A phase 1/2 randomized, double-blinded, placebo controlled ascending dose trial to assess the safety, tolerability and immunogenicity of ARCT-021 in healthy adults (preprint)

BackgroundThe pandemic of coronavirus disease-19 (Covid-19) continues to afflict the lives and livelihoods of many as global demand for vaccine supply remains unmet. MethodsPhase 1 of this trial (N=42) assessed the safety, tolerability and immunogenicity of ascending levels of one-dose ARCT-021, a self-amplifying mRNA vaccine against Covid-19. Phase 2 (N=64) tested two-doses of ARCT-021 given 28 days apart. Both young and older adults were enrolled. The primary safety outcomes were local and systemic solicited adverse events (AEs) reported immediately and up to 7 days post-inoculation and unsolicited events reported up to 56 days after inoculation. Secondary and exploratory outcomes were antibody and T cell responses to vaccination, respectively. ResultsARCT-021 was well tolerated up to one 7.5 g dose and two 5.0 g doses. Local solicited AEs, namely injection-site pain and tenderness, as well as systemic solicited AEs, such as fatigue, headache and myalgia, were more common in ARCT-021 than placebo recipients, and in younger than older adults. Seroconversion rate for anti-S IgG was 100% in all cohorts except for the 1 g one-dose in younger adults and the 7.5 g one-dose in older adults, which were each 80%. Neutralizing antibody titers increased with increasing dose although the responses following 5.0 g and 7.5 g ARCT-021 were similar. Anti-S IgG titers overlapped with those in Covid-19 convalescent plasma. ARCT-021 also elicited T-cell responses against the S glycoprotein. ConclusionTaken collectively, the favorable safety and immunogenicity profiles support further clinical development of ARCT-021.

Diversity of ACE2 and its interaction with SARS-CoV-2 receptor binding domain (preprint)

COVID-19, the clinical syndrome caused by the SARS-CoV-2 virus, has rapidly spread globally causing millions of infections and hundreds of thousands of deaths. The potential animal reservoirs for SARS-CoV-2 are currently unknown, however sequence analysis has provided plausible potential candidate species. SARS-CoV-2 binds to the angiotensin I converting enzyme 2 (ACE2) to enable its entry into host cells and establish infection. We analyzed the binding surface of ACE2 from several important animal species to begin to understand the parameters for the ACE2 recognition by the SARS-CoV-2 spike protein receptor binding domain (RBD). We employed Shannon entropy analysis to determine the variability of ACE2 across its sequence and particularly in its RBD interacting region, and assessed differences between various species ACE2 and human ACE2. As cattle are a known reservoir for coronaviruses with previous human zoonotic transfer, and has a relatively divergent ACE2 sequence, we compared the binding kinetics of bovine and human ACE2 to SARS-CoV-2 RBD. This revealed a nanomolar binding affinity for bovine ACE2 but an approximate ten-fold reduction of binding compared to human ACE2. Since cows have been experimentally infected by SARS-CoV-2, this lower affinity sets a threshold for sequences with lower homology to human ACE2 to be able to serve as a productive viral receptor for SARS-CoV-2.

The Fc-mediated effector functions of a potent SARS-CoV-2 neutralizing antibody, SC31, isolated from an early convalescent COVID-19 patient, are essential for the optimal therapeutic efficacy of the antibody (preprint)

SARS-CoV-2-neutralizing antibodies are promising therapeutics for COVID-19. However, little is known about the mechanisms of action of these antibodies or their effective dosing windows. We report the discovery and development of SC31, a potent SARS-CoV-2 neutralizing IgG1 antibody, originally isolated from a convalescent patient at day 27 after the onset of symptoms. Neutralization occurs via a binding epitope that maps within the ACE2 interface of the SARS-CoV-2 Spike protein, conserved across all common circulating SARS-CoV-2 mutants. In SARS-CoV-2 infected K18-human ACE2 transgenic mice, SC31 demonstrated potent survival benefit by dramatically reducing viral load concomitant with attenuated pro-inflammatory responses linked to severe systemic disease, such as IL-6. Comparison with a Fc-null LALA variant of SC31 demonstrated that optimal therapeutic efficacy of SC31 requires intact Fc-mediated effector functions that can further induce an IFN{gamma}-driven anti-viral immune response. Dose-dependent efficacy for SC31 was observed down to 5mg/kg when dosed before the activation of lung inflammatory responses. Importantly, despite Fc{gamma}R binding, no evidence of antibody dependent enhancement was observed with the Fc-competent SC31 even at sub-therapeutic doses. Therapeutic efficacy was confirmed in SARS-CoV-2-infected hamsters, where SC31 again significantly reduced viral load, decreased lung lesions and inhibited progression to severe disease manifestations. This study underlines the potential for significant COVID-19 patient benefit for the SC31 antibody that justifies rapid advancement to the clinic, as well as highlighting the importance of appropriate mechanistic and functional studies during development. One Sentence SummaryAnti-SARS-CoV-2 IgG1 antibody SC31 controls infection in vivo by blocking SP:ACE2 binding and triggering a Fc-mediated anti-viral response.