Monoclonal antibodies directed against SARS-CoV-2: synthetic neutralizing antibodies, the REGN-COV2 antibody cocktail

October 7, 2020

Robin E Ferner*†, Jeffrey K Aronson
On behalf of the Oxford COVID-19 Evidence Service Team
Centre for Evidence-Based Medicine
Nuffield Department of Primary Care Health Sciences
University of Oxford
*University of Birmingham
University College London
Correspondence to

Update added on 6 September 2021

A monoclonal antibody “cocktail”, REGN-COV2, reduced mortality in Ebola virus infection from 51% to 34%, and the current carefully chosen pair of monoclonal antibodies against SARS-CoV-2 reduced viral replication and lung damage in experimental animals. Only randomized controlled trials in humans with COVID-19 will show whether REGN-COV2 has clinically important benefits, with a favourable benefit to harm balance. Use outside clinical trials is currently generally unwarranted.

The development of monoclonal antibodies by Georges Köhler and César Milstein, in their attempt to understand the mechanisms by which cells of the human immune system are capable of producing so diverse a repertoire of antibodies, was recognized in the award of the Nobel Prize for Physiology or Medicine in 1984, shared with Niels Jerne. In the last two decades such antibodies have been used to treat a wide range of conditions, including malignancies and autoimmune disorders. New techniques for synthesizing monoclonal antibodies now make it possible to use transgenic mice to generate antibodies similar to human antibodies.

A virus infection, due to Ebola virus, has been treated in successful experiments using either humanized antibodies generated from mice or antibodies obtained from convalescent human patients. This suggests the possibility of treating COVID-19 with monoclonal antibodies directed against structural elements of the virus.

Fusion of coronaviruses with human cells requires “docking” of specialized glycoproteins, “spikes”, which protrude from the viral surface and interact with specific molecules on the surface of host cells. The virus binds, through the spike protein’s receptor binding domain, to a membrane bound enzyme, ACE-2, and its entry into the cell is activated by a transmembrane serine protease, TMPRSS2. The spike protein, whose sequence differs greatly from one type of coronavirus to another, presents a target for therapy by antiviral antibodies.

Production of REGN-COV2 monoclonal antibodies
A research group, almost all employees of the manufacturers, Regeneron Pharmaceuticals, studied antibodies to the SARS-CoV-2 coronavirus produced by mice with genetically humanized immune systems. Simultaneously, clones of single B-cells from convalescent patients allowed them to produce a very large library of different human antibodies that could bind to SARS-CoV-2.

One strategy to treat a virus infection is to develop antibodies that bind to a region of the virus that is highly conserved. However, this strategy cannot be used against the receptor binding domain, as mutations in the spike glycoprotein are frequent, making possible the evolution of mutants resistant to a single antibody. Over 7000 variants of the genome coding for the SARS-CoV-2 spike protein had been identified by April 2020. From the library the scientists had developed, they selected pairs of antibodies that could simultaneously bind to different parts of the receptor binding domain of the SARS-CoV-2 spike protein. In this way, they hoped to reduce the risk that viral mutants resistant to treatment would emerge. The antibodies were divided into four groups, according to the four epitopes in the spike protein to which they could bind. Two antibodies, REGN10933 and REGN10987, were considered likely to be more effective than either alone, because they bound to two different epitopes. 

The group then studied the activity of single and combined monoclonal antibodies against variants of the virus spike protein (“SARS-CoV-2 spike pseudoparticles”), produced in a vesicular stomatitis virus (VSV) engineered to contain the SARS-CoV-2 spike protein gene, and observed whether resistance occurred. In some cases, resistant mutants emerged readily—often as the result of a change in a single amino-acid—and were 10,000 to 100,000 times less sensitive to the antibody than their progenitors. One pair of antibodies, the combination (“cocktail”) of antibodies, REGN10933 and REGN10987 “did not rapidly select for mutants, presumably because escape would require the unlikely occurrence of simultaneous viral mutation at two distinct genetic sites, so as to ablate binding and neutralization by both antibodies in the cocktail.

REGN antibodies in Ebola virus infection
In a trial of four investigational therapies for Ebola virus infection in the Democratic Republic of Congo, 681 patients of any age who had a positive rt-PCR result for Ebola virus RNA were enrolled. They were randomly assigned to a triple chimeric monoclonal antibody ZMapp intravenously, or remdesivir, or a single monoclonal antibody MAb114, or a triple monoclonal antibody REGN-EB3. The primary end point was death at 28 days. After an in interim analysis of the results showed that MAb114 and REGN-EB3 were associated with lower mortality than ZMapp and remdesivir, only the first two were continued, on the recommendation of the data and safety monitoring board. At 28 days, there were 61 deaths in 174 patients (35%) who had been given MAb114 versus 84 of 169 (50%) in the ZMapp group (P = 0.007), and 52 of 155 (34%) in the REGN-EB3 group compared with 79 of 154 (51%) in a ZMapp subgroup (P = 0.002). A shorter duration of symptoms before admission and lower baseline viral loads correlated with improved survival. Three serious adverse events were reported, none of which was judged to be related to the interventions.

REGN antibodies in SARS-CoV-2 infection in experimental animals
A preprint, published in August 2020, described studies of the REGN10933 + REGN10987 antibody cocktail, now labelled REGN-COV2, in animal models of SARS-CoV-2 infection.

