High Bcl-2/Mcl-1 Gene Expression Ratio Identifies Molecular Sub-Groups of Multiple Myeloma Patients Extremely Sensitive to ABT-737

Abstract 4061

Multiple myeloma (MM) is a plasma cell malignancy presently incurable with heterogeneity in molecular abnormalities and treatment response. While the importance of Mcl-1 in maintaining survival of Human myeloma cell lines (HMCLs) has been well documented we recently identified a sub-group of HMCLs that highly expressed Bcl-2. This finding prompted us to investigate the sensitivity of HMCLs to ABT-737, a cell-permeant compound that binds with high affinity to Bcl-2 and antagonizes its anti-apoptotic function. The effect of ABT-737 on myeloma cell viability was assessed by APO2.7-PE staining on a large panel of HMCLs (n=26) representative of Multiple Myeloma molecular heterogeneity. We found 7 HMCLs highly sensitive to ABT-737 with an EC50 ranging from 7±0.4nM to 150±7.5nM. Myeloma cell death induced by ABT-737 occurred as early as 4 hours after treatment and led to Bax and Bak activation. While all these sensitive cell lines expressed a high level of Bcl-2, only Bcl-2/Mcl-1 gene expression ratio allowed the discrimination of highly ABT-737 sensitive from resistant HMCLs. High sensitivity (EC 50 <150 nM) was correlated to a ratio > 0.4. This result is in agreement with the high affinity of ABT-737 for Bcl-2 in contrast to Mcl-1. Induction of apoptosis may be due to the displacement of Bim, Puma or Bik from their endogenous complexes with Bcl-2, as demonstrated by immunoprecipitation assays. Interestingly, we demonstrated that all sensitive HMCLs harbored a t(11;14) while 77% (7 out of 9) of t(11;14) HMCLs are sensitive to ABT-737. Then, interrogation of the public expression database of MM patients showed that the Bcl-2/Mcl-1 ratio of both t(11;14) (median 0.29, range 0.05–0.71) and hyperdiploid (median 0.24, range 0.04–0.79) patients was significantly higher to all other groups of patients (p<.001). In order to fairly identify MM patients expressing high Bcl-2/Mcl-1 ratio we set up a flow cytometry assay. Preliminary results confirmed that primary myeloma cells with a high Bcl-2/Mcl-1 ratio are also highly sensitive to ABT-737. Altogether, the very high sensitivity of Bcl-2 ^high Mcl-1^weak HMCLs and primary myeloma cells to ABT-737 (similar to the one of CLL) supports the idea of a clinical trial using Bcl-2 antagonists in a selected subset of t(11;14) and hyperdiploid Multiple Myeloma patients.