The Role of Proteasome Inhibitors in Multiple Myeloma Bone Disease and Bone Metastasis: Effects on Osteoblasts and Osteocytes

The alterations of bone remodeling are typical of multiple myeloma (MM) patients where the uncoupled and unbalanced bone remodeling caused the onset of osteolytic lesions. Moreover, bone metastasis occurs in the majority of patients with breast and prostate cancer. Skeletal-related events negatively impact on quality of life by increasing the vulnerability to fractures. Several bone-targeting treatments have been developed to control bone pain and pathological fractures, including bisphosphonates and Denosumab. Nevertheless, these agents act by inhibiting osteoclast activity but do not improve bone formation. Proteasome inhibitors (PIs) have shown bone anabolic effects and encouraging results in stimulating osteoblast differentiation and bone healing. Among these, the first-in-class bortezomib and the second-generation PIs, carfilzomib, and ixazomib regulate the bone remodeling process by controlling the degradation of several bone proteins. PIs have been recently proven to also be efficacious in blocking MM-induced osteocyte death providing new possible therapeutic use in the management of bone loss. PIs have significant side effects that limit their use as bone anabolic strategy. Multiple alternative approaches have been made. The conjugation of PIs with bisphosphonates, which can target them to bone, showed good results in terms of bone anabolic activity. However, the clinical implications of these effects require further investigations.