Engineered Toxin Bodies Specific for TROP2 Positive Cancers Authors: Garrett L. Cornelison, Ileana Pedraza, Kendra Garrison, Elizabeth M. Kapeel, Channing Pletka, Abdul Khan, Jessica Momb, Rebecca Martin, Adam Bartos, Joseph D. Dekker, Jay Zhao, John Majercak, Garrett L. Robinson Molecular Templates, Austin, TX Lanier Biotherapeutics, Bogart, GA Molecular Templates produces next generation immunotoxins called Engineered toxin bodies (ETBs). ETBs are comprised of a proprietarily engineered form of Shiga-like Toxin A subunit (SLT-A) genetically fused to antibody-like binding domains. ETBs work through novel mechanisms of action and are capable of forced internalization, undergoing retrograde translocation to the cytosol, and inducing potent cell-kill via the enzymatic and permanent inactivation of ribosomes resulting in the inhibition of protein synthesis and induction of apoptosis. In addition, Molecular Templates has expanded the ETB platform to include Antigen Seeding Technology (AST) to generate ETBs with the ability to deliver foreign protein antigen to targeted populations of tumor cells. This mechanism of action allows for the intracellular processing of antigen and subsequent surface MHC-I presentation required for activation of a re-directed T lymphocyte response and the capacity to restore a functional immune clearance program against the tumor. Three ETBs are in clinical development (MT-5111 targeting HER2, MT-0169 targeting CD38, and AST enabled MT-6402 targeting PD-L1). The novel mechanisms of action have potential benefit in different indications including in the relapsed setting, when disease has progressed after chemotherapies and other targeted therapies, and additionally may be able to combine with standard of care. ETBs are being developed that target other cell surface receptors expressed on solid tumors including tumor-associated calcium signal transducer 2 (TROP2). TROP2 is a clinically validated target in metastatic triple-negative breast cancer (mTNBC) and other cancers such as metastatic urothelial carcinoma (mUC) using antibody drug conjugate (ADC) therapies such as sacituzumab govitecan (Trodelvy®). In vitro, ETBs targeting TROP2 specifically and directly kill tumor cells expressing TROP2 with picomolar activity. ETBs can bind to TROP2 in the presence of the Trodelvy parent monoclonal antibody, sacituzumab, and ETBs retain potency on TROP2 positive cell lines in the presence of clinically relevant concentrations of sacituzumab. AST enabled Trop2 targeted ETBs retain direct cell killing activity and can deliver multiple viral antigens to induce cytokine secretion and T-cell mediated killing in a co-culture assay of TROP2 target cells with antigen matched HLA type and antigen specific T-cells. In vivo, TROP2 targeted ETBs demonstrate good tolerability in a murine HCC1806 triple-negative breast cancer xenograft model and significantly reduce tumor burden relative to vehicle control. These pre-clinical in vitro and in vivo data suggest AST enabled Trop2 targeted ETBs have the potential to deplete Trop2 positive malignancies through multiple unique mechanisms of action. Citation Format: Garrett L. Cornelison, Ileana Pedraza, Kendra Garrison, Elizabeth M. Kapeel, Channing Pletka, Abdul Khan, Jessica Momb, Rebecca Martin, Adam Bartos, Joseph D. Dekker, Jay Zhao, John Majercak, Garrett L. Robinson. Engineered Toxin Bodies Specific for TROP2 Positive Cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-08-18.

Engineered toxin bodies (ETBs) are comprised of a proprietarily engineered form of Shiga-like Toxin A subunit (SLT-A) genetically fused to antibody-like binding domains. ETBs work through novel mechanisms of action and are capable of forced internalization, undergoing retrograde translocation to the cytosol, and inducing potent cell-kill via the enzymatic and permanent inactivation of ribosomes resulting in the inhibition of protein synthesis and induction of apoptosis through ribotoxic stress mechanisms. Additionally, Molecular Templates has developed the Antigen Seeding Technology (AST) platform to generate ETBs with the unique ability to deliver foreign protein antigen to targeted populations of tumor cells. This MOA allows for the intracellular processing of antigen and subsequent surface MHC-I presentation required for activation of a re-directed T lymphocyte response and the capacity to restore a functional immune clearance program against the tumor. Three ETBs are in clinical development (MT-5111 targeting HER2, MT-0169 targeting CD38, and AST enabled MT-6402 targeting PD-L1). The novel mechanisms of action have potential benefit in different indications including in the relapsed setting, when disease has progressed after chemotherapies and other targeted therapies, and additionally may be able to combine with standard of care. ETBs are being developed that target other cell surface receptors expressed on solid tumors including tumor-associated calcium signal transducer 2 (Trop2). Trop2 is a clinically validated target of antibody drug conjugate (ADC) therapy in metastatic triple-negative breast cancer (mTNBC) and other cancers such as metastatic urothelial carcinoma (mUC). In vitro, tumor cells expressing Trop2 are effectively, specifically, and directly killed with picomolar activity by targeted ETBs. AST enabled Trop2 targeted ETBs are capable of delivering viral antigens for multiple HLA types and inducing cytokine secretion and T-cell mediated killing in a co-culture assay of Trop2 target cells with antigen matched HLA type and antigen specific T-cells. These pre-clinical in vitro data suggest AST enabled Trop2 targeted ETBs have the potential to deplete Trop2 positive malignancies through multiple unique mechanisms of action. Citation Format: Garrett L. Cornelison, Adam Bartos, Brigitte Brieschke, Jessica Momb, Ileana Pedraza, Elizabeth M. Kapeel, Rebecca Martin, Channing Pletka, Adrian Gonzalez, Joseph D. Dekker, Jay Zhao, John Majercak, Garrett L. Robinson. Engineered toxin bodies (ETBs) targeting Trop2 [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 326.