The pharmacology of bisphosphonates and new insights into their mechanisms of action.

Bisphosphonates are chemically stable analogs of inorganic pyrophosphate, which are resistant to breakdown by enzymatic hydrolysis. The biological effects of bisphosphonates on calcium metabolism were originally ascribed to their physico-chemical effects on hydroxyapatite crystals. Although such effects may contribute to their overall action, their effects on cells are probably of greater importance, particularly for the more potent compounds. Remarkable progress has been made in increasing the potency of bisphosphonates as inhibitors of bone resorption, and the most potent compounds in current use are characterized by the presence of a nitrogen atom at critical positions in the side chain which, together with the bisphosphonate moiety itself, seems to be essential for maximal activity. As a class the bisphosphonates offer a very effective means of treating Paget's disease.

Bisphosphonates: preclinical aspects and use in osteoporosis.

The bisphosphonates are synthetic compounds characterized by a P-C-P bond. They have a strong affinity to calcium phosphates and hence to bone mineral. In vitro they inhibit both formation and dissolution of the latter. Many of the bisphosphonates inhibit bone resorption, the newest compounds being 10,000 times more active than etidronate, the first bisphosphonate described. The antiresorbing effect is cell mediated, partly by a direct action on the osteoclasts, partly through the osteoblasts, which produce an inhibitor of osteoclastic recruitment. When given in large amounts, some bisphosphonates can also inhibit normal and ectopic mineralization through a physical-chemical inhibition of crystal growth. In the growing rat the inhibition of resorption is accompanied by an increase in intestinal absorption and an increased balance of calcium. Bisphosphonates also prevent various types of experimental osteoporosis, such as after immobilization, ovariectomy, orchidectomy, administration of corticosteroids, or low calcium diet. The P-C-P bond of the bisphosphonates is completely resistant to enzymatic hydrolysis. The bisphosphonates studied up to now, such as etidronate, clodronate, pamidronate, and alendronate, are absorbed, stored, and excreted unaltered. The intestinal absorption of the bisphosphonates is low, between 1% or less and 10% of the amount ingested. The newer bisphosphonates are at the lower end of the scale. The absorption diminishes when the compounds are given with food, especially in the presence of calcium. Bisphosphonates are rapidly cleared from plasma, 20%-80% being deposited in bone and the remainder excreted in the urine. In bone, they deposit at sites of mineralization as well as under the osteoclasts. In contrast to plasma, the half-life in bone is very long, partially as long as the half-life of the bone in which they are deposited. In humans, bisphosphonates are used successfully in diseases with increased bone turnover, such as Paget's disease, tumoural bone disease, as well as in osteoporosis. Various bisphosphonates, such as alendronate, clodronate, etidronate, ibandronate, pamidronate, and tiludronate, have been investigated in osteoporosis. All inhibit bone loss in postmenopausal women and increase bone mass. Furthermore, bisphosphonates are also effective in preventing bone loss both in corticosteroid-treated and in immobilized patients. The effect on the rate of fractures has recently been proven for alendronate. In humans, the adverse effects depend upon the compound and the amount given. For etidronate, practically the only adverse effect is an inhibition of mineralization. The aminoderivatives induce for a period of 2-3 days a syndrome with pyrexia, which shows a similitude with an acute phase reaction. The more potent compounds can induce gastrointestinal disturbances, sometimes oesophagitis, when given orally. Bisphosphonates are an important addition to the therapeutic possibilities in the prevention and treatment of osteoporosis.

c-Fos: a key regulator of osteoclast-macrophage lineage determination and bone remodeling.

Mice lacking the proto-oncogene c-fos develop the bone disease osteopetrosis. Fos mutant mice were found to have a block in the differentiation of bone-resorbing osteoclasts that was intrinsic to hematopoietic cells. Bone marrow transplantation rescued the osteopetrosis, and ectopic c-fos expression overcame this differentiation block. The lack of Fos also caused a lineage shift between osteoclasts and macrophages that resulted in increased numbers of bone marrow macrophages. These results identify Fos as a key regulator of osteoclast-macrophage lineage determination in vivo and provide insights into the molecular mechanisms underlying metabolic bone diseases.

Rapid intracellular degradation of newly synthesized collagen by bone cells. Effect of dichloromethylenebisphosphonate.

Experiments were carried out to determine whether bone cells isolated from rat calvaria degrade newly synthesized collagen intracellularly prior to secretion and to assess the effect of dichloromethylenebisphosphonate, a compound shown to stimulate collagen synthesis during this event. The findings indicate that isolated bone cells grown in culture degraded a proportion (average 16%) of newly synthesized collagen prior to secretion. This process was markedly reduced by exposure to dichloromethylenebisphosphonate in a dose-related manner. Concomitantly with the observed decrease of degradation, an increase of collagen synthesis was detected as determined by the incorporation of [3H]proline into collagenase-digestible proteins or by the conversion of [3H]proline into [3H]hydroxyproline. No similar enhancement on total non-collagenous protein synthesis was evident. Dichloromethylenebisphosphonate did not influence the extracellular degradation of collagen. Although the reduction in intracellular degradation accounted only for part of the bisphosphonate mediated increase in net collagen synthesis, it is conceivable that the rate of collagen synthesis is regulated, at least in part, by mechanisms that modulate the level of intracellular degradation.