ABSTRACT Multiple myeloma (MM) is a plasma cell malignancy. Treatment usually involves a proteasome inhibitor, dexamethasone, and an immunomodulatory or chemotherapeutic treatment, with or without autologous stem cell transplantation. Because of intrinsic or acquired medication resistance, all myeloma patients eventually relapse. The expression of the prosurvival members of the Bcl-2 family, particularly Mcl-1, is a key process in the survival of myeloma cells. Because Mcl-1 is a critical mediator of disease progression and an important mechanism in the acquisition of resistance to therapy it is reasonable to ask if MM patients, particularly those at relapse, would benefit from an Mcl-1-targeted therapy? A few selective Mcl-1 inhibitors are currently being developed or are being tested in clinical trials, but unfortunately, most of the previous studies were terminated because of undesirable side effects, especially cardiac toxicity. To address this need, we developed a novel Mcl-1 inhibitor, KS18, to investigate the mechanistic underpinning of Mcl-1 role in MM, and to determine feasibility of Mcl-1 inhibition as a therapeutic strategy. Therefore, the rationale for this research is to determine whether our novel Mcl-1 inhibitor can be used along with the existing chemotherapeutic agents to effectively kill resistant myeloma cells, thus, inhibit or prevent MM relapse. Our preliminary studies demonstrated that unlike chemotherapy, KS18 kills resistant cells, suggesting that it could be combined with traditional chemotherapy to treat MM while minimizing the possibility of resistance development more effectively. To evaluate the efficacy and the likely mechanism of action of KS18 in combination with existing MM treatments, we propose the following Specific Aims: First, is inhibiting Mcl-1 via KS18 sufficient to reverse the acquired chemotherapeutic resistance in MM in vitro? We will test KS18 alone and in combination with therapeutic agents used in treatment of MM, including with bortezomib, venetoclax or as a third agent in combination with bortezomib and dexamethasone, which is part of clinical standard of care for MM patients. Second, can Mcl-1 inhibition reverse bortezomib resistance in MM in vivo? We will determine how Mcl-1 inhibition modulates the acquired resistance to chemotherapeutic agents, and third, what is the efficacy of Mcl-1 inhibitor and bortezomib combination in refractory and relapsed patient samples? We will investigate the efficacy of the KS18 and bortezomib combination in patient samples. These significant discoveries would demonstrate the efficacy of targeting anti-apoptotic protein Mcl-1 in MM as well as other cancers for modulating relapse, providing needed preclinical validation necessary for clinical translation. This contribution will be significant because it is expected to have broad translational importance in the treatment of MM. Furthermore, this proposal will enhance the infrastructure of research and education at Co...