Bone tissue is subject to remodeling throughout the lifetime of an individual. Through a continuous remodeling cycle, actuated via the so-called 'bone remodeling units', old bone is resorbed by osteoclasts with the formation of cavities that are subsequently filled by osteoblasts. Bone loss observed in old age and in women after menopause is due to an imbalance between bone resorption and formation. Biochemical markers provide a dynamic view of the remodeling process, which covers rate of turnover and pathogenesis, and should improve fracture risk prediction. Furthermore, they can be used to monitor the short-term effects of therapy, and indicate if an excessive slowing of the remodeling process is occurring. When searching for markers of bone remodeling, biochemists have focused mainly on skeletal molecules that can be dosed in plasma and/or urine, as indicators of osteoblast function (i.e. bone alkaline phosphatase, osteocalcin, procollagene I C- and N-terminal propeptides) or osteoclast function (i.e. pyridinium crosslinks, collagen I C- and N-terminal telopeptides). The clinical significance of any marker for bone remodeling depends on two fundamental characteristics: specificity and variability. If the objective is to monitor therapeutic efficacy, it seems most rational to use a resorption marker for drugs that act principally on osteoclast, such as estrogens or bisphosphonates, while for drugs that act principally on osteoblast, such as PTH-peptides a marker for bone formation would be more appropriate.
