Abstract B020: Targeting the DNA damage response sensor Replication Protein A for first in class cancer therapy
Authors: Pawelczak Katherine , VanderVere-Carozza Pamela , Jordon Matthew , Gavande Navnath , Turchi John 
Published: January 2024

Abstract
The DNA Damage Response (DDR) is a DNA repair and cell signaling pathway crucial in maintaining genomic stability. The most successful DDR targeted therapies inhibit the DNA damage sensor, PARP, a protein that recognizes single strand DNA (ssDNA) breaks as well as other DNA secondary structures induced by single strand gaps that can initiate chromosome instability and cell death. Building on the initial clinical success of PARP inhibitors, development of DDR targeted therapeutics has become increasingly popular with the majority of new agents targeting protein kinases downstream of the DNA damage sensors. The clinical outcomes with these therapeutics have not met expectations and thus a new approach to targeting the DDR pathway is needed. NERx Biosciences has developed a novel strategy of targeting the DDR by intervening upstream of the DDR kinases and targeting specific DDR sensors. The human ssDNA binding protein, replication protein A (RPA), is a critical sensor for the DDR, serving to sense ssDNA at replication forks that arise from replication stress (RS) and is a novel target for cancer therapy. We have discovered, developed, and characterized a novel small molecule RPA inhibitor (RPAi) NERx-329 that blocks the RPA-DNA interaction and elicits a state of chemical RPA exhaustion that results in in vivo anticancer activity. Chemical optimization enabled in vitro and in vivo analyses to elucidate mechanism of action, cellular engagement, and therapeutic activity of RPA-targeted agents. Biochemical reconstitution of the ATR signaling pathway shows that NERx-329 disrupts ATR kinase activity, further suggesting a novel mechanism of action driven by RPA inhibition induced replication catastrophe. NERx-329 elicits single agent in vitro anticancer activity across a broad spectrum of cancers and the cellular response suggests existence of a threshold before chemical RPA exhaustion induces cell death. In vivo analysis reveals activity in a series of lung and ovarian PDX models of human cancer. Single agent activity results in a tumor growth delay, which can be enhanced in combination therapy with DDR inhibitors as well as traditional DNA damaging cancer therapeutics. Data demonstrate that tumor growth observed in vivo is a function of NERx329 bioavailability and not development of a specific resistance mechanism. In addition, specific genetic predictors of RPAi sensitivity have been identified and results demonstrate specific genetic alterations increase RPAi activity in vitro and in vivo. NERx-329 represents the first clinically viable agent to target the DDR pathway by disrupting the RPA-DNA interaction and holds potential for significant impact in cancer treatment. This is supported by data from a retrospective clinical assessment of RPA expression to assess treatment outcomes in lung cancer patients, revealing that high RPA expression is a negative prognostic biomarker correlating with worse overall patient survival. NERx 329 offers a novel therapeutic approach for these cancers that are not effectively treated with existing therapeutics.
Citation Format: Katherine Pawelczak, Pamela VanderVere-Carozza, Matthew Jordon, Navnath Gavande, John Turchi. Targeting the DNA damage response sensor Replication Protein A for first in class cancer therapy [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr B020.