data lor Patients Receiving Hectorol (doxercalciferol injection) Injection:

iPTH Level Study C Study D Combined Protocols
(n=28) (n=42) (n=70)
Baseline (Mean of Weeks -2, -1 and 0)
Mean (SE) 698 (60) 762 (65) 736 (46)
Median 562 648 634
On-treatment (Week 12*)
Mean (SE) 406 (63) 426 (60) 418 (43)
Median 311 292 292
Change from Baseline**
Mean (SE) -292 (55) -336 (41) -318 (33)
Median -274 -315 -304
P-value*** .004 .001 .001
*Values were carried forward for the two patients on study for 10 weeks  
**Treatment iPTH minus baseline iPTH
***Wilcoxon one-sample test

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 (doxercalciferol injection) 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.

Table 2 shows the numbers of patients who achieved iFTH levels below 300 pg/mL on one, two, or three or more non-consecutive occasions during the 12-week treatment period. Tliiriy-seven of 70 patients (53%) had plasma iPTH levels within the targeted range (150-300 pg/mL) during Weeks 10-12.

Table 2 shows the numbers of patients who achieved iFTH levels below 300 pg/mL on one, two, or three or more non-consecutive occasions during the 12-week treatment period. Tliiriy-seven of 70 patients (53%) had plasma iPTH levels within the targeted range (150-300 pg/mL) during Weeks 10-12.

Table 2: Number of times iPTH ≤ 300 pg/mL

1 2 ≥3
Study C 3/28 0/28 16/28
Study D 4/42 4/42 25/42

Last reviewed on RxList: 1/24/2011
This monograph has been modified to include the generic and brand name in many instances.

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