Voruciclib Sensitizes High Risk Diffuse Large B Cell Lymphoma to BCL2 Inhibition Mediated Cell Death and Tumor Regression

Diffuse Large B Cell lymphoma (DLBCL) is the most common form of non-Hodgkin lymphoma in adults. Although upfront chemotherapy leads to favorable survival outcomes, relapsed or refractory patients continue to have poor prognosis with limited treatment options. In DLBCL, evasion of apoptosis - a key hallmark of cancer is mediated by functionally redundant BCL family members: BCL2, BCLxL and MCL-1. The BCL2 specific inhibitor venetoclax is approved for treating high-risk CLL, but responses in DLBCL have been limited, potentially due to compensatory upregulation of MCL-1. Currently a well-tolerated drug for inhibition of MCL-1, is unavailable in the lymphoma clinic.

Voruciclib, is a novel clinical stage oral CDK inhibitor with potent activity (<10 nM) against CDKs 9, 4, 6 and 1. Multiple mechanisms for downregulation of MCL-1 activity have been described for CDK inhibitors. Arguably best characterized is transcriptional inhibition of MCL-1, a short half-life transcript, via inhibition of transcriptional regulator CDK9. We evaluated MCL-1 expression in the FFPE lymphatic tissues from 33 patients with DLBCL, and found that it was expressed in 52% of cases, of both GC (germinal center) and ABC (activated B-cell)-like type. We therefore investigated whether voruciclib could synergize with venetoclax in pre-clinical models of DLBCL via inhibition of MCL-1. In cell-based assays, exposure of DLBCL cells to voruciclib as a single agent resulted in apoptosis which was preceded by context-dependent downregulation of MCL-1. To further explore the impact of voruciclib on MCL-1 activity and DLBCL viability in vivo, we utilized Presage's CIVO tumor microinjection technology. CIVO enables investigation of multiple drugs and drug combinations simultaneously in a living tumor facilitating in vivo assessment of anti-tumor drug synergy (Klinghoffer et al. Sci. Transl Med. 2015; Dey et al. PLOS One 2016). Voruciclib was introduced as a single agent or in combination with venetoclax to DLBCL xenografts. Microinjection, resulting in localized tumor exposure to voruciclib, led to MCL-1 downregulation in vivo across multiple models of DLBCL. In contrast, tumor exposure to venetoclax led to MCL-1 upregulation. Co-exposure to voruciclib and venetoclax demonstrated that the ability of voruciclib to downregulate MCL-1 is dominant to the upregulation by venetoclax. Consistent with the hypothesis that MCL-1 compensates for loss of BCL2 function in DLBCL, synergistic cell death was observed when voruciclib was combined with venetoclax. Synergy between voruciclib and venetoclax was observed in vivo in models representing both ABC (RIVA: CI value 0.5) and GC subtypes (NUDHL1 and Toledo: CI values 0.4). Similar activity was noted when venetoclax was combined with A1210477, an investigational MCL-1 inhibitor thereby suggesting MCL-1 downregulation to play a role in the observed synergy between venetoclax and voruciclib. Consistent with these results, preliminary studies on xenografted mice have shown that systemic administration of a sub-efficacious dose of venetoclax in combination with voruciclib led to impediment of tumor growth which was greater than the effect observed with each single agent. Additional systemic studies are ongoing with venetoclax in combination with voruciclib in a panel of DLBCL models to further strengthen this observation. Based on the above findings, a Phase 1b clinical trial has been designed to evaluate the combination of voruciclib and venetoclax in patients with the goal of expediting future treatment options for relapsed/refractory DLBCL. We expect to initiate this trial at multiple centers in early 2017.