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THERAPEUTIC CLASS: FOSAMAX PLUS contains alendronate sodium and colecalciferol (vitamin D3).
Alendronate Sodium: Alendronate sodium is a bisphosphonate that acts as a potent, specific inhibitor of osteoclast-mediated bone resorption. Bisphosphonates are synthetic analogs of pyrophosphate that bind to the hydroxyapatite found in bone.
Colecalciferol: Vitamin D3 is a secosterol that is converted to 25-hydroxyvitamin D3 in the liver. Conversion to the active calcium-mobilizing hormone 1,25-dihydroxyvitamin D3 (calcitriol) in the kidney is stimulated by both parathyroid hormone and hypophosphatemia. The principal action of 1,25-dihydroxyvitamin D3 is to increase intestinal absorption of both calcium and phosphate as well as regulate serum calcium, renal calcium and phosphate excretion, bone formation and bone resorption.
Vitamin D3 is required for normal bone formation. Vitamin D insufficiency develops when both sunlight exposure and dietary intake are inadequate. Insufficiency is associated with negative calcium balance, bone loss, and increased risk of skeletal fracture. In severe cases, deficiency results in secondary hyperparathyroidism, hypophosphatemia, proximal muscle weakness and osteomalacia, further increasing the risk of falls and fractures in osteoporotic individuals.
PHARMACOLOGY: Pharmacodynamics: Alendronate is a bisphosphonate that binds to bone hydroxyapatite and specifically inhibits the activity of osteoclasts, the bone-resorbing cells. Alendronate reduces bone resorption with no direct effect on bone formation, although the latter process is ultimately reduced because bone resorption and formation are coupled during bone turnover.
Osteoporosis in postmenopausal women: Osteoporosis is characterized by low bone mass that leads to an increased risk of fracture. It occurs in both males and females but is most common among women following the menopause, when bone turnover increases and the rate of bone resorption exceeds that of bone formation. These changes result in progressive bone loss and lead to osteoporosis in a significant proportion of women over age 50. Fractures, usually of the spine, hip, and wrist, are the common consequences. From age 50 to age 90, the risk of hip fracture in women increases 50-fold and the risk of vertebral fracture 15- to 30-fold. It is estimated that approximately 40% of 50-year-old women will sustain one or more osteoporosis-related fractures of the spine, hip, or wrist during their remaining lifetimes. Hip fractures, in particular, are associated with substantial morbidity, disability, and mortality.
Daily oral doses of alendronate (5, 20, and 40 mg for six weeks) in postmenopausal women produced biochemical changes indicative of dose-dependent inhibition of bone resorption, including decreases in urinary calcium and urinary markers of bone collagen degradation (such as hydroxyproline, deoxypyridinoline, and cross-linked N-telopeptides of type I collagen). These biochemical changes returned toward baseline values as early as three weeks following the discontinuation of therapy with alendronate and did not differ from placebo after 7 months despite the long retention of alendronate in the skeleton.
Long-term treatment of osteoporosis with FOSAMAX 10 mg/day (for up to five years) reduced urinary excretion of markers of bone resorption, deoxypyridinoline and cross-linked N-telopeptides of type I collagen, by approximately 50% and 70%, respectively, to reach levels similar to those seen in healthy premenopausal women. The decrease in the rate of bone resorption indicated by these markers was evident as early as 1 month and at three to six months reached a plateau that was maintained for the entire duration of treatment with FOSAMAX. In osteoporosis treatment studies, FOSAMAX 10 mg/day decreased the markers of bone formation, osteocalcin and bone specific alkaline phosphatase by approximately 50%, and total serum alkaline phosphatase by approximately 25 to 30%, to reach a plateau after 6 to 12 months. Similar reductions in the rate of bone turnover were observed in postmenopausal women during a one-year study with FOSAMAX once weekly 70 mg for the treatment of osteoporosis. These data indicate that the rate of bone turnover reached a new steady-state, despite the progressive increase in the total amount of alendronate deposited within bone.
As a result of inhibition of bone resorption, asymptomatic reductions in serum calcium and phosphate concentrations were also observed following treatment with FOSAMAX. In the long-term studies, reductions from baseline in serum calcium (approximately 2%) and phosphate (approximately 4 to 6%) were evident the first month after the initiation of Fosamax 10 mg. No further decreases in serum calcium were observed for the five-year duration of treatment, however, serum phosphate returned toward prestudy levels during years 3 through 5. In one-year studies with Fosamax once weekly 70 mg, similar reductions were observed at 6 and 12 months. The reduction in serum phosphate may reflect not only the positive bone mineral balance due to Fosamax but also a decrease in renal phosphate reabsorption.
Osteoporosis in men: Even though osteoporosis is less prevalent in men than in postmenopausal women, a significant proportion of osteoporotic fractures occur in men. The prevalence of vertebral deformities appears to be similar in men and women. Treatment of men with osteoporosis with FOSAMAX 10 mg/day for two years reduced urinary excretion of cross-linked N-telopeptides of type I collagen by approximately 60% and bone-specific alkaline phosphatase by approximately 40%. Similar reductions were observed in a one-year study in men with osteoporosis receiving FOSAMAX once weekly 70 mg.
