Cell-dependent Cellular Cytotoxicity Research Articles

DR30318, a novel tri-specific T cell engager for Claudin 18.2 positive cancers immunotherapy

BackgroundClaudin 18.2 (CLDN18.2) is a highly anticipated target for solid tumor therapy, especially in advanced gastric carcinoma and pancreatic carcinoma. The T cell engager targeting CLDN18.2 represents a compelling strategy for enhancing anti-cancer efficacy.MethodsBased on the in-house screened anti-CLDN18.2 VHH, we have developed a novel tri-specific T cell engager targeting CLDN18.2 for gastric and pancreatic cancer immunotherapy. This tri-specific antibody was designed with binding to CLDN18.2, human serum albumin (HSA) and CD3 on T cells.ResultsThe DR30318 demonstrated binding affinity to CLDN18.2, HSA and CD3, and exhibited T cell-dependent cellular cytotoxicity (TDCC) activity in vitro. Pharmacokinetic analysis revealed a half-life of 22.2–28.6 h in rodents and 41.8 h in cynomolgus monkeys, respectively. The administration of DR30318 resulted in a slight increase in the levels of IL-6 and C-reactive protein (CRP) in cynomolgus monkeys. Furthermore, after incubation with human PBMCs and CLDN18.2 expressing cells, DR30318 induced TDCC activity and the production of interleukin-6 (IL-6) and interferon-gamma (IFN-γ). Notably, DR30318 demonstrated significant tumor suppression effects on gastric cancer xenograft models NUGC4/hCLDN18.2 and pancreatic cancer xenograft model BxPC3/hCLDN18.2 without affecting the body weight of mice.

Abstract 6348: Unveiling diverse trimeric forms of STEAP1 in human prostate cancer cells: Exploring STEAP1-STEAP2 interdependence for expression and plasma membrane localization

Abstract Six-Transmembrane Epithelial Antigen of the Prostate 1 (STEAP1) is an emerging therapeutic target that is a member of the STEAP1-4 family of metalloreductases. We recently reported the preclinical characterization of xaluritamig (AMG 509), a STEAP1 XmAb® 2+1 T-cell engager molecule that is being developed for the treatment of prostate cancer (Nolan-Stevaux et al, 2023). Xaluritamig is designed to preferentially target tumor cells that express high levels of STEAP1. However, prostate tumor cells also express high levels of STEAP2. Here, we characterize the interactions between STEAP1 and STEAP2 in prostate cancer cell lines and the impact to xaluritamig activity. Bulk mRNA as well as single-cell gene expression data (scRNAseq) reveal a strong correlation between STEAP1 and STEAP2 expression in human prostate cancer, suggesting that the STEAP proteins may form both homotrimers and heterotrimers in prostate tumors. To test this idea, we investigated the structural behavior of STEAP1/2 trimers in human prostate cancer cell lines that endogenously express STEAP1 and STEAP2. Using xaluritamig and the parental anti-STEAP1 antibody, we show that STEAP1 primarily forms heterotrimers with STEAP2 in prostate cancer cells but can also assemble as homotrimers in the absence of STEAP2. Both STEAP1/2 heterotrimers and STEAP1 homotrimers can be targeted by xaluritamig, leading to T cell-dependent cellular cytotoxicity. Similarly, we found that anti-STEAP2 antibodies detected STEAP2 homotrimers when STEAP1 expression was ablated in cells, and that both STEAP1/2 heterotrimers and STEAP2 homotrimers were targeted by a STEAP2-specific T-cell engager molecule. We also demonstrate that neither STEAP1 nor STEAP2 significantly influences the mRNA or protein expression of the other. Our data elucidates the versatility of STEAP family members in forming both homotrimers and heterotrimers on the plasma membrane of human prostate cancer cells and the potential for therapeutic targeting with STEAP1/2-directed therapies. Characterizing the capability of existing STEAP1 or STEAP2 targeting therapeutics to bind their respective targets, whether within a homotrimer or heterotrimer, is crucial for a comprehensive understanding of the therapeutic potential of these drugs. Further research will help delineate the mechanism underlying the formation of STEAP1/2 trimers in cancer cells and continue advancing our understanding of this intriguing biological relationship. Citation Format: Cong Li, Lingming Liang, Joonhee Park, Olga Pryshchep, Binnaz Staley, Peng Li, Ben Alba, Xuan Liu, Fei Li, Gregory L. Moore, Matthew J. Bernett, Umesh Muchhal, John Desjarlais, Angela Coxon, Julie M. Bailis. Unveiling diverse trimeric forms of STEAP1 in human prostate cancer cells: Exploring STEAP1-STEAP2 interdependence for expression and plasma membrane localization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6348.

Abstract 2372: ATG-102, a novel LILRB4 x CD3 T cell engager, targeting two nonoverlapping epitopes of LILRB4, for the treatment of monocytic AML

Abstract Background: Acute myeloid leukemia (AML) is the most common acute leukemia in adults, and the treatment of AML, especially monocytic AML subtypes still has a poor outcome. T cell engagers (TCE) have been well used for the treatment of hematological malignancies, such as B cell leukemia and multiple myeloma. However, lack of ideal target antigens that only express on AML and leukemic stem cells (LSCs) but not on normal hematopoietic stem cells (HSCs) hampers the development of TCE therapy for AML. Leukocyte immunoglobulin-like receptor B4 (LILRB4), an inhibitory receptor belonging to the LILR family, is highly and restrictedly expressed on monocytic M4/M5 AML subtypes and LSCs, but not on normal HSCs which makes it an ideal target for TCE. Here we report a novel LILRB4 x CD3 bispecific T cell engager based on AnTenGagerTM platform, ATG-102. It targets two distinct epitopes of LILRB4, inducing potent T cell-dependent cellular cytotoxicity (TDCC) with low CRS, resulting in a potent anti-tumor efficacy in vitro and in vivo. Method: ATG-102 was constructed by introducing a high affinity anti-CD3 single chain fragment variable (scFv) to the hinge region of one of the heavy chains of a LILRB4 monoclonal antibody. A scFv targeting another epitope of LILRB4 was connected to the C terminal of the same heavy chain by GSSS linker, enabling a trivalent, dual epitope recognition of LILRB4. ATG-102 was evaluated in a series of preclinical studies for binding epitope and affinity, T cell activation, T cell dependent cytotoxicity (TDCC), and cytokine release. The in vivo antitumor efficacy of ATG-102 was evaluated in a humanized PBMCs and THP-1-luciferase cells engrafted NCG mice model. Results: ATG-102 binds to LILRB4 positive cells with a single-digit nM affinity. It binds to two distinct and nonoverlapping epitopes of LILRB4. ATG-102 showed limited binding capability to CD3+ cells before LILRB4 crosslinking. However, with the presence of LILRB4 positive THP-1 cell, ATG-102 strongly activated primary T cells, upregulating early and later markers of T cell activation, CD69 and CD25, respectively. ATG-102 induced strong TDCC against LILRB4-high expressing THP-1 cells, and it induced more potent TDCC against LILRB4-low expressing MOLM-13 cells than a benchmark antibody. While compared with benchmark, ATG-102 induced significantly lower release of IL-6, which is one of the major cytokines involved in cytokine release syndrome (CRS). In addition, ATG-102 demonstrated more potent in vivo anti-tumor efficacy compared with the benchmark antibody at 1 mg/kg and 0.1 mg/kg dose level in PBMC-humanized NCG mice bearing THP-1-luciferase cells. Conclusion: ATG-102 demonstrates trivalent, dual epitope recognition of LILRB4, conditionally redirects and activates T cells to recognize and kill monocytic AML cells. It demonstrates potent in vitro and in vivo anti-tumor efficacy with low risk of CRS. Citation Format: Ao Sun, Mengshi Gao, Rong Guo, Huiling Liu, Zaoshun Hu, Enlin Zheng, Hui Yuwen, Peng Chen, Jay Mei, Bo Shan, Bing Hou. ATG-102, a novel LILRB4 x CD3 T cell engager, targeting two nonoverlapping epitopes of LILRB4, for the treatment of monocytic AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2372.

