To ascertain the exact anti-myeloma mechanism of thalidomide in vivo, we performed structural development studies of thalidomide, and obtained various analogues with specific molecular properties. Among these derivatives, we found that a new thalidomide analogue, 2-(2,6-diisopropylphenyl)-5-hydroxy-1H-isoindole-1,3-dione (5HPP-33) had the most potent anti-myeloma effect and tubulin-polymerization-inhibiting activity. 5HPP-33 directly inhibited the growth and survival of various myeloma cell lines (RPMI8226, U266, and IM9) in a dose-dependent manner with IC50 of 1-10 microM. In contrast, thalidomide itself did not inhibit cellular growth of RPMI8226 cells. Cultivation with 10 microM 5HPP-33 induced G2/M phase cell cycle arrest, followed by apoptosis of myeloma cells. Treatment with 5HPP-33 induced caspase-3 activity and PARP cleavage. A tubulin polymerization assay using microtubule protein from porcine brain revealed that 5HPP-33 showed potent tubulin-polymerization-inhibiting activity with IC50 of 8.1 microM, comparable to that of the known tubulin-polymerization inhibitor, rhizoxin. Moreover, its activity was more potent than that of a known thalidomide metabolite, 5-hydroxythalidomide. Notably, the structural requirement for its activity was critical, as other analogues and derivatives of 5HPP-33 showed only slight tubulin-polymerization-inhibiting activity. Our data suggest that 5HPP-33 is a promising candidates for a therapeutic agent of multiple myeloma. In addition, these results suggest that the tubulin-polymerization inhibiting activity of thalidomide might be a possible mechanism for inducing the apoptosis of myeloma cells by thalidomide.