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Multiple myeloma is a B-cell malignancy characterized by an excess of monotypic plasma cells in the bone marrow. The molecular mechanisms that are involved in disease progression depend on the interaction between the multiple myeloma cells and the bone microenvironment. Because these mechanisms have been well characterized, it is possible to develop regimens that are more specific to pathways involved in the pathogenesis of multiple myeloma than is typical for conventional chemotherapy in disease management. Thalidomide and immunomodulatory drugs (IMiDs) have now been shown to block several pathways important for disease progression in multiple myeloma. First established as agents with antiangiogenic properties, thalidomide and IMiDs inhibit the production of interleukin (IL)-6, which is a growth factor for the proliferation of myeloma cells. In addition, they activate apoptotic pathways through caspase 8-mediated cell death. At the mitochondrial level, they are responsible for c-jun terminal kinase (JNK)-dependent release of cytochrome-c and Smac into the cytosol of cells, where they regulate the activity of molecules that affect apoptosis. By activating T cells to produce IL-2, thalidomide and IMiDs alter natural killer (NK) cell numbers and function, thus augmenting the activity of NK-dependent cytotoxicity. Data delineating these events have been derived from experiments done in resistant and sensitive multiple myeloma cell lines. Although thalidomide and IMiDs demonstrate similar biologic activities, IMiDs are more potent than thalidomide and achieve responses at lower doses. Lenalidomide, a thalidomide derivative, has also been shown to have a different toxicity profile. Our understanding of the mechanism of action of these agents has provided a platform for exciting clinical trials evaluating combinations of thalidomide and lenalidomide with both conventional chemotherapy and newer targeted agents.
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