For the second time, a study testing an experimental antibody drug for COVID-19 has been paused to investigate a possible safety issue in hospitalized patients.

Regeneron Pharmaceuticals Inc. said Friday that independent monitors had recommended placing on hold enrollment of the most severely ill patients — those who need intense oxygen treatment or breathing machines — because of a potential safety problem and unfavorable balance of risks and benefits.

The study can continue to test the two-antibody drug combo in hospitalized patients who need little or no extra oxygen, the monitors said. Other studies in mild or moderately ill people also are continuing.

Antibodies are proteins the body makes when an infection occurs; they attach to a virus and help it be eliminated. But it can take several weeks for the most effective ones to form. The experimental drugs aim to help immediately, by supplying concentrated versions of one or two antibodies that worked best against the coronavirus in lab and animal tests.

Earlier this month, a different group of monitors recommended pausing enrollment in a U.S. National Institutes of Health study testing an Eli Lilly antibody drug to investigate a possible safety issue in hospitalized patients. On Monday, the NIH said no safety problem had been verified, but they stopped the study because the drug didn’t seem to work in that situation.

“These kinds of results are informing us about the timing of the benefit,” said Dr. Myron Cohen, a University of North Carolina virologist who advises the government on COVID-19 treatments.

Tests in animals suggest that antibody drugs work best when given early in infection to lower the amount of virus, he said. Once someone is very sick, the drugs may not help, but it’s too soon to know if that's the case, he said.

Doctors already know that timing can matter when it comes to COVID-19 treatments. Studies suggest that dexamethasone and other steroids can lower the risk of death when given to very sick patients to tamp down an over-active immune system, but they may be harmful for those who are only mildly ill.

Lilly and Regeneron have asked the Food and Drug Administration to allow emergency use of their experimental antibody drugs for mild and moderately ill patients who don’t need hospitalization. President Donald Trump got the Regeneron drug when he was sickened earlier this month.

Regeneron said it would share Friday's advice from independent monitors with the FDA and leaders of a separate study in the United Kingdom testing its drug in hospitalized patients.

By: Marilynn Marchione, AP Chief Medical Writer