Targeting BH3-Domain Anti-Apoptotic Proteins with GX15-070 Decreases DNA Synthesis, Induces Cell Death and Sensitizes Rituximab-Sensitive and Resistant Non-Hodgkin’s Lymphoma Cell Lines to the Anti-Tumor Activity of Chemotherapy Agents.

GX15-070 is a potent pan-bcl-2 inhibitor with known activity against chronic lymphocytic leukemia (CLL) cells and mantle cell lymphoma (MCL) cell lines, currently undergoing phase I testing. Bcl-2 over-expression is associated with chemotherapy resistance and correlates with a poor clinical outcome in diffuse large B-cell lymphoma. Recently, we demonstrated that acquisition of rituximab resistance is associated with deregulation of BH3-domain pro- and anti-apoptotic proteins. This leads to concomitant resistance to multiple chemotherapy agents. Targeting BH3-domain anti-apoptotic proteins with GX15-070 is an attractive strategy to potentially overcome acquired biologic and/or chemotherapy resistance. To this end we studied the effects of GX15-070 in a panel of rituximab-sensitive (RSCL) and rituximab-resistant cell lines (RRCL). Resistant clones were generated by chronic exposure of Raji, RL or DHL-4 cells to escalating doses of rituximab with (4RH) or without (2R) human complement. Functional assays (e.g. ADCC and CMC assays) demonstrated a significant decrease in rituximab sensitivity in RRCL. In addition, resistance to chemotherapy agents (CDDP, doxorubicin, vincristine, etc.) was demonstrated in RRCL. Lymphoma cells were exposed in vitro to GX15-070 (0 to 20mM) with or without CDDP (0 to 10mM) or Doxorubicin (0 to 1mM). Following a 24 and a 48 hour period of drug exposure: induction of apoptosis and the percentage of viability was determined by flow cytometric analysis, Western blotting and trypan-blue staining techniques. In addition, changes in DNA synthesis and cell proliferation following drug exposure were performed using standard [3H]-thymidine incorporation assays. GX15-070 induced a dose-dependent cell death and decrease in DNA synthesis in all the cell lines tested (both RSCL and RRCL). Up to 75% of cell death was observed in all cell lines exposed to 20mM of GX15-070. Anti-tumor activity was seen even at the lowest tested dose of GX15-070 (2mM, 20–25% death cells). Incubation of RSCL and RRCL with 2 or 5mM of GX15-070 induced synergistic cytotoxic and anti-proliferative effects when combined with CDDP and doxorubicin. The IC₅₀ of CDPP and doxorubicin was decrease ten-fold by the concomitant in vitro exposure to GX15-070 in all cell lines tested, including RRCL. Our data demonstrates that GX15-070 is active against both RSCL and RRCL and augments the anti-tumor activity of chemotherapy agents. These results strongly suggest that GX15-070 added to systemic chemotherapy is a potentially valuable and novel therapeutic strategy in the treatment of B-cell NHL.