BETP degradation simultaneously targets acute myelogenous leukemia stem cells and the microenvironment.

The antileukemic effect of inhibiting bromodomain and extra-terminal domain-containing (BET-containing) proteins (BETPs) such as BRD4 has largely been largely attributed to transcriptional downregulation of cellular anabolic and antiapoptotic processes, but its effect on the bone marrow microenvironment, a sanctuary favoring the persistence of leukemic stem/progenitor cells, is unexplored. Sustained degradation of BETP with the small-molecule BET proteolysis-targeting chimera (PROTAC) ARV-825 resulted in a marked downregulation of surface CXCR4 and CD44, key proteins in leukemia-microenvironment interactions, in acute myeloid leukemia (AML) cells. Abrogation of surface CXCR4 expression impaired SDF-1 alpha-directed migration and was mediated through transcriptional downregulation of PIM1 kinase, which in turn phosphorylates CXCR4 and facilitates its surface localization. Downregulation of CD44, including isoforms CD44v8-10 impaired cystine uptake, lowered intracellular reduced glutathione, and increased oxidative stress. More important, BETP degradation markedly decreased the CD34(+)CD38(-)CD90(-)CD45RA(+)leukemic stem cell population and, alone or in combination with cytarabine, prolonged survival in a mouse model of human leukemia that included AML patient-derived xenografts (AML-PDX). Gene expression profiling and single-cell proteomics confirmed a downregulation of the gene signatures associated with "stemness" in AML and Wnt/beta-catenin and Myc pathways. Hence, BETP degradation by ARV-825 simultaneously targets cell-intrinsic signaling, stromal interactions, and metabolism in AML.