BACKGROUND: The emergence of new SARS-CoV-2 variants of concern B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta) that harbor mutations in the viral S protein raised concern about activity of current vaccines and therapeutic antibodies. Independent studies have shown that mutant variants are partially or completely resistant against some of the therapeutic antibodies authorized for emergency use. METHODS: We employed hybridoma technology, ELISA-based and cell-based S-ACE2 interaction assays combined with authentic virus neutralization assays to develop second-generation antibodies, which were specifically selected for their ability to neutralize the new variants of SARS-CoV-2. FINDINGS: AX290 and AX677, two monoclonal antibodies with non-overlapping epitopes, exhibit subnanomolar or nanomolar affinities to the receptor binding domain of the viral Spike protein carrying amino acid substitutions N501Y, N439K, E484K, K417N, and a combination N501Y/E484K/K417N found in the circulating virus variants. The antibodies showed excellent neutralization of an authentic SARS-CoV-2 virus representing strains circulating in Europe in spring 2020 and also the variants of concern B.1.1.7 (Alpha), B.1.351 (Beta) and B.1.617.2 (Delta). In addition, AX677 is able to bind Omicron Spike protein just like the wild type Spike. The combination of the two antibodies prevented the appearance of escape mutations of the authentic SARS-CoV-2 virus. Prophylactic administration of AX290 and AX677, either individually or in combination, effectively reduced viral burden and inflammation in the lungs, and prevented disease in a mouse model of SARS-CoV-2 infection. INTERPRETATION: The virus-neutralizing properties were fully reproduced in chimeric mouse-human versions of the antibodies, which may represent a promising tool for COVID-19 therapy. FUNDING: The study was funded by AXON Neuroscience SE and AXON COVIDAX a.s.

AXON COVIDAX a s ; Bratislava 811 02 Slovakia; AXON Neuroscience R and D Services SE; Bratislava 811 02 Slovakia

AXON Neuroscience R and D Services SE; Bratislava 811 02 Slovakia

AXON Neuroscience R and D Services SE; Bratislava 811 02 Slovakia; Institute of Neuroimmunology Slovak Academy of Sciences; Bratislava 845 10 Slovakia

AXON NEUROSCIENCE SE; Larnaca 6016 Cyprus

Biomedical Research Center Institute of Virology Slovak Academy of Sciences; Bratislava 845 05 Slovakia

Biomedical Research Center Institute of Virology Slovak Academy of Sciences; Bratislava 845 05 Slovakia; Department of Microbiology and Virology Faculty of Natural Sciences Comenius University in Bratislava; Bratislava 842 15 Slovakia

Czech Centre of Phenogenomics Institute of Molecular Genetics ASCR v v i Prumyslova 595 252 50 Vestec Czech Republic

Department of Applied Informatics Faculty of Mathematics Physics and Informatics Comenius University in Bratislava; Bratislava 842 48 Slovakia

Department of Biochemistry Faculty of Natural Sciences Comenius University in Bratislava; Bratislava 842 15 Slovakia

Department of Computer Science Faculty of Mathematics Physics and Informatics Comenius University in Bratislava; Bratislava 842 48 Slovakia

Institute of Neuroimmunology Slovak Academy of Sciences; Bratislava 845 10 Slovakia

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branisovska 31 CZ 37005 Ceske Budejovice Czech Republic

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branisovska 31 CZ 37005 Ceske Budejovice Czech Republic; Veterinary Research Institute Hudcova 70 CZ 62100 Brno Czech Republic

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branisovska 31 CZ 37005 Ceske Budejovice Czech Republic; Veterinary Research Institute Hudcova 70 CZ 62100 Brno Czech Republic; Department of Experimental Biology Faculty of Science Masaryk University Kamenice 753 5 CZ 62500 Brno Czech Republic

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