Bone-bound bisphosphonate inhibits growth of adjacent non-bone cells

The conventional view of the mode of action of bisphosphonates is that they are taken up by bone surfaces and then ingested by bone-resorbing osteoclasts, the activity of which they inhibit through their actions on the enzyme, farnesyl pyrophosphate (FPP) synthase. This model suggests that these compounds should only have effects on osteoclasts, and does not provide an explanation for their other actions, such as the epithelial abnormalities seen in osteonecrosis of the jaw, and their possible prolongation of disease-free survival in some malignancies. The present studies set out to determine whether cells other than osteoclasts are affected by bone-bound bisphosphonates. Bone slices were incubated overnight in PBS or in solutions of bisphosphonates (100 μM), washed, then transferred to 96-well plates (1 slice/well). Cells from 2 cell lines were seeded onto the bone slices: Caco-2 human colorectal adenocarcinoma epithelial cells and Chinese hamster ovary (CHO) cells. Cell proliferation (cell numbers and thymidine incorporation) was assessed at 4-72 h. Cell adhesion at 4 h was normal on bone slices pre-treated with bisphosphonates, but there were progressive reductions in cell numbers from 48 h and even greater reductions in thymidine incorporation from 24 h (>90% with zoledronate at 72 h). Growth inhibition was related to the clinical potency of the bisphosphonate used. There was no evidence of increased apoptosis in cells grown on bisphosphonate-coated bone, but levels of unprenylated Rap1A were increased, indicating inhibition of FPP synthase. Similar growth inhibition was observed in primary cultures of rat osteoblasts on bone, indicating that this was not specific to transformed cells. It is concluded that bisphosphonates bound to a bone surface can act on adjacent non-bone cells and inhibit their growth. This greatly widens the range of potential target cells for these drugs.