1effects on bone resorption and bone formation
As described in chapter 1, bone is constantly undergoing remodeling at bone remodeling units in which osteoclasts are resorbing bone or osteoblasts are forming new bone. Patients lose bone when resorption exceeds formation, and these changes may be magnified by the overall rate of bone turnover, which is determined by the number of active remodeling units. Estrogen deficiency in early post menopausal women leads to increased osteoclast activity and increased bone resorption that is particularly noticeable in the vertebral trabecular bone. The more gradual decrease in osteoblastic activity that occurs with aging may result in weakness of cortical as well as trabecular bone.
The drugs to be discussed have been classified as to their ability to decrease bone resorption(antiresorptive) or increase bone formation. Most of the currently available drugs decrease bone resorption, but owing to the coupling of bone resorption and formation, those drugs may also decrease bone formation. The efficacy of antiresorptive therapy most likely depends on the more rapid effects on bone resorption compared to bone formation, allowing a period of decreased resorption prior to the onset of decreased formation, leading to small increases in bone mass.
Studies have suggested that baseline bone turnover might predict drug efficacy. For example, the vertebral bone density was found to increase in a subset of postmenopausal women with elevated bone turnover treated with parenteral salmon calcitonin. This finding leg some clinicians to select patients for calcitonin therapy using measurements of bone turnover markers. However, although these markers are of great value in studying the mechanism of drug action and in following large groups of patients, the benefit of this approach has not been proven in the individual patient given the variability and cost of bone turnover markers. The recent advance in bone turnover marker technology may lead to new proposals for algorithms for selecting patients for drug treatment and following their responses to therapy.