Researchers led by scientists at the National Institute of Allergy and Infectious Diseases (NIAID) have developed antibodies that can simultaneously bind to two different antigens, targeting multiple regions of the SARS-CoV-2 spike protein. These bispecific antibodies successfully neutralize the original virus as well as the Alpha, Beta, Gamma, and Delta variants.

Emerging SARS-CoV-2 variants of concern threaten the efficacy of existing vaccines. Additional potent antibody-based therapeutics that target multiple sites of the spike protein are critically needed to neutralize emerging variants in infected patients and guide vaccine design.

“In the face of rapidly emerging SARS-CoV-2 variants that challenge our efforts to end the pandemic, our findings support the further exploration of bispecific antibodies that strategically combine antibody pairs as new tools to treat COVID-19,” the authors noted.

These findings are published this week in the Science Translational Medicine article, “Bispecific antibodies targeting distinct regions of the spike protein potently neutralize SARS-CoV-2 variants of concern.”

At present, COVID-19 antibody treatments inject a cocktail of individual monoclonal antibodies each targeting a specific region on the virus. Most investigators developing SARS-CoV-2-specific monoclonal antibodies have used antigen probe-based methods to isolate memory B cells or a mixture of plasmablasts and memory B cells.

The current study led by NIAID scientists Peter Crompton, MD, and Joshua Tan, PhD, shows combining selected monoclonal antibodies into bispecific antibodies can create stronger antibodies that are more potent than the monoclonal cocktails. One bispecific antibody that the authors generated demonstrates a potency 100 times greater than that of a cocktail of its monoclonal parents in vitro.

Moreover, the current study does not rely on antigen probe-based cell sorting to generate a large panel of monoclonal antibodies from plasmablasts and memory B lymphocytes of patients who have recovered from COVID-19. Instead, they combine potent monoclonal antibodies that bind to non-overlapping regions of the viral spike protein to generate bispecific antibodies.

The team showed two of their bispecific antibodies neutralized the original virus as well as the Alpha, Beta, Gamma, and Delta variants. Three of the most potent bispecific antibodies that they generated target distinct regions of the receptor-binding domain of the spike protein, and all three neutralized the SARS-CoV-2 Alpha and Beta variants.

Their most potent bispecific antibody (CV503) binds to the ridge region of the SARS-CoV-2 receptor-binding domain and competes with the angiotensin-converting enzyme 2 receptor, the host protein that binds to the viral spike protein. Analyzing crystal structures, the authors showed that CV503 had minimal contact with the key variant residues: K417, E484, and N501.

To demonstrate potential therapeutic applicability, the authors showed two of their bispecific antibodies protected hamsters from the clinical disease at a dose of 2.5 mg/kg body weight. One bispecific antibody that neutralized the Beta variant in vitro, the authors showed, protected hamsters against SARS-CoV-2 expressing the E484K mutation.

The team developed the bispecific antibodies from a pool of 216 monoclonal antibodies that target SARS-CoV-2. They collected these monoclonal antibodies from plasmablasts and memory B cells of convalescing COVID-19 patients and screened them for potency against the virus. They identified five bispecific antibodies that inhibit the virus at concentrations of less than 1 ng/mL.

Part of the reason why these bispecific antibodies are especially effective against SARS-CoV-2 variants is that they are designed to bind non-overlapping areas of the viral spike and do not encroach on regions of the viral spike protein that are subject to change due to new mutations in emerging variants.

“Bispecific antibodies represent a promising next-generation countermeasure against SARS-CoV-2 variants of concern,” the authors noted.