Abstract 2928: TROP2 ProTriTAC™, a protease-activated T cell engager prodrug targeting TROP2 for the treatment of solid tumors
Authors: Rocha Sony S. , Lin Regina , Dayao Maria R. , Banzon Raphaela Rose , Thothathri Subramanian , Wright Kevin J. , Aaron Wade , Xiao Yinghua , Bergo Nick , To Linh , Bush Mabel , Barath Manasi , Yu Timothy , Kwong Willis , Situ Hubert , Bragg Eric , O'Rear Jessica , Carlin Kevin , Yu Stephen , Solorio Maritza , Lemon Bryan , Austin Richard , Wesche Holger , Lin S. Jack 
Published: April 2023

Abstract
Tumor-associated calcium signal transducer 2 (Trop2) is a cell surface glycoprotein that promotes cell renewal, proliferation and tumorigenesis. Trop2 is homogenously overexpressed in numerous solid tumor types, including breast, head and neck, prostate, bladder, and non-small cell carcinoma; however, Trop2 is also expressed in several normal tissues, including skin, respiratory tract, and female reproductive organs. Although Trop2-targeted antibody drug conjugates (ADCs) have demonstrated clinical activity, with regulatory approvals in subsets of breast and bladder cancers, their broader utility is still hampered by off-target payload-mediated toxicities. To address this issue, and to avoid on-target off-tumor toxicities, we developed a Trop2-targeting ProTriTAC™ (Protease-activated Tri-specific T cell Activating Construct), a T cell engager prodrug which is engineered to be preferentially activated in tumor tissue. ProTriTACs consists of three humanized antibody-derived binding domains on a single polypeptide chain: anti-albumin for half-life extension, anti-CD3 for T cell engagement, and anti-target antigen for tumor cell engagement. The anti-albumin domain, bearing a masking moiety and a protease-cleavable linker, keeps the prodrug inert by inhibiting binding of the adjacent anti-CD3 domain to T cells. Cleavage of the linker by tumor-associated proteases removes the anti-albumin domain along with the masking moiety to reveal a potent active drug that directs T cell killing within the tumor. As designed, in vitro binding and functional assays show that the protease-activated T cell engager has &gt;1000-fold improved binding to primary human T cells and ~100-fold improved T cell killing compared to its intact masked prodrug. In immunodeficient mice reconstituted with human T cells, anti-Trop2 ProTriTAC demonstrated robust anti-tumor activity in multiple xenograft tumor models, including HCC70 (breast), CAL27 (head and neck), and HPAFII (pancreatic), with complete tumor regression as low as 30 μg/kg. To determine the potential toxicity and toxicokinetic characteristics of anti-Trop2 ProTriTAC, an intra-animal dose escalation was conducted in cynomolgus monkeys until individual maximum tolerated dose (MTD). The starting dose was 20 μg/kg with weekly 3-fold dose escalations until MTD. The one-month study revealed that anti-Trop2 ProTriTAC was tolerated at the 180 μg/kg dose level, but not at the next higher dose level of 540 μg/kg. Toxicokinetics, gross pathology, histopathology, and cytokine data will be presented. The preclinical activity in rodent tumor models, coupled to its tolerability in cynomolgus monkeys, support the development of anti-Trop2 ProTriTAC as a therapeutic in a broad range of Trop2-expressing solid tumors.
Citation Format: Sony S. Rocha, Regina Lin, Maria R. Dayao, Raphaela Rose Banzon, Subramanian Thothathri, Kevin J. Wright, Wade Aaron, Yinghua Xiao, Nick Bergo, Linh To, Mabel Bush, Manasi Barath, Timothy Yu, Willis Kwong, Hubert Situ, Eric Bragg, Jessica O'Rear, Kevin Carlin, Stephen Yu, Maritza Solorio, Bryan Lemon, Richard Austin, Holger Wesche, S. Jack Lin. TROP2 ProTriTAC™, a protease-activated T cell engager prodrug targeting TROP2 for the treatment of solid tumors [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 2928.