Bone remodeling, normal and abnormal: a biological basis for the understanding of cancer-related bone disease and its treatment

Abstract

Remodeling the cyclical replacement of old bone by new, serves to maintain its mechanical and metabolic functions. In each cycle a circumscribed volume of bone is removed by osteoclastic resorption and subsequently replaced by osteoblastic formation at the same location. Remodeling is carried out by elongated structures known as basic multicellular units (BMU) that travel through or across the surface of bone. Each BMU lasts about six months, with continued sequential recruitment of new osteoclasts and osteoblasts. Abnormal bone remodeling involves some combination of loss of directional control, increase in number of remodeling cycles and incomplete replacement. In metastatic bone disease, tumor cells find the hematopoietic bone marrow conducive to their survival and growth, because they can manipulate the local cytokine network to increase recruitment of osteoclasts from local precursors and so increase bone resorption. The effect on bone formation is biphasic; an initial increase is due partly to the normal evolution of the BMU, and partly to the induction of reparative woven bone formation. Later, normal BMU-based bone formation may fall to subnormal levels. In some tumors, a generalized increase in osteoclast recruitment and decline in bone formation are the systemic responses to one or more agents released by tumor cells into the circulation, of which the most frequent is parathyroid hormone-related peptide, but in both metastatic and non-metastatic disease, the cellular events in bone are essentially the same. Cancer-related bone disease is amenable to treatment with drugs that inhibit osteoclast recruitment, of which the bisphosphonates are the most effective. Treatment should be started before there has been irreparable damage to bone structure and before the onset of hypercalcemia. Although bisphosphonates remain in bone for a long time, adverse effects are very unlikely within the patient's lifetime.