Abstract B094: Suppressing ERG-driven oncogenesis by mRNA-based delivery of an intracellular mini-body targeting a unique protein methylation site in ERG fusion-positive prostate cancer

Abstract

Advanced prostate cancer is a leading cause of cancer-related deaths with limited effective targeted therapies for the different molecularly defined subtypes. About 50% of prostate cancers harbor ERG gene fusions, driving sustained expression of this oncogenic transcription factor associated with tumorigenesis and cancer progression. Despite its clinical relevance, ERG remains undruggable, highlighting a significant unmet need for novel therapeutic modalities. ERG mono-methylation at lysine 362 (K362) is a unique post-translational modification catalyzed by the protein methyltransferase EZH2, frequently upregulated and co-expressed in advanced ERG-fusion positive tumors (Zoma et al., 2021). Modifying the protein secondary structure and disrupting an auto-inhibitory domain, K362 methylation promotes ERG transcriptional and oncogenic activity in cancer cells. K362 methylation also provides an accessible, tumor-specific epitope for drug discovery. To exploit this unique vulnerability, we engineered a single-chain variable fragment (scFv) mini-antibody (mERG mini-body) that recognizes mono-methylated K362 ERG with remarkable affinity and selectivity, enabling specific targeting of ERG active oncogenic state. To perform proof-of-principle studies, the mERG mini-body was delivered intracellularly via DNA and mRNA expression systems formulated with lipid-polymeric nanoparticles (LPNP). Delivery efficiency, efficacy and mechanism of action were evaluated in multiple ERG-fusion positive models, including human and murine cell lines, cell line- and patient-derived organoids, murine allograft models derived from Pb-Cre4; Pten flox/flox ; Rosa26 ERG/ERG (ERG/PTEN) transgenic mice. DNA- and mRNA-based delivery in vitro demonstrated robust intracellular expression and nuclear localization of mERG mini-body, with effective and selective engagement of methylated ERG. K362 mutations in ERG abolished target recognition and binding, confirming the mERG mini-body specificity. Target engagement resulted in inhibition of ERG-driven transcriptional programs and suppression of oncogenic phenotypes, including reduced cell proliferation, organoid and tumor-sphere growth. Pharmacodynamic responses were durable and consistent across delivery systems, supporting platform flexibility. Systemic administration of LPNP-formulated mRNA by intravenous injection to tumor bearing mice achieved efficient and selective intratumoral delivery with sustained mERG mini-body expression, target engagement, and marked tumor growth inhibition in murine allografts, without evidence of systemic toxicity. These data establish the feasibility and efficacy of this novel mini-body-based strategy as a viable therapeutic modality for targeting an oncogenic transcription factor previously considered undruggable. With a defined biomarker strategy (ERG gene fusion and K362 methylation) and leveraging on validated mRNA delivery technologies, this work highlights the potential for this first-in-class therapeutic and supports further translational development for targeting ERG-driven prostate cancer. Carola Musumeci, Concetta Guerra, Domenico Albino, Elisa Storelli, Jacopo Sgrignani, Daniela Impellizieri, Simone Moro, Elisa Federici, Andrea Cavalli, Giuseppina MR. Carbone, Carlo V. Catapano. Suppressing ERG-driven oncogenesis by mRNA-based delivery of an intracellular mini-body targeting a unique protein methylation site in ERG fusion-positive prostate cancer [abstract]. In: Proceedings of AACR Drug Discovery and Development (AACR D3) Conference; 2026 Jul 21-24; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(14_Suppl):Abstract nr B094.