Purpose: MLN9708 (ixazomib citrate), which hydrolyzes to pharmacologically active MLN2238 (ixazomib), is a next-generation proteasome inhibitor with demonstrated preclinical and clinical antimyeloma activity, but yet with an unknown effect on myeloma bone disease. Here, we investigated its bone anabolic and antiresorptive effects in the myeloma setting and in comparison with bortezomib in preclinical models.Experimental Design: The in vitro effect of MLN2238 was tested on osteoclasts and osteoclast precursors from healthy donors and patients with myeloma, and on osteoprogenitors derived from bone marrow mesenchymal stem cells also from both origins. We used an in vivo model of bone marrow-disseminated human myeloma to evaluate MLN2238 antimyeloma and bone activities.Results: Clinically achievable concentrations of MLN2238 markedly inhibited in vitro osteoclastogenesis and osteoclast resorption; these effects involved blockade of RANKL (receptor activator of NF-kB ligand)-induced NF-kB activation, F-actin ring disruption, and diminished expression of aVb3 integrin. A similar range of MLN2238 concentrations promoted in vitro osteoblastogenesis and osteoblast activity (even in osteoprogenitors from patients with myeloma