Background: Bortezomib (BTZ), a proteasome inhibitor (PI), mainly targets the beta 5 subunit of the 20S proteasome, and is widely used in the treatment of multiple myeloma (MM). However, inhibitory effects on other subunits of the proteasome are not well understood. Therefore, we examined the anti-MM activity of novel syringolin analogs that inhibit the activity of both beta 5 and 2 subunits. After examination, we investigated the activity of compound 19a, developed as a syrbactin-class PI to improve cytotoxic activity and membrane permeability (Chiba T et al. Angew Chem Int Ed. 2014).

Materials and method: First, the cytotoxic and inhibitory effects of compound 19a on 20S proteasomes of MM cells, including BTZ-resistant MM cells and primary samples derived from MM patients, were examined. The primary MM specimens were collected after obtaining written informed consent at Nagoya City University Hospital, and the mechanism of antitumor activity of compound 19a on MM cells was evaluated by focusing on the unfolded protein response and endoplasmic reticulum (ER) stress. Finally, to evaluate the toxicity of compound 19a, BALB/c mice were intraperitoneally injected with either 19a or BTZ and body weight change was analyzed. These in vivo experiments were performed in accordance with the United Kingdom Coordinating Committee on Cancer Research Guidelines for the Welfare of Animals in Experimental Neoplasia, Second Edition, and were approved by the Ethics Committee of the Center for Experimental Animal Science, Nagoya City University Graduate School of Medical Sciences.

Results: The cytotoxic activity of compound 19a, observed in various MM cell lines at nanomolar concentrations, was found to be similar to the activity observed when BTZ-resistant MM and T-cell lymphoma cell lines were tested. More precisely, the two BTZ-resistant cell lines KMS-11/BTZ and OPM-2/BTZ showed 44.4-fold and 52.1-fold higher resistance, respectively, to BTZ than that shown by their parental cell lines KMS-11 and OPM-2. However, the two BTZ-resistant cells showed only 3.2-fold (IC50: 18.0 nM) and 4.3-fold (IC50: 5.1 nM) higher resistance to compound 19a than that shown by their parental cell lines KMS-11 (IC50: 5.7 nM) and OPM-2 (IC50: 1.2 nM), suggesting that compound 19a exhibits less cross-resistance to BTZ. Evaluation of 20S proteasome activity showed time-dependent inhibition of both beta 5 and 2 subunits in MM cells on treatment with compound 19a. Treatment with 10 nM 19a induced remarkable apoptosis of the MM cells, accompanied by elevated CHOP and NOXA expression, indicating excessive ER stress. A similar activity was also observed in primary MM samples derived from the patients. Furthermore, to clarify the effect of beta 2 inhibition on anti-MM activity, two MM cell lines, U266 and AMO1, and one T-cell lymphoma cell line, Hut78, were transfected with either siRNA-targeting PSMB7 encoding beta 2 subunit or control siRNA and were subjected to the analysis of cell growth and viability. Specific knocking down of PSMB7 was observed, which resulted in the progression of apoptosis of the two MM cell lines and Hut78 when compared with the control. In addition, knocking down of both PSMB7 and PSMB5 encoding beta 5 subunits triggered more potent apoptosis of U266 and Hut78 cells when compared with the knocking down of either PSMB7 or PSMB5 alone. This result suggests that dual-inhibitory activities of beta 5 and 2 subunits have an additive or a synergistic effect on cytotoxicity when compared with single inhibitory activity. Finally, to examine the toxicity of compound 19a, BALB/c mice were administrated with it in a dose-escalation manner and subjected to the analysis of alteration of body weight. No significant difference in loss of body weight was observed between 19a-treated and BTZ-treated mice when administered with the same dose. Study of in vivo cytotoxic activity of 19a on xenografted MM cells is currently underway.

Conclusion: We demonstrated the cytotoxic activity of a syringolin analog on various MM cells at nanomolar levels, which was attributed to the dual inhibition of beta 5 and 2 subunits of the 20S proteasome. Compound 19a, as a dual inhibitor of beta 2 and 5, was observed to be a more potent PI than BTZ, and could overcome the acquired BTZ resistance. The findings provide new insight into the treatment of relapse and/or refractory MM.


Ishida:Celgene KK: Research Funding; Kyowa Hakko Kirin, Co., Ltd.: Honoraria, Research Funding; Bayer Pharma AG: Research Funding. Iida:Janssen Pharmaceuticals: Honoraria, Research Funding; Celgene: Honoraria, Research Funding.

Author notes


Asterisk with author names denotes non-ASH members.