3 lab studies show antiviral Paxlovid effective against Omicron: Pfizer

US drug maker Pfizer has released three lab studies, not peer-reviewed yet, which demonstrate that its antiviral Paxlovid is effective against the Omicron variant of Covid-19.

In a statement, Pfizer noted that the in vitro studies suggest that Paxlovid has the potential to maintain plasma concentrations many-fold times higher than the amount required to prevent Omicron from replicating in cells.

“We specifically designed Paxlovid to retain its activity across coronaviruses, as well as current variants of concern with predominantly spike protein mutations,” said Mikael Dolsten, Chief Scientific Officer of Pfizer, in the statement.

“These data suggest that our oral Covid-19 therapy can be an important and effective tool in our continued battle against this devastating virus and current variants of concern, including the highly transmissible Omicron,” he added.

The clinical findings had earlier shown that Paxlovid reduces risk of hospitalisation or death by nearly 90 per cent compared to placebo for high-risk patients when treated within five days of symptom onset.

Paxlovid is administered as three tablets (two tablets of nirmatrelvir and one tablet of ritonavir) taken together orally twice daily for five days, for a total of 30 tablets.

It consists of nirmatrelvir, which inhibits a SARS-CoV-2 protein to stop the virus from replicating, and ritonavir, which slows down nirmatrelvir’s breakdown to help it remain in the body for a longer period at higher concentrations.

In the first in vitro study conducted by Pfizer, nirmatrelvir was tested against the Mpro — an enzyme that the coronavirus needs to replicate — from several SARS-CoV-2 variants of concern (VoCs), including Omicron, in a biochemical assay.

The results showed in all cases that nirmatrelvir was a potent inhibitor of its target. It could bind both the Omicron and the original Washington variant, indicating its continued ability to prevent in vitro viral replication.

In the second in vitro study by Pfizer, nirmatrelvir was tested against several SARS-CoV-2 VoCs, including Omicron, in an antiviral, cell-based assay.

The drug’s potency showed a concentration that is effective in producing 50 per cent of the maximal response for the Omicron variant, reaffirming its robust in vitro antiviral activity.

These results are in line with the values that have been observed for other VoCs (Alpha, Beta, Gamma, Delta, Lambda, and Mu) in this assay.

The third study, conducted by the Icahn School of Medicine at Mount Sinai (Icahn Mount Sinai) in collaboration with Pfizer, used a SARS-CoV-2-specific immunofluorescence-based assay.

In this assay, treatments were tested against the Alpha, Beta, Delta, and Omicron variants in two cell lines. The drug’s efficacy indicated that it had the concentration needed to inhibit infection by half.

These findings are consistent with several other studies and “provide confirmation to Pfizer’s findings that nirmatrelvir is the only orally administrable, authorised/approved compound which, to date, has been shown to have low nanomolar in vitro activity against Omicron,” the company said.

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