Inhibition of osteoblast function in vitro by aminobisphosphonates.

Bisphosphonates are analogues of pyrophosphate, a key physicochemical inhibitor of mineralisation. We examined the direct actions of bisphosphonates on the function of cultured osteoblasts derived from rat calvariae. Treatment with zoledronate, the most potent bisphosphonate studied, reduced osteoblast number at concentrations > or = 100 nM and was strongly toxic at 10 microM, causing a threefold decrease in osteoblast viability after 2 days and a 90% decrease in cell numbers after 14 days. In control osteoblast cultures on plastic, abundant formation of 'trabecular' mineralised bone matrix nodules began after 10 days. Continuous exposure to zoledronate inhibited bone mineralisation at concentrations as low as 10 nM. Pamidronate and clodronate exerted similar effects but at higher doses > or = 1 and > or = 10 microM, respectively). Short-term or intermittent exposure of osteoblasts to zoledronate and pamidronate (1-10 microM) was sufficient to inhibit bone mineralisation by > or = 85%. Zoledronate but not pamidronate or clodronate also strongly inhibited osteoblast alkaline phosphatase activity at concentrations > or = 100 nM and soluble collagen production at concentrations > or = 1 microM. We additionally studied the effects of zoledronate on osteoblasts cultured on dentine, a bone-like mineralised substrate, observing similar inhibitory effects, although at concentrations 10-100-fold higher; this shift presumably reflected adsorption of zoledronate to dentine mineral. Thus, zoledronate blocked bone formation in two ways: first, a relatively non-toxic, selective inhibition of mineralisation at concentrations in the low nanomolar range and second, a cytotoxic inhibition of osteoblast growth and function at concentrations > or = 1 microM. Although no data are available on the bisphosphonate concentrations that osteoblasts could be exposed to in vivo, our results are consistent with earlier observations that bisphosphonates may inhibit bone formation.