BET protein bromodomain inhibitor-based combinations are highly active against post-myeloproliferative neoplasm secondary AML cells

Myeloproliferative Neoplasms with myelofibrosis (MPN-MF) demonstrate constitutive activation of JAK-STAT signaling, which responds to treatment with the JAK1 & 2 kinase inhibitor (JAKi) ruxolitinib. However, MPN-MF often progresses (~20%) to secondary AML (sAML), where standard induction chemotherapy or ruxolitinib is relatively ineffective, necessitating the development of novel therapeutic approaches. In the present studies, we demonstrate that treatment with BET (bromodomain and extra terminal) protein inhibitor (BETi), e.g., JQ1, inhibits growth and induces apoptosis of cultured and primary, patient-derived (PD), post-MPN sAML blast progenitor cells. Reverse-phase protein array, mass-cytometry and Western analyses revealed that 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 and BIM in the sAML cells. Co-treatment with BETi and ruxolitinib synergistically induced apoptosis of cultured and PD sAML cells, as well as significantly improved survival of immune-depleted mice engrafted with human sAML cells. While BETi or heat shock protein (HSP) 90 inhibitor alone exerted lethal activity, co-treatment with BETi and HSP90i was synergistically lethal against the ruxolitinib-persister or ruxolitinib-resistant sAML cells. Collectively, these findings further support in vivo testing of BETi-based combinations with JAKi and HSP90i against post-MPN sAML cells.