Abstract 1237: IGM-2644, a CD38xCD3 bispecific IgM T cell engager, shows enhanced anti-tumor activity compared to daratumumab in preclinical models of multiple myeloma

Abstract Multiple myeloma (MM) is a hematologic malignancy characterized by the accumulation of monoclonal plasma cells in the bone marrow. CD38 is an attractive therapeutic target for MM due to its high expression on plasma cells, but it is also expressed on multiple subsets of normal immune cells. Approved therapies including anti-CD38 monoclonal antibodies such as daratumumab are available, however resistance inevitably develops, and toxicities such as reduced immune cell counts put patients at increased risk for infections. To address this and other issues associated with current therapies, we are developing IGM-2644, an engineered high affinity, high avidity bispecific anti-CD38xCD3 IgM T cell engager (TCE) antibody for the treatment of plasma cell disorders. IGM-2644 kills CD38-expressing tumor cells through complement-dependent cytotoxicity (CDC) and T cell-dependent cellular cytotoxicity (TDCC). Live cell imaging of tumor cells in the presence of normal human serum as a source of complement has shown that IGM-2644, compared to daratumumab, has exhibited superior CDC with faster kinetics and higher maximal killing, as well as increased activity on tumor cell lines that express low levels of CD38. IGM-2644 also has displayed enhanced cellular dependent cytotoxicity over daratumumab across tumor cell lines with a range of CD38 expression levels in vitro. In ex vivo assays with MM patient bone marrow mononuclear cells (BMMC), IGM-2644 has shown superior tumor cell depletion compared to daratumumab even with low T cell to tumor cell ratios. Furthermore, IGM-2644 can deplete tumor cells in BMMC from daratumumab-refractory MM patients. Similarly, using in vivo humanized xenograft tumor mouse models with various levels of CD38 expression, greater anti-tumor activity with IGM-2644 over daratumumab was observed. When compared to daratumumab, IGM-2644 demonstrated lower immune cell fratricide in an in vivo xenograft tumor mouse model, suggesting a potentially differentiated safety profile. In summary, IGM-2644 is a bispecific anti-CD38xCD3 IgM TCE antibody that demonstrates superior tumor cell killing when compared to daratumumab in vitro and especially in xenograft tumor models in vivo with low CD38 expression while having low immune cell depletion, potentially reducing the risk of infections. The safety and efficacy of IGM-2644 has been initially studied in a phase 1 clinical trial for relapsed/refractory (r/r) MM. Applicability of IGM-2644 for the treatment of other plasma cell disorders is currently being explored, including autoimmune diseases. Citation Format: Kevin C. Hart, Daniel Santos, Keyu Li, Rui Yun, Min Chai, Andras Kabai, Gene Li, Poonam Yakkundi, Roel P. Funke, Yinghui Guan, Thomas Manley, Liqin Liu, Angus M. Sinclair, Albert F. Candia, Bruce Keyt, Maya F. Kotturi. IGM-2644, a CD38xCD3 bispecific IgM T cell engager, shows enhanced anti-tumor activity compared to daratumumab in preclinical models of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1237.

AML Immunotherapy Using a Novel Tcrm-Based Bispecific Antibody That Targets a Leader Sequence Peptide Derived from Cathepsin G

Introduction: Myeloid azurophil granules provide a rich source of intracellular leukemia antigens. Cathepsin G (CG) is a serine protease that has higher expression in AML blasts in comparison to normal myeloid progenitors. Based on the unique biology of HLA-A*0201 (HLA-A2), in which presentation of leader sequence (LS)-derived peptides is favored, we focused on the LS-CG-derived peptide CG1 (FLLPTGAEA). LS peptides are naturally processed and loaded on HLA-A2, providing an abundant source of surface peptide/HLA (pHLA) targets. We eluted CG1 from the surface of primary HLA-A2 + AML blasts and AML cell lines, and demonstrated CG1-targeting immunity in leukemia patients following allogeneic stem cell transplant, hence highlighting the potential for CG1 to be a promising immunotherapeutic target in AML. Numerous platforms have been developed to target cell surface pHLA complexes. T cell receptor (TCR) mimic (m) antibodies are immunotherapeutic antibodies that target pHLA, which are the natural ligands for the TCR. TCRm antibodies have been a breakthrough in immunotherapy, as they: (1) target intracellular antigens expressed on HLA; (2) are engineered to have high affinity for HLA; (3) can elicit their cytotoxic effect independent of the TCR; and (4) can be produced in large quantities providing an off-the shelf-product for patient use. Here we report on the engineering, preclinical efficacy, and safety evaluation of a novel CG1-targeting, T cell-engager, bispecific antibody (CG1xCD3e), which incorporates a novel TCRm construct that recognizes cell surface CG1/HLA-A2 complexes. Methods: To generate the CG1/HLA-A2-binding monoclonal antibodies, H2L2 human transgenic mice were immunized with CG1/HLA-A2 monomers subcutaneously. The resulting paired heavy and light chains were cloned from the antigen-sorted memory B cells into the single pcDNA3.1(+) vector with both human (h) immunoglobulin (Ig)G1 and hKappa for subsequent ExpiCHO high-throughput transient expression. Antibody binding was assessed by Bio-Layer Interferometry (BLI) and T2 cellular binding assays. Xencor anti-CD3e scFv and Knobs-in-holes technology was applied for the bispecific antibody engineering for xMab format. In vitro activity of CG1xCD3e and T cell cytokine secretion was confirmed using flow cytometry-based T cell-dependent cellular cytotoxicity (TDCC). The specificity of CG1xCD3e for leukemia was assessed using colony forming unit assays. In vivo efficacy was evaluated in NSG mice engrafted with AML cell lines and treated with CG1xCD3e with normal donor peripheral blood mononuclear cells (ND-PBMC). Results: Using BLI, binding avidity of the CG1xCD3e to CD3-Fc, KD= 7.52 x 10-10 M, and affinity to CG1/HLA-A2, KD= 1.28 x 10-10 M were calculated. Using flow cytometry, we confirmed high affinity binding of CG1xCD3e to CG1/HLA on the CG1- expressing EM2 HLA-A2 + cell line, and also demonstrated binding of CG1xCD3e to CD3 using jurkat T cell line. After co-culturing GFP/Luciferase + HLA-A2 + AML (U937-A2 and ML-2) or CML (EM2) cell lines with ND-PBMC and different concentrations of CG1xCD3e bispecific antibody for 24 or 48 hours, flow cytometry analysis demonstrated highly specific CG1/HLA-A2 dependent killing of AML cells by T cells cultured with CG1xCD3e bispecific antibody, in comparison with control bispecific antibody with a disabled CG1 binding arm. This correlated with both tumor- and bispecific antibody- dependent T cell activation and cytokine secretion. To study the in vivo anti-leukemia activity of CG1xCD3e, we engrafted NSG mice with the HLA-A2 + ML-2 and U937-A2 AML cell lines, and treated mice after tumor engraftment with ND-PBMC (1 x 107 cells) and weekly intraperitoneal injections of CG1xCD3e bispecific antibody (0.01 mg/Kg, 0.05 mg/Kg, and 0.1 mg/Kg) or PBS. Bioluminescence imaging demonstrated that the mice treated with ND-PBMC + CG1xCD3e had significantly lower levels of leukemia burden compared to the mice treated with ND-PBMC alone (Figure 1). Lastly CFU assays using HLA-A2 + normal donor marrow in a semi-solid matrix of methylcellulose in the presence of T cells and either CG1xCD3e bispecific antibody or isotype antibody, confirmed specificity of CG1xCD3e bispecific antibody for leukemia. Conclusion: Our study provides strong pre-clinical evidence supporting the targeting of LS- derived peptides, specifically CG1, in the setting of AML using a novel TCRm-based bispecific antibody.

