Drug Resistance in Multiple Myeloma: Tumor-Intrinsic Mechanisms and the Bone Marrow Microenvironment.

Multiple myeloma (MM) is a malignant plasma cell disorder, and despite substantial improvements in prognosis achieved through chemotherapy, immunotherapy, and autologous stem cell transplantation, most patients ultimately develop relapsed or refractory disease. Drug resistance (DR) is increasingly recognized as a dynamically evolving ecosystem shaped by tumor-intrinsic plasticity and continuous remodeling of the bone marrow microenvironment (BMME), rather than as a single molecular lesion. This review summarizes the major mechanisms of resistance across key drug classes, including alterations in drug targets and signaling nodes, rewiring of apoptotic, proteostatic, and metabolic circuits, and BMME-dependent protection. We highlight how these processes converge on a limited set of survival hubs and collectively raise the apoptotic threshold under therapeutic pressure. The key to overcoming DR is to conceptualize it as an evolving ecosystem, thereby enabling rational, mechanism-based combination and sequencing strategies that may prolong progression-free survival and move MM closer to a functional cure.