Bisphosphonates promote apoptosis in murine osteoclasts in vitro and in vivo

TLDR

Bisphosphonates suppress bone resorption and are used for high‑turnover bone disorders, but their exact mechanisms are unclear, with hypotheses ranging from blocking osteoclast formation to directly killing mature osteoclasts. The study aimed to develop a novel in vitro model of osteoclast survival to investigate how bisphosphonates affect osteoclast viability. Using this model, the authors demonstrated that bisphosphonates trigger programmed cell death in osteoclasts, providing both in vitro and in vivo evidence of apoptosis induction. Treatment with risedronate, pamidronate, and clodronate increased apoptotic osteoclasts 4‑ to 24‑fold in vitro and similarly in vivo, with risedronate being the most potent, suggesting apoptosis as a key mechanism of bisphosphonate action and a potential therapeutic target.

Abstract

Bisphosphonates inhibit bone resorption and are therapeutically effective in diseases of increased bone turnover, such as Paget's disease and hypercalcemia of malignancy. The mechanisms by which they act remain unclear. Proposed mechanisms include inhibition of osteoclast formation from precursors and inhibitory or toxic effect on mature osteoclasts. We have developed a new in vitro model to study osteoclast survival and in this paper present in vitro and in vivo evidence that may explain both the observed reduction in osteoclast numbers and in bone resorption by mature osteoclasts, namely that bisphosphonates induce programmed cell death (apoptosis). Three bisphosphonates (risedronate, pamidronate, and clodronate) caused a 4- to 24-fold increase in the proportion of osteoclasts showing the characteristic morphology of apoptosis in vitro. This observation was confirmed in vivo in normal mice, in mice with increased bone resorption, and in nude mice with osteolytic cancer metastases, with similar-fold increases to those observed in vitro. Of the three compounds, risedronate, the most potent inhibitor of bone resorption in vivo, was the strongest inducer of osteoclast apoptosis in vitro. Osteoclast apoptosis may therefore be a major mechanism whereby bisphosphonates reduce osteoclast numbers and activity, and induction of apoptosis could be a therapeutic goal for new antiosteoclast drugs.