Abstract 2959: Novel CD38xCD3 bispecific IgM T cell engager, IGM-2644, potently kills multiple myeloma cells though complement and T cell dependent mechanisms

Abstract Multiple myeloma (MM), a cancer of plasma cells, occurs in ~34,000 new patients every year in the USA. Although therapeutic regimens, including anti-CD38 monospecific IgG antibodies such as daratumumab and isatuximab in combination with chemotherapies, demonstrate clinical efficacy, most patients eventually develop resistance. Several bispecific (CD38xCD3) or trispecific (CD38xCD3xCD28) T cell engager (TCE) antibody therapies are currently in development to improve upon the efficacy of CD38 targeted therapies by leveraging T cell dependent cellular cytotoxicity (TDCC) of MM cells. However, CD38 is also expressed on some normal hematopoietic cells which could potentially lead to undesired pharmacological activity such as depleting CD38+ immune cells, including activated cytotoxic T cells (i.e., fratricide). IGM-2644 is a novel, pentameric IgM antibody engineered to have ten CD38 binding sites, and an anti-CD3ε single chain Fv domain fused to a joining chain to engage CD3 on T cells, and retains the potential for complement-dependent cytotoxicity (CDC). Here we report the functional characterization of IGM-2644 using in vitro, ex vivo and in vivo anti-tumor efficacy studies with a safety evaluation of this novel IgM TCE. In vitro, IGM-2644 demonstrated significantly improved CDC activity in comparison with daratumumab and isatuximab, with >30-fold increased potency on CD38+ MM and lymphoma cell lines. IGM-2644 induced TDCC similar to a bispecific CD38xCD3 IgG on low CD38 expressing cell lines resistant to daratumumab, while demonstrating significantly lower levels of cytokine release than the bispecific IgG. In ex vivo colony forming unit (CFU) assays, IGM-2644 was able to reduce MM CFUs using primary MM patient bone marrow samples containing autologous T cells and myeloma cells, while no effect was observed on erythroid, granulocyte and macrophage CFUs in normal bone marrow samples.​ IGM-2644 dose dependently inhibited tumor growth in humanized xenograft models of CD38+ NCI-H929 (myeloma) and Raji (lymphoma). Importantly, IGM-2644 also demonstrated significantly reduced T cell fratricide compared to bispecific IgGs both ex vivo and in vivo. Significantly reduced activity of IGM-2644 on normal CD38+ innate immune cells was observed in ex vivo studies. CD38 expression has also been reported on human RBCs and platelets. However, minimal IGM-2644 binding was observed and at levels lower than daratumumab. In summary, IGM-2644 is a novel, potent, bispecific IgM TCE that has both CDC and TDCC mechanisms of cytotoxicity with the potential to be active in daratumumab resistant tumors. The balance of potent TDCC and CDC cytotoxic activity, along with an improved preclinical safety profile compared to other CD38xCD3 bispecific IgG TCEs, supports the clinical development of IGM-2644 in MM. Citation Format: Keyu Li, Rui Yun, Min Chai, Poonam Yakkundi, Rodnie Rosete, Gene Li, Liqin Liu, Mandy Li, Daniel Santos, Kevin C. Hart, Dean Ng, Paul R. Hinton, Umesh Muchhal, Thomas Manley, Maya F. Kotturi, Stephen F. Carroll, Angus M. Sinclair, Bruce A. Keyt. Novel CD38xCD3 bispecific IgM T cell engager, IGM-2644, potently kills multiple myeloma cells though complement and T cell dependent mechanisms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2959.

Igm-2644, a Novel CD38xCD3 Bispecific IgM T Cell Engager Demonstrates Potent Efficacy on Myeloma Cells with an Improved Preclinical Safety Profile

Multiple myeloma (MM) is a cancer of plasma cells affecting > 30,000 new patients every year in the United States. Despite improved treatment options, including anti-CD38 monospecific IgG antibodies such as daratumumab and isatuximab in combination with chemotherapies, resistance eventually develops in most patients. Several bispecific (CD38xCD3) or trispecific (CD38xCD3xCD28) T cell engager antibody therapies are currently in development to further improve upon the efficacy of CD38 targeted therapies by leveraging T cell dependent cellular cytotoxicity (TDCC) of MM cells. However, a unique challenge for anti-CD38 T cell engagers is to potently kill myeloma cells without depleting CD38+ immune cells, including activated cytotoxic T cells (i.e., fratricide), while also avoiding dangerous cytokine release syndrome associated with T cell engaging therapies. Encouraged by the favorable safety and tolerability profile of imvotamab (IGM-2323), our CD20xCD3 IgM T cell engager, we have developed a novel CD38xCD3 bispecific IgM T cell engager, IGM-2644. It has 10 binding sites for human CD38, and a single anti-CD3 scFv fused to the joining (J) chain. Here, we have evaluated the potency and potential safety profile of IGM-2644 in a panel of in vitro immunoassays and in vivo models. IGM-2644 demonstrated significantly improved complement-dependent cytotoxicity (CDC) activity compared to daratumumab and isatuximab, with over 30-fold increased potency on MM cell lines, and depletion of these cells was further enhanced when PBMCs were used as effector cells. IGM-2644 was able to achieve TDCC activities similar to bispecific CD38xCD3 IgGs on low CD38 expressing cell lines that are resistant to daratumumab, while showing much lower levels of cytokine release than bispecific IgGs. In a short-term in vitro colony forming unit (CFU) assay, IGM-2644 was able to reduce MM colonies using primary MM patient bone marrow samples containing autologous T cells and myeloma cells, while no effect was observed on the colony formation of erythroid, granulocyte and macrophage using bone marrow samples from normal donors. IGM-2644 was able to show in vivo efficacy and suppress CD38+ NCI-H929 (myeloma) and Raji (lymphoma) xenograft tumor growth in humanized mouse models. Importantly, IGM-2644 also demonstrated reduced T cell fratricide compared to bispecific IgGs both in vitro and in vivo. Taken together, our data demonstrates IGM-2644 is a potent molecule with both CDC and TDCC activities, and the potential to be active in daratumumab resistant tumors. It has an improved preclinical safety profile compared to other CD38xCD3 bispecific T cell engagers due to lower cytokine release and reduced T cell fratricide. A Phase I clinical study evaluating the activity and safety of IGM-2644 is planned.

