Despite the conventional and high-dose chemotherapy with hematopoietic stem cell transplantation, multiple myeloma eventually relapses, resulting in an incurable hematological malignancy. Therefore, novel therapeutic approaches in clinical settings are desired. Recently, thalidomide was introduced for the treatment of myeloma, and many clinical trials have since confirmed its efficacy in patients with relapsed/refractory or newly diagnosed multiple myeloma. Multiple mechanisms have been proposed to explain thalidomides antimyeloma activity. However, the precise mechanism underlying this activity remains unclear, because thalidomide rapidly undergoes spontaneous, nonenzymatic, hydrolytic cleavage to numerous metabolites in vivo. To elucidate the exact anti-myeloma mechanism of thalidomide in vivo, we have performed structural development studies of thalidomide, and obtained various analogs with specific molecular properties. Among these derivatives, we found that a new thalidomide analog, 2-(2,6-diisopropylphenyl)-5-hydroxy-1H-isoindole- 1,3-dione (5HPP-33), has the most potent anti-myeloma effect with tubulin polymerization inhibiting activity. 5HPP-33 directly inhibited the growth and survival of various myeloma cells in a dose-dependent manner with IC50 of 1-10 μM. In contrast, thalidomide itself did not inhibit RPMI8226 cell growth. A tubulin polymerization assay using microtubule protein from porcine brain revealed that 5HPP-33 had potent tubulin polymerization inhibiting activity with IC50 of 8.1 μM, comparable to that of rhizoxin, a known tubulin polymerization inhibitor. Moreover, its activity was more potent than that of a known thalidomide metabolite, 5-hydroxythalidomide. Our data suggest that 5HPP-33 is a promising candidate as a therapeutic agent for multiple myeloma. In addition, the results suggest that thalidomides tubulin polymerization inhibiting activity might be the mechanism underlying the induction of apoptosis in myeloma cells.