Novel BET protein proteolysis-targeting chimera exerts superior lethal activity than bromodomain inhibitor (BETi) against post-myeloproliferative neoplasm secondary (s) AML cells.

The PROTAC (proteolysis-targeting chimera) ARV-825 recruits bromodomain and extraterminal (BET) proteins to the E3 ubiquitin ligase cereblon, leading to degradation of BET proteins, including BRD4. Although the BET-protein inhibitor (BETi) OTX015 caused accumulation of BRD4, treatment with equimolar concentrations of ARV-825 caused sustained and profound depletion (>90%) of BRD4 and induced significantly more apoptosis in cultured and patient-derived (PD) CD34+ post-MPN sAML cells, while relatively sparing the CD34+ normal hematopoietic progenitor cells. RNA-Seq, Reverse Phase Protein Array and mass cytometry 'CyTOF' analyses demonstrated that ARV-825 caused greater perturbations in messenger RNA (mRNA) and protein expressions than OTX015 in sAML cells. Specifically, compared with OTX015, ARV-825 treatment caused more robust and sustained depletion of c-Myc, CDK4/6, JAK2, p-STAT3/5, PIM1 and Bcl-xL, while increasing the levels of p21 and p27. Compared with OTX015, PROTAC ARV-771 treatment caused greater reduction in leukemia burden and further improved survival of NSG mice engrafted with luciferase-expressing HEL92.1.7 cells. Co-treatment with ARV-825 and JAK inhibitor ruxolitinib was synergistically lethal against established and PD CD34+ sAML cells. Notably, ARV-825 induced high levels of apoptosis in the in vitro generated ruxolitinib-persister or ruxolitinib-resistant sAML cells. These findings strongly support the in vivo testing of the BRD4-PROTAC based combinations against post-MPN sAML.