Proteostasis as a Therapeutic Backbone: Proteasome Inhibitors and Emerging Regimens.

Multiple myeloma cells are uniquely dependent on proteostasis due to their exceptionally high immunoglobulin synthesis and protein turnover, rendering them hypersensitive to disruptions in protein degradation. Proteasome inhibitors targeting the 20 S catalytic core exploit this vulnerability and remain a therapeutic cornerstone in newly diagnosed and relapsed/refractory settings, anchoring both triplet and quadruplet regimens that incorporate anti-CD38 monoclonal antibodies, immunomodulatory agents, antibody-drug conjugates, and nuclear export inhibitors. Beyond the 20 S core, emerging targets, including the 19 S regulatory particle, deubiquitinating enzymes, and the E1-E2-E3 ubiquitination cascade, offer opportunities to intensify proteotoxic stress and overcome resistance. Targeted protein degradation platforms, including CELMoDs, proteolysis targeting chimeras, molecular glues, and proteasome cap-recruiting chimeras, further expand the therapeutic repertoire. Compensatory stress responses involving autophagy, the unfolded protein response, and the bone marrow microenvironment modulate therapeutic vulnerability. Collectively, these advances position proteostasis not as a single druggable target but as an integrated therapeutic network amenable to rational, biomarker-informed combination strategies in multiple myeloma.