Discovery and Development of a Safe and Efficient COVID-19 mRNA Vaccine, STP2104, Using a Novel Capping Library Screening (CLS) Method

Kanghyun Choi¹Joo-Young Lee¹Uk-Il Kim¹So-Hyun Park²So-Hee Hong³Yoo-Jin Bang⁴Yun-Ho Hwang⁵Jong-Eun Lee⁶Suhyun Park¹Soochong Kim²Sunhee Lee¹Ye-Jin Yun¹Tae-Gi Uhm¹Inyoung Lee¹Minju Kwon¹Eunji Kwon¹Suyeon Song¹Yongkyoung Kweon¹Heejene Kim¹Eun-Young Oh²Jae-Yong Kim⁷Hyeran Won⁵Jung-Ah Lee⁵Tae- Young Lee⁵Jinah Yeo⁵Seo-Yeon Kim⁵Se-Eun Kim⁵You-Jin Kim⁵Seonock Lee⁶Ruijing Sun⁶Eun-Joo Lee⁶Gamin Kim⁶Hanah Lim⁶Byungkyun Kim¹Jungho Kim⁶Dokeun Kim⁵Jae-Hwan Nam⁷Sang-Myeong Lee²Kyungjin Kim^1*Joo-Sung Yang^1*

• ¹ST Pharm Co., Ltd., Seoul, Republic of Korea
• ²College of Veterinary Medicine, Chungbuk National University, Cheongju, North Chungcheong, Republic of Korea
• ³Department of Microbiology, College of Medicine, Ewha Womens University, Seoul, Republic of Korea
• ⁴Department of Medical and Biological Sciences, The Catholic University of Korea, Bucheo-si, Republic of Korea
• ⁵Division of Infectious Disease Vaccine Research, Center for Vaccine Research, National Institute of Infectious Diseases, National Institute of Health, Korea Disease Control and Prevention Agency, CheongJu-si, Republic of Korea
• ⁶Department of Life Science, Sogang University, Seoul, Republic of Korea
• ⁷Department of Medical and Biological Sciences, The Catholic University of Korea, Bucheon-si, Republic of Korea

Messenger RNA (mRNA) vaccines represent a critical avenue for coronavirus disease 2019 prevention. We developed a COVID-19 mRNA vaccine encoding a codon-optimized full-length ancestral spike (S) protein with a signal peptide, which employs our novel patented co-transcriptional 5′-capping reagent, SmartCap ® . From the screening capping library of SmartCap ® , an SC101 cap was selected to derive a novel mRNA vaccine, STP2104. An in vitro study of STP2104 incorporating SC101 revealed enhanced protein expression in both the cell lysate and culture medium, and an in vivo immunogenicity study revealed strong humoral and cell-mediated immune responses. STP2104 further displayed potent neutralizing activity in immunized mice as derived via the PRNT50 assay using the wild-type virus. We evaluated the protection efficacy of STP2104 using human ACE2 transgenic mice immunized and challenged with SARS-CoV-2 to acquire the survival rate, virus titration, and histopathology study data. These studies proved that STP2104 is potent enough to induce protective immunity. A novel capping library screening (CLS) method was successfully utilized for exploring the optimal 5′-cap reagent, which improves S gene expression with mRNA stability. The clinical phase 1 studies of STP2104 will prove its safety, tolerability, and immunogenicity as well as the safety of the novel 5′-cap analogue SC101 in humans.