Speaking of neutralizing antibodies, everyone must be familiar with it.
At the beginning of the outbreak of the new crown epidemic, many scientific research teams began to look for "special drugs" for treating the new crown in the plasma of the recovered people - neutralizing antibodies. Today, a number of monoclonal neutralizing antibodies at home and abroad have been approved for the treatment and prevention of new crown pneumonia.
To understand why monoclonal neutralizing antibodies are the "all-rounder" in the fight against COVID-19, we have to start 132 years ago.
It can fight against incurable cancers and can deal with the most intense viruses
In 1890, the 36-year-old Emil Adolf von Behring published a landmark study in a German journal[1]: plasma from animals immune to diphtheria can be used to treat diphtheria infections. Emile Adolf von Belling was awarded the first Nobel Prize in Physiology or Medicine in 1901 for his work.
It is also this research that lays the foundation for the study of humoral immunity, indicating the presence of neutralizing antibodies against pathogenic microorganisms in the plasma of convalescent patients. In other words, human cognition of antibodies begins with anti-infection.
▲Screenshot of the official website of the Nobel Prize
Convalescent plasma therapy (CPT) has been used to treat influenza virus, respiratory syncytial virus (RSV), Ebola virus, and other coronavirus infections [3]. Similarly, convalescent plasma is also used in the treatment of COVID-19.
Theoretically, convalescent plasma can respond quickly to outbreaks of novel viruses. However, convalescent plasma also has some deficiencies, such as poor stability and accessibility, lack of large randomized controlled clinical studies to demonstrate the benefits and risks it poses to patients, etc. [3]. The above deficiencies limit the clinical application of convalescent plasma to a certain extent.
Fortunately, science has been advancing. In the 85 years since the publication of Emil Adolf von Belling's paper, scientists have successively discovered B cells that produce antibodies, and have also figured out the structure of antibodies at atomic-level resolution, making our understanding of antibodies clearer and clearer.
By 1975, the technology to make the antibody truly a drug emerged. That year, Georges K hler and César Milstein invented the technology to produce monoclonal antibodies,[4] for which they were awarded the Nobel Prize in Physiology or Medicine. The birth of this revolutionary technology has made monoclonal antibodies a rising star in the pharmaceutical industry.
In the field of oncology that we are best known for, targeted therapy or immunotherapy antibody drugs for targets such as CD20, HER2, EGFR and PD-1/L1 have emerged, and their birth has rewritten the therapeutic paradigm of cancer.
In the field of antivirals, monoclonal antibodies also play an important role.
According to statistics, as of March 2021, at least 21 neutralizing antibodies in nine infectious disease areas, including respiratory syncytial virus infection and Ebola virus infection, have entered the late clinical stage or been approved for marketing [3]. Among them, the first approved antiviral neutralizing antibody dates back to 1998, when the US FDA approved the monoanthan antibody palivizumab for the prevention of respiratory syncytial virus infection in children.
It is worth mentioning that the first approved therapeutic drug for Ebola virus, a highly toxic virus with a biosafety level of 4, is also neutralizing antibodies. In October 2020, the U.S. FDA approved the monoclonal antibody combination Inmazeb (atoltivimab, maftimab, and odesivimab-ebgn) for the treatment of adults and children infected with Ebola virus [6]. Just two months later, the FDA approved a second drug to treat Ebola virus infection, the neutralizing antibody Ebanga (Ansuvimab-zykl) [7]. To this day, the approved anti-Ebola virus drugs still only have these two neutralizing antibody combinations, which shows the important position of neutralizing antibodies in the field of antivirals.
Screening and mechanism of action of monoclonal neutralizing antibodies
So how exactly did the antiviral monoclonal neutralizing antibody develop?
From the research of Emile Adolf von Behring more than one hundred and thirty years ago, it is not difficult to find that there are antiviral neutralizing antibodies in the plasma of infected people who have recovered from infection. However, it is not easy to find the most powerful neutralizing antibody against viruses.
