Abstract 4575: Efficacy proof of concept studies with universal mRNA vaccines in various mouse tumor models
Authors: Wu Chenyan , Zhang Tina Tingting , Yan Xuefei , Jin Ying , Li Henry Q. X. , Ouyang Davy Xuesong 
Published: September 2020

Abstract
Introduction: Cancer immunotherapies have rapidly advanced in recent years and are revolutionizing cancer care. Among them, cancer vaccines are an attractive treatment approach because they may represent a specific, safe, and well tolerated therapy as well as the potential to overcome drug resistance and achieve durable response. Currently, personalized mRNA cancer vaccines are being actively pursued in clinical trials. However, only a few universal mRNA cancer vaccines have been investigated. This study assessed the antitumor efficacy of universal mRNA cancer vaccines targeting three tumor-associated antigens (TAAs) in CT26.WT-CEA syngeneic tumors, as well as KPC (KrasG12D; Trp53KO; Pdx1-cre) MuPrime™ tumors.
Methods: We have designed 27 epitopes against 3 TAAs, cloned into 4 DNA constructs in different combinations for mRNA synthesis by in vitro transcription (IVT). Prophylactic and therapeutic efficacy studies were carried out using CT26.WT-CEA tumor bearing mice with vaccine monotherapy or its combination with an anti-PD-1 antibody. At the end of the efficacy phase, mouse splenocytes were collected from half of the treated animals to measure peptide-specific IFN-γ production by ELISpot assay. In the remaining tumor-free mice, we re-grafted CT26.WT-CEA and parental CT26.WT (1st re-challenge) or a different syngeneic line H22 (2nd re-challenge), which shares two common antigens with CT26.WT. Finally, we selected the most efficacious mRNA vaccine to assess its efficacy in treating pancreatic KPC tumors.
Results: While anti-PD-1 monotherapy led to ~80% TGI, its combination with mRNA vaccines resulted in complete eradication of CT26.WT-CEA tumors in both prevention and intervention settings. In the ELISpot assay, splenocytes from vaccine treated mice showed significantly increased IFN-γ secreting cells after co-culture with specific peptides, in comparison to splenocytes from treatment naïve mice indicating existence of antigen specific CD8+ T cells. In the serial re-challenge studies, the vaccine treated, tumor-free mice showed no tumor growth in 2 rounds of re-challenge, either with the same tumor line (CT26.WT-CEA), its parental line (CT26.WT), or a different syngeneic line (H22), suggesting durable tumor growth inhibition by memory T cells after vaccine therapy. This universal mRNA cancer vaccine also demonstrated robust TGI against KPC tumors.
Conclusions: We demonstrated the effectiveness of universal mRNA cancer vaccines targeting different epitopes of multiple common tumor antigens in various mouse tumor models, including hard-to-treat KPC tumors. We are now performing toxicology studies using rats and NHPs to move the universal mRNA cancer vaccines toward IND filing.
Citation Format: Chenyan Wu, Tina Tingting Zhang, Xuefei Yan, Ying Jin, Henry Q. X. Li, Davy Xuesong Ouyang. Efficacy proof of concept studies with universal mRNA vaccines in various mouse tumor models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4575.