Synergistic Effect of Venetoclax for Antibody Dependent Cell Cytotoxicity By Daratumumab

The prognosis of multiple myeloma (MM) has improved following the development of new drugs such as proteasome inhibitors and immunomodulatory drugs, but many patients are refractory to existing therapy. Venetoclax is a selective, orally bioavailable inhibitor of BCL2, showing efficacy in not only the monotherapy but also as the combination therapy of MM patients, especially those harboring t (11;14). Herein, we aimed to evaluate the synergistic cytotoxicity of venetoclax and daratumumab in vitro using KMS12PE and SKMM1 cell lines. The cells were treated with venetoclax and daratumumab targeting BCL2 and CD38, respectively. Flow cytometry showed that both cells were good therapeutic targets for daratumumab with the positive ratio of CD38 being 100% and 75.7% in KMS12PE and SKMM1, respectively (Figure 1). Western blotting showed that BCL2 expression was strongly detected in KMS12PE but barely in SKMM1 cells (Figure 2). Furthermore, using the WST1 assay for evaluating the cytotoxicity of venetoclax, we observed that KMS12PE cells were sensitive to 100 nM of venetoclax while SKMM1 cells were not sensitive up to 10 uM, which corresponded with the expression levels of BCL2 in both the cells (Figure 3). Then, the cytotoxicities of venetoclax, daratumumab, and their combination, were compared using flow cytometry with annexin V detection; the results indicated that the combination therapy exhibited higher cytotoxicity than the monotherapies. This experiment was repeated 3 times, and the average value with standard deviation (SD) is shown in Figure 4. Collectively, these results indicate that the antibody-dependent cell-mediated cytotoxicity was enhanced in KMS12PE cells harboring t (11;14) with high BCL2 expression, suggesting that the combination of venetoclax and daratumumab displays a synergistic effect in treating MM harboring t (11;14) with high BCL2 expression. Our data support the results of a recent phase І/II clinical trial for MM (Bahlis N et al. Blood 2019;925-925.), wherein 24 patients with R/R MM were treated with a combination therapy of venetoclax, daratumumab and dexamethasone, and the efficacy and safety of the therapy were evaluated. The ORR of the patients with t (11;14) treated with the combination therapy was 92%, which demonstrated the significant efficacy of the combination treatment. Thus, while the efficacy of the combination therapy of venetoclax and daratumumab has already been confirmed in a clinical trial, our data further shows the synergistic cytotoxic effect of venetoclax and daratumumab in vitro. Our in vitro analysis could, therefore, serve useful in the estimation of the effective dosage of each drug for the treatment of MM using clinical samples.