[Bisphosphonates: the molecular targets and mechanisms of action]

Bisphosphonates directly act on osteoclasts to inhibit bone resorption and used most widely to treat osteoporosis. The compounds can be classified into two groups with different modes of action. Nitrogen containing bisphosphonates exert their effects by inhibiting a key enzyme in the mevalonate pathway. The specific target is the isoprenoid biosynthetic enzyme, farnesyl pyrophosphate synthase, which is indispensable for protein prenylation and activation of intracellular signalling proteins, including the small GTPases Rho, Rac, Cdc42 and Ras. The disruption of the function of these key enzymes may explain the loss of osteoclast activity and induction of apoptosis. Whereas the first generation of bisphosphonates such as etidronate and clodronate (nitrogen deficient bisphosphonates) can be incorporated into nonhydrolysable analogues of ATP that may inhibit ATP-dependent intracellular events. Bisphosphonates are highly effective to inhibit bone resorption and increase bone mineral density, although their precise mechanisms of molecular action are not completely understood.