Preclinical characterization of a Fab-like CD3/CLDN18.2 XFab® bispecific antibody against solid tumors

The claudin 18.2(CLDN18.2) antigen is highly expressed in gastric mucosa epithelial cells and frequently expressed in malignant tumors. Positive clinical outcomes have popularized claudin 18.2 as a novel cellular and antibody therapeutic. Here, we designed a bispecific antibody-ZWB67 using the XFab® platform, aimed at redirecting CD3+ effector T cells to CLDN18.2+ target cells or tissues. Physicochemical characterization, binding properties, T cell stimulatory activity, and T cell-dependent cellular cytotoxicity of ZWB67 were evaluated in dosage intervals using antigens of CD3 and target cells expressing CLDN18.2 or CD3. Then, the anti-tumor activity was assessed in humanized CD3EDG mice bearing MC-38-hCLDN18.2 tumors. Our data demonstrate that ZWB67 specifically binds to the human CD3e antigen (KD = 1.04E−08 M) and binds more strongly to CLDN18.2+ cells than to CD3+ cells (4.3- to 9.2-fold difference). ZWB67 showed good activity in the luciferase reporter system and exhibited dose-dependent activation, cytotoxicity of T cells, and cytokine release when co-cultured with CLDN18.2+ cells and CD3+ T cells. ZWB67 also exhibited high in vivo efficacy in the MC-38-hCLDN18.2 xenograft mouse model. In conclusion, the novel anti-CLDN18.2 × anti-CD3 bispecific antibody exhibited low affinity for anti-CD3, highly specific binding, potent cytotoxicity, and anti-tumor activity. These data provide a basis for future preclinical and clinical development of this therapeutic strategy.

Target cell line characterization reveals changes in expression of a key antigen that impacts T cell dependent cellular cytotoxicity assay performance

Cell lines are important tools regularly used for in-vitro potency assays employed in Good Manufacturing Process (GMP) lot release and stability testing of different therapeutic modalities. Characterization of these cell lines is key to understanding their performance.Bispecific T cell engager (BiTE®) molecules are an exciting modality in Amgen's therapeutic product portfolio. BiTE® molecules are engineered as two linked, single chain antibody domains. One domain targets the cluster of differentiation 3 (CD3) domain of T cells, and the other domain targets a specific antigen. The mechanism of action of the BiTE® molecule is to bring T cells into close proximity of tumor cells to facilitate tumor cell killing. One BiTE® molecule in development, AMG 757, is directed against delta-like ligand 3 (DLL3), which is expressed in small cell lung cancer tumor cells. AMG 757 is a half-life extended bispecific T cell engager (HLE BiTE®) construct. The bioassay employed to demonstrate the mechanism of action of AMG 757 is a T cell dependent cellular cytotoxicity (TDCC) cell-based assay requiring two cell lines, the effector T cell line HuT-78, and engineered tumor target cells, SHP-77-Luc. During the course of development of this bioassay, characterization of the SHP-77-Luc line showed an increase in the luminescence assay signal and Maximum-to-Minimum (Max-to-Min) ratio of the dose response curve as the passage number of the cells increased. Our research revealed an increase in not only luciferase expression but also an increase in the cell surface and intracellular expression levels of DLL3 over time in culture, which ultimately resulted in the increased assay signal window.

Abstract 4179: High valency of IGM-2323, a CD20xCD3 IgM bispecific T cell engager, displaces rituximab binding and induces potent B lymphoma cell killing

Abstract Rituximab-containing treatment regimens are the standard of care for patients with non-Hodgkin’s lymphoma (NHL). However, the majority of patients ultimately experience disease relapse or progression indicating resistance to rituximab therapy. IGM-2323 is an engineered high-affinity, high-avidity anti-CD20 pentameric IgM antibody with an anti-CD3 scFv fused to the joining chain. IGM-2323 offers a novel treatment strategy in NHL through two mechanisms: 1) the recruitment of T cells to kill CD20-expressing tumor cells through T cell dependent cellular cytotoxicity (TDCC) and, 2) complement-dependent cytotoxicity (CDC). We evaluated the activity of IGM-2323 in the presence of rituximab since rituximab can persist in patients after treatment discontinuation, and it can bind to an overlapping epitope on CD20 as IGM-2323. We hypothesized that the high valency of IGM-2323 could displace rituximab, thus enabling potent B cell killing by IGM-2323 even in the presence of high concentrations of rituximab. The affinity of IGM-2323 and its corresponding bivalent anti-CD20 IgG antibody to recombinant human CD20 protein were measured by surface plasmon resonance. IGM-2323 bound to human CD20 with an apparent 300-fold higher binding affinity (KD) and ~100-fold slower off-rate (kdis) than the bivalent anti-CD20 IgG. Human B cell lines with a range of CD20 expression levels, including a CD20-low rituximab-resistant Ramos cell variant, were pre-treated with escalating concentrations of rituximab, and subsequently evaluated in vitro for cell binding, TDCC, and CDC by IGM-2323. At high concentrations of rituximab, which correspond to reported peak serum concentrations (Cmax) found in rituximab-treated patients, IGM-2323 displaced the binding of rituximab on human B cell lines. In contrast, binding of a bispecific CD20xCD3 IgG was severely inhibited by the Cmax of rituximab. In TDCC assays with healthy donor effector T cells, pre-treatment with high concentrations of rituximab only modestly inhibited IGM-2323 activity. Furthermore, only a minor impact to the maximum killing activity (Emax) of IGM-2323 was observed at the Cmax of rituximab. In contrast, rituximab pre-treatment resulted in a distinctly lower Emax of a bispecific CD20xCD3 IgG. Live cell imaging of CDC kinetics was utilized to quantify the extent of CDC by IGM-2323 with or without rituximab. Pre-treatment with rituximab enhanced CDC of IGM-2323 compared to single agent activity. Our preclinical data indicate that IGM-2323 maintains activity in the presence of rituximab. IGM-2323 is currently being studied in a phase 1 clinical trial in relapsed/refractory NHL, where it has been generally well tolerated, with both complete and partial responses observed (NCT04082936). Clinical studies will continue to evaluate these findings, including the treatment of patients with circulating serum levels of rituximab. Citation Format: Kevin C. Hart, Kathryn Logronio, Mandy Li, Poonam Yakkundi, Marigold Manlusoc, Keyu Li, Paul R. Hinton, Dean Ng, Maya K. Leabman, Genevive Hernandez, Thomas Manley, Angus M. Sinclair, Stephen F. Carroll, Bruce A. Keyt, Maya F. Kotturi. High valency of IGM-2323, a CD20xCD3 IgM bispecific T cell engager, displaces rituximab binding and induces potent B lymphoma cell killing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4179.

