key: cord-0848144-p8xog6lk
authors: Zhou, Zeqi; Wang, Xiangbin; Fu, Yankai; Zhang, Xiqing; Liu, Changxiao
title: Neutralizing antibodies for the treatment of COVID-19
date: 2020-11-16
journal: Acta Pharm Sin B
DOI: 10.1016/j.apsb.2020.10.025
sha: 4bc19f42b0ab6aa8e26d69a571855ff2068d0bf6
doc_id: 848144
cord_uid: p8xog6lk

nan

To the Editor:

Coronavirus disease 2019 (COVID-19) is a global epidemic caused by the infection of severe acute respiratory coronavirus 2 (SARS-CoV-2) and its mortality rate was up to 20 times higher than that of influenza. The virus is infectious, strange and variable. The treatment of it must be urgent and targeted. It is not enough to wait for a vaccine available to contain and prevent its spreading, we need to take all kinds of feasible measures to deal with the severe challenge from the virus transmission and mutation.

Using convalescent plasma to treat COVID-19 patients is an emergency plan in an early stage of the outbreak. Up to date, more evidences have been provided that COVID-19 convalescent plasma contains neutralizing antibodies against the virus infection, so the "plasma therapy" has been already applied in some countries and has achieved promising outcomes. A Chinese research team treated 5 COVID-19 critical illness patients with the convalescent plasma and achieved an effective treatment indicating the neutralizing antibody has played a therapeutic role.

Although the plasma antibody therapy has demonstrated its efficacy, it has the limitations as:

1) the consent of the convalescent must be obtained before a blood donation; 2) the amount of the plasma is difficult to meet the clinical needs; 3) the plasma must undergo strict blood biosafety test and inactivation of the virus; 4) the endogenous non-specific antibodies in the plasma may cause the antibody dependent enhancement (ADE).

However, using modern bioengineering technologies to develop the neutralizing antibodies for the treatment of COVID-19 could make up for the defects of the plasma treatment scheme to a great extent.

Currently, a number of pharmaceutical and academic teams have explored multiple optional techniques, including but not limited to single B cell sorting, phage display, humanized animals, and high throughput sequencing (HTS), etc. A typical development process is provided in the paper 1 . Those technologies have the following advantages: 1) the antibody with extremely high affinity and superior neutralizing function can be selected from a huge pool; 2) the selected antibodies can be engineered to remove or change the sites related to potential side effect; 3) the selected antibodies can be constructed into multivalent or multi-specific neutralizing antibodies, aiming at improving affinity or avidity, and competing with the endogenous antibodies or the naïve receptors of the virus; 4) the selected antibodies can be produced massively to meet their great clinic demands. The updates of the development of the neutralizing antibodies by means of the above technologies are summarized in Table 1 and more clinical applications can be found in published article 2 . Table 1 Based on the above updates, the humanized antibody drugs have been tested in the animal J o u r n a l P r e -p r o o f studies and clinical trials and demonstrated promising therapeutic potentials, and no significant side effects found yet. The following points of view are presented for discussions:

As mentioned in the Table 1 It has been shown that the recently published nano-bodies were bound to those unique epitopes on the S protein RBD to block the interactions of ACE2 or non-neutralizing antibody.

This implies further evidence in support of the possibility for the neutralizing antibody drug to block the binding site of the endogenous antibodies.

In general, after the human body is infected by the virus, it elicits a large quantity of the neutralizing antibodies and nonspecific endogenous ones. Once the specific neutralizing antibodies are identified, their affinity competition analysis needs to be done for screening the specific neutralizing antibodies with the affinity constants [K D ] between 5-39 nmol/L 4 . Those antibodies with a high affinity and/or avidity could block the interference of the endogenous antibodies. Furthermore, a US team reported that the naïve Llamas single-domain antibody library-derived neutralizing nano-bodies blocked the interactions of S protein RBD with ACE2, indicating the antibodies could neutralize the virus. Additionally, they used cryo-EM to reveal that nano-bodies bind to all of three S protein RBDs. The crystal structures of the nano-body-RBD complex showed how both nano-bodies recognized the same epitope, which partly overlapped with the ACE2 binding sites, and blocked RBD-ACE2 interaction 4 .

Once the neutralizing antibody is developed, preclinical research should be performed before the clinical study. This has been illustrated by the following work 3 : 1) the in vitro neutralization ability of CA1 and CB6 against SARS-CoV-2 was investigated using pseudovirus and the live virus; 2) CB6 in vivo was tested in a Rhesus macaque model of SARS-CoV-2 infection in both prophylactic and treatment settings, and a strong protection effect of the antibody was observed.

During the process of selecting the high affinity antibody, the rare antibody recognizing the epitope of interest can be picked up and functionally assayed. Endogenous antibodies are polyclonal antibodies that can recognize either neutralizing or non-neutralizing antibodies with a binding affinity at the different levels. The antibody recognizing non-neutralizing epitopes or with a low or an ordinary affinity does not have a neutralizing function. The specific neutralizing antibodies with a high affinity or avidity work well by competing with ACE2 or the endogenous antibodies at the same epitopes or recognizing the unique different epitopes. This process provides an alternative route to developing therapeutic neutralizing antibody drugs.

As presented in Table 1 Regarding the ADE effect, the bio-engineering technologies can be used to change the Fc molecular conformation of the neutralizing antibody, and avoid its combining with FcR complex, then remove ADE effect before the clinical use (Fig. 1) . For example, the LALA mutation of CB6 monoclonal antibody could lower the risk of Fc-mediated acute lung injury. It is a key to develop the neutralizing antibodies with the high affinity and avidity. They can be promisingly used for the treatment of COVID-19 5 .

Insert Fig. 1 The novel SARS-CoV-2 is more transmissible than the previous virus and causes more serious illness than influenza. In addition, the virus mutates rapidly. One of the vigorous approaches is to develop those antibodies with a high affinity and avidity to treat COVID-19 by using bioengineering technologies.

In conclusion, we have the confidence that it is feasible to use the neutralizing antibodies for clinical application in the treatment of COVID-19. The evidences have shown that they are safe, effective and no significant allergy. Several teams in the world have been developing a number of candidates of the neutralizing antibody drugs which have entered the preclinical studies and clinical trials. It is expected the some of them could be launched for the therapy of COVID-19 in the near future.

J o u r n a l P r e -p r o o f Zeqi Zhou was responsible for writing the letter. Xiangbin Wang, Yankai Fu and Xuqing Zhang provided the data. Changxiao Liu reviewed the letter.

There are no conflicts of interest. 

Human neutralizing antibodies elicited by SARS-CoV-2 infection

Fruitful neutralizing antibody pipeline brings hope to defeat SARS-CoV-2

A human neutralizing antibody targets the receptor-binding site of SARS-CoV-2

Neutralizing nanobodies bind SARS-CoV-2 spike RBD and block interaction with ACE2

COVID-19: immunopathology and its implications for therapy