Pfizer unveils its oral SARS-CoV-2 inhibitor

Drugmaker Pfizer revealed its oral COVID-19 antiviral clinical candidate PF-07321332 on Tuesday at the American Chemical Society Spring 2021 meeting. The compound, which is currently in Phase 1 clinical trials, is the first orally administered compound in the clinic that targets the main protease (also called the 3CL protease) of SARS-CoV-2, the virus that causes COVID-19. By inhibiting the main protease, PF-07321332 prevents the virus from cleaving long protein chains into the parts it needs to reproduce itself. Dafydd Owen, director of medicinal chemistry at Pfizer, presented the compound in a symposium of the Division of Medicinal Chemistry.

Last year, Pfizer reported PF-07304814, a different small molecule inhibitor of SARS-CoV-2’s main protease. The work to develop that compound began during the 2002-2003 outbreak of SARS-CoV, severe acute respiratory syndrome. But that molecule can only be given intravenously, which limits its use to hospital settings.

Because PF-07321332 can be taken orally, as a pill or capsule, it could be given outside of hospitals if it proves to be safe and effective. People who have been exposed to SARS-CoV-2 could take it as a preventative measure, for example.

“For the foreseeable future, we will expect to see continued outbreaks from COVID-19. And therefore, as with all viral pandemics, it’s important we have a full toolbox on how to address it,” Charlotte Allerton, Pfizer’s head of medicine design, told C&EN.

PF-07321332 was developed from scratch during the current pandemic. It’s a reversible covalent inhibitor that reacts with one of the main protease’s cysteine residues. Owen also discussed the chemistry involved in scaling up the compound. The first 7 mg of the compound were synthesized in late July 2020. Encouraged by the early biological data, the Pfizer team aimed to scale up the synthesis. By late October, they’d made 100 g of the compound. Just two weeks later, the chemists had scaled up the synthesis to more than 1 kg. Owen said 210 researchers had worked on the project.

Ana Martinez, who studies COVID-19 treatments at the Spanish National Research Council CSIC and also presented during the symposium, told C&EN that having a COVID-19 antiviral is of critical importance. She eagerly anticipates the safety and efficacy data from the trials of PF-07321332. “Hopefully we will have a new drug to fight against COVID-19,” Martinez said. And because the molecule targets the main protease, she said that it might be useful for fighting other coronaviruses and preventing future pandemics.