Abstract 6251: Evaluation of a dual-targeting BCMA-CS1 HLE BiTE® molecule for multiple myeloma

Abstract Multiple myeloma (MM) is a cancer of the antibody-producing plasma cells (PC). MM is invariably fatal due to frequent disease relapse and/or treatment refractoriness, and therapies that provide deeper and more durable responses are needed. BiTE® (Bispecific T Cell Engager) molecules are immunotherapy agents that redirect a patient’s T cells to lyse tumor cells by simultaneously engaging a tumor associated antigen (TAA) on cancer cells and CD3ε on T cells. Clinical activity has been observed in MM using BiTE® molecules and other T cell engagers that target B-cell maturation antigen (BCMA). BCMA is an attractive MM TAA due to its broad prevalence and restricted normal tissue profile, mainly PCs and plasmablasts. However, heterogenous expression in MM cancer cells, potential antigen loss, and the presence of high levels of soluble BCMA in MM patient sera present challenges that may prevent BCMA-only-targeted therapies from achieving their full potential. To address these issues, we generated a BiTE® molecule capable of engaging BCMA and a second MM TAA, CS1 (also known as SLAMF7). This BiTE® molecule also contains an Fc-based domain to provide half-life extension (HLE). Here, we evaluated the in vitro and in vivo properties of this BCMA-CS1 HLE BiTE® molecule. The BCMA-CS1 HLE BiTE® molecule had nanomolar binding affinity for human BCMA, CS1, and CD3ε and the non-human primate (NHP) orthologues. In vitro, we observed picomolar activity against human MM cell lines with a range of BCMA and CS1 expression in a T cell dependent cellular cytotoxicity (TDCC) assay using human T cells or NHP peripheral blood mononuclear cells. The BCMA-CS1 HLE BiTE® molecule retained TDCC activity in the presence of soluble BCMA up to 2500 ng/mL as well as against human MM cells engineered to express only BCMA or CS1. In vivo, this BiTE® molecule inhibited tumor growth in a MM xenograft model. We also evaluated the BCMA-CS1 HLE BiTE® molecule in NHP over 15 days (IV dosing; intra-animal dose escalation from 60→240 μg/kg or 180→540 μg/kg on days 1 and 8). We observed hallmarks of BiTE® molecule activity in all groups, including transient decreases in circulating lymphocytes and moderate increases in cytokines like MCP-1. We measured the PC-specific transcripts BCMA and J-chain in NHP bone marrow and blood as surrogates for PC levels using ddPCR. These transcripts were reduced in both treatment groups (≥90%) with the strongest effects occurring in the 180→540 μg/kg group. Lastly, we showed that CS1 is highly and broadly expressed in MM patient samples and is restricted to a few normal hematopoietic cell types including PCs, NK cells, T cells, and some monocytes. These data suggest that the BCMA-CS1 HLE BiTE® molecule has potent in vitro and in vivo activity and may provide therapeutic benefit for MM patients by expanding the population of MM cancer cells that can be eliminated by a BiTE® molecule while overcoming common mechanisms that can impair BCMA-only-targeted MM therapies. Citation Format: Elizabeth T. Andrews, Stephanie C. Casey, Mohammad Farhad Amani, Grit Lorenczewski, Mozhgan Farshbaf, Lisa Winkel, Matthias Klinger, John M. Harrold, Famke Aeffner, Ana Goyos, Matthias Friedrich, Tara Arvedson, Matthew G. Chun. Evaluation of a dual-targeting BCMA-CS1 HLE BiTE® molecule for multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6251.

Abstract 2884: Optimized CD28 bispecific antibodies for targeted activation of T cells within the tumor microenvironment

Abstract Tumor-targeted CD28 bispecific antibodies (bsAbs) are designed to co-stimulate T cells specifically within the tumor microenvironment. By bridging T cells to malignant cells expressing a selected tumor-associated antigen (TAA), CD28 bsAbs deliver the so-called signal 2 to T cells unleashing their full cytotoxic potential. Contrary to bivalent CD28 monoclonal antibody (mAb) superagonists, the activation of CD28 with monovalent CD28 bispecifics requires the binding of a TAA expressed by the cancer cells to induce CD28 clustering. Preclinical studies have shown the benefit of adding costimulatory CD28 bsAbs for the treatment of solid tumors, boosting the efficacy of bispecific T cell engagers (CD3 bsAbs) or PD-(L)1 checkpoint inhibitors. Using our proprietary fully human κλ body antibody platform, which relies on a common heavy chain and on two distinct light chains driving specificity and affinity (i.e., one kappa and one lambda), we have identified a panel of agonist anti-CD28 kappa arms. Using our existing lambda anti-TAA arms (e.g., targeting CD19, MSLN, HER2, EGFR), an array of TAA-CD28 κλ bodies targeting different tumors were generated. The capacity of these CD28 κλ bodies to provide signal 2 to T cells was assessed both in vitro and in vivo in combination with CD3 bsAbs. In the presence of cancer cells and corresponding TAA-CD3 bsAbs, the CD28 κλ bodies enabled the activation of primary human CD4 and CD8 T cells, leading to T cell proliferation and secretion of cytokines such as IL-2 and IFNγ, and granzyme B. Importantly, when used alone or in combination with an untargeted CD3 bsAb (that is, in the absence of signal 1), CD28 κλ bodies showed no sign of T cell activation, potentially limiting peripheral toxicity. The resulting change in the T cell activation status led to a potent in vitro T cell-dependent cellular cytotoxicity (TDCC) against TAA-expressing cancer cell lines. CD28 κλ bodies targeting gastrointestinal tumors were tested in combination with CD3 bsAbs in xenogeneic tumor models and synergistically improved the antitumoral activity of T cell engagers, leading to an enhanced tumor control. Thus, via the identification of optimal agonist anti CD28 arms, CD28 bsAbs targeting multiple TAAs were designed for tumor-specific activation of the immune system. The resulting TAA-CD28 κλ bodies can enhance the antitumor response induced by CD3-retargeting bsAbs via the induction of T cell proliferation and activation, increased cytokine secretion and boosted anti-tumoral cytotoxicity. Other anti-TAA arms are being explored, to expand the panel of TAAs that could be easily paired with our anti-CD28 platform arms. Citation Format: Sara Majocchi, Pauline Lloveras, Coline Burnet-Merlin, Lise Nouveau, Nicolas Pleche, Pauline Malinge, Claudia Batista, Christelle Daviet, Maud Charreton, Guillemette Pontini, Franck Gueneau, Valery Moine, Giovanni Magistrelli, Limin Shang, Walter Ferlin, Krzysztof Masternak. Optimized CD28 bispecific antibodies for targeted activation of T cells within the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2884.

