On Day time 15, 0.1?mL PF-4136309 blood was sampled and assayed for CD45+CD33+ MOLM-13 tumor cells via flow cytometry. B-ALL, with low and limited expression on myeloid dendritic cells and HSCs. Methods We developed and characterized CLN-049, a T cell-activating bsAb targeting CD3 and FLT3, constructed as an IgG heavy chain/scFv fusion. CLN-049 binds the membrane proximal extracellular domain name of the FLT3 protein tyrosine kinase, which facilitates the targeting of leukemic blasts regardless of FLT3 mutational status. CLN-049 was evaluated for Mouse monoclonal antibody to Tubulin beta. Microtubules are cylindrical tubes of 20-25 nm in diameter. They are composed of protofilamentswhich are in turn composed of alpha- and beta-tubulin polymers. Each microtubule is polarized,at one end alpha-subunits are exposed (-) and at the other beta-subunits are exposed (+).Microtubules act as a scaffold to determine cell shape, and provide a backbone for cellorganelles and vesicles to move on, a process that requires motor proteins. The majormicrotubule motor proteins are kinesin, which generally moves towards the (+) end of themicrotubule, and dynein, which generally moves towards the (-) end. Microtubules also form thespindle fibers for separating chromosomes during mitosis preclinical safety and efficacy in vitro and in vivo. Results CLN-049 induced target-restricted activation of CD4+ and?CD8+ T cells. AML cell lines expressing a broad range of surface levels of FLT3 were efficiently lysed on treatment with subnanomolar concentrations of CLN-049, whereas FLT3-expressing hematopoietic progenitor cells and dendritic cells were not sensitive to CLN-049 killing. Treatment with CLN-049 also induced lysis of AML and B-ALL patient blasts by autologous T cells at the low effector-to-target ratios typically observed in patients with overt disease. Lysis of leukemic cells was not affected by supraphysiological levels of soluble FLT3 or FLT3 ligand. In mouse xenograft models, CLN-049 was highly active against human leukemic cell lines and patient-derived AML and B-ALL blasts. Conclusions CLN-049 has a favorable efficacy and safety profile in preclinical models, warranting evaluation of its antileukemic activity in the clinic. Keywords: immunity, immunotherapy, T-Lymphocytes Background Acute myeloid leukemia (AML) is the most frequent form of acute leukemia in adults, affecting 4C5 per 100?000 population.1 2 While several new treatments have recently been approved, including hypomethylating brokers, the antibody drug conjugate gemtuzumab ozogamicin and various small molecule kinase inhibitors,3 induction and consolidation chemotherapy followed by allogeneic hematopoietic stem cell transplantation (HSCT) remains the most widely used curative treatment approach. However, more than 50% of patients are not eligible for the intensive pre-treatment chemotherapy regimens. In addition, remission-inducing chemotherapy and HSCT are associated with substantial treatment-associated morbidity and mortality, and many patients experience a PF-4136309 subsequent relapse. Accordingly, prognosis of AML remains dismal with overall 5-year survival rates below 15%.4 Strategies to mobilize T cells against tumor cells via bispecific antibodies (bsAbs) or directly engineering T cells to express chimeric antigen receptors have achieved remarkable success in lymphoid malignancies, including acute lymphoid leukemia (B-ALL) and multiple myeloma.5C7 However, T cell engaging strategies for myeloid-derived neoplasias like AML are not yet clinically established. A major challenge in AML is usually target selection. Antibody-based therapeutics currently in development for AML focus primarily on CD33, CD123 and CLEC12A as target antigens.8 However, beyond their well-documented expression on malignant cells, these targets are also found at similar levels on normal myeloid cells and hematopoietic stem cells (HSCs), raising concerns regarding a narrow therapeutic window.9 CD33 is expressed on approximately 30% of healthy bone PF-4136309 marrow (BM) myeloid progenitors.10 11 CD123 is also expressed on many healthy cells such as myeloid progenitors, plasmacytoid dendritic cells (pDCs), monocytes, and basophils.12 Finally, CLEC12A is also expressed on normal myeloid cells including granulocytes, monocytes, macrophages, and DCs as well as granulocyte-macrophage progenitors.9 13 14 The receptor tyrosine kinase FLT3 is an attractive AML target with frequent expression on both blasts and leukemic stem cells, whereas expression on healthy myeloid cells is low and limited to myeloid DCs and HSCs. 15C17 FLT3 is usually a proto-oncogene that plays a key role in promoting leukemic cell proliferation and survival, and expression is usually preserved in AML after relapse.18 Its essential role in disease progression has been validated by the clinical benefit of tyrosine kinase inhibitors PF-4136309 that target mutant FLT3.19 20 Unlike kinase inhibitors that are specific to a particular FLT3 mutational context, antibody-based therapies that target the extracellular domain of FLT3 are independent of FLT3 mutations, allowing for treatment of a broader patient population. We recently developed an Fc-optimized monoclonal antibody (mAb) targeting FLT3, FLYSYN, which potently induces antibody-dependent cellular PF-4136309 cytotoxicity. FLYSYN showed promising safety and preliminary efficacy in AML patients with minimal residual.