Preclinical efficacy of CDK7 inhibitor-based combinations against myeloproliferative neoplasms transformed to AML.

Rising blast percentage or secondary acute myeloid leukemia (sAML) transformation in myeloproliferative neoplasms (MPNs) leads to JAK1/2 inhibitor (JAKi) therapy resistance and poor survival. Here, we demonstrate that treatment with the CDK7 inhibitor (CDK7i) SY-5609 depletes phenotypically characterized post-MPN sAML stem/progenitor cells. In cultured post-MPN sAML SET2, HEL and patient-derived (PD) post-MPN sAML cells, SY-5609 treatment inhibited growth and induced lethality while sparing normal cells. RNA-sequencing analysis after SY-5609 treatment reduced mRNA expression of MYC, MYB, CDK4/6, PIM1, and CCND1 but increased expression of CDKN1A and BCL2L1. Mass spectrometry of SY-5609-treated MPN-sAML cells also reduced c-Myc, c-Myb, PIM1, and CDK4/6 but increased p21, caspase-9, and BAD protein levels. CRISPR-mediated CDK7 depletion also reduced cell viability of HEL cells. Cytometry by time of flight (CyTOF) analysis of SY-5609-treated PD post-MPN sAML stem/progenitor cells showed reduced c-Myc, CDK6, and PU.1 but increased protein levels of CD11b, p21, and cleaved caspase-3. Cotreatment with SY-5609 and ruxolitinib was synergistically lethal in HEL, SET2, and PD post-MPN sAML cells. A CRISPR screen in sAML cells revealed BRD4, CBP, and p300 as codependencies with CDK7i. Accordingly, cotreatment with SY-5609 and the bromodomain and extra-terminal protein inhibitor (BETi) OTX015 or pelabresib or the CBP/p300 inhibitor GNE-049 was synergistically lethal in MPN-sAML cells (including those exhibiting TP53 loss). Finally, in the HEL-Luc/GFP xenograft model, compared with each agent alone, cotreatment with SY-5609 and OTX015 reduced sAML burden and improved survival without host toxicity. These findings demonstrate promising preclinical activity of CDK7i-based combinations with BETi or CBP/p300 inhibitor against advanced MPNs, including post-MPN sAML.