Abstract 2861: TriTAC-XR: An extended-release T cell engager platform designed to minimize cytokine release syndrome

Abstract CD3-targeted T cell engagers are potent anti-tumor therapies, but their development often requires management of cytokine release syndrome (CRS). One strategy to reduce CRS is subcutaneous dosing, which is hypothesized to mitigate CRS by reducing the maximum drug concentration (Cmax) and preserve efficacy by maintaining the same minimum drug concentration (Cmin) as intravenous dosing. Although promising for mitigating CRS, this approach is limited by its increased immunogenicity risks. A T cell engager designed to be dosed intravenously but engineered to mimic the PK properties of subcutaneous dosing could alleviate CRS without increasing immunogenicity. Here we describe TriTAC-XR, a platform of T cell engager prodrugs designed to become slowly activated in systemic circulation. This extended-release mechanism results in a slow build-up of circulating active drug and minimizes the Cmax/Cmin ratio, similar to subcutaneous dosing. TriTAC-XR prodrugs were engineered by adding a peptide mask and protease-cleavable linker to the N-terminus of a TriTAC, a constitutively active and half-life extended T cell engager. The mask binds to the anti-CD3ε domain and inhibits T cell binding. Upon cleavage by systemic proteases, active T cell engager is released. In vitro, TriTAC-XR has markedly reduced binding to recombinant CD3ε protein and to primary T cells as well as reduced potency in functional T cell-dependent cellular cytotoxicity (TDCC) assays compared to its unmasked active drug. In cynomolgus monkeys, TriTAC-XRs targeting multiple tumor antigens resulted in a gradual build-up of active drug during the first week post dose and significantly lower Cmax/Cmin ratios than comparable constitutively active TriTACs. Modeling based on these PK data predicts that TriTAC-XR dosed intravenously will result in a slower build-up of active drug and smaller Cmax/Cmin ratios than TriTAC dosed intravenously or subcutaneously. Cytokine release and target cell depletion in cynomolgus monkeys were used to compare the therapeutic index of TriTAC-XR to TriTAC. A single dose of FLT3-targeting TriTAC-XR resulted in 100-fold protection in cytokine release and similar FLT3 expressing cell depletion when compared to an equivalent FLT3-targeting TriTAC. Similarly, repeat dosing of a TriTAC-XR targeting B cells resulted in complete B cell depletion with substantially lower cytokines than a comparable TriTAC. TriTAC-XR is expected to improve the safety of T cell engagers by reducing CRS and may increase clinical dosing convenience by enabling higher doses that will extend dosing intervals. Citation Format: Kathryn Strobel Kwant, Sony S. Rocha, Timothy Yu, Katrina Stephenson, Raphaela Rose Banzon, Sydney Vollhardt, Golzar Hemmati, Evan Callihan, Wade H. Aaron, Subramanian Thothathri, Jessica O'Rear, Eric Bragg, Willis Kwong, Hubert Situ, Avneel Hundal, Stephen Yu, Taggra Jackson, Kevin Carlin, Yinghua Xiao, Maria Rosalyn Dayao, Linh To, Nick Bergo, Kevin Wright, Richard Austin, Holger Wesche, Bryan Lemon, S. Jack Lin. TriTAC-XR: An extended-release T cell engager platform designed to minimize cytokine release syndrome [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2861.

CRISPR/Cas9 disruption of EpCAM Exon 2 results in cell-surface expression of a truncated protein targeted by an EpCAM specific T cell engager

CRISPR/Cas9 gene-editing technology allows researchers to study protein function by specifically introducing double-stranded breaks in the gene of interest then analyze its subsequent loss in sensitive biological assays. To help characterize one of a series of highly potent, conditionally active, T cell engaging bispecific molecules called COBRA™, the human EpCAM gene was disrupted in HT29 cells using CRISPR/Cas9 and guide RNA targeting its Exon 2. Although a commercially available antibody indicated loss of cell-surface expression, the EpCAM targeting bispecific COBRA was still able to lyse these cells in a T cell dependent cellular cytotoxicity assay. RT-PCR sequence analysis of these cells showed a major alternative transcript generated after CRISPR/Cas9, with Exon 1 and 3 spliced together in-frame, skipping Exon 2 completely, to express a truncated cell-surface receptor recognized by the EpCAM-COBRA. Researchers who use CRISPR/Cas9 must be cognizant of this potential to express alternative versions of their proteins and use sensitive orthogonal detection methods to ensure complete gene disruption.

The Effects of BCMA Expression, Soluble BCMA, and Combination Therapeutics on the Anti-Tumor Activity of HPN217, a BCMA-Targeting Tri-Specific T Cell Engager Against Multiple Myeloma

IntroductionB-cell maturation antigen (BCMA) is a cell surface receptor highly and selectively expressed on normal plasma cells and transformed plasma cells in multiple myeloma (MM) patients. Upon ligand binding, BCMA initiates signals that promote the survival of MM cells and the production of immunosuppressive factors. Therapeutics that target BCMA are being investigated in the clinic, with encouraging preliminary results. HPN217 is a Tri-specific T Cell-Activating Construct (TriTAC) specific to BCMA, to serum albumin for half-life extension, and to CD3ε for redirecting T cells against MM cells. It is currently being evaluated in a phase 1 /2 clinical trial for relapsed or refractory MM (NCT04184050). Herein, we describe translational studies to examine factors that may impact the therapeutic efficacy of HPN217, including the target BCMA, in membrane-bound or soluble form, and concomitant or combination therapeutics such as γ-secretase inhibitor (GSI) and dexamethasone.ResultsTo evaluate the effects of HPN217 against primary MM cells, we used a patient-derived 3D-culture system (3DTEBM) designed to recapitulate the biology within the bone marrow microenvironment. 3DTEBM seeded with bone marrow accessory cells and autologous plasma recreate niches along an oxygen gradient that enable the survival and expansion of autologous MM cells without additional nutrient supplements. 3DTEBM's were established from 5 MM patients with varying ratios of autologous CD3+ T cells to MM cells (0.15-0.6). Although the functional competence of the T cells was unknown, HPN217 was able to mediate MM cell killing in 80% of the cultures with up to 71% of MM cells eliminated at a T cell/MM cell ratio of 0.45. The anti-tumor efficacy of HPN217 correlated strongly (R 2 = 0.99) with BCMA expression on the MM cells as measured by flow cytometry, suggesting the number of target receptors can be a limiting factor in efficacy. Consistent with this result, pre-incubation of target cells with 1 or 10 μg/mL anti-BCMA reduced the activity of HPN217 in T cell-dependent cellular cytotoxicity (TDCC) assays using healthy donor T cells and MM cell lines.Soluble BCMA (sBCMA) is produced when the extracellular domain of BCMA is cleaved by γ-secretase. It may act as a sink for HPN217. There was no correlation between the activity of HPN217 and the quantity of sBCMA in 3DTEBM. However, in TDCC assays, the addition of 6.25, 25 and 100 nM recombinant BCMA respectively led to 4-, 9- and 28-fold increases in the EC 50 of HPN217. Taken together, these data underscore the importance of preserving BCMA on MM cells and reducing sBCMA in circulation. Interestingly, treatment of MM cell line RPMI8226 with the GSI LY-3039478 for 24 hours increased the cell surface expression of BCMA by 3.6 folds. Using RPMI8226 as target cells in the 3DTEBM system, LY-3039478 increased the killing efficacy of HPN217-redirected primary T cells by 1.9 folds.Dexamethasone (Dex) is used with other therapeutics for treating MM. It is also commonly given to manage cytokine release syndrome (CRS) caused by T cell engagers. We conducted TDCC assays in the presence of 0.07-300 nM Dex to simulate plasma concentrations relevant to dose levels of Dex premedication for CRS. The highest Dex concentrations caused ≤3-fold increases in the EC 50 of HPN217. Considering this and the plasma half-life of i.v. injected Dex at <5 h, the suppressive effect of Dex on the anti-tumor activity of HPN217-redirected T cells may be limited.We then evaluated if MM.1S-Luc cell line xenografts in NCG mice would be a suitable model to extend the above in vitro findings to an in vivo setting. Lesions in the spine, skull and femur in NCG mice treated with vehicle could be detected by bioluminescent imaging. All mice succumbed to the disease within 40 days. By contrast, animals treated with HPN217 were protected in a dose-dependent manner. Mice that received the highest dose remained 100% disease-free at the end of the study (Figure 1).ConclusionsWe demonstrated HPN217 mediated BCMA-dependent primary MM cell killing by autologous T cells, and that the density of BCMA target on the surface of MM cells and sBCMA affected the efficacy of HPN217 in cultures. GSI, which increased the expression of BCMA on MM cells, enhanced the efficacy of HPN217. On the other hand, Dex had limited negative effect. HPN217 in combination with approved and experimental MM therapeutics is being evaluated in the 3DTEBM and MM.1S-Luc models. [Display omitted] DisclosuresNg: Harpoon Therapeutics: Current Employment, Current equity holder in publicly-traded company. Aaron: Harpoon Therapeutics: Current Employment, Current equity holder in publicly-traded company. Callihan: Harpoon Therapeutics: Current Employment, Current equity holder in publicly-traded company. Hemmati: Harpoon Therapeutics: Current Employment, Current equity holder in publicly-traded company. Law: Harpoon Therapeutics: Current Employment, Current equity holder in publicly-traded company. Azab: Cellatrix, LLC: Current Employment, Current holder of individual stocks in a privately-held company. Sun: Harpoon Therapeutics: Consultancy, Current equity holder in publicly-traded company, Ended employment in the past 24 months.

