Abstract 359: Dual BET/kinase inhibitors as a novel strategy for the treatment of multiple myeloma
Authors: Tauro Marilena , Ayaz Muhammad , Lawrence Harshani R. , Lawrence Nicholas J. , Shain Kenneth H. , Schonbrunn Ernst , Lynch Conor C. 
Published: August 2019

The bromodomain and extra-terminal (BET) family of proteins are important epigenetic regulators of critical oncogenes, including c-Myc. JQ1 is the most potent BRD4 inhibitor, currently used in several clinical trials and effective in preventing multiple myeloma progression in vivo.

Currently, clinical data is showing that patients often develop acquired resistance, indicating that single agent therapies targeting BRD4 such as JQ1 may not provide durable therapeutic responses. To overcome this limitation, we have developed a new class of dual inhibitors, capable to simultaneously target BRD4 and a panel of tyrosine kinases highly expressed in cancer including JAK2, FLT3, RET, FGFR1, ULK1 and ULK3. A preliminary in vitro screening identified SG3-014 as the lead compound, showing BRD4 and c-Myc inhibition activity similar to JQ1, but additionally, it is capable of inhibiting kinases that are over-activated in cancer and responsible for cytokine-mediated drug resistance pathways.

In vitro assays on MM cell lines reveals higher SG3-014 sensitivity (5TGM1, 0.85μM; U266, 0.99μM) compared to JQ1 (5TGM1, 4.7μM; U266, 16μM). In vivo studies using 1x106 5TGM1-Luc cells demonstrate that SG3-014 treatment (25mg/Kg) significantly contributes to overall survival compared to JQ1 and the vehicle control cohort (n=10/group) (median survival CTRL=40.5; SG3-014=50.5; JQ1=46 days). Post-study analyses demonstrated a significant reduction in myeloma induced bone disease in the SG3-014 treated mice as measured by X-ray/ μCT/ histomorphometry.

Clinical datasets identified ULK3 as the candidate kinase of SG3-014 targeting activity and correlated its expression with MM disease stages. Since ULK3 is involved in cellular senescence and autophagy regulation, a key mechanism by which several cancer cell lines protect themselves against apoptosis and become resistant to standard treatments, we explored the role of SG3-014 inhibition in the autophagy pathway.

Preliminary data confirm that SG3-014 is an autophagy inhibitor and, as opposed to JQ1, downregulates ATG proteins level, leading to dysfunctional autophagosome formation. We noted that within 6 h, SG3-014 completely shut down autophagy with decrease levels of ULK1, beclin-1, ATG16L-1, ATG12, ATG3, p62 and increased LC3I/II ratios (additionally confirmed by microscopy and flow cytometry). These effects were not noted with JQ1. Further investigation is clarifying the mechanism by which SG3-014 inhibitory activity on ULK3 is responsible for autophagy mediated cancer cell death.

In order to validate this target in the progression of the human disease, our future directions will analyze ex vivo multiple myeloma patient samples and examine the efficacy of the inhibitor in CD138 isolated multiple myeloma cells derived from patient biopsies.

We propose dual BRD4/autophagy inhibition as a resilient strategy to prevent multiple myeloma by-pass of single targeted therapies.

Citation Format: Marilena Tauro, Muhammad Ayaz, Harshani R. Lawrence, Nicholas J. Lawrence, Kenneth H. Shain, Ernst Schonbrunn, Conor C. Lynch. Dual BET/kinase inhibitors as a novel strategy for the treatment of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 359.