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Osteolytic bone lesions are commonly observed in patients with active multiple myeloma. For example, osteolytic bone is characterized by sparse trabeculae and is often associated with brittle bones.
These bone lesions result from an imbalance which favors bone resorption and osteoclast activity over that of the osteoblasts.
This imbalance is maintained through the activation or repression of multiple signaling pathways that are mediated by a variety of physical interactions between multiple myeloma plasma cells and bone marrow stromal cells.
For example, RANK ligand-mediated activation of osteoclasts is enhanced due to increased expression and secretion of RANK ligand from bone marrow stromal cells.
Additionally, the secretion of osteoprotegerin by osteoblasts and bone marrow stromal cells is decreased, thereby decreasing suppression of RANK-ligand signaling.
Many other cytokines are also released to stimulate osteoclasts and/or inhibit osteoblasts.
The identification of patients who may benefit from bone-directed therapy is often based on assessing the levels of some of these cytokines.
The class of drugs known as bisphosphonates has been developed to target and inhibit osteoclast activity.
Once administered to the patient, bisphosphonates are resorbed by the bone and then taken up by the osteoclasts.
Following drug uptake, osteoclast activity is inhibited, usually resulting in apoptosis.

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