Abstract B111: Characterization of APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR™ molecule for redirected T-cell cytotoxicity, in preclinical models of AML and nonhuman primates
Authors: Comeau Michael R. , Miller Robert E. , Bannink Jeannette , Johnson Starrla , Bader Robert , Gottschalk Rebecca , Misher Lynda , Mitchell Danielle , Parr Lara , DeFrancesco Melissa , Bienvenue David , McMahan Catherine J. , Hoyos Gabriela H. , Gross Jane A. 
Published: August 2018

Abstract
Introduction: CD123 has emerged as a promising target for T-cell directed immunotherapy in acute myeloid leukemia due to its high level of expression on leukemic blasts and stem cells and low frequency of expression on normal cell types. CD123 is expressed by rare normal leukocyte populations, including basophils and plasmacytoid dendritic cells in circulation and hematopoietic progenitor cells in the bone marrow. We have developed APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR molecule for redirecting T-cell cytotoxicity to CD123-expressing tumor cells. Results are presented that examine the in vitro and in vivo activity of APVO436 in preclinical models of AML, as well as pharmacokinetics and tolerability in nonhuman primates. Methods: CHO-produced APVO436 protein was used for in vitro functional studies with CD123+ AML tumor cell lines and primary human T-cell populations. Cytotoxic activity was determined using chromium release assays. On-cell binding, T-cell activation and proliferation were assessed using multicolor flow cytometry. Cytokine levels were measured using multiplex assay kits. In vivo studies to examine tumor growth inhibition activity were performed using NSG mice with established AML tumors followed by treatment with APVO436 coadministered with T cells. Tumor growth was assessed by bioluminescent imaging. In vivo studies to determine pharmacokinetics of APVO436 following single intravenous injections were performed in healthy BALB/c mice and cynomolgus monkeys. Results: APVO436 bound to CD123 and CD3 expressing cells with high affinity. APVO436 induced concentration-dependent lysis of CD123+ AML cell lines with primary human T cells at low effector-to-target ratios, accompanied by T-cell activation and proliferation. These activities were dependent on the expression of CD123 by the tumor target cells. Treatment of established disseminated tumors in mice with APVO436 resulted in a significant reduction in tumor burden. A single 10 mg/kg IV injection of APVO436 in normal BALB/c mice demonstrated an extended elimination serum half-life of approximately 300 hours. Single IV injections of APVO436 ranging from 0.25 mg/kg to 1 mg/kg were well tolerated in cynomolgus monkeys. The observed elimination half-life in cynomolgus serum was up to 84 hours with normal clearance and volume distribution. Conclusions: These data demonstrate APVO436 has potent in vitro activity against a number of CD123-expressing tumor cell lines and in vivo significantly reduces established tumor burden in xenograft models. APVO436 demonstrates an extended serum half-life in normal mice and cynomolgus monkeys. These data are supportive of further investigation of APVO436 as a potential treatment option for AML and other hematologic malignancies.
Citation Format: Michael R. Comeau, Robert E. Miller, Jeannette Bannink, Starrla Johnson, Robert Bader, Rebecca Gottschalk, Lynda Misher, Danielle Mitchell, Lara Parr, Melissa DeFrancesco, David Bienvenue, Catherine J. McMahan, Gabriela H. Hoyos, Jane A. Gross. Characterization of APVO436, a bispecific anti-CD123 x anti-CD3 ADAPTIR™ molecule for redirected T-cell cytotoxicity, in preclinical models of AML and nonhuman primates [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B111.