Safety, immunogenicity, and optimal dosing of VLPCOV-02, a SARS-CoV-2 saRNA vaccine with modified 5-methylcytosine base

Abstract

Variant-adapted vaccines are becoming increasingly important for continued COVID-19 prevention. Part 1 of the phase 1/2 study with VLPCOV-02, a lipid nanoparticle-encapsulated, self-amplifying RNA (saRNA) vaccine with a modified 5-methylcytosine (5 mC) base, demonstrated lower reactogenicity and incidence of adverse events, and induction of antibody responses. We report results of part 2 with an expanded number of participants (N = 323 [3 μg VLPCOV-02: 53 non-elderly and 54 elderly; 7.5 μg VLPCOV-02: 55 non-elderly and 55 elderly; 30 μg Comirnaty ready to use: 52 non-elderly and 54 elderly]) to determine the optimal dose level. VLPCOV-02 induced robust immunoglobulin G titers against receptor-binding domain and neutralizing antibody titers against all variants of SARS-CoV-2 tested, and induced CD4⁺ and CD8⁺ T cell responses. These results indicate that the incorporation of a modified 5 mC base improves the safety profile of the saRNA vaccine without compromising immunogenicity, supporting further development of this platform as a booster vaccine.

Keywords: Immunology.

Graphical abstract

Figure 1

Participant randomization Study schema to show participant enrollment and randomization. Participants underwent randomization between June 25, 2023, and July 21, 2023. At 4 weeks post-study drug administration, two participants discontinued the study. By 52 weeks, an additional 12 participants discontinued from the study. All other participants completed the study through 52 weeks.

Figure 2

Solicited adverse events reported up to 6 days after study drug administration (day 7) (A) The percentage of solicited local adverse events and their severity reported up to 6 days after study drug administration. (B) The percentage of systemic adverse events and their severity reported up to 6 days after study drug administration. Non-elderly (7.5 μg VLPCOV-02, n = 55; 3 μg VLPCOV-02, n = 53; 30 μg Comirnaty RTU, n = 52). Elderly (7.5 μg VLPCOV-02, n = 55; 3 μg VLPCOV-02, n = 54; 30 μg Comirnaty RTU, n = 54). The severity of solicited adverse events was graded as mild (grey color), moderate (turquoise color), or severe (orange color), assessed using Common Terminology Criteria in Solid Tumors version 5.0 and per United States Food and Drug Administration guidance (Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventive Clinical Trials).

Figure 3

Anti-RBD IgG responses Serum IgG antibody titers against SARS-CoV-2 RBD protein (Wild type) for non-elderly (18 to <65 years, left side) and elderly (≥65 years, right side) participants. Logarithmic values are reported as geometric mean titers. Bars indicate 95% confidence intervals. The upper limit of detection is 160,000, and the lower limit of detection is 6.8 AU/mL. Non-elderly (7.5 μg VLPCOV-02, n = 50; 3 μg VLPCOV-02, n = 48; 30 μg Comirnaty RTU, n = 50). Elderly (7.5 μg VLPCOV-02, n = 53; 3 μg VLPCOV-02, n = 51; 30 μg Comirnaty RTU, n = 49), also see Table S1.

Figure 4

Neutralizing antibody responses against SARS-CoV-2 variants Serum neutralizing antibody titers against (A) Wild type, (B) Gamma, (C) Omicron BA.5, and (D) Omicron XBB.1.5 variants for non-elderly (18 to <65 years, left side) and elderly (≥65 years, right side) participants. Logarithmic values are reported as geometric mean titers. Bars indicate 95% confidence intervals. The lower limit of detection is 20 ID50. Non-elderly (7.5 μg VLPCOV-02, n = 50; 3 μg VLPCOV-02, n = 48; 30 μg Comirnaty RTU, n = 50). Elderly (7.5 μg VLPCOV-02, n = 53; 3 μg VLPCOV-02, n = 51; 30 μg Comirnaty RTU, n = 49), also see Table S2.

Figure 5

CD4⁺ T cell responses against RBD Flow cytometric analysis was performed to analyze RBD-specific T cell responses. Responses to 3 μg or 7.5 μg of VLPCOV-02 or 30 μg of Comirnaty RTU are shown as fold-change from baseline (day 1) to week 4 (day 29) for each cohort. Panel A shows the activated CD4⁺ T cells, which were characterized by the expression of CD154⁺. Panel B shows the response in CD4⁺ Th1 cells, which was characterized by the expression of IL-2, TNF, and/or IFN-γ. Panel C shows the response in CD4⁺ Th2 cells, which was characterized by the expression of IL-4 and/or IL-13. Panel D shows the response in CD4⁺ Th17 cells, which was characterized by the expression of IL-17. Panel E shows the CD4⁺ IL-21⁺ cells, which were characterized by the expression of IL-21. The horizontal bars indicate median values. p values (two-sided) were calculated using the Mann-Whitney U-test. Non-elderly (7.5 μg VLPCOV-02, n = 32; 3 μg VLPCOV-02, n = 32; 30 μg Comirnaty RTU, n = 31). Elderly (7.5 μg VLPCOV-02, n = 37; 3 μg VLPCOV-02, n = 34; 30 μg Comirnaty RTU, n = 35). IFN, interferon; IL, interleukin; TNF, tumor necrosis factor, also see Figure S2.

Figure 6

CD8⁺ T cell responses against RBD Flow cytometric analysis was performed to analyze RBD-specific T cells. Responses to VLPCOV-02 or Comirnaty RTU are shown as fold change from baseline (day 1) to week 4 (day 29) for each cohort. RBD-specific CD8⁺ T cells were determined based on the expression of (A) IFN-γ, (B) TNF, (C) CD107a, (D) MIP-1β, or (E) IL-2, respectively. p values (two-sided) were calculated using the Mann-Whitney U-test. Non-elderly (7.5 μg VLPCOV-02, n = 32; 3 μg VLPCOV-02, n = 32; 30 μg Comirnaty RTU, n = 31). Elderly (7.5 μg VLPCOV-02, n = 37; 3 μg VLPCOV-02, n = 34; 30 μg Comirnaty RTU, n = 35). IFN, interferon; IL, interleukin; RBD, receptor-binding domain; TNF, tumor necrosis factor, also see Figure S5.