Pre-clinical activity of BET protein antagonist-based combinations against ibrutinib-sensitive or -resistant human Mantle Cell Lymphoma cells

Abstract

In Mantle Cell Lymphoma (MCL) cells, activated B cell receptor (BCR) and NFkB signaling exerts pro-growth and pro-survival activity. Treatment with the Bruton’s Tyrosine Kinase (BTK) inhibitor ibrutinib induces clinical responses, but long-term durable remissions and cure remains elusive. BET (bromodomain and extra-terminal) proteins, including BRD4, bind to acetylated lysine(s) on histones, assemble transcriptional co-regulators at the enhancers and promoters and increase the transcription of MCL-relevant oncogenes, e.g., MYC, BCL-2, CDK4/6, cyclin D1. Additionally, BRD4 is essential for NFkB activity. Here, we determined that BET protein antagonists JQ1 or I-BET151, which disrupt the binding of BRD4 to acetylated histones, dose-dependently inhibit the growth and induce apoptosis of cultured (MO2058 and Mino, Z138 and Jeko1) and primary MCL (pMCL) cells. Treatment with JQ1 attenuated the mRNA and protein expression of MYC, BCL2, MCL-1 and CDK4/6, while inducing the levels of HEXIM1, p21 and p27 in MCL cells. Importantly, JQ1 treatment also attenuated the nuclear RelA and inhibited the expression of several NFkB target genes, including XIAP, IkBα, cFLIP, cIAP2, as well as BTK. Compared to ibrutinib alone, co-treatment with JQ1 and ibrutinib markedly inhibited pBTK, BTK, pPLCγ2, pAKT and nuclear RelA levels and synergistically induced apoptosis of the cultured and pMCL cells. Following tail-vein infusion and engraftment of Mino cells in the bone marrow and spleen, co-treatment with JQ1 and ibrutinib, versus each agent alone, significantly improved the survival of NOD/SCID mice. By exposing parental Mino cells to escalating levels of ibrutinib, we isolated Ibrutinib-resistant Mino/IR cells (>20 fold resistant). Compared to Mino, Mino/IR cells demonstrated higher levels of BTK, AKT, XIAP, BCL-2 and CDK6, but reduced levels of NOXA, PUMA and p21. Co-treatment with JQ1 and the pan-histone deacetylase inhibitor panobinostat (PS) or the BCL-2 antagonist venetoclax (ABT199), or palbocyclib (CDK4/6 inhibitor) synergistically induced apoptosis of Mino and cultured pMCL, as well as of Mino/IR cells. These findings highlight promising BET protein (BA) antagonist-based combinations with ibrutinib, panobinostat, venetoclax or palbociclib against MCL cells. They also identify potential BA-based combinations against ibrutinib-resistant MCL cells.