We investigated the effects of bortezomib (PS-341) and immunomodulatory thalidomide analogs (immunomodulatory compounds; CC-4047, CC-6032, and CC-5013, or lenalidomide) on osteoblast and osteoclast differentiation in vitro using human mesenchymal stem cells (hMSC) to commit to osteoprogenitor cells and peripheral blood mononuclear cells (PBMCs) isolated from healthy donors, respectively. First, the concentration of bortezomib for an anti-myeloma effect was more than 1.0 nM in myeloma cells of multiple myeloma (MM) patients and more than 2.5 nM in myeloma cell lines. In contrast, anti-myeloma effects of immunomodulatory compounds on myeloma cells differed among myeloma cells and these compounds themselves. Subsequently, these agents (bortezomib; 0.5-5.0 nM, immunomodulatory compounds; 10 microM) were added to the osteoprogenitor cell culture media or the media for osteoclast differentiation. Low bortezomib concentrations (0.5 and 1.0 nM) increased ALP activity, and the delayed addition of bortezomib further increased ALP activity. Mineralized nodular formation with <2.5 nM bortezomib was not impaired. BMP2 expression on osteoprogenitor cells was found to increase in a time-dependent manner irrespective of treatment with bortezomib. On the other hand, the anti-osteoclast effect with low bortezomib concentration (< or =2.5 nM) depended on MM patients. In contrast, immunomodulatory compounds at 10 microM showed an anti-osteoclast effect without cytotoxicity to osteoblast differentiation, at which dose myeloma cells underwent apoptosis. These findings might improve the treatment strategy for MM patients without damaging BM stromal cells by combining bortezomib with immunomodulatory compounds.