Abstract 938: Pharmacologic targeting Mcl-1 with AZD5991 induces apoptosis, suppresses mitochondrial respiration and overcomes ibrutinib resistance in non-Hodgkin lymphoma cells

Bcl-2 family proteins determine cell fate and comprise pro-apoptotic “initiators” (Noxa, Bim, Puma), anti-apoptotic “guardians” (Bcl-2, Mcl-1, Bcl-xL) and pro-apoptotic effectors (Bax/Bak). Venetoclax, a Bcl-2 inhibitor, received regulatory approval in the therapy of chronic lymphocytic leukemia and acute myeloid leukemia. However, venetoclax is modestly effective in non-Hodgkin lymphoma (NHL). Mcl-1 is a short-lived pro-survival protein that is frequently overexpressed in NHL, leading to increased cell survival and drug resistance. Mcl-1 transgenic mice develop aggressive lymphoma. Thus, selective targeting Mcl-1 represents a promising pharmacologic strategy in NHL. AZD5991 is a selective small-molecule inhibitor with high potency against Mcl-1 which has entered clinical trials. Here we explored its pre-clinical activity in NHL. We detected Mcl-1 expression in ten diffuse large B-cell lymphoma (DLBCL) as well as in parental/ibrutinib-resistant JeKo-1 and Mino mantle cell lymphoma (MCL) cell lines. AZD5991 suppressed proliferation and induced apoptosis in DLBCL cells in a dose-dependent manner, as well as in parental/ibrutinib-resistant MCL cells. Mcl-1 inhibition led to mitochondrial dysfunction, including mitochondrial membrane depolarization, decreased mitochondrial mass, increased mitophagy, and increased levels of reactive oxygen species in both DLBCL and parental/ibrutinib-resistant MCL cells. Mcl-1 inhibition downmodulated basal and maximal respiratory capacity in sensitive, but not in resistant cells, as determined by the Seahorse assay, further indicating mitochondria compromise as a mechanism of susceptibility to Mcl-1 inhibition. An inhibitors screening assay identifies that AZD5991 demonstrated synergy with other BH3-mimetics. Co-treatment of DLBCL cells with Bcl-2/xL inhibitors AZD4320, venetoclax and navitoclax overcame resistance to AZD5991. Finally, Primary MCL cells were co-cultured with control, CD40L- or BAFF-expressing stroma which partially mimics the lymph node microenvironment and induces resistance to spontaneous and drug-induced apoptosis. Mcl-1 inhibition induced apoptosis of primary MCL cells in both control and BAFF-expressing conditions, accompanied by decreased mitochondrial respiration. Meanwhile, CD40L stroma rendered protection from AZD5991 which was partially overcome by co-treatment with venetoclax. In summary, Mcl-1 inhibition using selective BH3-mimetic AZD5991 restricts cell proliferation and induces apoptosis in a subset of DLBCL, parental/ibrutinib-resistant MCL cells and primary neoplastic B cells. Mcl-1 inhibition leads to mitochondrial dysfunction and inhibition of cellular respiration. Resistance to Mcl-1 inhibition may be overcome by concurrent targeting of alternative anti-apoptotic proteins (Bcl-2/xL) in NHL. Citation Format: Tingting Liu, Vi Lam, Elana Thieme, Xiaoguang Wang, Fei Xu, Steve Kurtz, Jeff Tyner, Olga V. Danilova, Alexey Danilov. Pharmacologic targeting Mcl-1 with AZD5991 induces apoptosis, suppresses mitochondrial respiration and overcomes ibrutinib resistance in non-Hodgkin lymphoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 938.