Intranasal nanoparticle STING agonist with immunoantiviral activity against SARS-CoV-2 variants

By Neha MathurApr 21 2022Reviewed by Danielle Ellis, B.Sc.

In a recent study posted to the bioRxiv* preprint server, researchers developed an immunoantiviral, NanoSTING, and demonstrated its broad-spectrum antiviral properties in hamsters and mice.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

NanoSTING is a nanoparticle formulation of cyclic guanosine monophosphate–adenosine monophosphate (cGAMP), a potent natural immunotransmitter. The researchers demonstrated that it broadly neutralized multiple respiratory viruses, including drug-resistant strains of influenza A and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

The stimulator of the interferon genes (STING) pathway enables a broad innate immune response against viruses, including SARS-CoV-2. Mechanistically, STING activation involves type I and III interferons (IFN-I and IFN-III) and controls early SARS-CoV-2 infection in the upper respiratory tract.

There is an unmet medical need for prophylactics and early post-exposure therapeutics to prevent coronavirus disease 2019 (COVID-19) and reduce SARS-CoV-2 transmission. Although vaccines are the preferred and necessary means of protection against SARS-CoV-2, they are inadequate to combat the SARS-CoV-2 for several reasons, as follows:

i) the mutational plasticity of SARS-CoV-2 facilitates its evolution, and vaccines cannot be custom manufactured for each newer variant.

ii) vaccines are inequitably distributed globally, making immunization of the entire human population impossible.

iii) as with influenza, eventually, people will stop taking additional booster shots facilitating the spread of SARS-CoV-2.

iv) Lastly, but most importantly, vaccines could prevent COVID-19 but not SARS-CoV-2 transmission.

Similarly, monoclonal antibodies (mAbs) are expensive, administered only in clinical settings, and largely ineffective against many SARS-CoV-2 variants, including Omicron. Oral antivirals, such as paxlovid and molnupiravir, are approved for use in humans but are susceptible to SARS-CoV-2 evolution and resistance.

Immunomodulators, such as defective interfering particles (DIPs), inhibit the replication of the wild-type SARS-CoV-2 and have demonstrated efficacy in small animal models; however, these need to be generated for each virus individually. Likewise, defective viral genomes (DVGs) need to replicate in vivo after administration limiting their therapeutic potential. The issue with aerosolized interferons, such as IFN-α2b, is that the timing of its administration is critical to achieving maximum efficacy.

Lipid nanoparticles in complex with the defective genomes can mitigate all the aforementioned concerns, and NanoSTING constituted of naturally occurring lipids has already been tested in humans.

About the study

In the present study, researchers tested the efficacy of a nanoparticle formulation of cGAMP, termed NanoSTING, against multiple VOCs of SARS-CoV-2 in hamsters and influenza A in mice. They showed the high translational potential of small amounts of NanoSTING. Notably, intranasal delivery of NanoSTING enabled the sustained release of cGAMP to the nasal compartment and the lungs of test animals for up to 48 hours, activating multiple antiviral pathways and facilitating IFN-I and IFN-III-mediated responses.

The team modified a spontaneously immortalized monocyte-like cell line (THP-1) and stimulated these cells with NanoSTING at a dose from 2.5-10.0 µg to confirm their ability to induce interferon responses. They observed a low level of luciferase activity at six hours, and secretion was maximal at 24 hours with five µg and 10 µg NanoSTING. Further, they performed kinetic measurements for 24 hours by measuring the luciferase activity in the supernatant.

Although cGAMP is a potent natural activator of the STING pathway, its in vivo half-life is ~35 minutes. Therefore, the researchers delivered varying amounts of NanoSTING (10-40 μg) intranasally to groups of BALB/c mice. Next, they harvested the nasal turbinates and lungs of the test mice. They performed a quantitative enzyme-linked immunosorbent assay (ELISA) to assay cGAMP.

Likewise, the team investigated the impact of intranasal NanoSTING administration in Syrian Golden hamsters.

Study findings

Dynamic light scattering (DLS) analysis revealed that the NanoSTING had a mean particle diameter of 98 nm, with a polydispersity index of 25.1% and a zeta potential of -40 mV. Furthermore, the authors found that NanoSTING was immunologically active and remained stable even without refrigeration.

The authors observed a dose-dependent increase in the concentration of cGAMP in the nasal turbinates of test mice. At a 10 μg dose, cGAMP was detected for up to 12 hours returning to baseline at 24 hours, whereas at the higher doses (20- 40 μg), they detected cGAMP for 24 hours and its reversion at 48 hours.

While in the mice lungs, cGAMP was detectable at the higher doses (20 and 40 μg); however, it was not undetectable at any timepoints in the sera of the test animals, even at the 40 μg dose. Together, these findings confirmed that the NanoSTING transported cGAMP to nasal passages in a concentration and time-dependent manner without systemic exposure.

In the hamster model, the transport volume of liquid formulations of NanoSTING did not impact its biodistribution to the lung and nasal compartments.

Hamsters receiving 60 µg of NanoSTING daily for four consecutive days showed no changes in clinical signs, such as temperature or body weight, compared to controls. In the lungs of these hamsters, NanoSTING efficiently engaged both interferon-dependent and interferon-independent antiviral pathways. In vivo, relative interferon ratios (RIR) of 0.27 and 0.67 sufficiently achieved a 50% and 100% reduction in SARS-CoV-2 titer, respectively, compared to peak interferon responses observed upon SARS-CoV-2 infection.

Conclusions                                                                                          

In the current study, a single dose of intranasal NanoSTING remarkably worked as a prophylactic and therapeutic against multiple respiratory viruses, including neutralization-resistant SARS-CoV-2 variants such as Omicron. Moreover, it conferred durable protection against highly pathogenic SARS-CoV-2 variants, Alpha and Delta, without any need for retreatment.

Overall, NanoSTING showed the potential to prevent respiratory viral diseases in susceptible populations. Therefore, it could be used for rapid intervention in respiratory infections much before disease etiology is determined.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.