Simply put, the process can be divided into five steps: obtaining the plasma of the recovered, finding the B cells that produce neutralizing antibodies, identification, antibody characterization and screening (some antibodies are further modified). Each step involves multiple methods and evaluation parameters.
▲Neutralizing antibody screening process[3]
Take the anti-new coronavirus monoclonal neutralizing antibody combination of Tempex Bo Pharmaceutical combhaclesumab/romimavir monoclonal antibody combination (BRII-196/198), which were screened and modified from the plasma of 8 Shenzhen new crown infected people in nearly two months by researchers from Tengsheng Bo Pharmaceutical, Tsinghua University and Shenzhen Third People's Hospital.
According to the research paper, they isolated a total of 206 monoclonal antibodies that can specifically bind to the binding region (RBD) of the new coronavirus spike protein receptor, and analyzed the characteristics of binding to RBD in detail, and finally screened out a pair of neutralizing antibodies with the highest activity and super complementarity [8,9]. Based on this pair of antibodies, the researchers engineered a longer half-life combination of ampavirinumab/romizumab.
As a passive immune mode, the mechanism of action of monoclonal neutralizing antibodies for the treatment of new crown infection mainly includes the following aspects.
First, block the binding of the new coronavirus RBD to human cellular angiotensin-converting enzyme 2 (ACE2) to achieve the purpose of preventing the new coronavirus from infecting human cells; second, the neutralizing antibody can bind to the infected cells expressing viral antigens, promote macrophage phagocytosis, activate antibodies and complement-dependent apoptosis, and accelerate the clearance of infected cells; third, after the neutralizing antibody binds to the virus, it can promote the virus to be engulfed and cleared by macrophages [3].
▲Neutralizing antibody antiviral mechanism[3]
Treatment of COVID-19 infection with an unlimited range of use and a longer window period
It can be seen from the above that in the outbreak of new infectious diseases, screening strong neutralizing antibodies from the body of the recovered person is a rapid and effective measure for human beings to respond to the epidemic. The neutralizing antibodies approved during the COVID-19 pandemic are prime examples.
In fact, since the end of 2020, a number of neutralizing antibodies have been granted emergency use authorization (EUA) by the US FDA for the prevention and treatment of new crown pneumonia. For example, Lilly's bamlanivimab/etesevimab reduced the risk of hospitalization and death in patients with mild and moderate illness at high risk by 87%[10], and the regenerative casirivimab/imdevimab reduced the risk of hospitalization and death by 70% [11], compared with sotrovimab's 79% [12,13]. Although another Eli Lilly neutralizing antibody, bebtelovimab, has not yet published data for phase III clinical studies, it has also been granted emergency use authorization by the U.S. FDA [14].
The first anti-coronavirus drug approved by the State Drug Administration of China (NMPA) was also a neutralizing antibody, ampavirinumab/romimab [15]. From the final analysis of the Phase III study, patients in the ampavirusumab/romimab-treated group had a 80 percent lower risk of hospitalization and death compared with the placebo group (p-value and significant benefit within 10 days of symptom onset [16]. It is also based on the above data that ambavir monoclonal antibody/romizumab is also included as an antiviral program in the newly released Diagnosis and Treatment Plan for Novel Coronavirus Pneumonia (Trial Ninth Edition) [17].
▲New coronavirus (Image source: NIAID-RML)
Since last November, the Omilon (B.1.1.529 or BA.1) mutant strain has gradually swept the world. The loss or decrease in activity of many neutralizing antibodies due to the presence of more than 30 mutations on the Omilon mutant strain spike protein, although the neutralizing activity of both sorovimab and ambavir/romizumab combination was not significantly affected [18,19].
Earlier this year, when scientists monitored the evolution of the Omiljung (BA.1) mutant strain, they discovered a new subtype of BA.2. In subsequent studies, scientists found that Omiljung BA.2 was more transmissible than BA.1. Due to the differences in RBD between BA.2 and BA.1[21], the preventive and therapeutic effects of vaccines and neutralizing antibodies are challenged.
