On December 8, 2021, China's first anti-COVID-19 drug - ambavir monoclonal antibody / romizumab (BRII-196/198) - was approved[1], so that China's anti-epidemic epidemic entered a new stage of "seedlings and drugs", and the news of the approval of this monoclonal antibody combination was once on the hot search of major media platforms.
Unexpectedly, the Omicron (B.1.1.529) strain that appeared in South Africa on November 9 quietly landed in Tianjin on the day after the approval of ambavirumab/romizumab Chinese mainland, bringing great pressure to China's anti-epidemic work.
In mid-November, more than 30 mutations were found in the Omiljung spike (S) protein, including 15 mutations in the receptor binding region (RBD) necessary to infect human cells alone[2]. Therefore, many researchers believe that Omilon will inevitably seriously affect the effectiveness of vaccines and monoclonal antibody drugs.
▲ Mutation on the S protein of Omi kerong (Image source: https://www.gisaid.org/)
It was also true that studies soon found that some of the approved neutralizing antibodies showed varying degrees of failure [3]. For a while, many people were worried about the fate of China's first approved anti-COVID-19 antibody combination.
The good news is that on the evening of December 12, Temsheng Bo Pharmaceutical released the news that laboratory neutralization data show that the combination of ampavirinumab/romizumab has maintained neutral activity against the novel coronavirus variant Omikeron [4]. Two days later, the team of he Dayi, a well-known virologist and professor at Columbia University School of Medicine, found that in the neutralization experiment against the Omiljung mutant strain, the neutralizing activity of 19 antibodies except for Sotrovamab and romimab was not affected, and the neutralization activity of other antibodies was reduced [5].
This news undoubtedly gave the Chinese people a "shot in the arm".
So why is it that the combination of ambavir monoclonal antibody/romizumab remains neutral to Omikeron despite most antibody failures?
From the combination of ambavir monoclonal antibody/romisvir monoclonal antibody alone, we think there are at least two reasons. Let's take a look at them.
Super complementary
At the beginning of the research and development, the Tengsheng Bo pharmaceutical team, which has experience in the field of antivirals, chose the neutralizing antibody combination therapy and chose a pair of neutralizing antibodies with super complementarity. The purpose of this is actually to avoid drug resistance.
In fact, in June 2020, a team found that non-competitive or partially competitive neutralizing antibody combinations could avoid the escape of THE NEW CROWN mutant [6]. This is also the reason why the FDA approves basically antibody combinations.
▲New coronavirus S protein model (Image source: NIAID-RML)
If so, why is the neutralization effect of some combinations greatly reduced, while the neutralization effect of ambavir monoclonal antibody/romizumab is maintained well? To answer this question, we have to start with the complementarity and neutralization mechanism of antibodies.
As we all know, the current neutralizing antibodies are all RBDs that target the S protein of the new coronavirus. You know, although RBD is small, there are many sites that can be used to neutralize antibody binding. The location of the neutralizing antibody binding to RBD is different, which must have a certain impact on the effect of the neutralizing antibody.
As early as 2020, David Veesler's team at the University of Washington studied the sites at which the neutralizing antibody antigen binding segment (Fab) binds to the S protein trimer of the new coronavirus with the help of cryo-electron microscopy, and finally identified six different sites (Ia, Ib, IIa, IIb, IIc, and IV) on the RBD of S protein [7].
▲ 6 different binding sites (where the black box represents the binding site of ACE2 and RBD, and the other 6 colors represent the site of monoclonal antibody binding to RBD)
It is not difficult to find from the above figure that the two sites of Ia and Ib coincide with the receptor binding motif (RBM) of ACE2 and RBD, but the regions and modes of coincidence are different. Neutralizing antibodies that bind to the Ia site can bind to RBD only when it is turned on, and the region coinciding with RBM is larger than that of Ib antibodies. Neutralizing antibodies that bind to the IB site can bind to RBD on or off and partially cover the RBM region. Therefore, the neutralizing antibodies that combine these two loci are all by competing with ACE to prevent the new crown virus from infecting human cells [7,8]. In other words, antibodies that bind to the Ia or IB site are competitive and not highly complementary.
IIa, IIb, and IIc are all antibodies that bind to the RBD covert site and can only bind to RBD when it is turned on, where IIa partially coincides with RBM and IIb and IIc gradually move away from RBM [7,8]. Although the three do not compete with ACE2 in terms of sites alone, studies have found that neutralizing antibodies bind to these three sites, hindering the binding of ACE2 to RBD elsewhere or space [7,8].
