Abstract 5067: BET protein proteolysis targeting chimera (BETP-PROTACs) exert more potent activity than BETP bromodomain inhibitor (BETi) against post-myeloproliferative neoplasm (MPN) secondary (s) AML cells

Abstract

Abstract In BCR-ABL1-negative myeloproliferative neoplasms with myelofibrosis (MPN-MF) transformation to AML (sAML) occurs in up to 20% of patients. Ruxolitinib (R) is a type I, ATP-competitive, JAK1 & 2 inhibitor (JAKi), which is effective in the therapy of MPN-MF but does not significantly impact the clinical outcome in post-MPN sAML. We have previously reported that treatment with BETi, e.g. JQ1 or OTX015 inhibits growth and induces apoptosis of cultured sAML cells, including those that express JAK2 V617F and mutant TP53, e.g. HEL92.1.7 and SET2, as well as patient-derived (PD) CD34+ sAML cells. BETi treatment attenuated the protein expressions of c-MYC, p-STAT5, Bcl-xL, CDK4/6, PIM1 and IL-7R, while concomitantly inducing the levels of HEXIM1, p21, NOXA and BIM in the sAML cells. However, treatment with BETi leads to the accumulation of BETP, e.g. BRD4, which may reduce BETi-mediated repression of c-MYC, NFκB and BETP-regulated oncoproteins. In contrast, BETP-PROTACs (proteolysis targeting chimera) ARV-825 and ARV-771 (Arvinas Inc.) degrade BETPs (including BRD4) in the cultured and PD CD34+ sAML cells. At equimolar concentrations, BETP-PROTACs were significantly more potent than the BETi in inducing apoptosis of cultured and PD sAML cells (p < 0.05), while sparing the CD34+ normal hematopoietic progenitor cells. Notably, BETP-PROTACs caused efficient and prolonged depletion of the levels of BETPs, including BRD4 (> 90%) in the sAML cells. BETP-PROTAC treatment caused more up and down regulation of mRNA and protein expressions than BETi, as determined by RNA-Seq and reversed phase protein array (RPPA) analyses, respectively. As compared to treatment with BETi, BETP-PROTAC caused greater depletion of c-MYC, JAK2, p-STAT5, STAT5, p-STAT3, STAT3, PIM1 and Bcl-xL, whereas the protein levels of p21 and p27 were upregulated. CyTOF or mass-cytometry also showed that BETP-PROTAC, more than OTX015 treatment, reduced BRD4, c-MYC and p-Rb, while inducing p21 levels in the CD34+ sAML stem/progenitor cells expressing CD90, CD244, CD123 and TIM3-Fc. Compared to treatment with each agent alone, co-treatment with BETP-PROTAC and R was synergistically lethal against the cultured and PD CD34+ sAML cells. Additionally, co-treatment with BETP-PROTAC and HSP90 inhibitor AUY922 or BCL2/BcL-xL antagonist ABT263 was synergistically lethal against R-sensitive and R-resistant sAML cells. As compared to treatment with vehicle control, R treatment alone, treatment with BETP-PROTAC ARV-771 alone or in combination with R significantly reduced the in vivo sAML burden and improved the median survival of the immune-depleted mice engrafted with luciferase-transduced HEL92.1.7 cells. These findings strongly support further in vivo development of the novel BETP-PROTACs-based combinations against post-MPN sAML. Note: This abstract was not presented at the meeting. Citation Format: Dyana T. Saenz, Warren C. Fiskus, Kanak Raina, Taghi Manshouri, Kevin G. Coleman, Yimin Qian, Andrew P. Crew, Angela Shen, Christopher P. Mill, Baohua Sun, Misun Kim, Agnieszka J. Nowak, Srdan Verstovsek, Craig M. Crews, Kapil N. Bhalla. BET protein proteolysis targeting chimera (BETP-PROTACs) exert more potent activity than BETP bromodomain inhibitor (BETi) against post-myeloproliferative neoplasm (MPN) secondary (s) AML cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5067. doi:10.1158/1538-7445.AM2017-5067