Judging from the National Institutes of Health(NIH) updated treatment guidelines for COVID-19 on April 8[22], neutralizing antibodies are only recommended for non-hospitalized adult patients. Because basic studies have confirmed that the neutralizing activity of bebtelovimab against Omexon BA.2 is not affected [23]. However, key Phase III clinical data on bebtelovimab have not yet been released, so the NIH recommends a level of CIII for it [22].
▲NiH recommended treatment regimen
As for sotrovaimab, which maintained neutralization activity against the Omicron BA.1, it was not recommended by the NIH due to the announcement of deactivation by the US FDA on April 5. According to the information released by the US FDA[24], the reason why sotrovimab was discontinued was mainly due to the decline in neutralization activity and insufficient blood concentration.
Specifically[24], sotrovimab's EC50 for BA.2 decreased by a factor of 15.7 compared to the wild type, and two studies showed that sotrovimab's EC90 for BA.2 decreased by a factor of 48.1 (1.476 ug/ml) and 25.3 times (0.68 ug/ml), respectively. From the pharmacokinetic data of sorovimab, the maximum plasma concentration of sotrovimab after a single injection (500 mg) is 143 ug/ml, less than 10 times that of EC90 [24]. It is based on the above information that the FDA believes that the previously recommended dose of sotorovimab is not suitable for BA.2.
It should be noted that a study published by researchers at the University of Washington School of Medicine showed that the neutralizing activity of sotrovimab against BA.2 decreased, but it was still able to alleviate BA.2 infection with the lungs of human ACE2-susceptible mice by binding the Fc segment to effector cells [25].
The neutralizing activity of the combination of ambavir monoclonal antibody/romizumab against BA.2 has also decreased, and in vitro pseudovirus experiments have shown that the IC50 of this combination on BA.2 is 1.13 ug/ml (data provided by Temsex BoDrug). However, because the combination of intravenous administration (1000 mg/1000 mg), the plasma concentration can peak rapidly within about 5 hours, and even one week after injection, the plasma concentration of this combination can be maintained at (202 +154) ug/ml, which is about 220 times greater than that of BA.2 neutralizing the active IC50 [26]. Therefore, it is currently believed that this combination still has a therapeutic effect on BA.2.
Of course, whether it is bebtelovimab, or sorovimab, as well as the ampavirinumab/romimavir combination, how effective they are on BA.2 requires further clinical studies to confirm.
▲New coronavirus (Image source: NIAID-RML)
In clinical treatment, neutralizing antibodies also have several irreplaceable advantages.
First, the use of the small molecule antiviral drugs Paxlovid (nematvir/ritonavir tablets) and Remdesivir recommended by the NIH is affected by renal function in patients, and is not recommended or restricted if the patient's glomerular filtration rate (eGFR) < 30 mL/min [22]. These patients are more likely to develop breakthrough infections and progress to severe disease [27] and are appropriate for early administration of monoclonal neutralizing antibodies.
Second, the ritonavir contained in Paxlovid is a strong inhibitor of CYP3A, and many commonly used drugs are metabolized through CYP3A, which may increase the toxicity of these drugs. In addition, there are drugs that enhance the activity of CYP3A, and these drugs in combination with Paxlovid may reduce the blood concentration of nematvir. For example, commonly used cardiovascular drugs such as amiodarone, clopidogrel, rivaroxaban, etc. have interactions with ritonavir. In the latest updated treatment guidelines, the NIH also explicitly lists drugs that cannot be used with Paxlovid or require dose adjustment [22]. Monoclonal neutralizing antibodies also do not have this problem.
Third, among the currently approved antiviral drugs, the treatment window (10 days for symptoms) is the longest for both small molecules and neutralizing antibodies [9,10,12,15,22]. This also means that this combination can benefit more patients. Finally, long-life neutralizing antibodies are effective in maintaining blood concentrations and providing long-term protection for patients. We will expand on this issue in the next part.
To prevent infection with COVID-19, neutralizing antibodies are a powerful complement to vaccines
Another important role of neutralizing antibodies is to prevent COVID-19 infection.
