Abstract 4778: Development of self-amplifying RNA vaccines targeting prostate tissue-restricted and tumor-associated antigens

Abstract Prostate cancer is the most frequent cancer type in men, with increasing incidence at advanced age. As this cancer is frequently slow growing, opportunities for cancer interception exist, particularly among men with low-grade tumors who frequently undergo active surveillance. A well-tolerated vaccine intervention would be ideal for this population. Self-amplifying RNA (saRNA) vaccines built on a replication-deficient alphavirus backbone such as VEEV, offer several advantages for vaccine development. Sequences for relatively large antigens can be encoded and efficiently expressed, at low vaccine doses. Practical and efficient delivery of the saRNA is possible with advanced nanoparticles such as cationic inorganic lipid nanoparticles (LION). Among candidate antigens for a prostate cancer interception vaccine, we selected prostein (SLC45A3), a protein whose natural expression is restricted to the prostate gland and prostate cancer and ERG, a member of the ETS family of transcription factors frequently involved in chromosomal translocations in prostate cancer. We generated saRNA constructs with several design strategies and demonstrated that upon in vitro transfection, both antigens are expressed as assessed by mass spectrometry and Western blot analysis. This was achieved using commercial transfection reagents and LION. Prostein expression was particularly notable in that this eleven-transmembrane protein could be detected in HEK-293 cells using flow-cytometry and cell-surface staining. Nearly 50% of the cell population stained positively. Immunogenicity of the vaccines was tested in mice. Following formulation with LION particles i.m. injection and boosting, saRNAs coding for prostein elicited strong cellular immunity as demonstrated by IFN-γ ELISPOT analysis. Detailed analysis of intracellular cytokine expression indicated a strong CD8 response. saRNAs coding for ERG also showed antigen-specific ELISPOTs. Using commercially available recombinant ERG protein, an ELISA was developed and a strong humoral response to the ERG saRNA vaccine was demonstrated. The saRNA vaccines will be further credentialled with additional preclinical studies including cancer preventive efficacy studies in genetically engineered mouse models of prostate cancer. Citation Format: Robert H. Shoemaker, Jesse H. Erasmus, Peter Berglund, Jason D. Marshall, Yuri Koboziev, Bradley M. Beckman, Brian D. Cholewa, Steven G. Reed. Development of self-amplifying RNA vaccines targeting prostate tissue-restricted and tumor-associated antigens [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(6_Suppl):Abstract nr 4778.