Abstract 933: ProTriTAC is a modular and robust T cell engager prodrug platform with therapeutic index expansion observed across multiple tumor targets
Authors: Lin S. Jack , Rocha Sony S. , Kwant Kathryn , Dayao Maria Rosalyn , Ng Tessie M. , Banzon Raphaela Rose , Thothathri Subramanian , Aaron Wade , Callihan Evan , Hemmati Golzar , Wright Kevin J. , Barath Manasi , Xiao Yinghua , To Linh , Yu Timothy , O'Rear Jessica , Bragg Eric , Gatto Scott , Yu Stephen , Patnaik Purbasa , Hundal Avneel , Austin Richard J. , Lemon Bryan , Wesche Holger 
Published: July 2021

Abstract
T cell engagers have compelling clinical activity, but their use is limited by the availability of tumor targets with minimal normal tissue expression. Conditionally active T cell engagers designed to be active only in the tumor and to spare normal tissues could increase the number of addressable tumor targets. Here, we describe a T cell engager prodrug platform, named ProTriTAC, that is designed to be preferentially active in the tumor to reduce its on-target/off-tumor toxicity and improve the safety profile of T cell engagers targeting solid tumor antigens. ProTriTACs are based on the TriTAC platform with three binding domains on a single polypeptide: 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, can keep the prodrug inert with an extended half-life. Activation by tumor-associated proteases removes the anti-albumin domain along with the masking moiety to reveal a short-lived active drug to direct T cell killing within the tumor. Multiple in vitro and in vivo approaches were used to demonstrate that ProTriTAC has an expanded therapeutic index (TI). In vitro binding and functional assays show that the prodrug has &gt;1000x attenuated binding to primary human T cells and approximately 500x reduced T cell killing compared to its protease-activated active drug. Humanized rodent tumor xenograft models were used to simultaneously measure anti-tumor efficacy and clinically relevant toxicity endpoints with a ProTriTAC targeting the tumor antigen epithelial cell adhesion molecule (EpCAM). EpCAM ProTriTACs could be safely administered at a dose 10x higher than the minimal efficacious dose required for durable tumor regression. Higher doses of EpCAM ProTriTAC produced body weight loss, elevated ALT/AST and bilirubin, and evidence of tissue damage by histopathology. In contrast, a constitutively active EpCAM-targeting T cell engager could only be dosed safely up to its minimum efficacious dose. These data suggest that the conditionally active ProTriTAC confers a 10x TI expansion compared to the constitutively active T cell engager. Demonstration of TI expansion, in vitro and in vivo, for ProTriTACs targeting other tumor antigens will also be presented, demonstrating the robust applicability of the ProTriTAC platform across multiple tumor targets.
Citation Format: S. Jack Lin, Sony S. Rocha, Kathryn Kwant, Maria Rosalyn Dayao, Tessie M. Ng, Raphaela Rose Banzon, Subramanian Thothathri, Wade Aaron, Evan Callihan, Golzar Hemmati, Kevin J. Wright, Manasi Barath, Yinghua Xiao, Linh To, Timothy Yu, Jessica O'Rear, Eric Bragg, Scott Gatto, Stephen Yu, Purbasa Patnaik, Avneel Hundal, Richard J. Austin, Bryan Lemon, Holger Wesche. ProTriTAC is a modular and robust T cell engager prodrug platform with therapeutic index expansion observed across multiple tumor targets [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 933.