Targeting the Bcl-2 Family of Proteins in Hodgkin Lymphoma: In Vitro Cytotoxicity, Target Modulation and Drug Combination Studies of the BH3 Mimetic ABT-737

Introduction: Currently, patients with refractory Hodgkin lymphoma (HL) or those who relapse multiple times have an extremely poor prognosis. Even patients who do not relapse often experience late toxicities, including secondary cancer, heart failure, pulmonary dysfunction, and infertility. Therefore, new agents and novel therapeutic approaches are urgently needed. Anti-apoptotic proteins such as Bcl-2 and Bcl-xl have been found to be associated with the growth and survival of Hodgkin Reed–Sternberg cells and thus carry the potential to be effective targets for therapeutics. In this study we report the in vitro cytotoxicity, biological correlative findings and drug combination analysis of the novel BH3 mimetic ABT-737 [Abbott laboratories (Abbott Park, IL)] against HL cells.

Materials and Methods: HL cell lines (KMH2 and HDLM2) were cultured in the presence of increasing concentrations of ABT-737 or its enantiomer control. Normal bone marrow stromal cells were used as controls for non-specific cytotoxicity. Cell growth inhibition was measured by Alamar blue assay and the induction of apoptosis was demonstrated by an annexin specific staining technique. Time and drug concentration dependent changes in proteins involved in cell survival and apoptosis were investigated by Western blot analysis. The ability of ABT-737 to influence the anti-lymphoma activity of a panel of twenty distinct chemotherapeutic agents was evaluated by drug combination and cell growth inhibition studies. Combination indices (CI) were calculated to identify therapeutic enhancement of different agents in the presence of Bcl-2 inhibition by ABT-737.

Results: ABT-737 showed significant dose-dependent cytotoxicity and apoptotic activity against HL cells with an approximate IC₅₀ of 1mM with maximum cell death occurring at about 5mM. At the molecular level, increased levels of cleaved caspase3 and PARP, as well as annexin positivity, were noticeable within three hours of treatment with the agent. Drug combination studies have shown the ability of Bcl-2 inhibition to synergize with novel therapeutic agents that target histone deacetylase function (Apicidin, CI 0.35), Hsp90 stability (17-AAG, CI 0.3) and the activity of specific receptor tyrosine kinases (Sorafenib, CI 0.7 and Sunitinib CI 0.3 for HDLM2 and CI 0.9 for KMH2). Among conventional anti-neoplastic agents, ABT-737 showed significant synergistic activity with irinotecan and oxaliplatin (CI 0.6). Importantly, treatment with ABT-737 decreased the expression of the critical HL cell growth promoter, NF- B as determined by band densities on Western blot analysis: a two fold decrease in KMH2 and a five fold decrease in HDLM2 cells. Specific target modulation was demonstrated by changes in key apoptosis and cell survival regulators such as Bcl-X_S/L, Bcl-X, p53 and survivin by two to 10 fold decreases in Western blots.

Discussion: Data presented in this study support the hypothesis that Bcl-2 family of proteins can be an effective target for therapeutics in HL. We have shown that the BH3 mimetic ABT-737 induces apoptosis in these cells, characterized by the modulation of key components of cell growth and survival pathways. In addition, we have identified distinct classes of anti-lymphoma and anti-neoplastic agents whose activities are enhanced by concurrent inhibition of Bcl-2. These findings provide the rationale for further evaluation of ABT-737 and the subsequent clinical development of a targeted anti-Bcl-2 therapy for refractory Hodgkin lymphoma.