Alendronate (Fosamax®, Merck) is a bisphosphonate drug used for osteoporosis and several other bone diseases. It is marketed alone as well as in combination with vitamin D (2,800 U, under the name Fosavance).

Pharmacokinetics

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The systemic bioavailability after oral dosing is only 0.6 % as well in women and in men (fasting state). Intake together with meals and certain drinks (coffee, orange juice) further reduces the bioavailability. Soft tissues and bones are fastly reached by about 50%. After resorption in the bone alendronate has an estimated terminal halflife of 10 years; the remainder is excreted unchanged by the kidneys.

Pharmacology

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Alendronate blocks osteoclast-mediated bone-resorption. It is chemically related to etidronate and the N-containing bisphosphonates such as pamidronate, which with it shares the same mode of action. Its inhibition of bone-resorption is dose-dependent and 100 to 1,000 times stronger than the equimolar effect of etidronate. Theoretically, alendronate may also inhibit bone-mineralization but this effect is 6,000 times weaker than the inhibition of bone-resorption. Under therapy normal bone tissue develops and alendronate is deposited in the bone-matrix in pharmacologically inactive form. For optimal action enough calcium and vitamin D are needed in the body. Hypocalcemia should therefore be corrected before starting therapy.

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