Abstract 1786: APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR™ molecule for redirected T-cell cytotoxicity, induces potent T-cell activation, proliferation and cytotoxicity with limited cytokine release

Introduction: Depletion of CD123 over-expressing malignant cells provides a potential new treatment option which may improve patient outcomes in several hematological malignancies. CD123 is over-expressed in AML, MDS, ALL, CML, HCL and BPDCN and infrequently expressed by normal cells making it an attractive target which is being pursued using a number of different approaches including T-cell engaging immunotherapy. Cytokine release syndrome is a significant concern with T-cell activating therapeutics which has led to severe complications in clinical trials. We have developed APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR molecule for redirecting T-cell cytotoxicity to CD123-expressing tumor cells. A potential advantage of the ADAPTIR platform is reduced cytokine release upon T-cell engagement compared to other formats (Mol Cancer Ther. 2016 Sep;15(9):2155-65). Here we present in vitro and in vivo activity of APVO436 and compare the activity of APVO436 to another anti-CD123 x anti-CD3 bispecific containing the amino acid sequence of MGD006.Methods: Binding, T-cell activation and proliferation were assessed using multi-color flow cytometry. Cytotoxic activity was determined using chromium release assays and flow cytometry. PBMC samples were obtained from normal donors and AML patients. In vivo studies were performed using NSG mice transplanted with human PBMC9s. The CD123 and CD3 binding domain sequences for flotuzumab (MGD006) were obtained from patent W02015026892 engineered in Macrogenic9s dual-affinity re-targeting format as reported in Sci Transl Med. 2015 May 27;7(289):289ra82.Results: Dose-dependent cytotoxicity of CD123 expressing tumor cell lines and primary AML cells was induced by APVO436 at low effector to target ratios, accompanied by T-cell activation and proliferation. APVO436 induced significantly lower levels of several T-cell cytokines including IFNγ, IL-2, and TNFα compared to the molecule in the dual-affinity re-targeting format. In vivo, APVO436 significantly reduced established tumor burden in xenograft murine models.Conclusions: APVO436 potently induces T-cell activation, proliferation and CD123 + cell depletion with AML and normal donor samples and CD123 expressing tumor cell lines with limited levels of T-cell cytokine release compared to another CD123 x CD3 targeting bispecific format suggesting a potential safety advantage. APVO436 inhibits tumor-growth in sub-cutaneous tumor models with IV-implanted human T cells, indicating migration and engagement of T cells at the tumor site. These data are supportive of further investigation of APVO436 as a potential treatment option for AML and other hematological malignancies. GLP toxicology studies have been completed in non-human primates and APVO436 is advancing to clinical testing. Citation Format: Michael R. Comeau, Robert E. Miller, Robert Bader, Rebecca Gottschalk, Mollie Daugherty, Toddy Sewell, Lynda Misher, Lara Parr, Melissa DeFrancesco, David Bienvenue, Catherine J. McMahan, Gabriela H. Hoyos, Jane A. Gross. APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR™ molecule for redirected T-cell cytotoxicity, induces potent T-cell activation, proliferation and cytotoxicity with limited cytokine release [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 1786.