Abstract 723: MX69 induces apoptosis by inhibiting XIAP in both rituximab sensitive and resistant lymphomas

Background: X-linked inhibitor of apoptosis protein (XIAP) has been implicated in development of resistance to chemotherapy via its direct inhibition of caspases 3,7 and 9. It is overexpressed in several lymphoma cell lines including rituximab-resistant cell lines (RRCL), Raji4RH and RL4RH, produced by our group. We have previously demonstrated that XIAP is critical for chemotherapy-resistance and survival in RRCL by knocking down XIAP using siRNA interference which demonstrated that XIAP is critical for chemotherapy-sensitivity and survival in RRCL. MDM2 inhibition activates p53 transcription and inhibits translation of XIAP, thus promoting apoptosis of cancer cells. Here, we targeted XIAP at a translational level by inhibiting it with MX69, a dual inhibitor of MDM2 and XIAP. Materials and Methods: Cell lines representing Burkitt’s lymphoma (BL), activated B-cell like diffuse large B-cell lymphoma (ABC-DLBCL, germinal center B-cell like DLBCL (GCB-DLBCL), mantle cell lymphoma (MCL) and B-cell lymphoblastic leukemia were exposed to MX69 as a single agent (0-80 uM) over 24, 48 and 72 hrs and IC50 concentrations were calculated. Subsequently, Raji, Raji4RH, RL, RL4RH, HBL-2, TMD-8, Daudi and Reh cell lines were exposed to MX69 (0-80 uM), in combination with doxorubicin (0-1 uM), cytarabine (0-50 uM), vincristine (0-10 nM), etoposide (0-50 uM), carboplatin (0-10 uM), ixazomib (0-1.5 uM), ibrutinib (0-10 uM) and venetoclax (0-10 uM) for 48 hours. Cell viability was determined by Cell Titer-Glo. Coefficient of synergy was calculated using CalcuSyn. Induction of apoptosis was evaluated by flow cytometry (Annexin V/PI stain). MDM2, p53, XIAP and PARP protein expression was determined by Western blotting. Results: MX69 induced cell death in a dose- and time-dependent manner in all cell lines. Annexin/PI staining showed caspase-dependent apoptosis with MX69 in all cell lines. Western blotting confirmed significant inhibition of MDM2, XIAP and changes in p53 and PARP following exposure to MX69. MX69 demonstrated significant synergistic activity when combined with doxorubicin, ixazomib, ibrutinib or venetoclax; synergy was strongest for the MX69- venetoclax combination. Conclusion: Our data suggests that in vitro exposure to MX69 resulted in anti-tumor activity in a wide variety of B-cell lymphoma cells lines (including BL, DLBCL, MCL). Perhaps related to its anti-tumor effects, MX69 inhibited XIAP levels. These findings are similar to prior siRNA XIAP knockdown experiments. Strong synergistic activity was observed when XIAP was combined with various chemotherapy agents and small molecules inhibitors (such as venetoclax, ixazomib or ibrutinib). Ex vivo experiments using primary tumor cells isolated from lymphoma patients and lymphoma mouse models have been planned. Targeting MDM2 and XIAP can be an attractive therapeutic strategy in patients with rituximab-sensitive or -resistant B-cell lymphoma. Citation Format: Sumera Khan, Cory Mavis, Ahmad Hanif, Juan Gu, Pallawi Torka, Francisco Hernandez-Ilizaliturri. MX69 induces apoptosis by inhibiting XIAP in both rituximab sensitive and resistant lymphomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 723.