Abstract 5738: A pan-ras mRNA vaccine elicits specific immune responses and inhibits tumor growth in the mouse model of colon cancer
Authors: Chen Renxiang , Liu Wei , Brown David M. , Bong Yong-Sik , He Jiaxi , Shen Dong , Wang Cun Yu 
Published: April 2023

Abstract
KRAS mutations are present in about 25% of tumors, making them one of the most common mutated oncogenes among human cancers. Specifically, KRAS G12 mutation (89%) is the most common in tumors, followed by G13 (9%) and Q61 (1%). These mutations were once considered “undruggable” for many years until appearance of sotorasib. However, it was developed only for G12C mutation.
We designed a Pan-Ras neoantigen mRNA vaccine (mRNA-1521), which covers all prevalent RAS mutations to achieve broad-spectrum cancer therapy. We synthesized a multiepitope neoantigen KRAS mRNA vaccine (mRNA-1521) and examined its immune response and anti-tumor efficacy in the colon cancer of Balb/c mice. Two groups (A, B) of mice were immunized with mRNA-1521 on days 0, 21, and 49, while PBS was given to two other groups (C, D) as the control on the same days. All mice were inoculated with CT26 cells (colon carcinoma cell line harboring G12D mutation) on day 56. Afterward, anti-PD1 antibodies were administered to groups A and C every two days. Tumor sizes were measured every 3 days after inoculation until anyone exceeded 1500mm3 in the control group. The serum IgG titer and T cell response of mice were explored using ELISA and splenocyte ELISpot assay after euthanasia. Tumor tissues were harvested and embedded in paraffin for the immunohistochemistry (IHC) test. Next-generation sequencing (NGS) was also applied to investigate the whole transcriptome analysis.
Tumor growth rates in all treatment groups were slower than in the control group. Among them, the mean tumor size and weight in the combination group of mRNA-1521 and anti-PD1 (372mm3, 0.48g) were only 25% of the PBS group (1488mm3, 2g). On average, the anti-PD1 and vaccine groups had moderate tumor size and weight (1100mm3/1.5g and 886mm3/1.26g, respectively). The splenocytes from the mice in each group were collected. The IFN-γ secreting splenocytes in the combination group were 50% higher than the mRNA-1521 group. Furthermore, the total IgG titer of vaccine-immunized mice (OD450nm&gt;2) was 3 and 4 times higher than the anti-PD1 and control groups. The IHC showed that the tumor tissues in the vaccine groups had more CD8+ and granzyme B+ CD8+ T cell infiltrations than in the control group. Whole transcriptome analysis showed that RAS downstream gene expressions were significantly lower in the vaccine group, indicating that the RAS vaccine was effective in inhibiting signaling pathways accounting for tumor proliferation, progression, and migration.
In conclusion, prophylactic immunization of mRNA-1521 inhibited tumor growth in the mouse model of colon cancer. Its inhibitory effects on tumor growth were more remarkable in the combination of anti-PD1 antibodies. Moreover, the vaccine could elicit specific T and B cell responses, which are crucial in anti-cancer immunotherapy.
Citation Format: Renxiang Chen, Wei Liu, David M. Brown, Yong-Sik Bong, Jiaxi He, Dong Shen, Cun Yu Wang. A pan-ras mRNA vaccine elicits specific immune responses and inhibits tumor growth in the mouse model of colon cancer. [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 5738.