Rhesus macaques were given different doses of cocktail at different times before or after inoculation with different doses of SARS-CoV2 virus. At post mortem five days after infection, untreated monkeys showed lung injury, while treatment before or after infection reduced the incidence and intensity of pneumonia. Only two of four monkeys given the higher dose of REGN-COV2 developed pneumonitis (Table 1).

Table 1. Lung injury in macaque monkeys according to timing and dosage of REGN-COV2

 Dose in macaques Lung injury with interstitial pneumonia Lobes affected by inflammation in individual monkeys
Placebo 3 of 4* 0, 1, 2, 3
Prophylaxis 0.3 mg/kg 3 of 4 0, 2, 2, 3
Prophylaxis 50 mg/kg 1 of 4 0, 0, 0 ,1
Therapy 25 mg/kg 2 of 4 0, 1, 1, 0
Therapy 150 mg/kg 2 of 4 0, 1, 1, 0


The cocktail also reduced virus replication in golden hamsters, which suffer a more serious clinical illness from SARS-CoV-2 than macaques (Table 2).

Table 2. Viral RNA in the lungs of hamsters according to timing and dosage of REGN-COV2

Therapeutic dose in hamsters Log median genomic RNA/g Log median subgenomic RNA/g
Placebo 8.4 6.25
0.5 mg/kg 7.3 3.7
5 mg/kg 6.2 1.7
50 mg/kg 6.4 1.7


Current clinical trials
Regeneron registered two trial protocols on 11 June 2020 and one on 19 August 2020, titled:

  1. Safety, Tolerability, and Efficacy of Anti-Spike (S) SARS-CoV-2 Monoclonal Antibodies for the Treatment of Ambulatory Adult Patients With COVID-19
  2. Safety, Tolerability, and Efficacy of Anti-Spike (S) SARS-CoV-2 Monoclonal Antibodies for Hospitalized Adult Patients With COVID-1
  3. Study Assessing the Safety, Tolerability, Pharmacokinetics, and Immunogenicity of Repeated Subcutaneous Doses of Anti-Spike (S) SARS-CoV-2 Monoclonal Antibodies (REGN10933+REGN10987) in Adult Volunteers as Related to COVID-19


The intention in these trials is to randomize 5914 patients in all. The following description is taken from the first of these trials. They specified three phases:

Phase 1
To evaluate the safety and tolerability of REGN10933+REGN10987 compared to placebo.
To evaluate the virologic efficacy of REGN10933+REGN10987 compared to placebo in reducing viral shedding of SARS-CoV-2

Phase 2
To evaluate the virologic efficacy of REGN10933+REGN10987 compared to placebo in reducing viral shedding of SARS-CoV-2

Phase 3
To evaluate the clinical efficacy of REGN10933+REGN10987 compared to placebo”.

The trial is a multicentre, randomized, quadruple-masked, placebo-controlled comparison with low-dose and high-dose REGN COV2, intended to recruit 2104 patients, with a long list of relevant primary and secondary outcome effect measures, including all-cause mortality as a secondary outcome in phases 2 and 3. Pharmacokinetics and adverse events are also included.

In the UK REGN-COV2 has also been added to the protocol for the adaptive randomized controlled RECOVERY trial. Participants aged at least 12 years will be given a single dose of REGN10933 + REGN10987 8 g (4 g of each monoclonal antibody) in 250 ml of 0.9% saline infused intravenously over 60 minutes ± 15 minutes as soon as possible after randomization.

At least two other very small randomized, placebo-controlled, double-masked, phase 1 trials of monoclonal antibodies directed against structural elements of SARS-CoV-2 have also been registered: NCT04441918 and NCT04483375.

Update added on 6 September 2021

Since the early studies, summarized above, Regeneron Pharmaceuticals Inc have marketed a formulation, branded REGEN-COV® in the USA and Ronapreve® in the UK, containing a combination of two neutralizing IgG1 recombinant human monoclonal antibodies, casirivimab and imdevimab, which target different epitopes on the SARS-CoV-2 spike protein.

The antibodies are given by consecutive subcutaneous injection or combined intravenous infusion, and the formulation has recently been authorized by the UK regulator, the MHRA, “for the prophylaxis and treatment of acute Covid-19 infection”. The authorization is temporary. The FDA has also recently extended its Emergency Use Authorization for the cocktail, originally approved in November 2020, to cover “emergency use as post-exposure prophylaxis in certain adults and pediatric individuals”.

Prophylactic use of the combination antibody treatment by subcutaneous injection was investigated in a company-led, randomized placebo-controlled study of 1505 people over the age of 12 years; none had previously had covid-19, and one-third were at high risk of severe disease if they contracted it. They were treated within 96 hours of a diagnosis of SARS-CoV-2 infection in a household contact. The primary end-point was symptomatic infection confirmed by quantitative PCR.

Of 753 patients who received REGEN-COV, 11 (1.6%) developed symptomatic covid-19, compared with 59/752 (7.8%) who received placebo. The results were presented in terms of relative risk reduction, but the absolute risk reduction was 6.2%, equivalent to a number-needed-to treat (NNTB) of 16. The viral load was lower, and symptoms resolved more rapidly, in infected patients who had received the active treatment. There were few reported adverse events in either placebo or active treatment groups.

The UK’s Summary of Product Characteristics advises that “administration should be under conditions where management of severe hypersensitivity reactions, such as anaphylaxis, is possible”.

Disclaimer: This article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited should be checked. The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.