A Novel Inhibitor of BET Family Bromodomains Demonstrates In Vivo and In Vitro Potency in B-Cell Malignancies

Chronic Lymphocytic Leukemia (CLL) is a B-cell malignancy with aberrant activation of the B-cell receptor (BCR) pathway. Despite durable remissions with targeted therapies (e.g., ibrutinib) in CLL, it remains an incurable disease. Epigenetic modifications, including DNA methylation and dysregulation of chromatin regulators have been shown to contribute to the neoplastic phenotype and the differential biologic behavior of tumor cells, including leukemia. An additional layer of epigenetic complexity in cancer cells is the acquisition of super-enhancer regions enriched at genes with known oncogenic function including MYC and BCL2. Super-enhancers in multiple myeloma cells and other tumors have been found strongly enriched for binding of BRD4, a member of the human bromodomain and extraterminal (BET) domain family of proteins which includes BRD2, BRD3, BRD4, and the testis-specific member BRDT. BRD4 binds to acetylated lysines on histones and regulates the expression of important oncogenes (e.g., MYC and BCL2). We investigated the therapeutic benefit of BET inhibition in cell culture and in vivo disease models of leukemia/lymphoma using PLX51107, a novel BRD4 inhibitor with unique binding mode.

Results: We report that BRD4 is significantly overexpressed in CLL patient-derived B-cells compared to B-cells from healthy donors on both transcript and protein level (p < .001). RNA-seq analysis of 55 CLL patients revealed expression of various BRD4 isoforms with marked abundance of BRD4-long and BRD4-short. Next we sought to investigate the anti-tumor activity of PLX51107 in multiple malignant B-cell lines and patient-derived CLL cells. PLX51107 inhibited cell growth in MEC1, OCI-Ly2 and OCI-Ly6 (p < .001) dose-dependently with IC₅₀ of 1.0 ± 0.09, 1.2 ± 0.05, 1.8 ± 0.05 μM, respectively. Notably, PLX51107 antagonized CpG-induced increase in cell proliferation of primary CLL cells (p < .01) which was consistent with the downmodulation of MYC and MCL1 along with the accumulation of the cyclin-dependent kinase inhibitor p21 and IκBα (p < .005). Furthermore, the efficacy of PLX51107 to disrupt survival signaling from the microenvironment was investigated under co-culture conditions with two different bone marrow stroma cell lines, wherein PLX51107 treatment significantly induced cytotoxicity in B-CLL cells (p < .01) without affecting stromal cell viability. By employing microarray analysis we identified possible novel targets of BRD4 in CLL. Validation of those targets is currently ongoing. Particularly, Bruton's tyrosine kinase (BTK) and phospholipase C gamma 2 (PLCG2) were markedly decreased with PLX51107 treatment (p < .005), thereby signifying potential therapeutic effect(s) for dual targeting of BRD4 and BCR-associated kinases to achieve deeper and durable responses in relapsed/refractory B-cell malignancies. Lastly, anti-tumor effects of BRD4 inhibition were evaluated in vivo using Eμ-TCL1 and cMYC/TCL1 adoptive transfer models of leukemia and lymphoma, respectively. In the Eμ-TCL1 engraftment model of aggressive CLL, PLX51107 treatment resulted in prolonged survival (p < .001) accompanied with decreased disease burden, lymphocyte infiltration and proliferation when compared to vehicle-treated mice. Next, the cMYC/TCL1 adoptive transfer mouse model was used to evaluate BRD4 inhibition in a highly penetrant, malignant leukemia/lymphoma phenotype analogous to high grade lymphoma wherein PLX51107 prolonged survival (p < .0001), decreased peripheral lymphocyte counts and neoplastic cell infiltration and proliferation in both spleen and lymph nodes.

Conclusion: Collectively our findings reveal BRD4 as a valid and novel target for epigenetic therapy directed against core transcriptional programs in malignant/proliferating B-cells and provide support for use of PLX51107 as an effective treatment in clinical trials for relapsed/refractory CLL patients and related aggressive forms of B-cell malignancies, with the ultimate goal of improving the outcome of these patients.