PS1310 THE NOVEL BISPECIFIC CD47‐CD19 ANTIBODY TG‐1801 POTENTIATES THE ACTIVITY OF UBLITUXIMAB‐UMBRALISIB (U2) DRUG COMBINATION IN PRECLINICAL MODELS OF B‐NHL (2019)

Background: Umbralisib is an orally available PI3Kδ inhibitor that has demonstrated activity both in preclinical models and primary B cell non‐Hodgkin lymphoma (B‐NHL) cells, and in patients with B‐cell malignancies. The doublet therapy of umbralisib used in combination with the glycoengineered anti‐CD20 monoclonal antibody (mAb) ublituximab (“U2” regimen), provides a non‐chemotherapy backbone regimen on which several novel multidrug combinations have been explored clinically. TG‐1801 is a novel, bispecific antibody in Phase 1 testing that targets CD47 selectively on CD19+ B‐cells, sparing red blood cells or platelets, blocking the CD47‐SIRPα macrophage checkpoint on mature B cells. Aims: We explored in vitro and in vivo potential synergies between TG‐1801 and ublituximab, umbralisib, and the U2 combination in preclinical models of B‐NHL. Methods: Human B‐cell lymphoma Raji and Jeko‐1 cells were cultured in vitro in the presence of different stromal and immune cell types including bone marrow derived stromal cells (BMSCs), M2‐polarized primary macrophages, and primary circulating PBMCs. Cells were analyzed by proliferation assay, transcriptomic analysis (qPCR array and RNA sequencing followed by gene set enrichment analysis) and fluorimetric evaluation of antibody‐dependent cell death (ADCC) and antibody‐dependent cell phagocytosis (ADCP). In vivo , drug efficacy was determined in a Raji xenograft model, by dosing tumor‐bearing mice for 17 days with ublituximab (5 mg/kg, qw) +/‐ umbralisib (150 mg/kg, bid), the combination of both and/or TG‐1801 (5 mg/kg, qw). Results: In the context of the B‐NHL co‐culture system, U2 combination treatment led to the decrease of anti‐inflammatory cytokines genes (IL‐6, IL‐10, IL‐1RA) and upregulation of pro‐inflammatory IL‐2, accompanied by a 2 fold increase of M2 dependent ADCP. When U2 was combined with TG‐1801, this 2‐fold increase in ADCP was doubled, brought to a 4‐fold increase. In addition, TG‐1801 alone activated ADCC 4.1‐fold above control, and when combined with ublituximab was 7.2‐fold above control. As expected, umbralisib did not impact ADCC. Transcriptomic analysis of CD20+ cells further revealed that U2 treatment was mainly associated with modulation of redox processes in Raji co‐cultures (NES = −0.57, p = 0.016). Although TG‐1801 single agent did not significantly affect the transcriptome of these cells, combination with U2 provoked the down‐regulation of genes associated to cell architecture homeostasis, including cellular membranes (NES = −0.57, p = 0.006), cytoskeleton (NES = −0.51, p = 0.046), and cell proliferation (NES = −0.44, p = 0.018). In vivo , ublituximab alone displayed a tumor growth inhibition (TGI) of 88%, with 3/8 mice harboring a barely palpable tumor, while the umbralisib alone treatment arm showed a TGI of 50%, with 2/8 mice lacking detectable tumors. TG‐1801 exhibited a 76% TGI with 1/8 tumor free‐mouse. Most importantly, the combination of TG‐1801 with umbralisib alone, ublituximab alone, and U2 achieved TGI of 85%, 93% and 93% respectively, with more tumor‐free mice 35 days after the last dose in these 3 groups. Interestingly, this superior anti‐tumor effect of the different TG‐1801 combinations was associated with a higher infiltration of mouse macrophages within the tumors as assessed by F4/80 IHC labeling. Summary/Conclusion: The data presented here set the preclinical rationale and support a combination strategy of the novel CD47‐CD19 bispecific antibody TG‐1801 in B‐NHL with other B‐cell targeted mechanisms, including umbralisib and ublituximab.