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Abstract 871: BR101801: A first-in-class dual inhibitor of PI3Kδ and DNA-PK in non-Hodgkin's lymphoma

Introduction: Inhibition of PI3Kδ (idelialisib, TGR-1202) has been reported as having limited clinical benefits for the treatment of indolent non-Hodgkin9s lymphoma such as chronic lymphocytic leukemia, small lymphocytic lymphoma or follicular lymphoma but not aggressive NHL such as relapsed/refractory Diffuse Large B-Cell Lymphoma (RR DLBCL). Recently, up-regulation of γ-H2AX, a sensor of DNA damage, was reported to be caused by hyper-proliferation burden in aggressive NHL cells under protective action of the DNA repair system. This means that DNA repair system targeting therapy in aggressive NHL could lead to high genomic instability toward cell death. In this study, BR101801, a first-in-class dual inhibitor of DNA-PK and PI3Kδ, will be introduced as a novel candidate of RR DLBCL. Methods: Cell-based selectivity assay for PI3K isotypes was performed using a specific ligand-induced phospho-AKT in various cells. To assess the effect of BR101801 on DNA-PK in LoVo cells, cells were pre-incubated with etoposide for 2 hours and then exchanged into new media containing BR101801 or other PI3Kδ inhibitors without etoposide. Activity of BR101801 was evaluated based on prolongation of γ-H2AX using Western blotting. GI50 was tested in vitro by WST-based viability assay in various NHL cell lines. Results: BR101801 showed higher selectivity for p110δ and DNA-PK compared to the other PI3K class I enzymes by 7- to 100-fold (IC50 p110δ = 4 nM; IC50 DNA-PK = 7 nM; p110α, p110β, and p110γ IC50 values were 368, 398, and 52 nM, respectively), whereas no activity was observed for the other 394 human kinases at 1 μM. The inhibitory level of phospho-AKT by BR101801 was analyzed by stimulation with PDGF (p110α), LPA (p110β), IgM (p110δ), and C5a (p110γ), resulting in IC50 values of 146, 50, 1.2, and 110 nM, respectively. BR101801 prolonged etoposide-induced γ-H2AX but not idelalisib and showed potent growth inhibition in DLBCL cell lines compared to PI3Kδ inhibitors such as idelalisib and TGR-1202. SU-DHL-10 cells represented low sensitivity (IC50≒4.46 μM) when idelalisib was treated alone. However, a combination of idelalisib and NU7441 was used to improve the combination index (CI) to below 0.5 in a dose-dependent manner. This result indicates that the inhibition of both DNA-PK and PI3Kδ lead to synergistic effect through targeting each independent pathway like as BR101801. Next, several DNA-damaging inducers known to boost γ-H2AX, including bendamustine, doxorubicin, and etoposide, were combined with BR101801. In vitro synergy activity was confirmed based on a CI value less than 0.5, and in vivo synergy was assessed in a SU-DHL-10 xenograft model. Conclusions: BR101801 is a first-in-class dual inhibitor of PI3Kδ and DNA-PK. DNA-PK targeting has shown to be efficient with PI3Kδ inhibitor or DNA-damaging inducers. Therefore, the strategy of dual targeting with DNA-PK and PI3Kδ suggests further investigation as a novel option of RR DLBCL. Citation Format: Mi Kwon Son, Byeongwook Jeon, Jin Sang Wang, Baek kyung Kim, Bo Ram Lee, Yeon seo Choi, Nam-Hoon Kim, Jayhyuk Myung, Dal-Hyun Kim. BR101801: A first-in-class dual inhibitor of PI3Kδ and DNA-PK in non-Hodgkin9s lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 871.