Voruciclib plus venetoclax show high efficacy for CLL b cells on human stromal cells.

Abstract

e20009 Background: Although treating B-Chronic Lymphocytic Leukemia (CLL) with small molecule inhibitors has shown promise, a lasting cure for this disease has yet to be found. Further treatments directed at key molecular targets in the CLL B cell need development for patients with adverse prognostic factors, and relapsed/refractory patients. We evaluated the impact of two novel drugs with non-overlapping mechanisms of action on CLL B cells: Voruciclib, a cyclin dependent kinase (CDK) inhibitor targeting CDKs 9, 6, 4, 1 and Venetoclax a BCL-2 inhibitor. To mimic the protective effect of the CLL micro-environment these experiments were done with CLL cells cocultured in the presence of bone marrow mesenchymal stromal cells (BMSC). Methods: Voruciclib, (MEI Pharma) and Venetoclax were tested for killing activity alone and in combination against CLL cells cocultured with healthy human bone marrow MSC. CLL B cells were from untreated patients stratified on risk of disease progression. Low risk had mutated and high risk patients had unmutated IVGH genetic status. CLL cells were cultured on BMSC’s with a series of escalating doses of individual drugs and drug combinations at fixed molar ratios then Annexin/PI stained for viability testing by flow cytometry. Killing curves generated for each drug/combination were analyzed by the combination index (C.I.) approach of Chou and Talalay with Calcusyn, characterizing interactions as synergistic, additive or inhibitory. C.I. value hierarchy classes synergy as moderate (0.7-0.9), synergistic (0.3-0.7), strong (0.1-0.3) and very strong (below 0.1). Results: CLL cells with unmutated IGVH status were more sensitive to Voruciclib than mutated IGVH both with and without BMSC’s. The combination Voruciclib +Venetoclax showed synergism for all patients regardless of risk; half strongly to very strongly (Table). Similar synergistic effects were seen with relapsed/refractory patients. Conclusions: Based on these preclinical data, Voruciclib + Venetoclax is a very promising combination to improve treatment of CLL patients. We speculate that CDK9 inhibition, which regulates transcription of Mcl-1, with Bcl-2 inhibition results in potent apoptosis induction in CLL B cells. [Table: see text]