Preclinical immunogenicity and efficacy of a candidate COVID-19 vaccine based on a vesicular stomatitis virus-SARS-CoV-2 chimera.

BACKGROUND: To investigate a vaccine technology with potential to protect against coronavirus disease 2019 (COVID-19) and reduce transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with a single vaccine dose, we developed a SARS-CoV-2 candidate vaccine using the live vesicular stomatitis virus (VSV) chimeric virus approach previously used to develop a licensed Ebola virus vaccine. METHODS: We generated a replication-competent chimeric VSV-SARS-CoV-2 vaccine candidate by replacing the VSV glycoprotein (G) gene with coding sequence for the SARS-CoV-2 Spike glycoprotein (S). Immunogenicity of the lead vaccine candidate (VSV∆G-SARS-CoV-2) was evaluated in cotton rats and golden Syrian hamsters, and protection from SARS-CoV-2 infection also was assessed in hamsters. FINDINGS: VSV∆G-SARS-CoV-2 delivered with a single intramuscular (IM) injection was immunogenic in cotton rats and hamsters and protected hamsters from weight loss following SARS-CoV-2 challenge. When mucosal vaccination was evaluated, cotton rats did not respond to the vaccine, whereas mucosal administration of VSV∆G-SARS-CoV-2 was found to be more immunogenic than IM injection in hamsters and induced immunity that significantly reduced SARS-CoV-2 challenge virus loads in both lung and nasal tissues. INTERPRETATION: VSV∆G-SARS-CoV-2 delivered by IM injection or mucosal administration was immunogenic in golden Syrian hamsters, and both vaccination methods effectively protected the lung from SARS-CoV-2 infection. Hamsters vaccinated by mucosal application of VSV∆G-SARS-CoV-2 also developed immunity that controlled SARS-CoV-2 replication in nasal tissue. FUNDING: The study was funded by Merck Sharp & Dohme, Corp., a subsidiary of Merck & Co., Inc., Rahway, NJ, USA, and The International AIDS Vaccine Initiative, Inc. (IAVI), New York, USA. Parts of this research was supported by the Biomedical Advanced Research and Development Authority (BARDA) and the Defense Threat Reduction Agency (DTRA) of the US Department of Defense.

Therapeutic efficacy of the small molecule GS-5734 against Ebola virus in rhesus monkeys.

The most recent Ebola virus outbreak in West Africa, which was unprecedented in the number of cases and fatalities, geographic distribution, and number of nations affected, highlights the need for safe, effective, and readily available antiviral agents for treatment and prevention of acute Ebola virus (EBOV) disease (EVD) or sequelae. No antiviral therapeutics have yet received regulatory approval or demonstrated clinical efficacy. Here we report the discovery of a novel small molecule GS-5734, a monophosphoramidate prodrug of an adenosine analogue, with antiviral activity against EBOV. GS-5734 exhibits antiviral activity against multiple variants of EBOV and other filoviruses in cell-based assays. The pharmacologically active nucleoside triphosphate (NTP) is efficiently formed in multiple human cell types incubated with GS-5734 in vitro, and the NTP acts as an alternative substrate and RNA-chain terminator in primer-extension assays using a surrogate respiratory syncytial virus RNA polymerase. Intravenous administration of GS-5734 to nonhuman primates resulted in persistent NTP levels in peripheral blood mononuclear cells (half-life, 14 h) and distribution to sanctuary sites for viral replication including testes, eyes, and brain. In a rhesus monkey model of EVD, once-daily intravenous administration of 10 mg kg(-1) GS-5734 for 12 days resulted in profound suppression of EBOV replication and protected 100% of EBOV-infected animals against lethal disease, ameliorating clinical disease signs and pathophysiological markers, even when treatments were initiated three days after virus exposure when systemic viral RNA was detected in two out of six treated animals. These results show the first substantive post-exposure protection by a small-molecule antiviral compound against EBOV in nonhuman primates. The broad-spectrum antiviral activity of GS-5734 in vitro against other pathogenic RNA viruses, including filoviruses, arenaviruses, and coronaviruses, suggests the potential for wider medical use. GS-5734 is amenable to large-scale manufacturing, and clinical studies investigating the drug safety and pharmacokinetics are ongoing.

AVI-7288 for Marburg Virus in Nonhuman Primates and Humans.

