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Why do researchers think chloroquine or hydroxychloroquine may prevent coronavirus infections?

Both are antimalarial drugs that also have well-known antiviral activity against many viruses, including SARS and MERS. At least they work against those viruses in lab dishes.

In lab tests, hydroxychloroquine can also stop SARS-CoV-2, the coronavirus that causes COVID-19, from infecting cells and decreases replication of viruses that do get inside cells, researchers report March 18 in Cell Reports. A February 4 report in Cell Research found that chloroquine also inhibits the virus.

The drugs are thought to block viruses from entering cells by changing the pH, or acidity, of cellular compartments called lysosomes. That “creates a less friendly environment for the virus, so it might be more difficult for the virus to get into human cells in the first place,” says Michael Avidan, an anesthesiologist at Washington University School of Medicine in St. Louis. Avidan is involved in a clinical trial testing whether chloroquine can protect healthcare workers from infection or from developing serious disease.

In addition, hydroxychloroquine and chloroquine disrupt interactions between some of SARS-CoV-2’s proteins with proteins called sigma receptors in human cells, researchers report April 30 in Nature. Interrupting those protein interactions may make it difficult for the virus to replicate, says study coauthor Adolfo Garcia-Sastre, a microbiologist who directs the Global Health and Emerging Pathogens Institute of Icahn School of Medicine at Mount Sinai in New York City.

Together, those antiviral capabilities make the drugs attractive for use against the coronavirus. But there’s another important reason chloroquine and hydroxychloroquine were some of the first drugs pressed into action: They’re available. Doctors have been prescribing the drugs, already approved by the U.S. Food and Drug Administration, for decades and they’re generally safe, although there are some serious side effects.