US President Donald Trump is receiving an experimental treatment of synthetic antibodies for Covid-19, his doctor said Friday, and is "fatigued but in good spirits."
Trump received a single dose of Regeneron's antibody cocktail, according to a letter issued by White House physician Sean Conley.
The treatment is undergoing clinical trials but hasn't received any form of regulatory approval.
"He's being evaluated by a team of experts, and together we'll be making recommendations to the President and First Lady in regards to next best steps," Conley said.
Trump -- who has repeatedly cast doubt on the seriousness of the pandemic -- first announced in an overnight tweet that he and First Lady Melania Trump, 50, had tested positive and were going into quarantine.
Earlier this week, Regeneron announced results from one of its early-stage trials which showed its drug, which is infused intravenously, reduced viral load and recovery time in non-hospitalized Covid-19 patients.
"We have begun discussing our findings with regulatory authorities while continuing our ongoing trial," said George Yancopoulos, the company's president and chief scientific officer on Tuesday.
The US biotech firm is concurrently running late-stage trials for hospitalized Covid-19 patients and for the drug's potential use as a prophylaxis.
Antibodies are infection-fighting proteins made by the immune system that can bind to particular structures on the surfaces of pathogens and prevent them from invading cells.
Vaccines work by teaching the body to make its own antibodies, while scientists are also testing ready-made antibodies from the blood of recovered patients, called convalescent plasma.
But it is not possible to make convalescent plasma a mass treatment.
Researchers can also comb through the antibodies produced by recovered patients and select the most effective out of thousands, and then manufacture it at scale.
Regeneron's experimental Covid-19 drug, called REGN-COV2, is a combination of two antibodies, referred to as a "cocktail."
The idea is it will have a better chance at working if the virus mutates in order to evade the blocking action of a single antibody.
Last year, a triple-antibody cocktail developed by Regeneron was shown to be effective against the Ebola virus.