622MO An asymmetrical CLL1/CD3 bispecific antibody, ABL602, exhibits CLL1 binding-dependent CD3 binding/activation and antitumor activity in acute myeloid leukemia (AML) mouse model and leukemia blasts from AML patients

Abstract

C-type lectin-like molecule 1 (CLL1) is highly expressed in acute myeloid leukemia blast cells but not in normal hematopoietic stem cells (HSCs). This limited expression of CLL1 makes it as a promising target for the AML therapy compared with other targets such as CD33 and CD123, which are also expressed in HSCs. Previously we have shown potent CLL1-specific antitumor activity using two AML cell line/models. In this study, we determined whether binding affinity of ABL602 to CD3 can be augmented upon binding to CLL1 and expanded our study to various AML cell lines and blasts from AML patients. To determine CLL1 binding-dependent CD3 binding of ABL602, T cell binding was measured by flow cytometry in the presence of CLL1-spiked beads. ABL602-mediated antitumor activity and T cell activation were tested in CLL1 expressing AML cell lines and AML blasts from patients. In vivo antitumor activity was determined in humanized mice bearing AML tumors. ABL602 (2+1), with its CD3 binding arm sterically hindered by CLL1 binding arm, binds to CD3 with a lower affinity compared with 1+1 format in the absence of CLL1-expressing cells. By incubating T cells with beads spiked with various amounts of CLL1 in the presence of ABL602, we proved that ABL602’s binding affinity to CD3 was increased in a CLL1-dependent manner, reaching comparable level to that of 1+1. ABL602 induced superior tumor cell killing than 1+1 reference antibody JNJ-67571244. Moreover, ABL602 showed a dose-dependent anti-tumor activity with tumor regression in OCIAML2 xenograft and HL-60 orthotopic model. In AML patient blasts, CLL1 was highly expressed and ABL602 exhibited a strong T cell activation and tumor killing activity on CLL1+ AML blasts. This study supports the target-specific T cell activation and tumor killing activity of 2+1 heterodimeric structure of ABL602 on both AML cell lines and patient-originated AML blasts and warrants further pre-clinical development.