Abstract 1860: Bispecific claudin-6 x CD3 antibodies in a 2 + 1 format demonstrate selectivity and activity on human ovarian cancer cells

Abstract There is a large unmet need for targeted therapies to treat ovarian cancer and other solid tumors. A promising strategy is the use of T cell engaging bispecific antibodies that recruit T cells to kill cancer cells by simultaneously binding to tumor-associated antigens (TAA) on cancer cells and CD3 on T cells. Effective and safe use of these therapeutics depends on selective targeting of cancer cells over normal tissue, feasible when a TAA is more highly expressed on cancer cells versus normal tissues. Selectivity can also be improved by using a mixed valency “2 + 1” bispecific format, coupled with target affinity tuning, to achieve avid binding to the TAA. Claudin-6 (CLDN6) is a tetraspan membrane protein, a member of the claudin family of tight junction proteins, and has been identified as a promising TAA for ovarian cancer due to its high expression on ovarian and other cancer tissues and low expression on normal tissue. However, a complicating factor is that many proteins in the claudin family have high sequence identity. Most similar to CLDN6 is CLDN9, and the two proteins differ at only 3/76 residues in their extracellular loops, creating a challenge for the development of highly selective CLDN6 antibodies. We analyzed CLDN6 and CLDN9 expression using a combination of normal tissue gene expression data (GTEx), cancer cell genomics data (TCGA), and immunohistochemistry (IHC) on a panel of cancer and normal tissues. Data confirmed the tumor-specific expression pattern of CLDN6 but indicated high CLDN9 expression in normal tissues, suggesting that strong antibody selectivity for CLDN6 versus CLDN9 is critical. We created CLDN6-selective T cell engaging bispecific antibodies by starting with a highly selective monoclonal antibody humanized from a mouse hybridoma. Selectivity was further improved by engineering the CDR regions. Variant antibodies were screened for binding to transiently transfected HEK293E cells that highly expressed either CLDN6, CLDN9, or other homologous claudin family members. Selective variants were converted into the XmAb® 2 + 1 bispecific antibody format and tested in T cell-dependent cellular cytotoxicity (TDCC) assays using cancer cell lines expressing CLDN6, or HEK293E cells transfected with claudin homologs. A set of lead CLDN6 x CD3 bispecifics displaying a range of potencies on CLDN6+ cancer cell lines and high selectivity for CLDN6 over CLDN9 and other homologous claudins was identified. Tolerability and pharmacokinetics of these molecules are currently being assessed in non-human primates, and activity will be evaluated in humanized mouse models of ovarian cancer. Citation Format: Matthew S. Faber, Sung-Hyung Lee, Yoon Kyung Kim, Jing Qi, Kendra N. Avery, Duc-Hanh T. Nguyen, Rumana Rashid, Araz Eivazi, Seung Y. Chu, Juan E. Diaz, Connie Ardila, Ruschelle Love, Alex Nisthal, Norman J. Barlow, Christine Bonzon, Umesh S. Muchhal, Matthew J. Bernett, John R. Desjarlais. Bispecific claudin-6 x CD3 antibodies in a 2 + 1 format demonstrate selectivity and activity on human ovarian cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1860.

Abstract 1831: Affinity tuned XmAb®2+1 GPC3 x CD3 bispecific antibodies demonstrate selective activity in liver cancer models

Abstract Bispecific T cell engagers simultaneously bind CD3 on T cells and tumor-associated antigens to promote T cell-mediated killing of tumor cells. These agents provide synthetic immunity by expanding, activating, and redirecting T cells against a target of interest. While targeting lineage-restricted antigens such as CD19 or CD20 have found clinical success in hematopoietic cancers, targeting solid tumors requires high expressing tumor associated antigens (TAAs) with minimal normal tissue expression. Glypican 3 (GPC3), a lipid-anchored cell surface protein, is over-expressed in the majority of hepatocellular carcinoma (HCC) and a smaller subset of lung squamous cell carcinoma. It is considered an ideal target because its expression is not detected in adult tissues, and its function can promote tumor growth through the Wnt/beta-catenin pathway. Although GPC3 demonstrates good differential expression, bispecific T cell engagers are powerful immunomodulatory agents and careful tuning can improve the therapeutic window on all targets. Building upon the XmAb® heterodimeric Fc platform, we generated bispecific antibodies in an XmAb 2+1 Fab2-scFv-Fc format that are bivalent for GPC3 and monovalent for CD3. Reducing the affinity of GPC3 encouraged avid binding and strong killing activity only on high GPC3 expressing cell lines while minimizing reactivity on low expressing cell lines. We found modulating the CD3 affinity, either directly through mutation or indirectly by the molecular format, contributes to the selectivity of the bispecific antibodies. To ensure biologically valid antigen densities were being considered, IHC was conducted on paraffin embedded arrays of tumor tissue, normal tissue, and cell lines. Upon matching the staining intensity between the three sample types, we identified cell lines with corresponding GPC3 antigen density that could serve as proxies of tumor and normal tissue. In vitro T cell-dependent cellular cytotoxicity (TDCC) assays on the relevant cell lines confirmed selective potency on high versus low expressing cells. Citation Format: Alex Nisthal, Nargess Hassanzadeh-Kiabi, Kendra N. Avery, Rumana Rashid, Juan E. Diaz, Umesh S. Muchhal, Seung Y. Chu, Gregory L. Moore, Katrina Bykova, John R. Desjarlais. Affinity tuned XmAb®2+1 GPC3 x CD3 bispecific antibodies demonstrate selective activity in liver cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1831.