Pharmacokinetics: Absorption: Alendronate Sodium: Relative to an intravenous (IV) reference dose, the mean oral bioavailability of alendronate in women was 0.64% for doses ranging from 5 to 70 mg when administered after an overnight fast and two hours before a standardized breakfast. Oral bioavailability in men (0.6%) was similar to that in women.
The alendronate in the FOSAMAX PLUS (70 mg/2800 IU) and FOSAMAX PLUS (70 mg/5600 IU) tablets and the FOSAMAX 70 mg tablet is bioequivalent.
Bioavailability was decreased similarly (by approximately 40%) whether alendronate was administered one or one-half hour before a standardized breakfast. In osteoporosis studies, FOSAMAX was effective when administered at least 30 min before the first food or beverage of the day.
Bioavailability was negligible whether alendronate was administered with or up to two hours after a standardized breakfast. Concomitant administration of alendronate with coffee or orange juice reduced bioavailability by approximately 60%.
In healthy subjects, oral prednisone (20 mg three times daily for five days) did not produce a clinically meaningful change in the oral bioavailability of alendronate (a mean increase ranging from 20 to 44%).
Colecalciferol: Following administration of FOSAMAX PLUS (70 mg/2800 IU) after an overnight fast and two hours before a standard meal, the mean area under the serum-concentration-time curve (AUC0-120 hrs) for vitamin D3 (unadjusted for endogenous vitamin D3 levels) was 296.4 ng·hr/mL. The mean maximal serum concentration (Cmax) of vitamin D3 was 5.9 ng/mL, and the median time to maximal serum concentration (Tmax) was 12 hrs. Following administration of FOSAMAX PLUS (70 mg/5600 IU) after an overnight fast and two hours before a meal, the mean area under the serum-concentration-time curve (AUC0-80 hrs) for vitamin D3 (unadjusted for endogenous vitamin D3 levels) was 490.2 ng·hr/mL. The mean maximal serum concentration (Cmax) of vitamin D3 was 12.2 ng/mL and the median time to maximal serum concentration (Tmax) was 10.6 hours. The bioavailability of the vitamin D3 in FOSAMAX PLUS (70 mg/2800 IU) and FOSAMAX PLUS (70 mg/5600 IU) is similar to an equal dose of vitamin D3 administered alone.
Distribution: Alendronate Sodium: Studies in rats show that alendronate transiently distributes to soft tissues following 1 mg/kg IV administration but is then rapidly redistributed to bone or excreted in the urine. The mean steady state volume of distribution, exclusive of bone, is at least 28 L in humans. Concentrations of drug in plasma following therapeutic oral doses are too low for analytical detection (less than 5 ng/mL). Protein binding in human plasma is approximately 78%.
Colecalciferol: Following absorption, vitamin D3 enters the blood as part of chylomicrons. Vitamin D3 is rapidly distributed mostly to the liver where it undergoes metabolism to 25-hydroxyvitamin D3, the major storage form. Lesser amounts are distributed to adipose and muscle tissue and stored as vitamin D3 at these sites for later release into the circulation. Circulating vitamin D3 is bound to vitamin D-binding protein.
Metabolism: Alendronate Sodium: There is no evidence that alendronate is metabolized in animals or humans.
Colecalciferol: Vitamin D3 is rapidly metabolized by hydroxylation in the liver to 25-hydroxyvitamin D3, and subsequently metabolized in the kidney to 1,25-dihydroxyvitamin D3, which represents the biologically active form. Further hydroxylation occurs prior to elimination. A small percentage of vitamin D3 undergoes glucuronidation prior to elimination.
Elimination: Alendronate Sodium: Following a single IV dose of [14C]alendronate, approximately 50% of the radioactivity was excreted in the urine within 72 hours and little or no radioactivity was recovered in the feces. Following a single 10 mg IV dose, the renal clearance of alendronate was 71 mL/min, and systemic clearance did not exceed 200 mL/min. Plasma concentrations fell by more than 95% within 6 hours following IV administration. The terminal half-life in humans is estimated to exceed 10 years, reflecting release of alendronate from the skeleton.
Colecalciferol: When radioactive vitamin D3 was administered to healthy subjects, the mean urinary excretion of radioactivity after 48 hours was 2.4%, and the mean fecal excretion of radioactivity after 4 days was 4.9%. In both cases, the excreted radioactivity was almost exclusively as metabolites of the parent. The mean half-life of vitamin D3 in the serum following an oral dose of FOSAMAX PLUS (70 mg/2800 IU) is approximately 24 hours.
Characteristics in Patients: Preclinical studies show that the alendronate that is not deposited in bone is rapidly excreted in the urine. No evidence of saturation of bone uptake was found after chronic dosing with cumulative IV doses up to 35 mg/kg in animals. Although no clinical information is available, it is likely that, as in animals, elimination of alendronate via the kidney will be reduced in patients with impaired renal function. Therefore, somewhat greater accumulation of alendronate in bone might be expected in patients with impaired renal function (see DOSAGE & ADMINISTRATION).
