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SARS-CoV-2 Antibody Cocktail Prevents Rapid Emergence of Antibody Resistance
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Mark Mascolini
Combining anti-SARS-CoV-2 antibodies to treat COVID-19 appeared to stifle viral escape by mutation in meticulous cell studies by researchers at the US firm Regeneron Pharmaceuticals [1]. When the investigators assessed two antibodies that bind to nonoverlapping regions of the target RBD viral spike protein, the combination prevented antibody resistance by requiring the unlikely emergence of simultaneous mutations to two distinct genetic sites.
The Regeneron team noted that targeting the spike protein of SARS-CoV-2, which permits viral binding to target cells via ACE2 receptors, holds promise for treatment of COVID-19. But treatment with an individual antibody that homes to a single viral protein, like the RBD spike protein, can be overwhelmed if antispike mutations evolve and lead to antibody resistance.
Initial attempts to control HIV infection showed that single antiretroviral agent therapy led to prompt emergence of resistance mutations that quickly rendered treatment useless. Therapy succeeded only when investigators combined antiretrovirals that would allow HIV to escape control only by simultaneously mutating at multiple genetic positions. Such simultaneous mutation cannot occur if the combined antiretrovirals stop HIV replication.
Contemplating a similar approach with diverse anti-SARS-CoV-2 antibodies led Regeneron scientists to identify a large assortment of potent, fully human neutralizing antibodies targeting the viral RBD spike protein [2]. From this collection of neutralizing antibodies that can simultaneously bind to the RBD spike, they aimed to find partners for a therapeutic cocktail that could both control viral replication and "might also protect against antibody resistance due to virus escape mutants that could arise in response to selective pressure from single antibody treatments."
Using the VSV pseudoparticle system, the researchers determined that their top 8 neutralizing antibodies remained potent against all SARS-CoV-2 variants collected through the end of March 2020 representing over 7000 unique viral genomes. Next they forced the selection of escape mutants to single antibodies and antibody combinations in a replicating VSV-SARS-CoV-2-S virus. Some mutants quickly became fixed in the viral population and proved resistant to antibody concentrations up to 50 ug/mL (a concentration about 10,000 to 100,000 times greater than the 50% inhibitory concentration). Single amino acid changes prevented binding to antibodies the researchers had originally selected for breadth against all known RBD variants.
Analyzing 22,872 unique SARS-CoV-2 viral sequences collected through the end of May 2020, the Regeneron investigators determined that natural variants resistant to individual antiviral antibodies are rare in nature, "but these escape variants could easily be selected and amplified under the pressure of ongoing antibody treatment."
Then the researchers evaluated viral escape after combined treatment with 2 neutralizing antibodies (REGN10987 and REGN10933), a cocktail rationally designed to avoid mutation-driven escape by using two antibodies that bind to distinct, nonoverlapping regions of the RBD spike. Attempts to grow VSV-SARS-CoV-2-S virus during treatment with this 2-antibody cocktail did not result in emergence of escape mutants. Resistance mutations did not emerge, the Regeneron team suggested, "presumably because escape would require the unlikely occurrence of simultaneous viral mutation at two distinct genetic sites" to prevent binding and neutralizing by the two cocktail antibodies.
The research team also assessed treatment with (1) two combinations of antibodies that completely or partially compete for binding to the RBD protein (resulting in rapid emergence of escape mutants to one combination but not the other) and (2) a combination of antibodies with complete competition for binding to the RBD protein (resulting in ablation of antibody neutralization with a single amino acid substitution).
The investigators proposed that "a clinical candidate selection criterion for broad potency that includes functional assessment against naturally circulating sequence variants, as well as inclusion of multiple antibodies with non-overlapping epitopes, may provide enhanced protection against loss of efficacy."
References
1. Baum A, Fulton BO, Wloga E, et al. Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies. Science. 15 Jun 2020: eabd0831. Full-text: https://doi.org/10.1126/science.abd0831
2. Hansen J, Baum A, Pascal KE, et al. Studies in humanized mice and convalescent humans yield a SARSCoV-2 antibody cocktail. Science. 10.1126/science.abd0827 (2020). doi:10.1126/science.abd0827
The data described herein strongly support the notion that cocktail therapy may provide a powerful way to minimize mutational escape by SARS-CoV-2; in particular, our studies point to the potential value of antibody cocktails in which two antibodies were chosen so as to bind to distinct and non-overlapping regions of the viral target (in this case, the RBD of the spike protein), and thus require the unlikely occurrence of simultaneous mutations at two distinct genetic sites for viral escape. A clinical candidate selection criterion for broad potency that includes functional assessment against naturally circulating sequence variants, as well as inclusion of multiple antibodies with non-overlapping epitopes, may provide enhanced protection against loss of efficacy. Future in vivo animal and human clinical studies need to pay close attention to possible emergence of escape mutants and potential subsequent loss of drug efficacy.
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