Myeloproliferative neoplasm, myelofibrosis (MPN-MF), exhibits increased JAK-STAT signaling and often progresses (∼15-20%) to AML (sAML). JAK inhibitor (JAK-I) ruxolitinib (Rux) or standard induction chemotherapy is only modestly active against sAML, culminating in treatment-refractory relapse. Genetic alterations commonly documented in sAML include the co-occurrence of JAK2 V617F and mutant TP53. Here, we demonstrate that treatment with ARV-825 (Arvinas, Inc.), a BET (bromodomain and extraterminal) protein PROTAC (proteasome activating chimera) which degrades (through the proteasome) BET proteins by binding and recruiting E3 Ubiquitin ligase cereblon activity to them, caused efficient, and prolonged depletion of the levels of the BET protein BRD4. In contrast, treatment with the bromodomain inhibitors (BET-Is) JQ1 or OTX015 induced the levels of BRD4. ARV-825 treatment also mediated greater and more sustained attenuation than OTX-015 of the mRNA and protein expressions of BCL-xL, CDK4/6, PIM1, p-STAT5 and p-STAT3 levels, while concomitantly inducing p21 and p27 in the cultured sAML (HEL92.1.7 and SET2) cells. This correlated with high level of apoptosis in the cultured and patient-derived post-MPN-MF sAML cells, with relative sparing of the normal CD34+ progenitor cells. Compared to treatment with each agent alone, co-treatment with ARV-825 and the JAK-Is Rux (100 to 1000 nM) or pacritinib (250 to 1000 nM), was synergistically more lethal against the cultured sAML cells (CI of < 1.0 on the isobologram analyses). Co-treatment with ARV-825 and Rux caused a marked inhibition of p-STAT5, p-STAT3, MYC, CDK4/6, BCL-xL and PIM1 in the sAML cells. HEL92.1.7 and SET2 cells harbor JAK2 V617F as well as express mutant TP53 (M133K in HEL92.1.7 and R248W in SET2 cells). Co-treatment with ARV-825 or JQ1 and the heat shock protein (HSP) 90 inhibitor AUY922, which is known to down-regulate the levels of mutant-TP53, JAK2, c-RAF, p-STAT5, p-STAT3 and p-AKT, was synergistically lethal against HEL92.1.7 and SET2 cells. We have isolated JAK-I-resistant HEL92.1.7 cells (> 10-fold resistant to ruxolitinib; HEL/JIR cells) under the in vitro selection pressure of a continuous exposure to JAK-I. Notably, compared to the parental HEL92.1.7, HEL/JIR cells were highly and collaterally sensitive to both ARV-825 and AUY922. Furthermore, co-treatment with ARV-825 and AUY922 was also synergistically lethal against HEL/JIR cells (CI < 1.0). Taken together, these findings demonstrate that ARV-825 is a highly active agent alone or in combination with JAK-Is against post-MF sAML cells expressing mutant p53. Combined therapy with ARV-825 and an HSP90 inhibitor is synergistically lethal against sAML cells resistant to JAK-Is. Based on these finding, in vivo testing of the BRD4 PROTAC-based combinations against post-MF sAML cells is warranted.

Citation Format: Dyana T. Saenz, Warren C. Fiskus, Taghi Manshouri, Jing Lu, Yimin Qian, Kanak Raina, Baohua Sun, Stephanie S. Krieger, Simrit Parmar, Srdan Verstovsek, Kapil N. Bhalla. Superior lethal activity of single agent BET protein PROTAC compared to bromodomain inhibitor or in combination with JAK inhibitor against post-myelofibrosis secondary AML cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4699.