A Novel BRD4 Inhibitor CA2 Suppresses MM Cell Proliferation in an Orthotopic Myeloma Mouse Model

We previously demonstrated that a novel BRD4 inhibitor, CA2, suppressed MM cell proliferation in vitro in the 57th ASH Annual Meeting. In the present study, we investigated the inhibitory mechanisms of CA2 on myeloma cell proliferation in vitro and in vivo. CA2 has a unique quinolinone core and inhibited the proliferation of MM cell lines, MM.1s, AMO-1, NCI-H929, OPM-2, and IM-9. As expected, CA2 suppressed the expression of c-MYC mRNA and protein in a dose- and time-dependent manner. In addition, CA2 decreased BRD4 binding to c-MYC promoters and c-MYC enhancers in a ChIP assay using IM-9 cells and an anti-BRD4 antibody (Figure 1). Flow cytometric Annexin-V/propodium iodide staining analyses demonstrated that CA2 induced apoptosis in MM cells. Taken together, CA2 may induce apoptosis in MM cells by inhibiting the binding of BRD4 to promoters and enhancers of c-MYC gene thereby suppressing c-MYC expression. We next performed a pharmacokinetic study of CA2 in mice. Half-lives (t1/2) of CA2 following oral administration (50 mg/kg) and subcutaneous administration (10 mg/kg) were approximately 61 min and 139.8 min, respectively. The values of area under the curve for oral and subcutaneous administrations were 179,000 hr·ng/mL and 31,000 hr·ng/mL, respectively. Lastly, we assessed the in vivo effects of CA2 using orthotopic MM mouse model, where IM-9Luc cells were inoculated intravenously through the tail veins of SCID mice that had received 2 Gy of irradiation. The mice in the vehicle-control arm died of MM by approximately 30 days after transplantation; IM-9Luc cells engrafted in many organs including bone marrow of spine and femurs, kidneys, ovaries, and stomach. CA2 treatment consisting of 20 cycles of 100 mg/kg/day b.i.d. oral dosing resulted in reduced luminescence intensity of IM-9Luc cells and prolonged the survival of mice (mean 33.7 days) compared to the vehicle-control arm (mean 27.1 days; p=0.029, Figure 2). In conclusion, CA2 is a BRD4 inhibitor with a novel structure and could be a promising molecular targeting-agent against MM. Disclosures Yoshioka: ConverGene LLC: Employment. Kado:Japan Community Health Care Organization Kyoto Kuramaguchi Medical Center: Employment. Strovel:ConverGene LLC: Employment.