Abstract 2118: Development of RNAscope multiplex-based assay for exploratory pharmacodynamic biomarkers assessment in breast cancer patients from Phase I clinical trial of ORM-5029, a potent GSPT1 degrader

Abstract Breast cancer is the most common invasive cancer in women worldwide and remains a clinical challenge. Addressing the unmet need for better therapeutics, we used our Dual-Precision Targeted Protein Degradation (TPD² TM) approach that merges the power of protein degraders with precision of ADC technology to design ORM-5029. It delivers a highly potent and precise catalytic GSPT1 protein degrader molecule (SMol006) selectively to HER2-expressing tumor cells via conjugation to pertuzumab. GSPT1 is a protein associated with regulation of the translational termination complex, and its loss results in activation of the integrated stress response (ISR) which ultimately leads to apoptosis. Activation of ISR triggers overall reduction of protein synthesis and activation of stress response genes (e.g., ATF3 and DDIT3). ORM-5029 is currently being investigated for safety and tolerability in an open label, multicenter, phase 1, dose escalation and expansion study in patients with HER2-expressing advanced breast tumors (NCT05511844). Here, we evidence that the efficacy of ORM-5029 correlates with pharmacodynamic degradation of GSPT1 and activation of ATP3 and DDIT3 transcripts both in vitro and in tumor xenografts in mice. With the aim of evaluating GSPT1 degradation and consequent activation of ATF3 and DDIT3 in pre- and post-treatment tumor samples from HER2+ breast cancer patients, we are developing an RNAscope multiplex (Leica System) assay panel. This approach combines detection of GSPT1 protein with 20zz ISH probe-mediated detection of ATF3 and DDIT3 mRNA in tumor sections. As a part of method development, we treated BT474 cells with an efficacious concentration of GSPT1 degrader and optimized the immunofluorescence and immunohistochemistry protocol with our in-house generated GSPT1 antibody (MAb001). Data showed that GSPT1- MAb001 could be successfully used for both immunofluorescence and immunohistochemistry assays. The optimized protocol is currently being developed into RNAscope multiplex workflow using BT474 FFPE sections and tissue microarrays. In summary, RNAscope multiplex assay could be used as a method to determine pharmacodynamic response in HER2+ breast cancer patients treated with ORM-5029. Citation Format: Shikha Saini, Yeonjoon Kim, Wesley Wong, Tina Karunaratne, Olaf Christensen, Sujoy Dutta, Palash Bhar, Khuloud Takrouri, Nathan Fishkin, Dong-Ki Choi, Min-Soo Kim, Joanne Lim, Anna Skaletskaya, James Palacino, Peter Park. Development of RNAscope multiplex-based assay for exploratory pharmacodynamic biomarkers assessment in breast cancer patients from Phase I clinical trial of ORM-5029, a potent GSPT1 degrader [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 2118.