The IV site is the most distinctive, not only is it far from the core RBM of RBD, but studies have shown that antibodies bound to this site do not compete with ACE2 [7,9]. Moreover, whether RBD is turned on or off, the antibody can bind to this site.
▲Partial neutralization of the binding site of the antibody to RBD that has been approved for clinical application [7,8]
It is not difficult to see from the above description: the neutralizing antibody binding to the IV site is the most complementary to the neutralizing antibody binding to the two loci of Ia and IB. Coincidentally, the binding site of ampavirinumab is Ia, and the binding site of romizumab is IV [5,7,8], so the complementarity of the two is indeed super-strong.
Let's look at the neutralization mechanism of ambavir monoclonal antibody/romidavir monoclonal antibody.
Studies have shown that the binding epitope of amphavirmab to RBM and the binding epitope of ACE2 to RBM are highly consistent [10]. This highly coincident feature not only gives ampavirule a stronger competitive binding ability, but also makes it more resilient to viral escape due to mutations in some epitopes of RBD [11].
As for romimaclumab, it binds to the IV site and is not competitive with the ampaviconitumab binding to Ia. Not long ago, a research team found that antibodies binding to the IV site can inhibit the rearrangement of S proteins necessary for membrane fusion between viruses and cells, thereby inhibiting the occurrence of viral membrane fusion and preventing viruses from invading cells [12].
▲Schematic diagram of ambavir monoclonal antibody / romizumab combined with RBD (courtesy of Tengsheng Bo drug)
Overall, this super-binding site complementarity and different antiviral mechanisms may be one of the reasons why the combination of ampavirus monoclonal antibody/romizumab therapy maintains neutral activity on Omiqueron.
Ultra-high blood concentrations
So how well did the combination of ambavirumab/romimexumab maintain the neutralizing activity of Omexon? After all, the neutralization activity of ambavizumab binding to the Ia site has decreased [5].
Although in vitro experiments have shown that Omexon has some effect on the neutralization activity of ampavirine, the combined use of ampavirinumab/romisvirumab is not much affected [13]. In addition to the complementarity described earlier, blood concentrations are also a major contributor to maintaining neutral activity.
Let's first look at the blood concentration of the patient after injecting ampavirinumab/romizumab. In order to increase the blood drug concentration as soon as possible to enhance the antiviral effect, combined with the phase 1 clinical data, under the premise of ensuring safety, the researchers of Tengsheng Bo Pharmaceutical chose a single high-dose administration (1000mg/1000mg) method (NCT04518410).
After a single intravenous administration, it only takes about 5 hours for the concentration of blood antibodies to exceed 300 μg/ml, which is more than 1000 times that of IC50 and more than 100 times that of IC50 (unpublished data on Tengsheng Bo Medicine). That is to say, from the blood concentration after injection alone, ambavir monoclonal antibody / romizumab is sufficient to deal with the Omilkejung mutant strain.
Coronavirus (Image source: NIAID-RML)
In addition, it is worth mentioning that the crystallinetable segment (Fc) of ambavir monoclonal antibody / romizumab is modified. This modification has two benefits, on the one hand, the half-life of amphavir monoclonal antibody/romizumab to 2-3 times that of ordinary antibodies, extending from 21 days to 46-76 days [14,15], and on the other hand, significantly increasing the drug distribution of the target organ (lung) and increasing the concentration of lung antibodies [16]. Undoubtedly, these two points are also crucial for the maintenance of neutralizing antibody activity.
It is for the above two reasons that the neutralizing activity of ambavir maclonumab/romizumab on omiqueron is maintained. Of course, the specific mechanism behind it needs to be studied in more depth.
Overall, from the existing research, ampavirusumab and romizumab are combined with two completely different and non-interfering sites of RBD, which makes them truly functionally complementary. In addition, the ultra-high blood concentration and higher lung antibody concentration after treatment also helped to maintain neutralization activity of amphavir monoclonal antibody/romimab against all currently known variants.
It is understood that the research of Zhang Linqi's team at Tsinghua University on the neutralization of amphavir monoclonal antibody/romizumab and Omikejong is about to be published, and Professor Zhang is looking forward to bringing us more comprehensive answers.
bibliography:
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The author of this article 丨BioTalker