Abstract 1825: SAR443216, a novel trispecific T cell engager with potent T cell-dependent cytotoxicity for HER2-low tumors

Abstract Current HER2-targeted therapies have markedly improved the outcome of cancer patients with HER2 overexpressing tumors. However, these patients may eventually relapse or develop treatment resistance. In addition, HER2-low patients that are not eligible for treatment constitute a significant portion of the breast cancer patients. To address these unmet needs, we have developed a novel HER2-targeting T cell engager, SAR443216. This is a trispecific antibody with binding sites for HER2, CD3 and CD28, and containing a mutated IgG4-Fc which lacks effector functions. CD28 binding contributes to T cell activation, including activation of IL-2 and NFκB pathways, as well as induction of anti-apoptotic protein, Bcl-xL. In the presence of HER2-positive cancer cells, SAR443216 is able to activate primary human CD4 and CD8 T cells, resulting in T cell proliferation and secretion of cytokines and granzyme B. Moreover, it has potent in vitro T cell-dependent cellular cytotoxicity (TDCC) against a panel of HER2-expressing cancer cell lines, including those that are HER2-low. The potency of in vitro TDCC is largely correlated with HER2 surface expression in the target cells. Finally, in a HER2-low breast cancer xenograft model, SAR443216 also exhibited significant anti-tumor activity in immuno-deficient NSG mice reconstituted with primary human T cells. Thus, SAR443216 represents a promising new drug for cancer patients with HER2-expressing tumors, including those who are currently ineligible for stand-of-care therapy. Citation Format: Wenwen Sha, Sri Vadde, Zhili Song, Edward Seung, Zhen Xing, Liqing Chen, Virna Cortez-Retamozo, Sukhvinder Sidhu, Dinesh Bangari, Lan Wu, Ronnie Wei, Zhi-yong Yang, Gary Nabel, Vasiliki Pelekanou, Michele Sanicola-Nadel, Serena Masciari, Dmitri Wiederschain, Lily Pao. SAR443216, a novel trispecific T cell engager with potent T cell-dependent cytotoxicity for HER2-low tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1825.

Abstract CT111: Results of a phase 1 dose escalation study of ERY974, an anti-glypican 3 (GPC3)/CD3 bispecific antibody, in patients with advanced solid tumors

Abstract Background: ERY974, a bispecific T cell-redirecting antibody, redirects T cells to tumor cells by engaging the CD3 antigen on T cells and the glypican 3 (GPC3) antigen selectively expressed on tumors. ERY974 demonstrates T cell-dependent cellular cytotoxicity in vitro and transient cytokine elevations in preclinical toxicology studies (Ishiguro et al. 2017). The primary objective of this dose escalation (DE) study was to determine ERY974's maximum tolerated dose in patients with locally advanced or metastatic solid tumors expressing GPC3. Methods: The study included adult patients with advanced or metastatic solid tumors not amenable to standard therapy, histologically confirmed, with measurable disease and a life expectancy ≥ 3 months, including patients with ≤ 1cm and ≤ 1 brain metastasis. Patients with interstitial lung disease, or acute/active chronic infection were excluded. ERY974 was administered IV and dosed weekly. DE was initiated with an accelerated titration design of single patient cohorts followed by three patient cohorts. To mitigate for the toxicity of cytokine release syndrome (CRS), steroid prophylaxis and a flexible study design was implemented which included a two-step intra-patient escalation (regimen A), and a three-step intra-patient escalation (regimen B). Results: 29 patients were enrolled in dose levels ranging from 0.003 μg/kg to 0.81 μg/kg. Treatment-related adverse events that occurred in greater than 20% of patients included CRS and pyrexia. Dose level 0.81 μg/kg (regimen A) was confirmed not tolerable due to DLTs of Grade 3 CRS and Grade 2 CRS in two out of three patients (assessed according to Lee, et al. 2014). The Grade 3 CRS was associated with Grade 3 transaminitis and a Grade 3 elevation of bilirubin. Both CRS events led to dose delay and dose reduction. Increases in IL-6, IL-8 and IL-10 were observed in patients with the CRS. The severity and frequency of CRS in regimen B were similar to those observed in regimen A at the same dose level. One partial response (per modified RECIST criteria) was observed in a patient with esophageal cancer treated with 0.54 μg/kg (regimen B) and having 40% of the tumor tissue staining positive for GPC3 via immunohistochemistry. Stable disease lasting 3 months or longer was observed in four patients. Conclusions: The observed responses and CRS side effects are markers of ERY974 biologic activity. At doses below 0.81 μg/kg (regimen A), ERY974 was generally well tolerated with a manageable toxicity profile, including ERY-induced CRS which was manageable with steroid administration and anti-IL6R therapy. Further research is required to determine if combined prophylactic anti-IL6R and steroid therapy is a more effective strategy for managing CRS.

Abstract 1833: Development of anti-CLDN18.2-anti-CD3 bispecific antibody as cancer therapeutics

Abstract Claudin-18 (CLDN18) is a member of the large Claudin family of four-span transmembrane proteins, which are essential components of the mammalian tight junctions (TJs) in the epithelial cells. Claudin-18 has two splice variants, CLDN18.1 and CLDN18.2. While CLDN18.1 is specifically expressed in the lung tissue, CLDN18.2 expression in normal tissue is more restricted and is only detected in small patches of stomach mucosal. CLDN18.2 expression is elevated in many types of epithelial cancers including stomach, esophagus, pancreatic and ovarian cancers. Anti-CLDN18.2 monoclonal antibody Claudiximab (IMAB362) demonstrated promising results in clinical trials for gastric cancers. Using a combination of DNA, protein, and transfected cells as immunogens, we have generated a panel of mouse monoclonal antibodies which are highly specific for CLDN18.2 but do not cross-react to CLDN18.1. The lead antibody E9 had higher binding affinity for CLDN18.2, compared to the IMAB362. The humanized anti-CLDN18.2 antibody showed a strong antibody dependent cytotoxicity (ADCC) activity against Mia PaCa-2 cells overexpressing CLDN18.2. To redirect T-cell-mediated lysis toward CLDN18.2-overexpressed tumor cells, we generated an anti-CLDN18.2/anti-CD3 bispecific antibody. We demonstrated that the bispecific antibody binds to both human T cells and CLDN18.2-expressing tumor cells. Furthermore, the anti-CLDN18.2/anti-CD3 bispecific antibody had robust activity in killing Mia PaCa-2 cells overexpressing CLDN18.2 in the in vitro T cell-dependent cellular cytotoxicity (TDCC) assay. In addition, the bispecific antibody retained the in vitro ADCC activity against Mia PaCa-2 cells overexpressing CLDN18.2. Taken together, our data suggest that the anti-CLDN18.2/anti-CD3 bispecific antibody is a promising cancer therapeutic candidate utilizing multiple anti-tumor mechanisms of action, including ADCC and TDCC. Citation Format: Haishan Lin, Huey Li, Hunter Drobenaire, Veronica Pizzarella, Richard Zhang. Development of anti-CLDN18.2-anti-CD3 bispecific antibody as cancer therapeutics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1833.