Multiple myeloma (MM) is an incurable malignancy characterized by the uncontrolled expansion of plasma cells in the bone marrow. While proteasome inhibitors like bortezomib efficiently halt MM progression, drug resistance inevitably develop, and novel therapeutic approaches are needed. Here, we used a recently discovered Sec61 inhibitor, mycolactone, to assess the interest of disrupting MM proteostasis via protein translocation blockade. In human MM cell lines, mycolactone caused rapid defects in secretion of immunoglobulins and expression of pro‐survival interleukin (IL)‐6 receptor and CD40, whose activation stimulates IL‐6 production. Mycolactone also triggered pro‐apoptotic endoplasmic reticulum stress responses synergizing with bortezomib for induction of MM cell death and overriding acquired resistance to the proteasome inhibitor. Notably, the mycolactone–bortezomib combination rapidly killed patient‐derived MM cells ex vivo, but not normal mononuclear cells. In immunodeficient mice engrafted with MM cells, it demonstrated superior therapeutic efficacy over single drug treatments, without inducing toxic side effects. Collectively, these findings establish Sec61 blockers as novel anti‐MM agents and reveal the interest of targeting both the translocon and the proteasome in proteostasis‐addicted tumors.

Multiple Myeloma (MM) is an incurable neoplasm of mature B cells and the second most prevalent hematological malignancy worldwide. While combinations of proteasome inhibitors like bortezomib (Bz) and immunomodulators (IMiDs) like lenalinomide (Len) are generally effective in newly diagnosed patients, some do not respond to this first-line therapy, and all others will eventually become drug resistant. We previously reported that inhibiting the Sec61 translocon with mycolactone synergizes with Bz to induce terminal unfolded protein response in MM cells, irrespective of their resistance to proteasome inhibition. Here, we examined how Sec61 blockade interferes with IMiD action and whether it overrides resistance to Len. With this aim, we knocked out the IMiD target CRBN in the MM1S cell line and a Bz-resistant subclone to generate Len- and Len/Bz-resistant daughters, respectively. Both the Len- and Len/Bz-resistant clones were susceptible to mycolactone toxicity, especially the doubly resistant one. Notably, the synergy between mycolactone and Bz was maintained in these two clones, and mycolactone also synergized with Len in the two Len-susceptible ones. Further, mycolactone enhanced the therapeutic efficacy of the Bz/Len combination in both mice engrafted with parental or double drug resistant MM1S. Together, these data consolidate the interest of Sec61 blockers as new anti-MM agents and reveal their potential for treatment of refractory or relapsed MM.

Purpose Multiple Myeloma (MM) is an incurable neoplasm of mature B cells and the second most prevalent hematological malignancy worldwide. While combinations of proteasome inhibitors like bortezomib (Bz) and immunomodulators (IMIDs) like lenalinomide (Len) are generally effective in stopping MM development, treated patients will eventually become resistant to one or both of these drugs. Using MM cell lines and patient-derived tumors, we recently reported that blocking the Sec61 translocon with mycolactone triggers an atypical unfolded protein response synergizing with Bz to kill MM cells, and overriding resistance to Bz. To extend this work, here we examined how Sec61 blockade interferes with Len and whether it overcomes IMID resistance. Methods Using the MM1S model cell line and a previously established daughter with stable resistance to Bz, we genetically engineered two additional daughters with single and double resistance to Len and Bz. The four cell lines were then compared side by side for sensitivity to mycolactone, alone and combined to Bz and/or Len, in vitro and in vivo. Results The synergistic effect of mycolactone on Bz was maintained in both single and double drug resistant MM cells and notably, extended to Len. Double drug resistance even conferred MM cells with an increased sensitivity to Sec61 blockade. Notably, mycolactone enhanced the therapeutic efficacy of the Bz + Len combination in both mice engrafted with parental or double drug resistant MM1S. Conclusion These data support the interest of further evaluating Sec61 blockers in MM drug combinations and their potential for treatment of refractory or relapsed MM.

Multiple Myeloma (MM) is an incurable malignancy characterized by the uncontrolled expansion of plasma cells in the bone marrow. While proteasome inhibitors like bortezomib efficiently halt MM progression, drug resistance or toxicity inevitably develop. Here, we used a recently discovered inhibitor, mycolactone, to assess the interest of targeting the Sec61 translocon in MM. In human cell lines and tumors from MM patients, mycolactone triggered pro-apoptotic endoplasmic reticulum stress responses synergizing with bortezomib for induction of MM cell death, irrespective of their resistance to proteasome inhibition. Notably, this synergy was selective of cancer cells and extended to B cell acute lymphoblastic leukemia. Sec61 blockade also caused collateral defects in MM secretion of immunoglobulins and expression of pro-survival interleukin (IL)-6 receptor and CD40, whose activation stimulates IL-6 production. Further, the mycolactone-bortezomib combination demonstrated superior therapeutic efficacy over single drug treatments in immunodeficient mice engrafted with MM cells, without inducing toxic side effects. Collectively, these findings establish Sec61 blockers as novel anti-MM agents and reveal the interest of targeting both the translocon and the proteasome in proteostasis-addicted tumors.