[Bisphosphonates: pharmacology and use in the treatment of osteoporosis]

Bisphosphonates are attractive antiresorptive drugs for osteoporosis. The only compound available in Japan is the first-generation bisphosphonate, etidronate. Etidronate adsorbes to the surface of hydroxyapatite crystals, and can slow bone mineralization and inhibit bone resorption. It has a narrow therapeutic window between these two actions, so that long-term continuous administration is not feasible. The intermittent use of etidronate produces a positive effect on bone mass. But the response varies directly with the rate of bone turnover at baseline. In high turnover osteoporosis there could be a gain in bone mass, but it reaches a plateau after 2 to 3 years. In normal or low turnover osteoporosis bone mass is stabilized but does not increase. However, the long-term effect of bisphosphonates on bone strength is not known.