Abstract 6725: NTX-470, a novel multispecific T cell engager expressed from mRNA targets PSMA and STEAP1 prostate cancer antigens to generate enhanced functional activity

Abstract Prostate cancer is the most common cancer in American men and the second leading cause of cancer death in men in western countries. Most metastatic patients develop castration resistant prostate cancer, which despite advances in immunotherapy, is limited in treatment options and represents a critical unmet need. Bi-specific T cell engagers (TCEs) targeting prostate cancer antigens represent a modality that has demonstrated preliminary clinical activity. Clinically validated antigens that are highly expressed in prostate cancers include Prostate-Specific Membrane Antigen (PSMA) and Six Transmembrane Epithelial Antigen of the Prostate 1 (STEAP1). TCEs targeting either PSMA or STEAP1 individually have shown promising clinical activity, but challenges with durability and toxicity have been reported, potentially attributable to antigen escape, target-independent T cell activation (bystander activation) and specificity issues. Contrary to its name, PSMA expression is not exclusive to prostate cells and can be found in several other tissues and/or conditions, including normal nonprostatic epithelial cells, inflammation/infection, nonprostatic neoplastic cells and nonprostatic tumor-associated neovasculature. The widespread expression of PSMA may limit the therapeutic window of PSMA-specific TCEs and drive immunotoxicity by increased on-target, off-tumor binding. To address these issues, Nutcracker Therapeutics (NTX) has developed novel and highly complex multispecific TCE molecules that target both PSMA and STEAP1, which can be expressed using NTX’s proprietary mRNA therapeutics platform. A wide library of molecules was designed with attenuated PSMA and CD3 binding to mitigate on-target, off-tumor binding and target-independent T cell activation. Molecules were screened and selected based on their propensity to cause bystander T cell activation and the strength of PSMA binding. We identified a small number of molecules that are capable of engaging CD3 T cells but display minimal activity in the absence of STEAP1 or PSMA expressing target cells, including molecules that retain low bystander activity and attenuated PSMA binding yet maintain robust target cell killing. These molecules include NTX-470, which targets both STEAP1 and PSMA on tumor cells with minimal bystander activity. Citation Format: Chris Rae, Sudha Adusumilli, Srinivasa Bandi, Shruti Lal, Pragyesh Dhungel, Kimmy Ferry, Shima Gholizadeh, Anushtha Sharma, Adrienne Sallets, Weiqun Liu, Evan McCartney-Melstad, Pei-Ken Hsu, Ray Low, Colin McKinlay, Meredith Leong, David Y. Oh, Lawrence Fong, Guna Kannan, Samuel Deutsch, Ole Haabeth. NTX-470, a novel multispecific T cell engager expressed from mRNA targets PSMA and STEAP1 prostate cancer antigens to generate enhanced functional activity 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 (6Suppl): Abstract nr 6725.