While public health measures and vaccines are important means of preventing COVID-19, both are deficient. In the case of vaccination, the first problem it faces is that aging, immunocompromise, the presence of underlying diseases, and viral variants can affect the effectiveness of vaccines [28,29]; the second problem is that the protective efficacy of vaccines also decreases with the duration of vaccination .[30] Therefore, in addition to the two, an effective means of prevention is needed to make up for the shortcomings of the two.
▲Comparison of antibody titers at peaks in different populations and at the end of the trial[29]
Clinical research data prove that long-term neutralizing antibodies are an important long-term passive immune tool.
For example, results of a Phase 3 clinical trial of post-exposure prophylaxib/imdevimab in which the neutralizing antibody combination Casirivimab/imdevimab were conducted in a single dose subcutaneously reduced the risk of symptomatic COVID-19 infection by 81% [31], while data from the phase 3 clinical study of long-acting neutralizing antibody combination titxagevimab/cilgavimab showed median follow-up of 83 days, compared with placebo. This combination reduces the risk of developing symptoms of COVID-19 infection by 77% [32].
Currently, the above two preventive measures are authorized by the FDA for post-exposure prophylaxis [33] and pre-exposure prophylaxis [34] emergency use, respectively. In addition, the bamlanivimab/etesevimab combination[35] is also authorized by the FDA for emergency use for post-exposure prevention.
From the FDA-designated tixagevimab/cilgavimab population[36], people who are moderately to severely impaired due to medical reasons, or who are unable to respond adequately to the COVID-19 vaccine due to receiving immunosuppressive drugs or treatments, and who do not recommend that any approved or authorized COVID-19 vaccine be vaccinated because of a history of severe adverse reactions (such as severe allergic reactions) to the COVID-19 vaccine and/or the components of the COVID-19 vaccine, benefit from the preventive function of the neutralizing antibodies.
▲New coronavirus (Image source: NIAID-RML)
However, after the Omikejong BA.2 mutant strain became the dominant epidemic strain, only the long-acting neutralizing antibody combination tixagevimab/cilgavimab was still recommended by the FDA for pre-exposure prophylaxis of COVID-19.
From the instructions published by the FDA, the semi-maximum effect concentration (EC50) of tixagevimab/cilgavimab (150 mg/150 mg) on BA.1 decreased by 12-30 times compared with the wild-type reference strain, so the FDA doubled the dose of tixagevimab/cilgavimab (300 mg/300 mg) according to the phase I pharmacokinetic study. This combination reduced the semi-maximum effect concentration (EC50) of BA.2 by a factor of 5.2, still maintaining its effectiveness against BA.2, and the current FDA-recommended dose of tixagevimab/cilgavimab is still 300 mg/300 mg [37].
It is worth mentioning that the neutralizing antibody combination of tixagevimab/cilgavimab provides long-term protection because the Fc segment of tixagevimab/cilgavimab has been modified by YTE and the half-life has been increased to more than 80 days [38]. The Fc segment of the mainland approved ambavir monoclonal antibody/romizumab has also undergone YTE modifications, significantly increasing the half-life [26], giving the combination the potential to provide long-term protection.
The above content shows that the new crown neutralizing antibody is a powerful supplement to the new crown vaccine, which can effectively protect people who cannot be vaccinated or have poor vaccine effect. For high-risk people who are already infected, they also have the potential to provide long-term protection against re-infection after application.
In general, antibodies as a drug have been widely used in the clinic, and the birth of monoclonal antibody drugs has rewritten the treatment paradigm of many diseases, including cancer, and many patients have been reborn.
In the field of antivirals, monoclonal neutralizing antibodies, as a drug that can be quickly obtained and can be used for the treatment and prevention of infectious diseases, show its anti-epidemic "all-rounder" characteristics.
▲Monoclonal neutralizing antibodies can be used for multiple stages of COVID-19 prevention and treatment
It is conceivable that in the process of human combat against the new crown epidemic, monoclonal neutralizing antibodies will become a powerful complement to the new crown vaccine, while working with small molecule antiviral drugs to save more patients from hospitalization or death, and provide long-term immune protection for users.
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The author of this article 丨BioTalker