The geminal
bisphosphonates are characterized by a PCP bond and are therefore analogs of
pyrophosphate. They bind strongly to
hydroxyapatite crystals and in vitro inhibit both crystal formation and dissolution. In vivo they inhibit soft tissue calcification and when given in large amounts also normal calcification. This effect is due to the inhibition of
calcium phosphate crystal growth. Furthermore, the
bisphosphonates are very potent inhibitors of
bone resorption. The mechanism(s) of action is not yet known but is likely to be at a cellular level. The extent of the
biological activity of each compound depends on the specific chemical structure, so that each individual
bisphosphonate must be considered as a separate compound. The only common characteristic is the PCP group, which gives the compound its high affinity to bone. The individual effects, however, are determined by the side groups on the
carbon atom. This opens interesting possibilities for the development of new compounds. No
bisphosphonate analyzed so far can be degraded in vivo; all are either deposited in the skeleton, where they remain for years until the bone is destroyed, or are excreted in the urine. The high affinity for bone explains the specificity of the compounds for bone and the fact that they have relatively few nonosseous effects.
Bisphosphonates are used in man to inhibit ectopic calcification, including dental
tartar and
ectopic ossification. Furthermore, they are used to inhibit
bone resorption, especially in diseases such as Paget's disease and tumoral
osteolysis. Finally, when linked to 99nTc,
bisphosphonates are employed as bone scanning agents.