Phase I interim results of a phase I/II study of the IgG-Fc fusion COVID-19 subunit vaccine, AKS-452

Abstract

To address the coronavirus disease 2019 (COVID-19) pandemic caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a recombinant subunit vaccine, AKS-452, is being developed comprising an Fc fusion protein of the SARS-CoV-2 viral spike protein receptor binding domain (SP/RBD) antigen and human IgG1 Fc emulsified in the water-in-oil adjuvant, Montanide™ ISA 720. A single-center, open-label, phase I dose-finding and safety study was conducted with 60 healthy adults (18-65 years) receiving one or two doses 28 days apart of 22.5 µg, 45 µg, or 90 µg of AKS-452 (i.e., six cohorts, N = 10 subjects per cohort). Primary endpoints were safety and reactogenicity and secondary endpoints were immunogenicity assessments. No AEs ≥ 3, no SAEs attributable to AKS-452, and no SARS-CoV-2 viral infections occurred during the study. Seroconversion rates of anti-SARS-CoV-2 SP/RBD IgG titers in the 22.5, 45, and 90 µg cohorts at day 28 were 70%, 90%, and 100%, respectively, which all increased to 100% at day 56 (except 89% for the single-dose 22.5 µg cohort). All IgG titers were Th1-isotype skewed and efficiently bound mutant SP/RBD from several SARS-CoV-2 variants with strong neutralization potencies of live virus infection of cells (including alpha and delta variants). The favorable safety and immunogenicity profiles of this phase I study (ClinicalTrials.gov: NCT04681092) support phase II initiation of this room-temperature stable vaccine that can be rapidly and inexpensively manufactured to serve vaccination at a global scale without the need of a complex distribution or cold chain.

Keywords: AKS-452; COVID-19; Coronavirus; Fc-fusion; Infectious disease; Pandemic; Prophylaxis; SARS-CoV-2; Vaccine.

Fig. 1

AKS-452 pH 1/2 clinical study design. Phase 1 was a 6 × 3 (Sentinel) + 6 × 7 (Expansion) design. The first 3 (sentinel) subjects of cohorts 1, 3 and 5 were vaccinated in a consecutive order, in which the first subject was observed 2 h post-vaccination before the second subject of the respective cohort received vaccination, and the third subject was treated in a similar fashion. After the third subject in a cohort had been vaccinated and observed for 2 h, the next 7 subjects of the respective cohort were vaccinated simultaneously and in a shorter period (total n = 10). Subjects of cohorts 2, 4 and 6 received injections simultaneously with the expansion group of the respective single-dose cohort, because dosages had been considered safe. Based on safety and immunogenicity (seroconversion/titers) of Phase 1 cohorts, optimal dose levels for one-dose and two-dose strategy will be selected for Phase 2 cohorts.

Fig. 2

AKS-452 Phase 1 study immunogenicity. (A) Serum samples were obtained at Day 0, 28, and 56 of initial vaccine dose and assessed for anti-SP/RBD IgG binding titers via ELISA and presented per subject (all Day 0 samples were < lower limit of quantitation; not shown). Seroconversion was defined as > 2.42 µg/mL IgG (derived from validation studies with COVID-19 naïve subject samples; see Methods). Human Convalescent serum (HCS) was used as a comparator for samples from vaccinated subjects. Statistical comparisons between Day for each cohort and between cohorts were performed using a model with cohort and Day as fixed effects and a random subject effect. p-values were adjusted for multiplicity. (B) % Inhibition at a 1:40 dilution of sera of Day 56 serum samples were derived via a human ACE2 binding to SP/RBD ELISA. Statistical comparisons between cohorts were performed using a model with cohort as a fixed effect. p-values were adjusted for multiplicity. (C) Comparison of IgG titer vs. inhibitory potency (inhibitory dilution 50% in ACE2 binding assay, ID₅₀) of Day 56 sera for vaccinated subjects and HCS. Linear regression was performed on log transformed data. The slope for HCS was found to be significantly higher (p < 0.0001). (D) Anti-SP/RBD IgG isotype titers were obtained via isotype-specific ELISAs (mean µg/mL ± SD).

Fig. 3

AKS-452 Phase 1 study mutant SP/RBD immunogenicity. Serum samples obtained on Day 56 of initial vaccine dose and human convalescent sera (HCS) were assessed for titers of IgG binding to different mutant SARS-CoV-2 SP/RBD recombinant proteins via ELISA presented via combined cohorts of one vs. two doses (A). Statistical comparison of group means within a particular SP/RBD variant ELISA was performed with the 2-tailed t-test using unequal variance (GraphPad Prism). (B) Ratios of mutant/wild-type (WT, Washington) strain titers are presented in which the mean ratios of each of the 6 cohorts were used to calculate the grand mean and SEM for each relative mutant titer.

Fig. 4

AKS-452 Phase 1 study mutant SP/RBD immunogenicity and viral neutralization. Serum samples obtained on Day 56 of initial vaccine dose and human convalescent sera (HCS) were assessed for % neutralization (at 1:40 dilution of serum) of wild-type (Washington) and mutant (alpha and delta) live virus strains to infect live VERO E6 cells via the Plaque Reduction Neutralization Test. Statistical comparison of group means within a particular SP/RBD variant assay was performed with the 2-tailed t-test using unequal variance (GraphPad Prism). *denotes p < 0.05; all cohort means within a viral strain were significantly different from the respective HCS mean (p < 0.05) unless otherwise denoted by the “not significant” designation, ns.