One of the experimental drugs that President Trump received while he was battling the coronavirus has been approved for emergency use by the Food and Drug Administration. The drug, made by the biotech company Regeneron, is the second antibody treatment to win emergency use approval from the FDA.

The treatment combines two antibodies — casirivimab and imdevimab — and administers them together by IV. In a clinical trial of about 800 people, the combination was shown to significantly reduce virus levels within days of treatment.

In its authorization on Saturday, the FDA made clear that the drug is only for the treatment of mild to moderate COVID-19 in people 12 years and older who are at high risk of developing more severe symptoms. It's not for patients who are hospitalized because of COVID-19 or who require oxygen therapy because of the virus.

"The emergency authorization of these monoclonal antibodies administered together offers health care providers another tool in combating the pandemic," said Dr. Patrizia Cavazzoni, acting director of the FDA's Center for Drug Evaluation and Research. "We will continue to facilitate the development, evaluation and availability of COVID-19 therapies."

Both Regeneron's drug and another approved treatment, made by Eli Lilly, are synthetic versions of human antibodies that mimic the immune system's ability to fight off harmful pathogens. They bind to the coronavirus and prevent it from invading cells.

The White House celebrated the "promising results" and noted that the administration has spent close to half a billion dollars to support large-scale manufacturing of Regeneron's antibody treatment for delivery to U.S. hospitals.

"Due to advancements in quality care, information for medical personnel to better treat patients, and life-saving vaccines rapidly advancing toward approval faster than ever before, the United States has never been more prepared to confront the coronavirus and save millions of lives as we are now," White House spokesman Michael Bars said in a statement.

Regeneron's drug, called REGEN-COV2, "is designed to mimic what a well-functioning immune system does by using very potent antibodies to neutralize the virus," Regeneron's chief scientific officer, Dr. George Yancopoulos, said in a statement. "We are encouraged that no variants resistant to the cocktail were identified in the clinical trial analyses to date."

Both Eli Lilly and Regeneron applied for FDA authorization in October. Eli Lilly's product got the green light first, and its first batch has been distributed to hospitals around the United States. Regeneron's product will now be available to boost the supply, and the company estimates that 300,000 doses will be given to patients at no cost, though together there still won't be nearly enough to serve all the people who could qualify for them.

Neither drug has yet gotten full FDA approval, which entails much more rigorous reviews that take longer to complete. Emergency use authorizations are granted during public health emergencies when a treatment could be effective.

On Thursday, the FDA granted emergency use authorization for baricitinib, which when used with remdesivir has shown to be effective in battling more severe cases of COVID-19 requiring hospitalization or a ventilator.

NPR's Richard Harris contributed to this report.

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