Increased serum CRBN levels are associated with improved survival in MM patients

therapeutic interventions impacting outcome.Methods: To answer this question and delineate the mechanisms that contribute to the development of an acquired drug-tolerant/resistant phenotype, we exposed venetoclax-sensitive myeloma cells to high dose treatment and generated monoclonal drug-tolerant expanded persister (DTEP) clones with a three-to ten-fold increase in IC50 compared to the parental cells.These cells were subjected to an integrated analysis including WGS, RNA-seq and proteomics, as well as drug screening.Results: Parental and resistant cells showed no marked differences in mutational frequencies, including the absence of genomic alterations in the BCL-2 gene such as the Gly101Val BCL-2 mutation.Instead, these clones exhibited reduced mitochondrial priming and had increased protein expression of anti-apoptotic regulators, including MCL-1, BCL-XL, and BCL-W, resulting in the sequestration of the pro-apoptotic ligand BIM.Since these data suggested a functional substitution between anti-apoptotic BCL-2 family members, we next evaluated if MCL1 or BCL-XL represent a co-dependency in resistant cells that can be therapeutically targeted to overcome resistance.Simultaneous inhibition of MCL1 (via S63845) or BCL-XL (via A155463) and BCL2 (via venetoclax) increased BIM release and enhanced cell death in the resistant clones compared to single agents, with combination index (CI) values < 0.3 in all doses tested.We next evaluated if the altered mitochondrial priming observed in the resistant cells was also responsible for a broader resistance to anticancer agents.Indeed, we found that most targeted agents, regardless of the mechanism of action, demonstrated a reduced ability to induce cell death in venetoclax-resistant cells compared with parental cells, suggesting a broad resistance to anti-cancer agents due to general selection for reduced apoptotic signaling.Conclusions: In conclusion, we report that resistance to Venetoclax in in vitro models of MM evolves from the outgrowth of persister clones with a shift in mitochondrial dependency that confers broad resistance to all antitumor agents, including standard-of-care myeloma drugs.