Mechanisms of bone metastasis.

Solid cancers metastasize to bone by a multistep process that involves interactions between tumor cells and normal host cells. Some tumors, most notably breast and prostate carcinomas, grow avidly in bone because the bone microenvironment provides a favorable soil. In the case of breast carcinoma, the final step in bone metastasis (namely bone destruction) is mediated by osteoclasts that are stimulated by local production of the tumor peptide parathyroid hormone-related peptide (PTH-rP), whereas prostate carcinomas stimulate osteoblasts to make new bone. Production of PTH-rP by breast carcinoma cells in bone is enhanced by growth factors produced as a consequence of normal bone remodeling, particularly activated transforming growth factor-beta (TGF-beta). Thus, a vicious cycle exists in bone between production by the tumor cells of mediators such as PTH-rP and subsequent production by bone of growth factors such as TGF-beta, which enhance PTH-rP production. The metastatic process can be interrupted either by neutralization of PTH-rP or by rendering the tumor cells unresponsive to TGF-beta, both of which can be accomplished experimentally. The osteoclast is another available site for therapeutic intervention in the bone metastatic process. Osteoclasts can be inhibited by drugs such as the new-generation bisphosphonates; as a consequence of this inhibition, there is a marked reduction in the skeletal events associated with metastatic cancer to bone, such as pain, fracture, and hypercalcemia. However and possibly even more importantly, there is also a reduction of tumor burden in bone. In experimental situations, this has clearly been shown to affect not only morbidity but also survival. The precise mechanism by which bisphosphonates inhibit osteoclasts is still unclear and may represent a combination of inhibition of osteoclast formation as well as increased apoptosis in mature osteoclasts. However, studies with potent bisphosphonates such as ibandronate, pamidronate, and risedronate have clearly documented that reduction of bone turnover and osteoclast activity leads to beneficial effects not only on skeletal complications associated with metastatic cancer, but also on tumor burden in bone.