The Pan-Bcl-2 Inhibitor GX15-O70 Induces Apoptosis in Human Myeloma Cells by Noxa Induction and Strongly Enhances Melphalan, Bortezomib or TRAIL-R1 Antibody Apoptotic Effect.

GX15-070 belongs to small molecule pan-Bcl-2 inhibitors that mimic BH3-only proteins, therefore it may bind the groove of anti-apoptotic Bcl-2 family members and induce cell death. Mcl-1, an anti-apoptotic protein from the Bcl-2 family, is an essential survival protein for Multiple Myeloma (MM) cells. Mcl-1 displays specific affinity for some BH3-only proteins. In viable MM cells, Mcl-1 strongly interacts with BH3-only Bim protein, keeping in check its pro-apoptotic properties. Consequently, we tested a panel of human MM cell lines (HMCL) (n=9) for GX15-070 apoptosis induction. Apoptosis of HMCL was induced at doses ranging from 0,15 μM to 5 μM. HMCL were differentially sensitive to GX15-07. For most HMCL the maximal apoptotic effect was observed at 2,5 μM of GX15-070. Thus we found 2 very sensitive HMCL (> 60% of death cells), 3 intermediate sensitive HMCL (>30% <60%) and 4 resistant HMCL (<30%). Sensitivity to GX15-070 was not correlated with IL-6 dependence. For further studies 4 HMCL were selected according to GX15-070 apoptosis induction, namely RPMI-8826 (71% ± 4), MDN (64% ± 4), LP1 (43% ± 3) and U266 (22%± 2). After 24 hours treatment, no substantial modifications of the anti-apoptotic proteins (Bcl-2, Bcl-xL and Mcl-1) were induced. Among the BH3-only pro-apopototic proteins examined, only Noxa was consistently induced by GX15-070 in all sensitive HMCL. In contrast, Noxa was not induced in the GX15-070 resistant U266 cell line. Pro-apoptotic multidomain molecules Bax and Bak were not modified by GX15-070.

We also evaluated GX15-070 in combination with other drugs currently used in MM treatment. We wondered if GX15-070 could sensitize HMCL LP1 and/or L363 cells to either Melphalan or PS-341 since both HMCL are weakly sensitive to both agents. HMCL were pre-incubated with GX15-070 for 7h at doses ranging from 0,15 to 5 μM followed by either melphalan or Bortezomib during 24h. GX15-070 displayed an additive apoptotic effect either in combination with melphalan or with Bortezomib for both HMCL. Interestingly, GX15-070 had a synergistic apoptotic effect in combination with a fully human antibody directed against TRAIL-R1, Mapatumumab, Human Genome Sciences, Rockville, MD. Of note, GX15-070/Mapatumumab combination exhibited a synergistic apoptotic effect on the GX15-070 resistant U266 cell line. Ongoing studies are conducted to unravel the contribution of GX15-070 either on the intrinsic or the extrinsic apoptotic pathways and to explore the feasibility of GX15-070 as a MM therapeutic agent.