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 (ABCs). ABC-DLBCL cells are sensitive to Bruton tyrosine kinase intervention. However, patients with relapsed or refractory ABCDLBCL had overall response rates from 33% to 37% for Bruton tyrosine kinase inhibitors, suggesting the evaluation of combination-based treatment for improved efficacy. We investigated the efficacy and mechanism of the bromodomain and extraterminal 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 2 bromodomains of BRD4. Adding AZD5153 to acalabrutinib demonstrated combination benefits in ABC-DLBCL cell line and patient-derived xenograft models. Differential expression analyses revealed 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 for perpetually active BCR signaling in ABC-DLBCL. Our analyses further indicated significant alterations in BCR, RELB/alternative NF-& kappa;B, and toll like receptor/interferon signaling. Validation of these results mapped a positive-feedback signaling loop regulated by PAX5. We demonstrated that AZD5153 decreased PAX5 expression, whereas acalabrutinib disruption of BCR signaling inhibited PAX5 activation. Furthermore, several interferon levels were decreased by AZD5153 and acalabrutinib in tumors. Adding interferon-beta1 (IFN & beta;1) to cells treated with acalabrutinib partially rescued PAX5 activation. Our results demonstrate that AZD5153 enhances the efficacy of acalabrutinib through PAX5 and BCR mechanisms that are critical for ABC-DLBCL.