Abstract 1745: Application of BET/CBP/p300 multi-bromodomain inhibitors as a novel therapeutic strategy for MLL-rearranged acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common form of childhood leukemia. While the prognosis of patients with ALL has improved significantly, ALL with chromosomal rearrangements involving mixed-lineage leukemia (MLL) gene remains an incurable disease. Thus, development of a safe and efficacious therapeutic agent based on the biology of MLL rearrangement is of high importance to address this unmet medical need. Bromodomains are protein motifs that bind to acetylated histones and are recognized as new epigenetic drug targets in recent years. Bromodomain and extra-terminal motif proteins (“BET proteins”) are epigenetic ‘reader' proteins that are characterized by containing two of such bromodomains. BRD4, a member of BET protein family, is a key component of multi-protein complex that induce oncogene transcription at Super-Enhancers. Currently, small-molecule BET inhibitors are being investigated in the clinic as potential therapeutics for hematological cancers. CBP and p300, two paralogous histone acetyltransferases, also contain bromodomains and are considered as drug targets for cancer. Because BET and CBP/p300 proteins are key components of the etiology of MLL-rearranged (MLL-r) leukemia, an agent that simultaneously target both classes of proteins may have therapeutic advantages. We previously demonstrated that a novel BET inhibitor, CG13250, suppressed multiple myeloma cell proliferation in vitro and in vivo (Biochem Biophys Res Commun, 484:262-268, 2017) and subsequently synthesized additional BET inhibitors (Bioorg Med Chem Lett, 29:1220-1226, 2019). In the present study, we demonstrate that some of our BET inhibitors can also inhibit bromodomains of CBP/p300 proteins. One such compound, CN470, potently suppresses BRD4, CBP, and p300 with BROMOscan KD values of 33, 32, and 20 nM, respectively, and suppressed mRNA and protein expression of c-MYC as expected. CN470 potently inhibited the growth of MLL-r ALL cell lines in a dose-dependent manner and flow cytometric analyses showed that CN470 caused apoptosis in MLL-r ALL cells following a cell cycle arrest at the G1 phase. Moreover, co-immunoprecipitation assay demonstrated that CN470 reduced the binding of BRD4 to acetylated H3 (H3K27), an effect absent with a reference BET inhibitor, OTX-015. Taken together, CN470 may represent an effective strategy against MLL-r ALL cells. As a confirmatory study, we investigated the in vivo effects of CN470 by using SEMLuc/GFP cells in an orthotopic mouse mod