A promising new study, released as a preprint on the bioRxiv* server, reports on the high neutralizing efficacy of a polyclonal swine antibody against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen responsible for the novel coronavirus disease 2019 (COVID-19). By demonstrating the ability of this antibody to neutralize all currently circulating viral variants, the study suggests that it is ideal for development as a new and very effective therapy for COVID-19.
Antibodies against SARS-CoV2
In the absence of effective antiviral drugs, or vaccines, until the latter became available late in the pandemic, passive immunization by means of convalescent plasma (CP), or synthetic monoclonal antibodies (mAbs), was resorted to in an attempt to help seriously ill patients fight the virus until their own immune response kicked in.
As of now, the use of CP with high titers of virus-specific antibodies, given within 72 hours of symptom onset in mildly ill patients, is associated with a reduced risk of progression to severe disease to approximately a fourth. This result is linked to CP with a titer of 1:3,200, and CP with lower antibody titers offers about 70% immunity.
Another 50 mAbs are being synthesized, with neutralizing activity against the virus. These target the spike protein. Some cocktails (from Eli Lilly and Regeneron) have shown their protective efficacy against severe disease, reducing the risk by about 70% if given early enough, in patients with mild to moderate symptoms who are at high risk for progression to severe COVID-19.
These cocktails have received emergency use authorization in both the USA and Europe.
However, a new threat is now being posed by the emergence of escape mutations in novel SARS-CoV-2 variants. The UK variant cannot be neutralized by most anti-NTD antibodies, and requires a higher concentration of several anti-RBD antibodies for the expected efficacy.
The protection offered by CP works only with the wildtype virus. Studies have shown poor CP efficacy against the South African variant, also called variant of concern (VOC) 501Y.V2.
The lack of neutralization activity for half the CP recipients is due to changes at three mutations within the viral spike protein. These mutations comprise K417N, E484K, and N501Y, of which the second seems to affect binding and neutralization most.
All three are in the receptor-binding domain (RBD) of the viral spike protein, which engages the host cell receptor and mediates viral entry.
The 501Y.V2 lineage also carries some NTD mutations, such as L18F, D80A, D215G, Δ242-244, and R246I, which allow the spike to evade neutralization, at least partly, by multiple mAbs. This could encourage the emergence of escape variants by exerting a positive selection pressure.
Polyclonal antibodies, which recognize multiple epitopes on the spike protein, have been explored as another therapeutic option, since their breadth of epitope binding makes them more likely to overcome the effects of alterations in one or a few viral antigens.
These have been shown to have efficacy against the virus in vitro. These were raised against SARS-CoV-2 in humanized or glycohumanized animals. Unlike wildtype polyclonal antibodies, these proved to be well tolerated in humans. The former are known to cause serum sickness or allergic reactions in up to a third of users unless immunomodulating drugs are also used.
XAV-19 produces complete neutralization
The current study examines a glycohumanized swine polyclonal antibody, called XAV-19, that neutralizes the Wuhan D614G strain of the virus, for its ability to show the same activity against the UK and South African (SA) variants as well.
The researchers used batches of XAV-219 with neutralizing activity against the viral RBD ranging from a 50% inhibitory concentration (IC50) of 2 to 12 μg/ml. These were then tested against wildtype spike, as well as spike mutants containing N501Y, N439K, Y453F or E484K. Finally, spike mutants with the combinations of spike alterations found in the UK and SA variants were assayed for neutralization.
All single mutations were fully neutralized at IC50, similar to that of the Wuhan type, albeit slightly lower for the E484K mutant. The antibody completely inhibited both UK- and SA-type spike proteins, with the IC50 being 6.4, 4.0 μg/ml, respectively, compared to 4.5 μg/ml for the Wuhan spike.
In contrast, banlanivimab, an earlier mAb from Eli Lilly, was also able to inhibit both the Wuhan and the UK variants with a low IC50 of 0.01μg/ml, but could not inhibit the SA spike.
Complete neutralization of live virus infection of human Vero cells was also demonstrated with a concentration above 5 μg/ml of XAV-19, but zero activity below 1.5 μg/ml. The IC50 against Wuhan, UK and SA strains was 2.2 μg/ml for the first two, and 3.2 μg/ml, respectively.
Again, bamlanivimab showed a low IC50 of 0.01 μg/ml against the Wuhan and UK lineages but could not neutralize the SA lineage at any concentration.
The researchers also carried out two separate experiments to find the TCID100 (tissue culture infective dose 100, which is the concentration required to produce 100% inhibition of the cytopathogenic effect (CPE). For the UK strain, it was 0.78 μg/ml, compared to 1.56 μg/ml with the Wuhan strain. Partial neutralization was observed below 0.78 μg/ml, down to 0.1 μg/ml, for the UK strain, but not for the Wuhan strain.
What are the implications?
This preprint is authored by employees of the company Xenothera which produces such glycohumanized polyclonal antibodies.
The study shows that this polyclonal swine antibody can completely neutralize all three variants of SARS-CoV-2, namely, the Wuhan, UK and SA strains, at concentrations between 1 to 5 μg/ml. The SA variant is less susceptible to neutralization at lower concentrations, and the UK lineage is more sensitive to this antibody.
Further research will show if this difference truly exists, and if so, its cause. Nonetheless, a clear range of concentrations is demonstrable, within which the UK and SA variants can be neutralized completely, a clear difference being observable from bamlanivimab, which shows little neutralizing activity on the SA variant.
The neutralizing activity shown in the ELISA (enzyme-linked immunosorbent assay) format can be used as a predictor of activity against SARS-CoV-2 variants of concern. This finding is useful in predicting the activity of numerous mAbs and more than half a dozen polyclonal antibodies raised against the virus, which are undergoing clinical trials.
Most of these were raised against the Wuhan strain, and must be re-tested for neutralizing capacity against the newer lineages.
XAV-19 is now under testing in the POLYCOR trial, having already passed phase 2a safety testing at a median serum concentration of 50 μg/ml. The current study shows a high neutralization capacity of all variants at 5 μg/ml.
These findings indicate that XAV-19 “can provide high and sustained therapeutic activity with an inhibitory quotient well above 10.” Its uniformly high activity against both point and grouped mutations in the new variants indicate it is robust to such changes.
The study may indicate that XAV-19 deserves further attention, in the current scenario where the UK variant is becoming globally dominant, with other variants of concern emerging.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.