Abstract 143: Preclinical studies on potential therapeutic combination partners for the potent and selective PI3Kδ inhibitor INCB050465 in DLBCL

The delta isoform of PI3K (PI3Kδ) plays an essential role in B-cell development and function by mediating the signaling of key receptors on B cells. Increased malignant B cell proliferation and survival has also been associated with aberrant activation of PI3Kδ, making selective inhibition of this isoform an attractive therapeutic approach for the treatment of B cell malignancies. INCB050465 is a potent inhibitor of PI3Kδ, with a >20,000 fold selectivity over other PI3K isoforms. Emerging clinical data indicate that INCB050465 monotherapy is well tolerated and results in promising clinical responses in patients with various lymphoma histologies, including those with DLBCL. We therefore sought to explore rational combination strategies for INCB050465 using mouse xenograft models of ABC-subtype (HBL-1), GCB-subtype (Pfeiffer), and GCB/double-hit (WILL-2) human DLBCL, evaluating standard of care agents such as bendamustine and rituximab, as well as with targeted agents. PIM inhibition is a logical addition to PI3Kδ inhibition as a therapeutic approach as both kinases play a critical role in the AKT signaling pathway, having overlapping substrates. Likewise BET inhibition is a rational addition to PI3Kδ inhibition in “double-hit” DLBCL due to de-regulation of MYC transcriptional activity. In vivo studies performed in the Pfeiffer xenograft model demonstrate that INCB050465 combined with the pan-PIM inhibitor INCB053914 yielded complete tumor regressions. This profound decrease in tumor cell survival was due in part to the significant reduction in pBAD levels resulting from dual PIM and PI3Kδ inhibition. Despite modest single agent activity in vivo, the combination of INCB050465 with BET inhibitors, INCB054329 or INCB057643, resulted in significant anti-tumor efficacy in all of the DLBCL models studied, and caused a marked repression in tumor MYC expression. To study the transcriptional effects of combining PI3Kδ and BET inhibitors in this lymphoma model, WILL-2 xenograft tumors from mice treated with single dose INCB050465, INCB054329, the combination, or vehicle control were analyzed by RNAseq. INCB050465 enhanced the ability of INCB054329 to repress a MYC-driven transcriptional program, and the combination also regulated multiple developmental and inflammatory pathways. Together, these data support the clinical evaluation of the PI3Kδ inhibitor INCB050465 as part of a combination regimen with PIM or BET inhibitors for the treatment of DLBCL. Citation Format: Matthew C. Stubbs, Robert Collins, Leslie Hall, Alla Volgina, Holly Koblish, Sang Hyun Lee, Timothy Burn, Phillip C. Liu, Jin Lu, Eddy Yue, Yun-Long Li, Andrew P. Combs, Wenqing Yao, Gregory Hollis, Reid Huber, Bruce Ruggeri, Peggy Scherle. Preclinical studies on potential therapeutic combination partners for the potent and selective PI3Kδ inhibitor INCB050465 in DLBCL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 143. doi:10.1158/1538-7445.AM2017-143