Abstract 4713: BET bromodomain degradation as a therapeutic strategy in drug-resistant multiple myeloma

Multiple myeloma (MM) is an incurable malignancy with an unmet need for novel therapeutic modalities. Moreover, acquired or de novo resistance to established or novel therapeutics remains a major challenge in this, and other, neoplasias. BET Bromodomain inhibitors (BBIs), including JQ1, have potent anti-MM activity in vitro and in in vivo, but do not provide curative outcome and do not induce apoptosis in most cell types. We sought to investigate dBET, a class of BBIs that induce degradation of BET Bromodomains (BRDs) through CRBN-mediated ubiquitination and proteasomal degradation, in drug-resistant MM. Additionally, we posited that resistance to dBET treatment would emerge through genetic perturbations and wished to uncover potential mechanisms prior to its clinical utilization. To address this, we compared effects of optimized lead compound, dBET6, with JQ1 on a panel of MM cell lines, including clones resistant to JQ1 or bortezomib and assessed viability using CS-BLI/CTG assay and BRD/c-MYC expression by western blot. Using an open-ended unbiased genome-wide CRISPR (clustered regularly interspaced short palindromic repeats)-associated Cas9 approach, we examined whether we could uncover genes associated with resistance to dBET6. MM1.S cells were transduced with Cas9 and pooled lentiviral particles of the GeCKO library, consisting of 2 pooled sgRNA sub-libraries (∼120,000 sgRNAs; targeting ∼19,000 genes and ∼1800 miRNAs). Using this CRISPR/Cas9-based approach we sought to expedite the isolation of MM cells resistant to dBET6. We treated the pool of cells thrice with dBET (≥IC80), allowing regrowth between treatments and maintaining a coverage of 1000 cells/sgRNA. dBET6-resistant cells were processed to quantify sgRNA enrichment or depletion, using deep sequencing. We observed dBET6 to have significantly greater potency against MM cells than JQ1, or its combination with lenalidomide, and that MM1S.CRBN-/- cells were resistant to dBET6. Resistance to neither JQ1 nor Bortezomib conferred resistance to dBET6. We observed dBET6 to induce rapid ( Citation Format: Geoffrey M. Matthews, Yiguo Hu, Michal Sheffer, Paul J. Hengeveld, Dennis L. Buckley, Megan A. Bariteau, Haley Poarch, James E. Bradner, Constantine S. Mitsiades. BET bromodomain degradation as a therapeutic strategy in drug-resistant multiple myeloma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4713.