The Shape-Shifting Myeloma: Adaptive Plasticity as a Hallmark of Relapse and Refractoriness.

Relapsed and refractory multiple myeloma remains the principal cause of myeloma-related mortality despite major advances in therapeutic options, including proteasome inhibitors, immunomodulatory drugs, monoclonal antibodies, and T-cell-based immunotherapies. Conventional resistance models based on linear genetic clonal evolution inadequately explain reversible drug resistance, heterogeneous responses, and relapse after deep remissions. Emerging data from longitudinal genomics, single-cell analyses, and functional studies support a paradigm in which relapsed and refractory multiple myeloma is driven by adaptive plasticity rather than irreversible genetic change alone. Myeloma cells undergo reversible cell state transitions through transcriptional, epigenetic, metabolic, and proteostatic reprogramming, shaped by bone marrow microenvironmental cues and immune pressure. This narrative review integrates current evidence supporting adaptive plasticity as a central driver of therapeutic failure in relapsed and refractory multiple myeloma and discusses clinical and translational implications, highlighting adaptive treatment strategies and approaches targeting phenotypic flexibility to improve durability of response.