Remodeling of the bone marrow vasculature induced by venetoclax and azacitidine damage

The Bcl2 inhibitor venetoclax in combination with the hypomethylating agent azacitidine (ven/aza) has become increasingly used clinically for the treatment of many hematological malignancies. Although its effects on malignant cells have been extensively studied, its impact on the surrounding bone marrow (BM) microenvironment (BME) remains unexplored. In this study, we report that ven/aza therapy causes significant damage to the BME of mice. Comparatively high Bcl2 expression in the sinusoidal endothelial cell (EC; SEC) compartment among all stromal subtypes, results in high sensitivity to ven/aza treatment, causing selective depletion of SECs and breakdown in cell-cell communication pathways in the EC network, leading to vascular leakiness in the BM. Furthermore, our detailed transcriptomic and imaging studies reveals significant downregulation of essential adhesion molecules in residual SECs, leading to significant defects in human hematopoietic stem/progenitor cell (HSPC) homing and engraftment of hematopoietic stem cells (HSCs) after ven/aza treatment. To conclude, our study showcases that maintaining SEC integrity in response to ven/aza therapy may play a key factor in achieving effective engraftment of donor-derived HSCs.