BET inhibition targets ABC-DLBCL constitutive B-cell receptor signaling through PAX5.

B-cell receptor (BCR) signaling is essential for the diffuse large B-cell lymphoma (DLBCL) subtype that originates from activated B-cells (ABC). ABC-DLBCL cells are sensitive to Bruton tyrosine kinase intervention. However, relapsed or refractory ABC-DLBCL patients had overall response rates of 33-37% for Bruton tyrosine kinase inhibitors, suggesting the evaluation of combination-based treatment for improved efficacy. We investigated the efficacy and mechanism of bromodomain and extra-terminal motif (BET) inhibitor AZD5153 combined with the Bruton tyrosine kinase inhibitor acalabrutinib in ABC-DLBCL preclinical models. AZD5153 is a bivalent BET inhibitor that simultaneously engages the two bromodomains of BRD4. Adding AZD5153 to acalabrutinib demonstrated combination benefits in ABC-DLBCL cell line and patient-derived xenograft models. Differential expression analyses identified PAX5 transcriptional activity as a novel downstream effector of this drug combination. PAX5 is a transcription factor for BCR signaling genes and may be critical to the perpetually active BCR signaling in ABC-DLBCL. Our analyses further indicated significant alterations to BCR, RELB/alternative NFκB, and toll-like receptor/interferon signaling. Validation of these results mapped a positive feedback signaling loop regulated by PAX5. We demonstrate that AZD5153 decreases PAX5 expression, while acalabrutinib disruption to BCR signaling inhibits PAX5 activation. Furthermore, several interferons were decreased by AZD5153 and acalabrutinib in tumors. Adding IFNß1 to cells treated with acalabrutinib partially rescued PAX5 activation. Our results demonstrate AZD5153 enhances the efficacy of acalabrutinib through PAX5 and BCR mechanisms that are critical for ABC-DLBCL.