BRD4 degraders produce long-lasting loss of BRD4 Ppotein and robust efficacy in Burkitt’s lymphoma cells.

8557 Background: We have created specific BRD4 degraders using PROTAC (PROteolysis TArgeting Chimera) technology. This involves creating bi-functional molecules, with one end having a ligand for BRD4 and the other end a recruiting element for the E3 ligase cereblon. These PROTACs promote the interaction of BRD4 with cereblon, resulting in its degradation via the proteasome degradation machinery. Methods: Treatment of NAMALWA, Ramos, CA-46 and Daudi Burkitt’s lymphoma cells with BRD4 PROTACs leads to rapid loss of BRD4, with near complete loss observed within 4 hours. This effect is potent and long lasting, with the most effective molecules having DC50s in the pM range, and with 1 hour treatment of cells resulting in BRD4 loss over a 3 day period. We examined the functional effects of BRD4 PROTACs in several Burkitt’s lymphoma cell lines, comparing their effects to those observed with the clinical BRD4 inhibitor OTX015. Results: We found that treatment of cells with the BRD4 inhibitor OTX015 led to a rapid and robust hyper-accumulation of BRD4 that, together with the reversible nature of binding to BRD4, may have accounted for the observed moderate suppression of MYC expression, modest inhibition of cell proliferation and lack of cellular apoptosis. In contrast, BRD4 PROTACs maintained suppression of BRD4 protein throughout the several day experiment, maintained near complete MYC suppression and caused both pronounced suppression of proliferation and robust apoptotic responses, as measured by PARP cleavage. Conclusions: These data imply that degradation of BRD4 can provide a more robust therapeutic approach to MYC-driven hematological cancers.