ABT-737 Sensitizes B Cell Tumors for Killing by CD19-Retargeted T Cells,

T cells expressing tumor-targeting chimeric antigen receptors are showing promise in clinical trials for patients with B cell leukemia and lymphoma. However, increased levels of anti-apoptotic proteins, a common trait among B-cell tumors, may hamper treatment efficacy. ABT-737 is a small molecule inhibitor of anti-apoptotic proteins such as BCL-2, BCL-xL, BCL-w, and MCL-1, which induces apoptosis via the intrinsic apoptosis pathway in contrast to T-cells that utilize the extrinsic pathway controlled by death receptors and their ligands. ABT-737 has been shown to efficiently promote apoptosis in B-cell tumors as exemplified in models of pre-B-ALL. Recently, ABT-737 was shown to synergize with TRAIL to induce apoptosis. This prompted us to investigate if ABT-737 could be combined with T-cell therapy to enhance tumor cell death.

PBMCs from healthy donors and patients with pre-B-ALL was genetically engineered with a second generation chimeric antigen receptor (CAR) targeting CD19 on B-cells. The T-cells and ABT-737 were tested both individually, and in combination, for their cytotoxic capacity in in vitro assays such as flow cytometry and the Caspase-Glo® 3/7 assay. The effects were studied in a panel of B-cell tumor cell lines (Daudi, U698, Karpas422, DG75, Nall-1) since they may exhibit different apoptosis resistance profiles. The expression of anti-apoptosis molecules in these cell lines was investigated by PCR.

PCR confirmed expression of BCL family proteins in the cell lines tested. CD19-targeting T-cells specifically induced apoptosis in CD19+ tumor cells. Similarly, but less efficiently, ABT-737 as single agent increased apoptosis in the various tumor cell lines. When combining T-cell and ABT-737 therapy, the tumor cell death was significantly increased to that of single agent treatment. The effect varied from additive to synergistic effects. The tumor cell lines did not change the level of antigen presenting molecules (MHC I and II), death receptors (Fas) or adhesion or costimulatory molecules (ICAM-I, CD80, CD86) upon ABT-737 treatment. Hence, the effect did not likely represent increased killing by enhanced physical interaction between T-cells and tumors but rather simultaneous engagement of both intrinsic and extrinsic apoptosis pathways.

The apoptosis inducer ABT-737 is potently enhancing CD19-targeting T-cell therapy. By triggering both intrinsic and extrinsic apoptosis pathways also resistant tumors may succumb to treatment.

Simonsson:Novartis, BMS, Merck, Pfizer: Consultancy, Honoraria.