Abstract PR13: The anti-CD47 antibody Hu5F9-G4 is a novel immune checkpoint inhibitor with synergistic efficacy in combination with clinically active cancer targeting antibodies
Authors: Chao Mark P. , McKenna Kelly M. , Cha Adriel , Feng Dongdong , Liu Jie , Sikic Branimir I. , Majeti Ravindra , Weissman Irving L. , Takimoto Chris , Volkmer Jens-Peter 
Published: November 2016

Abstract
We have recently discovered that CD47 is a checkpoint of the innate immune system. CD47 is highly expressed on many cancers, allowing evasion of immune surveillance. By binding the inhibitory receptor SIRPα on macrophages and other phagocytes, CD47 serves as a “don't eat me” signal and innate immune checkpoint. In pre-clinical models, CD47-blocking monoclonal antibodies (mAbs) inhibit human cancer growth and metastasis in many hematologic malignancies and solid tumors. Anti-CD47 mAb therapy also enables macrophages to prime and activate CD8+ T cells, which limit tumor growth even after treatment cessation. These findings formed the basis for developing an anti-CD47 mAb (Hu5F9-G4) as a new class of cancer immunotherapy that is being tested in phase 1 trials.
In addition to monotherapy efficacy, we demonstrate here in pre-clinical models that Hu5F9-G4 treatment leads to profound anti-tumor synergy with mAbs targeting tumor-specific antigens, including rituximab (anti-CD20) for non-Hodgkin's lymphoma, trastuzumab (anti-Her2) for breast cancer, and anti-EGFR mAbs for colorectal cancer (CRC). In patient-derived xenograft (PDX) models, Hu5F9-G4 combined with these cancer-specific mAbs led to synergistic tumor elimination compared to either antibody alone. This synergy is due to the simultaneous supply of a strong pro-phagocytic signal through Fc receptor-mediated antibody dependent cellular phagocytosis by the cancer targeting mAb, in combination with blockade of the anti-phagocytic signal CD47 by Hu5F9-G4. We next demonstrate that combination efficacy with Hu5F9-G4 and cancer targeting mAbs is independent of intrinsic tumor signaling pathways and resistance. Using CRC PDX models, we show that Hu5F9-G4 combined with either the anti-EGFR mAb cetuximab or panitumumab leads to synergistic tumor elimination in RAS wildtype tumors. While anti-EGFR mAbs are clinically approved in RAS wildtype metastatic CRC, they are not effective in patients with RAS mutated CRC, which represent approximately 40% of all CRC. Remarkably, the combination treatment was also synergistic against CRCs with RAS pathway mutations, in which anti-EGFR mAbs alone were not effective. Thus, the combination of Hu5F9-G4 with anti-EGFR mAbs can rescue efficacy of EGFR mAb activity in RAS mutant CRC and that synergistic efficacy is independent of mutations in intrinsic tumor signaling pathways. This strategy could be applied to other tumor types that develop signal pathway resistance. Based on these findings we have initiated a clinical development program to explore the efficacy of Hu5F9-G4 monotherapy efficacy and in combination with cancer targeting mAbs.
We are currently conducting a first-in-class, first-in-human Phase 1 dose escalation trial of Hu5F9-G4 in advanced solid tumors (NCT02216409) to determine the safety, tolerability and recommended phase 2 dose of Hu5F9-G4. In over 20 patients treated, Hu5F9-G4 has been well tolerated in the presence of multiple dose escalations. Mild anemia has been observed, which is an expected on-target effect due to CD47-mediated clearance of aged red blood cells. This anemia occurs with the first dose only, is transient, and is well managed through a priming and maintenance dose strategy. To date, no grade 3 or 4 anemias have been observed, with no patients requiring blood transfusions. Current dose levels of Hu5F9-G4 are associated with drug serum levels that achieve anti-tumor efficacy in pre-clinical models, thus demonstrating effective saturation of the internal CD47 receptor tissue sink. Future trials of Hu5F9-G4 in combination with cancer targeting mAbs are being planned.
In summary, Hu5F9-G4 represents a novel immune checkpoint therapy that engages both innate and adaptive immune systems, displays widespread synergistic efficacy in combination with cancer targeting mAbs, and can overcome tumor signaling pathway resistance. The clinical activity of Hu5F9-G4 alone and in combination is being explored.
Citation Format: Mark P. Chao, Kelly M. McKenna, Adriel Cha, Dongdong Feng, Jie Liu, Branimir I. Sikic, Ravindra Majeti, Irving L. Weissman, Chris Takimoto, Jens-Peter Volkmer. The anti-CD47 antibody Hu5F9-G4 is a novel immune checkpoint inhibitor with synergistic efficacy in combination with clinically active cancer targeting antibodies [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr PR13.