BACKGROUND: AVI-7288 is a phosphorodiamidate morpholino oligomer with positive charges that targets the viral messenger RNA that encodes Marburg virus (MARV) nucleoprotein. Its safety in humans is undetermined. METHODS: We assessed the efficacy of AVI-7288 in a series of studies involving a lethal challenge with MARV in nonhuman primates. The safety of AVI-7288 was evaluated in a randomized, multiple-ascending-dose study in which 40 healthy humans (8 humans per dose group) received 14 once-daily infusions of AVI-7288 (1 mg, 4 mg, 8 mg, 12 mg, or 16 mg per kilogram of body weight) or placebo, in a 3:1 ratio. We estimated the protective dose in humans by comparing pharmacokinetic variables in infected nonhuman primates, uninfected nonhuman primates, and uninfected humans. RESULTS: Survival in infected nonhuman primates was dose-dependent, with survival rates of 0%, 30%, 59%, 87%, 100%, and 100% among monkeys treated with 0 mg, 3.75 mg, 7.5 mg, 15 mg, 20 mg, and 30 mg of AVI-7288 per kilogram, respectively (P<0.001 with the use of the log-rank test for the comparison of survival across groups). No safety concern was identified at doses up to 16 mg per kilogram per day in humans. No serious adverse events were reported. Drug exposure (the area under the curve) was dose-dependent in both nonhuman primates and humans; drug clearance was independent of dose but was higher in nonhuman primates than in humans. The protective dose in humans was initially estimated, on the basis of exposure, to be 9.6 mg per kilogram per day (95% confidence interval, 6.6 to 12.5) for 14 days. Monte Carlo simulations supported a dose of 11 mg per kilogram per day to match the geometric mean protective exposure in nonhuman primates. CONCLUSIONS: This study shows that, on the basis of efficacy in nonhuman primates and pharmacokinetic data in humans, AVI-7288 has potential as postexposure prophylaxis for MARV infection in humans. (Funded by the Department of Defense; ClinicalTrials.gov number, NCT01566877.).

Protection against filovirus diseases by a novel broad-spectrum nucleoside analogue BCX4430.

Filoviruses are emerging pathogens and causative agents of viral haemorrhagic fever. Case fatality rates of filovirus disease outbreaks are among the highest reported for any human pathogen, exceeding 90% (ref. 1). Licensed therapeutic or vaccine products are not available to treat filovirus diseases. Candidate therapeutics previously shown to be efficacious in non-human primate disease models are based on virus-specific designs and have limited broad-spectrum antiviral potential. Here we show that BCX4430, a novel synthetic adenosine analogue, inhibits infection of distinct filoviruses in human cells. Biochemical, reporter-based and primer-extension assays indicate that BCX4430 inhibits viral RNA polymerase function, acting as a non-obligate RNA chain terminator. Post-exposure intramuscular administration of BCX4430 protects against Ebola virus and Marburg virus disease in rodent models. Most importantly, BCX4430 completely protects cynomolgus macaques from Marburg virus infection when administered as late as 48 hours after infection. In addition, BCX4430 exhibits broad-spectrum antiviral activity against numerous viruses, including bunyaviruses, arenaviruses, paramyxoviruses, coronaviruses and flaviviruses. This is the first report, to our knowledge, of non-human primate protection from filovirus disease by a synthetic drug-like small molecule. We provide additional pharmacological characterizations supporting the potential development of BCX4430 as a countermeasure against human filovirus diseases and other viral diseases representing major public health threats.

Discovery and early development of AVI-7537 and AVI-7288 for the treatment of Ebola virus and Marburg virus infections.

There are no currently approved treatments for filovirus infections. In this study we report the discovery process which led to the development of antisense Phosphorodiamidate Morpholino Oligomers (PMOs) AVI-6002 (composed of AVI-7357 and AVI-7539) and AVI-6003 (composed of AVI-7287 and AVI-7288) targeting Ebola virus and Marburg virus respectively. The discovery process involved identification of optimal transcript binding sites for PMO based RNA-therapeutics followed by screening for effective viral gene target in mouse and guinea pig models utilizing adapted viral isolates. An evolution of chemical modifications were tested, beginning with simple Phosphorodiamidate Morpholino Oligomers (PMO) transitioning to cell penetrating peptide conjugated PMOs (PPMO) and ending with PMOplus containing a limited number of positively charged linkages in the PMO structure. The initial lead compounds were combinations of two agents targeting separate genes. In the final analysis, a single agent for treatment of each virus was selected, AVI-7537 targeting the VP24 gene of Ebola virus and AVI-7288 targeting NP of Marburg virus, and are now progressing into late stage clinical development as the optimal therapeutic candidates.