Abstract 2927: ITGB6 ProTriTAC™, a protease-activated T cell engager prodrug targeting Integrin-β6 for the treatment of solid tumors

Abstract Integrin-β6 (ITGB6) is a transmembrane protein that partners exclusively with the integrin-αV subunit to form the integrin-αVβ6 heterodimer. Due to its roles in tissue remodeling and cancer pathogenesis, therapeutic targeting of integrin-αVβ6 has been investigated for both fibrotic diseases and cancers. ITGB6 is homogenously overexpressed in numerous solid tumor types, including breast, head and neck, pancreatic, and non-small cell lung cancers. Additionally, its expression is an unfavorable prognostic marker in multiple cancers. However, past attempts to therapeutically target ITGB6 have been hindered by its expression in several normal tissues, including bladder, skin, lung, kidney, and muscle. For example, clinical testing of an integrin αVβ6-targeting monoclonal antibody was discontinued reportedly due to toxicity concerns. To reduce on-target/off-tumor toxicities from ITGB6 expression in normal tissues and to enable safe targeting of ITGB6 in solid tumors, we developed an ITGB6-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 >1000-fold improved binding to human T cells and ~100-fold improved T cell killing compared to the intact masked prodrug. In immunodeficient mice reconstituted with human T cells, anti-ITGB6 ProTriTAC demonstrated robust anti-tumor activity in multiple ITGB6-expressing 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-ITGB6 ProTriTAC, an intra-animal dose escalation was conducted in cynomolgus monkeys. Starting at 20 μg/kg with weekly 3-fold dose escalations, anti-ITGB6 ProTriTAC was well tolerated up to 540 μg/kg, which was the highest dose tested. The preclinical activity in rodent tumor models, coupled to its tolerability in cynomolgus monkeys, support the development of anti-ITGB6 ProTriTAC as a therapeutic in a broad range of ITGB6-expressing solid tumors. Citation Format: Regina J. Lin, Sony S. Rocha, Maria R. Dayao, Subramanian Thothathri, Raphaela Rose Banzon, Kevin J. Wright, Wade Aaron, Yinghua Xiao, Nick Bergo, Linh To, Mabel Bush, Manasi Barath, Yi Yang, 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. ITGB6 ProTriTAC™, a protease-activated T cell engager prodrug targeting Integrin-β6 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 2927.