In a recent study posted to the bioRxiv* preprint server, researchers used aerosolized receptor binding domain (RBD)-62 with 1,000-fold enhanced binding affinity for angiotensin converting enzyme-2 (ACE-2) to treat rhesus macaque models challenged with the Delta variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Study: RBD-based high affinity ACE2 antagonist limits SARS-CoV-2 replication in upper and lower airways. Image Credit: CoronaBorealis tudio/Shutterstock.com
*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.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
After the onset of the coronavirus disease 2019 (COVID-19) pandemic, the rapid development of various types of vaccines, including the messenger ribonucleic acid (mRNA) vaccines, significantly reduced the morbidity and mortality associated with SARS-CoV-2 infections.
However, the emergence of the Omicron variant and its sub-variants has challenged the immunity induced by primary and booster vaccine doses and previous SARS-CoV-2 infections.
While many of the variants of SARS-CoV-2 have contained mutations in the RBD region of the spike protein, the Omicron variants are known to have the largest number of mutations in the spike protein region, enabling immune escape.
While antivirals such as molnupiravir and Paxlovid have been effective in reducing the severity of SARS-CoV-2 infections and decreasing the number of hospitalizations, the use of antivirals has been judicious since it poses the risk of drug-resistant mutations developing in subsequent SARS-CoV-2 variants.
This also increases the need for alternative therapeutic agents that target the disease in a variant-agnostic approach. In this respect, therapies that target the host cell and prevent the virus from successfully infecting the cell remain more relevant in the face of emerging SARS-CoV-2 variants with novel mutations.
About the study
In the present study, the researchers tested the efficacy of a mutated SARS-CoV-2 RBD developed in vitro, which binds to the ACE-2 receptor with high affinity without inhibiting the enzymatic activity of ACE-2.
This mutated protein, RBD-62, has a 1,000-fold higher binding affinity for the ACE-2 receptor than the RBD of the ancestral Wuhan-Hu-1 strain.
Furthermore, in vitro, experiments have demonstrated that a half-maximal inhibitory concentration (IC50) of 18 pM of RBD-62 can effectively block SARS-CoV-2 Beta variant infection.
Experiments with Syrian hamster models have also demonstrated that inhaled RBD-62 can protect against weight loss during infection with the USA-WA1/2020 strain of SARS-CoV-2.
The present study used rhesus macaque models and tested the efficacy of RBD-62 in protecting against infection with the Delta variant, which is believed to be the most pathogenic of all the SARS-CoV-2 variants to the macaques.
The macaques were treated daily for five days with aerosolized RBD-62 administered via the airways using a nebulizer. The treatment was stopped after the fifth day to observe the viral rebound kinetics during the infection.
Bronchoalveolar lavage samples and nasal swabs were collected at different time points during the study for ribonucleic acid (RNA) extraction and detection of viral replication based on the amplification of the sub-genomic RNA coding for the transcript of the nucleocapsid protein.
A tissue culture infectious dose assay was also conducted to measure the culturable virus and determine the transmissibility potential of the virus.
The mucosal and serum levels of immunoglobulin g (IgG) binding titers against various RBDs, including those from the wild type, Delta, and Omicron BA.1 variant, were also evaluated to determine the impact of the treatment on the development of primary or secondary immune responses.
Results
The results reported that treatment with RBD-62 provided equal protection in both lower and upper airways, which had not been observed for the immunity induced by any of the clinically approved COVID-19 vaccines.
Furthermore, while RBD-62 treatment was successful in protecting against infection with the SARS-CoV-2 Delta variant, it did not result in the development of drug resistance or hinder the formation of memory responses against the Delta variant.
The optimized aerosol delivery of the RBD-62 treatment improved drug delivery, specifically into the respiratory tract and lungs, significantly decreasing the viral replication rates during the treatment.
Furthermore, even after the cessation of the treatment after the fifth day, the viral titers in the bronchoalveolar lavage samples were found to be low, and the responses of the mucosal and serum IgG were not found to be affected by the RBD-62 treatment.
In addition, the B- and T-cell immunity also exhibited no signs of anti-drug immunity, indicating that RBD-62 could be reused during subsequent infections.
Conclusions
Overall, the findings reported that treatment of SARS-CoV-2 Delta infection in rhesus macaques with aerosolized RBD-62, which binds to the ACE-2 receptor of the host cells, protected the virus in both lower and upper airways.
Additionally, RBD-62 treatment did not result in anti-drug resistance or impede the formation of secondary immune responses.
The results suggested that RBD-62 could reduce disease severity during SARS-CoV-2 infections, irrespective of the variant.
*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.
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