In 2019 and 2020, Pfizer's (NYSE:PFE) top-selling product was pneumococcal vaccine Prevnar 13. It generated over $5.8 billion in sales in both years. That's a lot of money for a very successful product.

But Prevnar 13 is no longer Pfizer's top-selling product. The COVID-19 vaccine BNT162b2, developed by Pfizer and BioNTech (NASDAQ:BNTX), generated sales of $7.8 billion in the first half of 2021 alone. Pfizer expects the vaccine will rake in $33.5 billion over the entire year. Even with the company splitting profits with BioNTech, Pfizer should conservatively make in the ballpark of $17 billion from BNT162b2 this year.

Even more money could be on the way. Pfizer is busy working on its next potential COVID-19 blockbuster -- and it isn't a vaccine.

A capsule with COVID-19 printed on it on top of a face mask.

Image source: Getty Images.

Stopping the coronavirus in its tracks

Pfizer has stated for a while that it wouldn't restrict its efforts to fight COVID-19 to only vaccines. It has also focused on developing potential treatments for the infectious disease. The company provided an update on one especially promising therapy in its second-quarter conference call last week.

Protease inhibitors are a class of antiviral drugs that have been effective in treating HIV and hepatitis C. These therapies bind to protease enzymes in viruses and prevent the virus from replicating. 

Pfizer initiated an early stage clinical study evaluating oral protease inhibitor PF-07321332 in March of this year. The company had good news to report from that study in its Q2 update.

Chief scientific officer Mikael Dolsten said that in the phase 1 study, PF-07321332 exceeded the level predicted to inhibit coronavirus viral replication by more than fivefold. Dolsten also stated that the experimental protease inhibitor showed powerful antiviral activity in preclinical testing that could be effective against "all currently known COVID-19 variants."

So far, the experimental antiviral drug appears to have a good safety profile. Dolsten said that there haven't been any safety issues in giving doses of up to 500 milligrams twice per day over a 10-day period.

Based on these encouraging results, Pfizer advanced the oral protease inhibitor into phase 2/3 testing in July. The company will evaluate PF-07321332 in five-day and 10-day treatments for individuals who have been in close contact with someone with COVID-19.

A big market opportunity

Pfizer estimates that the addressable market for its protease inhibitor could be in the hundreds of millions of patients. That doesn't seem farfetched considering the rapid spread of COVID-19 exposure due to the delta variant. 

The U.S. Food and Drug Administration (FDA) has already granted Emergency Use Authorization (EUA) to Regeneron's (NASDAQ:REGN) antibody cocktail REGEN-COV as a treatment for hospitalized COVID-19 patients and for post-exposure prophylaxis. However, there are a few drawbacks to Regeneron's therapy. 

First, REGEN-COV is expensive -- more than $2,000 per dose. Second, it must be administered via infusion or subcutaneous injection. Third, the current U.S. EUA for post-exposure prophylaxis only applies to individuals who have been exposed to COVID-19 who have a high risk of developing COVID-19 and who haven't been fully vaccinated.

Pfizer's PF-07321332 would be much more convenient than REGEN-COV since it's taken orally. Although the big drugmaker hasn't given any hints about what the pricing for the antiviral therapy might be should it win EUA or approval, a lower price tag could open up a wide market that might include lower-risk individuals who are exposed to COVID-19. 

Coming soon?

There shouldn't be a long wait for Pfizer's next potential COVID-19 blockbuster. Assuming the phase 2/3 testing goes well, the company thinks that it will be able to file for U.S. EUA in the fourth quarter of this year.

Pfizer CEO Albert Bourla said in the company's Q2 call that he's given the green light to manufacture "significant quantities" of the oral protease inhibitor so that large volumes of doses will be available if EUA is granted. He added that Pfizer is absorbing the risk of making this investment because "it is the right thing to do."

PF-07321332 probably won't be as big a catalyst for the big pharma stock as the Pfizer-BioNTech COVID-19 vaccine. However, Pfizer won't have to split the profits on the oral therapy as it does with BNT162b2. If the company's late-stage testing of the COVID-19 drug is successful, Pfizer seems very likely to have another blockbuster on its hands in 2022.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.