By Jonathan D. Rockoff

Regeneron Pharmaceuticals Inc. said Thursday it is beginning to test in humans a potential coronavirus drug, the latest therapy to enter clinical trials.

The company plans to evaluate whether the drug -- a combination of two antibodies -- can treat Covid-19 patients and possibly even prevent infections. If the drug proves to work safely, it could be available for emergency use as early as the fall, Regeneron Chief Scientific Officer George Yancopoulos said.

"It could really have a major impact to both prevent and treat disease in humans," Dr. Yancopoulos said in an interview.

Only one drug, the antiviral remdesivir from Gilead Sciences Inc., has proven to work against the virus, albeit modestly.

Researchers are seeking to develop more-powerful agents. Dr. Yancopoulos said antibody drugs could be more potent than an antiviral because they borrow from the body's own approach to defending against viruses.

Antibodies are the soldiers of the immune system, which deploys the proteins to fight off foreign invaders such as viruses.

The intent of using antibodies against the new coronavirus is to block it from entering a person's cells by binding to the spike proteins jutting from the surface of the virus that it uses to latch onto the cells. If the virus can't latch onto and penetrate the cells, other molecular defenses circulating in the body known as macrophages would have time to hunt down and destroy the virus.

By contrast, antiviral drugs are designed to attack the virus after it enters a cell.

Researchers are hopeful that an antibody drug can foil the new coronavirus because antibodies that patients developed naturally have shown the ability to neutralize the pathogen, Dr. Yancopoulos said.

"We're all learning that natural immunity using antibodies is working against this virus like it does against other viruses," Dr. Yancopoulos said.

The drug may even prevent Covid-19 because the antibodies injected would be capable of neutralizing the virus, though the protective effect might require regular dosing, perhaps every month, Dr. Yancopoulos said.

Earlier this month Eli Lilly & Co. said it and partner AbCellera Biologics Inc. began studying their antibody drug derived from the blood of an early U.S. Covid-19 survivor. Other drugmakers are also working on antibody coronavirus therapies.

Researchers at Regeneron, based in Tarrytown, N.Y., discovered the two antibodies that make up the company's drug after examining the proteins produced by genetically modified mice and by patients who had recovered from Covid-19.

The company hopes the combination, or cocktail, of two antibodies binding to different parts of the coronavirus will prove more effective than one alone. The company is betting that the virus won't be able to evade the cocktail as easily as it would if it developed resistance to one of the antibodies.

The journal Science plans to publish articles soon by Regeneron researchers discussing how they discovered their coronavirus drug and how their cocktail approach could thwart the virus, Dr. Yancopoulos said.

Regeneron designed the trial in consultation with the U.S. Food and Drug Administration so it could proceed rapidly should the company's drug produce positive results at each step along the way, Dr. Yancopoulos said.

Typically, drugs go through three stages of testing, each requiring its own separate study. Under the "adaptive" trial design Regeneron is using, the drug would be tested in a single study that progresses through all three phases provided it keeps showing it is safely clearing the virus, Dr. Yancopoulos said.

The placebo-controlled trial will start by examining the drug in about 50 subjects, Dr. Yancopoulos said. The subjects will either be in an early stage of infection -- not so serious as to require hospitalization, for instance -- or are hospitalized and need help breathing.

If the drug shows promise in infected patients, Regeneron plans to study the drug in people who aren't infected but are at high risk, such as health-care workers or a family member of an infected patient, to see if the injections could prevent infection, Dr. Yancopoulos said.

Write to Jonathan D. Rockoff at Jonathan.Rockoff@wsj.com