Abstract 2792: Development of pharmacodynamic assays for quantifying SMARCA2 protein degradation and target gene expression in response to a SMARCA2 degrader (PRT3789)
Authors: Moore Andrew , Bachner Carly , Srinivasan Lalitha , Kurup Pradeep , Grego Alex , Vitkovitsky Caroline , Ito Koichi , Bhagwat Neha , Scherle Peggy 
Published: April 2023

Abstract
The SWI/SNF complex plays an important role in controlling gene expression via chromatin remodeling. One of its catalytic subunits, SMARCA4, is frequently mutated in multiple tumor types, and SMARCA4-deficient cells are highly dependent on the other catalytic subunit, SMARCA2, for survival. Therefore, targeting SMARCA2 with selective protein degraders has therapeutic potential in SMARCA4 deficient human cancers. We have previously described the development of a potent and selective SMARCA2 targeted degrader, PRT3789, that demonstrates robust degradation of SMARCA2 protein and excellent preclinical efficacy in SMARCA4-del models (Hulse, et al. 2022). PRT3789 will be evaluated in a Phase 1 clinical trial in patients with SMARCA4-mutant cancer. In order to assess target engagement following clinical administration of PRT3789, we developed two pharmacodynamic (PD) assays to measure SMARCA2 protein degradation and changes in SMARCA2 target gene expression in human peripheral mononuclear cells (PBMCs). We used the MSD® S-PLEX platform for the development of a sensitive and quantitative, plate-based immunoassay for determining the protein concentration of SMARCA2 in human PBMC lysates. This assay exhibits a wide dynamic range of detection for SMARCA2 with an LLOD at 1.5 pg/mL, as well as exhibiting acceptable spike recovery and minimal cross-reactivity to SMARCA4. In addition, we optimized the PBMC isolation protocol and lysis conditions to enable sensitive detection of SMARCA2 protein. In order to demonstrate downstream gene expression changes as a consequence of SMARCA2 protein degradation, we developed a secondary qPCR assay. PBMCs from healthy human donors were treated ex vivo with PRT3789 for 24 - 48 hours, followed by RNA-sequencing to identify differentially expressed genes. We refined an 8-gene panel that showed robust expression in PBMCs, as well as consistent and dose-dependent changes in response to PRT3789 treatment. We further calculated a differential gene expression (DGE) score based on expression of this gene panel that correlates with SMARCA2 protein degradation. In summary, we describe the development and characterization of two independent clinically relevant PD assays that can be used to evaluate target engagement in blood samples following PRT3789 administration.
Citation Format: Andrew Moore, Carly Bachner, Lalitha Srinivasan, Pradeep Kurup, Alex Grego, Caroline Vitkovitsky, Koichi Ito, Neha Bhagwat, Peggy Scherle. Development of pharmacodynamic assays for quantifying SMARCA2 protein degradation and target gene expression in response to a SMARCA2 degrader (PRT3789) [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 2792.