Merck antiviral drug molnupiravir received emergency authorization, joining Pfizer’s Paxlovid as the only authorized oral antiviral drugs for treating Covid-19. Though the Merck and Pfizer antivirals appear to work against the omicron variant, FDA officials stressed that these drugs are authorized only for certain patients and they are not a substitute for vaccination.
Merck’s oral antiviral drug molnupiravir now has FDA emergency use authorization for treating mild-to-moderate Covid-19, a Thursday decision that comes one day after the regulator authorized use of a pill from Pfizer. The FDA actions come as the highly infectious omicron variant becomes the dominant strain of the novel coronavirus, fueling a rise in Covid-19 cases that is pushing hospitals to capacity across the country.
The Merck and Pfizer antivirals both work by interfering with the virus’s ability to replicate, though in different ways. Molnupiravir introduces errors into the genetic code of SARS-CoV-2, while Paxlovid targets a key viral enzyme called main protease. Patrizia Cavazzoni, director of the FDA’s Center for Drug Evaluation and Research, said both drugs should work against omicron.
“The available data that we have indicate that both Paxlovid and molnupiravir are effective against omicron,” Cavazzoni said, speaking during a Thursday morning media briefing. “Both drugs interfere with aspects of the virus’s replication apparatus, and that apparatus is preserved across variants.”
Molnupiravir’s authorization, which comes three weeks after an FDA advisory committee meeting for the drug, covers its use in adults diagnosed with Covid-19 who are at a high risk of their disease progressing to hospitalization or death. The Merck drug is to be used when other treatment options are not accessible or appropriate. It can only be prescribed to those 18 and older because the drug can affect bone and cartilage growth.
Authorized use of Pfizer’s antiviral, Paxlovid, extends to pediatric patients. The FDA’s guidelines state the drug may be used for treating Covid-19 patients 12 and older weighing at least 40 kg (about 88 pounds). The FDA did not convene an advisory meeting for the Pfizer drug. John Farley, director of FDA’s Office of Infectious Diseases, said there were no pressing scientific questions about Paxlovid that would benefit from an advisory committee discussion.
Both molunupiravir and Paxlovid are available only by prescription. The FDA said treatment with these drugs should begin as soon as possible after a positive Covid-19 diagnosis, and within five days of the start of symptoms. These drugs aren’t for patients who are already hospitalized. Earlier this year, Merck stopped testing molnupiravir in those who are hospitalized after an early look at Phase 2 trial data indicated that the drug was unlikely to help these patients.
Authorization of Merck’s antiviral is based on a placebo-controlled clinical trial enrolling non-hospitalized adults with Covid-19 who were at high risk of progressing to severe disease or hospitalization. These higher risk patients include those who have a chronic medical condition or those who had not or could not receive a Covid-19 vaccine. The main goal was to measure the percentage of people hospitalized or dead from any cause during the 29 days after the course of treatment. The FDA said that of the 709 people in the study who received molnupiravir, 6.8% were hospitalized or died. By comparison, 9.7% of the 699 people given a placebo were hospitalized or died. During the follow-up period, one patient treated with molnupiravir died compared to nine of those given a placebo. Side effects reported include diarrhea, nausea, and dizziness.
In the clinical trial for Pfizer’s antiviral, Paxlovid led to an 88% reduction in Covid-19-related hospitalization or death from any cause compared to placebo. Of the 1,039 patients treated with Paxlovid, 0.8% of patients were hospitalized or died during the 28-day follow-up period, compared to 6% of those given a placebo.
Farley said that patients should consult with their physician to determine the best treatment. While molnupiravir is indicated for those who don’t have other treatment options, there are some groups of patients in which Paxlovid would not be appropriate. Examples include patients taking other medications that could interact with the Pfizer drug, Farley said. Paxlovid is also not recommended for patients who have severe kidney problems or cirrhosis of the liver. The key feature of both drugs is that they are oral, which enables patients to take these medication at home. Authorized antibody therapies from Regeneron, Eli Lilly, partners Vir Biotechnology and GlaxoSmithKline, and Roche, are infusions that must be administered in a clinical setting.
Patients take Merck’s molnupiravir as four pills, twice a day. Patients prescribed Pfizer’s Paxlovid must take more pills. The drug is taken with ritonavir, a drug that slows the breakdown of Paxlovid and helps it remain in the body for a longer period of time. Dosing of the Pfizer drug requires two tablets of Paxlovid and one of ritonavir, twice daily. The duration of treatment for both the Merck and Pfizer antivirals is five days.