Abstract 913: FLT3-targeting TriTACs are T cell engagers for treatment of acute myeloid leukemia

Abstract

Abstract FLT3 (fms related receptor tyrosine kinase 3) is a cell surface protein expressed in hematopoietic stem and progenitor cell populations1. FLT3 RNA is overexpressed in greater than 95% of acute myeloid leukemia (AML) samples2, and FLT3 mutations are found in 25% to 35% of AML samples1. In 2020 in the United States, an estimated 19,940 new AML cases and 11,180 AML-related deaths are expected3, exemplifying that better therapies are needed to treat this disease. Given the frequency of FLT3 expression in AML and unmet medical need for patients, FLT3 represents an attractive target for T cell-redirecting immunotherapy in AML. FLT3-targeting Tri-specific T cell Activating Constructs (FLT3 TriTACs) are designed to simultaneously engage FLT3 on a target AML cell and CD3 on a T cell, resulting in T cell activation, proliferation, and lysis of the AML cell. FLT3 TriTACs are ~53 kDa molecules that consist of three binding domains: an N-terminal single domain antibody (sdAb) that binds to human FLT3, a middle sdAb that binds to human serum albumin (HSA) to extend the half-life, and a C-terminal single chain Fv (scFv) that binds to CD3ϵ of the T cell receptor (TCR). Biophysical analyses of these molecules indicate they are stable with high monomer content. FLT3 TriTACs bind to human and cynomolgus monkey FLT3, CD3ϵ and albumin with similar affinities. Flow cytometry analysis of T cells from various normal donors and a panel of FLT3-positive and FLT3-negative tumor cell lines confirmed binding of FLT3 TriTACs to their targets expressed on the cell surface. FLT3 TriTACs induce potent killing of FLT3-expressing AML cell lines in vitro. In co-cultures of T cells from normal human donors, target tumor cells, and HSA, these TriTACs mediated dose-dependent and FLT3-dependent cytotoxicity. Cell lines engineered to express cynomolgus FLT3 were also readily killed by T cells with FLT3 TriTACs. Preclinical evaluation of FLT3 TriTACs in vitro, in mouse tumor models, and cynomolgus monkey pharmacokinetic studies will be presented.