12.1 Mechanism of Action Alendronate Sodium Animal studies have indicated the following mode of action. At the cellular level, alendronate shows preferential localization to sites of bone resorption, specifically under osteoclasts. The osteoclasts adhere normally to the bone surface but lack the ruffled border that is indicative of active resorption. Alendronate does not interfere with osteoclast recruitment or attachment, but it does inhibit osteoclast activity. Studies in mice on the localization of radioactive [ 3 H]alendronate in bone showed about 10-fold higher uptake on osteoclast surfaces than on osteoblast surfaces. Bones examined 6 and 49 days after [ 3 H]alendronate administration in rats and mice, respectively, showed that normal bone was formed on top of the alendronate, which was incorporated inside the matrix. While incorporated in bone matrix, alendronate is not pharmacologically active. Thus, alendronate must be continuously administered to suppress osteoclasts on newly formed resorption surfaces. Histomorphometry in baboons and rats showed that alendronate treatment reduces bone turnover (i.e., the number of sites at which bone is remodeled). In addition, bone formation exceeds bone resorption at these remodeling sites, leading to progressive gains in bone mass. Cholecalciferol Vitamin D 3 is produced in the skin by photochemical conversion of 7-dehydrocholesterol to previtamin D 3 by ultraviolet light. This is followed by non-enzymatic isomerization to vitamin D 3 . In the absence of adequate sunlight exposure, vitamin D 3 is an essential dietary nutrient. Vitamin D 3 in skin and dietary vitamin D 3 (absorbed into chylomicrons) is converted to 25-hydroxyvitamin D 3 in the liver. Conversion to the active calcium-mobilizing hormone 1,25-dihydroxyvitamin D 3 (calcitriol) in the kidney is stimulated by both parathyroid hormone and hypophosphatemia. The principal action of 1,25-dihydroxyvitamin D 3 is to increase intestinal absorption of both calcium an

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Osteoporosis

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marketed

(alendronate sodium + cholecalciferol)

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